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1.  Leaf area and light use efficiency patterns of Norway spruce under different thinning regimes and age classes 
Highlights
► Absorbed light was only marginally superior to predict volume increment than leaf area. ► Larger trees were consistently more efficient than smaller trees. ► Thinning increased the efficiency of an average tree.
Silviculture focuses on establishing forest stand conditions that improve the stand increment. Knowledge about the efficiency of an individual tree is essential to be able to establish stand structures that increase tree resource use efficiency and stand level production. Efficiency is often expressed as stem growth per unit leaf area (leaf area efficiency), or per unit of light absorbed (light use efficiency). We tested the hypotheses that: (1) volume increment relates more closely with crown light absorption than leaf area, since one unit of leaf area can receive different amounts of light due to competition with neighboring trees and self-shading, (2) dominant trees use light more efficiently than suppressed trees and (3) thinning increases the efficiency of light use by residual trees, partially accounting for commonly observed increases in post-thinning growth. We investigated eight even-aged Norway spruce (Picea abies (L.) Karst.) stands at Bärnkopf, Austria, spanning three age classes (mature, immature and pole-stage) and two thinning regimes (thinned and unthinned). Individual leaf area was calculated with allometric equations and absorbed photosynthetically active radiation was estimated for each tree using the three-dimensional crown model Maestra. Absorbed photosynthetically active radiation was only a slightly better predictor of volume increment than leaf area. Light use efficiency increased with increasing tree size in all stands, supporting the second hypothesis. At a given tree size, trees from the unthinned plots were more efficient, however, due to generally larger tree sizes in the thinned stands, an average tree from the thinned treatment was superior (not congruent in all plots, thus only partly supporting the third hypothesis).
doi:10.1016/j.foreco.2011.11.044
PMCID: PMC4268600  PMID: 25540477
Picea abies; APAR; Maestra
2.  Do individual-tree growth models correctly represent height:diameter ratios of Norway spruce and Scots pine? 
Forest Ecology and Management  2010;260(10):1735-1753.
Research highlights
▶ We examined four individual tree models in Europe: BWIN, Moses, Silva and Prognaus. ▶ We simulated growth of open-grown trees and on research plots for 15 or 30 years. ▶ Height:diameter ratios were correctly predicted by all four models. ▶ Height:diameter ratios were within the bounds of open grown trees and dense stands. ▶ They decreased with age and density; dominant trees had lower ratios than mean trees.
Height:diameter ratios are an important measure of stand stability. Because of the importance of height:diameter ratios for forest management, individual-tree growth models should correctly depict height:diameter ratios. In particular, (i) height:diameter ratios should not exceed that of very dense stands, (ii) height:diameter ratios should not fall below that of open-grown trees, (iii) height:diameter ratios should decrease with increasing spacing, (iv) height:diameter ratios for suppressed trees should be higher than ratios for dominant trees. We evaluated the prediction of height:diameter ratios by running four commonly used individual-tree growth models in central Europe: BWIN, Moses, Silva and Prognaus. They represent different subtypes of individual-tree growth models, namely models with and without an explicit growth potential and models that are either distance-dependent (spatial) or distance-independent (non-spatial). Note that none of these simulators predict height:diameter ratios directly. We began by building a generic simulator that contained the relevant equations for diameter increment, height increment, and crown size for each of the four simulators. The relevant measures of competition, site characteristics, and stand statistics were also coded. The advantage of this simulator was that it ensured that no additional constraint was being imposed on the growth equations, and that initial conditions were identical. We then simulated growth for a 15- and 30-year period for Austrian permanent research plots in Arnoldstein and in Litschau, which represent stands at different age-classes and densities. We also simulated growth of open-grown trees and compared the results to the literature. We found that the general pattern of height:diameter ratios was correctly predicted by all four individual-tree growth models, with height:diameter ratios above that of open-grown trees and below that of very dense stands. All models showed a decrease of height:diameter ratios with age and an increase with stand density. Also, the height:diameter ratios of dominant trees were always lower than that of mean trees. Although in some cases the observed and predicted height:diameter ratios matched well, there were cases where discrepancies between observed and predicted height:diameter ratios would be unacceptable for practical management predictions.
doi:10.1016/j.foreco.2010.07.055
PMCID: PMC2987550  PMID: 21151352
Stand stability; Height:diameter ratio; Individual-tree growth model; Model evaluation; Open-grown trees
3.  A functional–structural model for radiata pine (Pinus radiata) focusing on tree architecture and wood quality 
Annals of Botany  2011;108(6):1155-1178.
Backgrounds and Aims
Functional–structural models are interesting tools to relate environmental and management conditions with forest growth. Their three-dimensional images can reveal important characteristics of wood used for industrial products. Like virtual laboratories, they can be used to evaluate relationships among species, sites and management, and to support silvicultural design and decision processes. Our aim was to develop a functional–structural model for radiata pine (Pinus radiata) given its economic importance in many countries.
Methods
The plant model uses the L-system language. The structure of the model is based on operational units, which obey particular rules, and execute photosynthesis, respiration and morphogenesis, according to their particular characteristics. Plant allometry is adhered to so that harmonic growth and plant development are achieved. Environmental signals for morphogenesis are used. Dynamic turnover guides the normal evolution of the tree. Monthly steps allow for detailed information of wood characteristics. The model is independent of traditional forest inventory relationships and is conceived as a mechanistic model. For model parameterization, three databases which generated new information relating to P. radiata were analysed and incorporated.
Key Results
Simulations under different and contrasting environmental and management conditions were run and statistically tested. The model was validated against forest inventory data for the same sites and times and against true crown architectural data. The performance of the model for 6-year-old trees was encouraging. Total height, diameter and lengths of growth units were adequately estimated. Branch diameters were slightly overestimated. Wood density values were not satisfactory, but the cyclical pattern and increase of growth rings were reasonably well modelled.
Conclusions
The model was able to reproduce the development and growth of the species based on mechanistic formulations. It may be valuable in assessing stand behaviour under different environmental and management conditions, assisting in decision-making with regard to management, and as a research tool to formulate hypothesis regarding forest tree growth and development.
doi:10.1093/aob/mcr156
PMCID: PMC3189843  PMID: 21987452
Functional–structural plant model; wood quality; internodes; knots; wood density; growth ring; photosynthesis; respiration; allometry; plant architecture; carbon allocation; Pinus radiata
4.  Crown Plasticity and Competition for Canopy Space: A New Spatially Implicit Model Parameterized for 250 North American Tree Species 
PLoS ONE  2007;2(9):e870.
Background
Canopy structure, which can be defined as the sum of the sizes, shapes and relative placements of the tree crowns in a forest stand, is central to all aspects of forest ecology. But there is no accepted method for deriving canopy structure from the sizes, species and biomechanical properties of the individual trees in a stand. Any such method must capture the fact that trees are highly plastic in their growth, forming tessellating crown shapes that fill all or most of the canopy space.
Methodology/Principal Findings
We introduce a new, simple and rapidly-implemented model–the Ideal Tree Distribution, ITD–with tree form (height allometry and crown shape), growth plasticity, and space-filling, at its core. The ITD predicts the canopy status (in or out of canopy), crown depth, and total and exposed crown area of the trees in a stand, given their species, sizes and potential crown shapes. We use maximum likelihood methods, in conjunction with data from over 100,000 trees taken from forests across the coterminous US, to estimate ITD model parameters for 250 North American tree species. With only two free parameters per species–one aggregate parameter to describe crown shape, and one parameter to set the so-called depth bias–the model captures between-species patterns in average canopy status, crown radius, and crown depth, and within-species means of these metrics vs stem diameter. The model also predicts much of the variation in these metrics for a tree of a given species and size, resulting solely from deterministic responses to variation in stand structure.
Conclusions/Significance
This new model, with parameters for US tree species, opens up new possibilities for understanding and modeling forest dynamics at local and regional scales, and may provide a new way to interpret remote sensing data of forest canopies, including LIDAR and aerial photography.
doi:10.1371/journal.pone.0000870
PMCID: PMC1964803  PMID: 17849000
5.  Applicability of non-destructive substitutes for leaf area in different stands of Norway spruce (Picea abies L. Karst.) focusing on traditional forest crown measures 
Forest Ecology and Management  2010;260(9):1498-1506.
Research highlights
▶ Determination of individual tree leaf area usually is only possible destructively. ▶ Surrogates which can be assessed non-destructively are investigated. ▶ From about 150 trees leaf area is estimated by 3P-branch sampling. ▶ These estimates are best correlated with crown surface area. ▶ Equations to determine individual tree leaf area non-destructively are presented.
Since individual tree leaf area is an important measure for productivity as well as for site occupancy, it is of high interest in many studies about forest growth. The exact determination of leaf area is nearly impossible. Thus, a common way to get information about leaf area is to use substitutes. These substitutes are often variables which are collected in a destructive way which is not feasible for long term studies. Therefore, this study aimed at testing the applicability of using substitutes for leaf area which could be collected in a non-destructive way, namely crown surface area and crown projection area. In 8 stands of Norway spruce (Picea abies L. Karst.), divided into three age classes and two thinning treatments, a total of 156 trees were felled in order to test the relationship between leaf area and crown surface area and crown projection area, respectively. Individual tree leaf area of the felled sample trees was estimated by 3P-branch sampling with an accuracy of ±10%. Crown projection area and crown surface area were compared with other, more commonly used, but destructive predictors of leaf area, namely sapwood area at different heights on the bole. Our investigations confirmed findings of several studies that sapwood area is the most precise measure for leaf area because of the high correlation between sapwood area and the leaf area. But behind sapwood area at crown base and sapwood area at three tenth of the tree height the predictive ability of crown surface area was ranked third and even better than that of sapwood area at breast height (R2 = 0.656 compared with 0.600). Within the stands leaf area is proportional to crown surface area. Using the pooled data of all stands a mixed model approach showed that additionally to crown surface area dominant height and diameter at breast height (dbh) improved the leaf area estimates. Thus, taking dominant height and dbh into account, crown surface area can be recommended for estimating the leaf area of individual trees. The resulting model was in line with many other findings on the leaf area and leaf mass relationships with crown size. From the additional influence of dominant height and dbh in the leaf area model we conclude that the used crown model could be improved by estimating the position of the maximum crown width and the crown width at the base of the crown depending on these two variables.
doi:10.1016/j.foreco.2010.07.048
PMCID: PMC2954308  PMID: 21072126
Norway spruce; Leaf area; Crown projection area; Crown surface area; Age; Thinning treatment
6.  Tree Growth and Competition in a Betula platyphylla–Larix cajanderi Post-fire Forest in Central Kamchatka 
Annals of Botany  2004;94(3):333-343.
• Background and Aims Fire is the dominant disturbance in central Kamchatka boreal forests, yet patterns and mechanisms of stand recovery have not been investigated.
• Methods Measurements were made of 1433 stems ≥1·3 m height and annual radial increments of 225 randomly selected trees in a 0·4-ha plot of a 53-year-old fire-origin mixed-species stand to examine the spatio-temporal variation in establishment, growth, size inequality and the mode of competition among individual trees. Growth variations were related to tree size, age and local interference with neighbours.
• Key Results Betula platyphylla formed the main canopy following a fire in 1947, with Larix cajanderi and Pinus pumila progressively reinvading the lower tree and shrub stratum. Most B. platyphylla originated from sprouts in small patches (polycormons) during the first 15 post-fire years. Betula platyphylla had normal distributions of diameter and age classes, but negatively skewed height distribution, as expected from shade-intolerant, pioneer species. Larix cajanderi had fewer tall and many short individuals. The smaller and younger B. platyphylla grew disproportionately more in diameter than larger trees from 1950 to 1975, and hence stem size inequalities decreased. The reverse trend was observed from 1995 to 2000: larger trees grew more, indicating an increasing asymmetry of competition for light. Betula platyphylla had steady diameter growth in the first 25 post-fire years, after which the growth declined in smaller trees. Neighbourhood analysis showed that the decline resulted from increased competition from taller neighbours.
• Conclusions The observed growth patterns suggest that mode of interactions altered during stand development from early stages of weak competition for soil resources released by fire to later stages of asymmetric competition for light. Asymmetric crown competition started later than reported in other studies, which can be attributed to the lower stem density leaving much space for individual growth, greater relative importance of below-ground competition in this site of nutrient-poor volcanic soil, and the vegetative origin of B. platyphylla. Larix cajanderi growing under B. platyphylla had steady diameter growth during the first 20 years, after which growth declined. It is suggested that early succession fits the tolerance model of succession, while inhibition dominates in later stages.
doi:10.1093/aob/mch149
PMCID: PMC4242174  PMID: 15256381
Size-dependent growth; individual-based spatial competition model; Ripley's K-function; stem size variability; competitive asymmetry; Richards model; stem allometry
7.  Effects of Dwarf Mistletoe on Stand Structure of Lodgepole Pine Forests 21-28 Years Post-Mountain Pine Beetle Epidemic in Central Oregon 
PLoS ONE  2014;9(9):e107532.
Lodgepole pine (Pinus contorta) forests are widely distributed throughout North America and are subject to mountain pine beetle (Dendroctonus ponderosae) epidemics, which have caused mortality over millions of hectares of mature trees in recent decades. Mountain pine beetle is known to influence stand structure, and has the ability to impact many forest processes. Dwarf mistletoe (Arceuthobium americanum) also influences stand structure and occurs frequently in post-mountain pine beetle epidemic lodgepole pine forests. Few studies have incorporated both disturbances simultaneously although they co-occur frequently on the landscape. The aim of this study is to investigate the stand structure of lodgepole pine forests 21–28 years after a mountain pine beetle epidemic with varying levels of dwarf mistletoe infection in the Deschutes National Forest in central Oregon. We compared stand density, stand basal area, canopy volume, proportion of the stand in dominant/codominant, intermediate, and suppressed cohorts, average height and average diameter of each cohort, across the range of dwarf mistletoe ratings to address differences in stand structure. We found strong evidence of a decrease in canopy volume, suppressed cohort height, and dominant/codominant cohort diameter with increasing stand-level dwarf mistletoe rating. There was strong evidence that as dwarf mistletoe rating increases, proportion of the stand in the dominant/codominant cohort decreases while proportion of the stand in the suppressed cohort increases. Structural differences associated with variable dwarf mistletoe severity create heterogeneity in this forest type and may have a significant influence on stand productivity and the resistance and resilience of these stands to future biotic and abiotic disturbances. Our findings show that it is imperative to incorporate dwarf mistletoe when studying stand productivity and ecosystem recovery processes in lodgepole pine forests because of its potential to influence stand structure.
doi:10.1371/journal.pone.0107532
PMCID: PMC4164639  PMID: 25221963
8.  Accounting for density reduction and structural loss in standing dead trees: Implications for forest biomass and carbon stock estimates in the United States 
Background
Standing dead trees are one component of forest ecosystem dead wood carbon (C) pools, whose national stock is estimated by the U.S. as required by the United Nations Framework Convention on Climate Change. Historically, standing dead tree C has been estimated as a function of live tree growing stock volume in the U.S.'s National Greenhouse Gas Inventory. Initiated in 1998, the USDA Forest Service's Forest Inventory and Analysis program (responsible for compiling the Nation's forest C estimates) began consistent nationwide sampling of standing dead trees, which may now supplant previous purely model-based approaches to standing dead biomass and C stock estimation. A substantial hurdle to estimating standing dead tree biomass and C attributes is that traditional estimation procedures are based on merchantability paradigms that may not reflect density reductions or structural loss due to decomposition common in standing dead trees. The goal of this study was to incorporate standing dead tree adjustments into the current estimation procedures and assess how biomass and C stocks change at multiple spatial scales.
Results
Accounting for decay and structural loss in standing dead trees significantly decreased tree- and plot-level C stock estimates (and subsequent C stocks) by decay class and tree component. At a regional scale, incorporating adjustment factors decreased standing dead quaking aspen biomass estimates by almost 50 percent in the Lake States and Douglas-fir estimates by more than 36 percent in the Pacific Northwest.
Conclusions
Substantial overestimates of standing dead tree biomass and C stocks occur when one does not account for density reductions or structural loss. Forest inventory estimation procedures that are descended from merchantability standards may need to be revised toward a more holistic approach to determining standing dead tree biomass and C attributes (i.e., attributes of tree biomass outside of sawlog portions). Incorporating density reductions and structural loss adjustments reduces uncertainty associated with standing dead tree biomass and C while improving consistency with field methods and documentation.
doi:10.1186/1750-0680-6-14
PMCID: PMC3283479  PMID: 22115425
carbon accounting; forest inventory; greenhouse gas; dead wood; snag; standing dead
9.  Effects of soil chemistry on tropical forest biomass and productivity at different elevations in the equatorial Andes 
Oecologia  2012;170(1):263-274.
The dependence of aboveground biomass and productivity of tropical forests on soil fertility is not fully understood, since previous studies yielded contrasting results. Here, we quantify aboveground biomass (AGB) and stem wood production, and examine the impact of soil chemistry on these parameters in mature tropical forest stands of the equatorial Andes in Ecuador. In 80 plots of 0.04 ha at four elevation levels (500, 1,000, 1,500 and 2,000 m a.s.l., total sample area = 3.2 ha), we measured ten important soil chemical parameters, inventoried all trees ≥10 cm dbh and monitored stem diameter growth with dendrometer tapes in 32 plots. Top canopy height and stem density significantly decreased from 500 to 2,000 m, while tree basal area increased and AGB remained invariant (344 ± 17 Mg DM ha−1, mean ± SE) with elevation. Wood specific gravity (WSG) showed a significant, but small, decrease. Stem wood production decreased from 4.5 to 3.2 Mg DM ha−1 year−1 along the transect, indicating a higher biomass turnover at lower elevations. The only soil variable that covaried with AGB was exchangeable K in the topsoil. WSG increased with decreases in N mineralisation rate, soil pH and extractable Ca and P concentrations. Structural equation modelling (SEM) revealed that nitrogen availability acts on stem wood production only indirectly through a negative relation between N mineralisation rate and WSG, and a positive effect of a lowered WSG on stem growth. The SEM analysis showed neither direct nor indirect effects of resin-extractable P on wood production, but a negative P influence on AGB. We conclude that nitrogen availability significantly influences productivity in these Andean forests, but both N and P are affecting wood production mainly indirectly through alterations in WSG and stem density; the growth-promoting effect of N is apparently larger than that of P.
Electronic supplementary material
The online version of this article (doi:10.1007/s00442-012-2295-y) contains supplementary material, which is available to authorized users.
doi:10.1007/s00442-012-2295-y
PMCID: PMC3422456  PMID: 22410639
Aboveground biomass; Ecuador; Soil nutrients; Tree growth; Wood production
10.  Multi-decade biomass dynamics in an old-growth hemlock-northern hardwood forest, Michigan, USA 
PeerJ  2014;2:e598.
Trends in living aboveground biomass and inputs to the pool of coarse woody debris (CWD) in an undisturbed, old-growth hemlock-northern hardwood forest in northern MI were estimated from multi-decade observations of permanent plots. Growth and demographic data from seven plot censuses over 47 years (1962–2009), combined with one-time measurement of CWD pools, help assess biomass/carbon status of this landscape. Are trends consistent with traditional notions of late-successional forests as equilibrial ecosystems? Specifically, do biomass pools and CWD inputs show consistent long-term trends and relationships, and can living and dead biomass pools and trends be related to forest composition and history? Aboveground living biomass densities, estimated using standard allometric relationships, range from 360–450 Mg/ha among sampled stands and types; these values are among the highest recorded for northeastern North American forests. Biomass densities showed significant decade-scale variation, but no consistent trends over the full study period (one stand, originating following an 1830 fire, showed an aggrading trend during the first 25 years of the study). Even though total above-ground biomass pools are neither increasing nor decreasing, they have been increasingly dominated, over the full study period, by very large (>70 cm dbh) stems and by the most shade-tolerant species (Acer saccharum and Tsuga canadensis).
CWD pools measured in 2007 averaged 151 m3/ha, with highest values in Acer-dominated stands. Snag densities averaged 27/ha, but varied nearly ten-fold with canopy composition (highest in Tsuga-dominated stands, lowest in Acer-dominated); snags constituted 10–50% of CWD biomass. Annualized CWD inputs from tree mortality over the full study period averaged 1.9–3.2 Mg/ha/yr, depending on stand and species composition. CWD input rates tended to increase over the course of the study. Input rates may be expected to increase over longer-term observations because, (a) living biomass is increasingly dominated by very large trees whose dead trunks have longer residence time in the CWD pool, and (b) infrequent major disturbances, thought to be important in the dynamics of these forests, have not occurred during the study period but would be expected to produce major, episodic pulses in CWD input.
Few fragments of old-growth cool-temperate forests remain, but such forests can constitute a very large carbon pool on a per-area basis. The carbon sink/source status of these forests remains unclear. While aboveground living biomass at this study site shows no strong aggrading or declining trend over the last half-century, this remains a modest span in the innate time-scale of late-successional forest. The effects of rare disturbances, long-term shifts in composition and size structure, and changes in soil carbon and CWD pools may all influence long-term carbon status.
doi:10.7717/peerj.598
PMCID: PMC4183956  PMID: 25289184
Long-term studies; Old-growth forest; Forest carbon pools; Northern hardwood forest; Temperate forest
11.  Impact of Future Climate on Radial Growth of Four Major Boreal Tree Species in the Eastern Canadian Boreal Forest 
PLoS ONE  2013;8(2):e56758.
Immediate phenotypic variation and the lagged effect of evolutionary adaptation to climate change appear to be two key processes in tree responses to climate warming. This study examines these components in two types of growth models for predicting the 2010–2099 diameter growth change of four major boreal species Betula papyrifera, Pinus banksiana, Picea mariana, and Populus tremuloides along a broad latitudinal gradient in eastern Canada under future climate projections. Climate-growth response models for 34 stands over nine latitudes were calibrated and cross-validated. An adaptive response model (A-model), in which the climate-growth relationship varies over time, and a fixed response model (F-model), in which the relationship is constant over time, were constructed to predict future growth. For the former, we examined how future growth of stands in northern latitudes could be forecasted using growth-climate equations derived from stands currently growing in southern latitudes assuming that current climate in southern locations provide an analogue for future conditions in the north. For the latter, we tested if future growth of stands would be maximally predicted using the growth-climate equation obtained from the given local stand assuming a lagged response to climate due to genetic constraints. Both models predicted a large growth increase in northern stands due to more benign temperatures, whereas there was a minimal growth change in southern stands due to potentially warm-temperature induced drought-stress. The A-model demonstrates a changing environment whereas the F-model highlights a constant growth response to future warming. As time elapses we can predict a gradual transition between a response to climate associated with the current conditions (F-model) to a more adapted response to future climate (A-model). Our modeling approach provides a template to predict tree growth response to climate warming at mid-high latitudes of the Northern Hemisphere.
doi:10.1371/journal.pone.0056758
PMCID: PMC3585260  PMID: 23468879
12.  Tree thinning as an option to increase herbaceous yield of an encroached semi-arid savanna in South Africa 
BMC Ecology  2005;5:4.
Background
The investigation was conducted in a savanna area covered by what was considered an undesirably dense stand of Colophospermum mopane trees, mainly because such a dense stand of trees often results in the suppression of herbaceous plants. The objectives of this study were to determine the influence of intensity of tree thinning on the dry matter yield of herbaceous plants (notably grasses) and to investigate differences in herbaceous species composition between defined subhabitats (under tree canopies, between tree canopies and where trees have been removed). Seven plots (65 × 180 m) were subjected to different intensities of tree thinning, ranging from a totally cleared plot (0 %) to plots thinned to the equivalent of 10 %, 20%, 35 %, 50% and 75 % of the leaf biomass of a control plot (100 %) with a tree density of 2711 plants ha-1. The establishment of herbaceous plants (grasses and forbs) in response to reduced competition from the woody plants was measured during three full growing seasons following the thinning treatments.
Results
The grass component reacted positively to the tree thinning in terms of total dry matter (DM) yield, but forbs were negatively influenced. Rainfall interacted with tree density and the differences between grass DM yields in thinned plots during years of below average rainfall were substantially higher than those of the control. At high tree densities, yields differed little between seasons of varying rainfall. The relation between grass DM yield and tree biomass was curvilinear, best described by the exponential regression equation. Subhabitat differentiation by C. mopane trees did provide some qualitative benefits, with certain desirable grass species showing a preference for the subhabitat under tree canopies.
Conclusion
While it can be concluded from this study that high tree densities suppress herbaceous production, the decision to clear/thin the C. mopane trees should include additional considerations. Thinning of C. mopane with the exclusive objective of increasing productivity of the grass layer would thus invariably involve a compromise situation where some trees should be left for the sake of the qualitative benefits on the herbaceous layer, soil enrichment, provision of browse and stability of the ecosystem.
doi:10.1186/1472-6785-5-4
PMCID: PMC1164409  PMID: 15921528
13.  Cambial activity related to tree size in a mature silver-fir plantation 
Annals of Botany  2011;108(3):429-438.
Background and Aims
Our knowledge about the influences of environmental factors on tree growth is principally based on the study of dominant trees. However, tree social status may influence intra-annual dynamics of growth, leading to differential responses to environmental conditions. The aim was to determine whether within-stand differences in stem diameters of trees belonging to different crown classes resulted from variations in the length of the growing period or in the rate of cell production.
Methods
Cambial activity was monitored weekly in 2006 for three crown classes in a 40-year-old silver-fir (Abies alba) plantation near Nancy (France). Timings, duration and rate of tracheid production were assessed from anatomical observations of the developing xylem.
Key Results
Cambial activity started earlier, stopped later and lasted longer in dominant trees than in intermediate and suppressed ones. The onset of cambial activity was estimated to have taken 3 weeks to spread to 90 % of the trees in the stand, while the cessation needed 6 weeks. Cambial activity was more intense in dominant trees than in intermediate and suppressed ones. It was estimated that about 75 % of tree-ring width variability was attributable to the rate of cell production and only 25 % to its duration. Moreover, growth duration was correlated to tree height, while growth rate was better correlated to crown area.
Conclusions
These results show that, in a closed conifer forest, stem diameter variations resulted principally from differences in the rate of xylem cell production rather than in its duration. Tree size interacts with environmental factors to control the timings, duration and rate of cambial activity through functional processes involving source–sink relationships principally, but also hormonal controls.
doi:10.1093/aob/mcr168
PMCID: PMC3158687  PMID: 21816842
Cambial activity; forest-stand structure; silver fir (Abies alba); tree-ring formation; tree-to-tree competition; social status; wood anatomy; xylem cell differentiation
14.  How the timberline formed: altitudinal changes in stand structure and dynamics around the timberline in central Japan 
Annals of Botany  2012;109(6):1165-1174.
Background and Aims
Altitudinal timberlines are thought to move upward by global warming, a crucial topic in ecology. Tall tree species (the conifer Abies mariesii and the deciduous broad-leaved Betula ermanii) dominate the sub-alpine zone between 1600 and 2500 m a.s.l., the timberline, on Mount Norikura in central Japan. Dwarf pine Pinus pumila dominates above the timberline to near the summit (3026 m a.s.l.). This study evaluated how the timberline formed on Mount Norikura by examining altitudinal changes in stand structure and dynamics around the timberline.
Methods
One hundred and twenty-five plots of 10 m × 10 m were established around the timberline (2350–2600 m a.s.l.). Trunk diameter growth rate during 6 years was examined for A. mariesii, B. ermanii and P. pumila. Mortality during this period and mechanical damage scars on the trunks and branches due to strong wind and snow were examined for A. mariesii only.
Key Results
The density, maximum trunk height and diameter of A. mariesii in plots decreased with altitude. The maximum trunk height of B. ermanii decreased with altitude, but density and maximum trunk diameter did not decrease. In contrast, the density of P. pumila abruptly increased from around the timberline. A strong negative correlation was found between the densities of P. pumila and tall tree species, indicating their interspecific competition. Trunk diameter growth rates of A. mariesii and B. ermanii did not decrease with altitude, suggesting that these two tall tree species can grow at the timberline. The ratio of trees with mechanical damage scars increased with altitude for A. mariesii, a tendency more conspicuous for larger trees. The mortality of larger A. mariesii was also greater at higher altitude. Tall tree species may not increase their trunk height and survive around the timberline because of mechanical damage.
Conclusions
This study suggests that the altitudinal location of the timberline is mainly affected by mechanical damage due to strong wind and snow rather than by growth limitation due to low temperature. Therefore, the timberline would not move upward even under global warming if these growth and mortality characteristics do not change for a long time.
doi:10.1093/aob/mcs043
PMCID: PMC3336954  PMID: 22451598
Abies mariesii; Betula ermanii; climate change; disturbance; global warming; Pinus pumila; timberline
15.  Structure and Function of Shisham Forests in Central Himalaya, India: Dry Matter Dynamics 
Annals of Botany  2002;89(1):41-54.
The biomass and net primary productivity (NPP) of 5‐ to 15‐year‐old Shisham (Dalbergia sissoo Roxb.) forests growing in central Himalaya were estimated. Allometric equations were developed for all above‐ and below‐ground components of trees and shrubs for each stand. Understorey forest floor biomass and litter fall were also estimated in forest stands. The biomass (dry matter), forest floor biomass (standing crop litter), tree litter fall and NPP of trees and shrubs increased with increasing age of the forest stand, whereas the dry matter and herb NPP decreased significantly (P < 0·001) with increasing age of the forest. Total forest biomass and NPP ranged from 58·7 (5‐year‐old stand) to 136·1 t ha–1 (15‐year‐old stand) and 12·6 (5‐year‐old stand) to 20·3 t ha–1 year–1 (15‐year‐old stand), respectively. Of these values, tree biomass accounted for 85·7 (5‐year‐old stand) to 90·1 % (15‐year‐old) of total forest biomass, and tree NPP for 72·2 (5‐year‐old) to 82·3 % (15‐year‐old) of total forest NPP. The biomass accumulation ratio (BAR) of the bole component (bole wood + bole bark) increased with increasing age of the forest stand. The bole BAR was 5·8 (5‐year‐old stand) to 7·9 (15‐year‐old stand). However, total BAR of the forest stand ranged from 5·5 (5‐year‐old) to 7·5 (15‐year‐old).
doi:10.1093/aob/mcf004
PMCID: PMC4233771  PMID: 12096818
Dalbergiasissoo Roxb; biomass; net primary productivity; litter input; forest floor biomass; turnover rate; Tarai belt; Central Himalaya; India
16.  Estimating Leaf Area Index in Southeast Alaska: A Comparison of Two Techniques 
PLoS ONE  2013;8(11):e77642.
The relationship between canopy structure and light transmission to the forest floor is of particular interest for studying the effects of succession, timber harvest, and silviculture prescriptions on understory plants and trees. Indirect measurements of leaf area index (LAI) estimated using gap fraction analysis with linear and hemispheric sensors have been commonly used to assess radiation interception by the canopy, although the two methods often yield inconsistent results. We compared simultaneously obtained measurements of LAI from a linear ceptometer and digital hemispheric photography in 21 forest stands on Prince of Wales Island, Alaska. We assessed the relationship between these estimates and allometric LAI based on tree diameter at breast height (LAIDBH). LAI values measured at 79 stations in thinned, un-thinned controls, old-growth and clearcut stands were highly correlated between the linear sensor (AccuPAR) and hemispheric photography, but the latter was more negatively biased compared to LAIDBH. In contrast, AccuPAR values were more similar to LAIDBH in all stands with basal area less than 30 m2ha−1. Values produced by integrating hemispheric photographs over the zenith angles 0–75° (Ring 5) were highly correlated with those integrated over the zenith angles 0–60° (Ring 4), although the discrepancies between the two measures were significant. On average, the AccuPAR estimates were 53% higher than those derived from Ring 5, with most of the differences in closed canopy stands (unthinned controls and old-growth) and less so in clearcuts. Following typical patterns of canopy closure, AccuPAR LAI values were higher in dense control stands than in old-growth, whereas the opposite was derived from Ring 5 analyses. Based on our results we advocate the preferential use of linear sensors where canopy openness is low, canopies are tall, and leaf distributions are clumped and angles are variable, as is common in the conifer forests of coastal Alaska.
doi:10.1371/journal.pone.0077642
PMCID: PMC3817193  PMID: 24223718
17.  Intra-annual variability of anatomical structure and δ13C values within tree rings of spruce and pine in alpine, temperate and boreal Europe 
Oecologia  2009;161(4):729-745.
Tree-ring width, wood density, anatomical structure and 13C/12C ratios expressed as δ13C-values of whole wood of Picea abies were investigated for trees growing in closed canopy forest stands. Samples were collected from the alpine Renon site in North Italy, the lowland Hainich site in Central Germany and the boreal Flakaliden site in North Sweden. In addition, Pinus cembra was studied at the alpine site and Pinus sylvestris at the boreal site. The density profiles of tree rings were measured using the DENDRO-2003 densitometer, δ13C was measured using high-resolution laser-ablation-combustion-gas chromatography-infra-red mass spectrometry and anatomical characteristics of tree rings (tracheid diameter, cell-wall thickness, cell-wall area and cell-lumen area) were measured using an image analyzer. Based on long-term statistics, climatic variables, such as temperature, precipitation, solar radiation and vapor pressure deficit, explained <20% of the variation in tree-ring width and wood density over consecutive years, while 29–58% of the variation in tree-ring width were explained by autocorrelation between tree rings. An intensive study of tree rings between 1999 and 2003 revealed that tree ring width and δ13C-values of whole wood were significantly correlated with length of the growing season, net radiation and vapor pressure deficit. The δ13C-values were not correlated with precipitation or temperature. A highly significant correlation was also found between δ13C of the early wood of one year and the late wood of the previous year, indicating a carry-over effect of the growing conditions of the previous season on current wood production. This latter effect may explain the high autocorrelation of long-term tree-ring statistics. The pattern, however, was complex, showing stepwise decreases as well as stepwise increases in the δ13C between late wood and early wood. The results are interpreted in the context of the biochemistry of wood formation and its linkage to storage products. It is clear that the relations between δ13C and tree-ring width and climate are multi-factorial in seasonal climates.
doi:10.1007/s00442-009-1421-y
PMCID: PMC2744769  PMID: 19653008
Dendrochonology; Carbohydrate storage; Picea abies; Pinus cembra; Pinus sylvestris; Tracheid lumen area; Wood density
18.  Relationships of tree height and diameter at breast height revisited: analyses of stem growth using 20-year data of an even-aged Chamaecyparis obtusa stand 
Tree Physiology  2013;33(1):106-118.
Stem diameter at breast height (DBH) and tree height (H) are commonly used measures of tree growth. We examined patterns of height growth and diameter growth along a stem using a 20-year record of an even-aged hinoki cypress (Chamaecyparis obtusa (Siebold & Zucc.) Endl.) stand. In the region of the stem below the crown (except for the butt swell), diameter growth rates (ΔD) at different heights tended to increase slightly from breast height upwards. This increasing trend was pronounced in suppressed trees, but not as much as the variation in ΔD among individual trees. Hence, ΔD below the crown can be regarded as generally being represented by the DBH growth rate (ΔDBH) of a tree. Accordingly, the growth rate of the stem cross-sectional area increased along the stem upwards in suppressed trees, but decreased in dominant trees. The stem diameter just below the crown base (DCB), the square of which is an index of the amount of leaves on a tree, was an important factor affecting ΔDBH. DCB also had a strong positive relationship with crown length. Hence, long-term changes in the DCB of a tree were associated with long-term changes in crown length, determined by the balance between the height growth rate (ΔH) and the rising rate of the crown base (ΔHCB). Within the crown, ΔD's were generally greater than the rates below the crown. Even dying trees (ΔD ≈ 0 below the crown) maintained ΔD > 0 within the crown and ΔH > 0 until about 5 years before death. This growth within the crown may be related to the need to produce new leaves to compensate for leaves lost owing to the longevity of the lower crown. These results explain the different time trajectories in DBH–H relationships among individual trees, and also the long-term changes in the DBH–H relationships. The view that a rise in the crown base is strongly related to leaf turnover helps to interpret DBH–H relationships.
doi:10.1093/treephys/tps127
PMCID: PMC3556985  PMID: 23303367
allometry; crown rise; linear mixed models; pipe model theory; stem form; stem taper
19.  Computing Competition for Light in the GREENLAB Model of Plant Growth: A Contribution to the Study of the Effects of Density on Resource Acquisition and Architectural Development 
Annals of Botany  2007;101(8):1207-1219.
Background and Aims
The dynamical system of plant growth GREENLAB was originally developed for individual plants, without explicitly taking into account interplant competition for light. Inspired by the competition models developed in the context of forest science for mono-specific stands, we propose to adapt the method of crown projection onto the x–y plane to GREENLAB, in order to study the effects of density on resource acquisition and on architectural development.
Methods
The empirical production equation of GREENLAB is extrapolated to stands by computing the exposed photosynthetic foliage area of each plant. The computation is based on the combination of Poisson models of leaf distribution for all the neighbouring plants whose crown projection surfaces overlap. To study the effects of density on architectural development, we link the proposed competition model to the model of interaction between functional growth and structural development introduced by Mathieu (2006, PhD Thesis, Ecole Centrale de Paris, France).
Key Results and Conclusions
The model is applied to mono-specific field crops and forest stands. For high-density crops at full cover, the model is shown to be equivalent to the classical equation of field crop production ( Howell and Musick, 1985, in Les besoins en eau des cultures; Paris: INRA Editions). However, our method is more accurate at the early stages of growth (before cover) or in the case of intermediate densities. It may potentially account for local effects, such as uneven spacing, variation in the time of plant emergence or variation in seed biomass. The application of the model to trees illustrates the expression of plant plasticity in response to competition for light. Density strongly impacts on tree architectural development through interactions with the source–sink balances during growth. The effects of density on tree height and radial growth that are commonly observed in real stands appear as emerging properties of the model.
doi:10.1093/aob/mcm272
PMCID: PMC2710279  PMID: 18037666
Functional–structural plant models; GREENLAB; competition for light; Beer–Lambert Law; plant plasticity; dynamical system
20.  Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots. 
Several widespread changes in the ecology of old-growth tropical forests have recently been documented for the late twentieth century, in particular an increase in stem turnover (pan-tropical), and an increase in above-ground biomass (neotropical). Whether these changes are synchronous and whether changes in growth are also occurring is not known. We analysed stand-level changes within 50 long-term monitoring plots from across South America spanning 1971-2002. We show that: (i) basal area (BA: sum of the cross-sectional areas of all trees in a plot) increased significantly over time (by 0.10 +/- 0.04 m2 ha(-1) yr(-1), mean +/- 95% CI); as did both (ii) stand-level BA growth rates (sum of the increments of BA of surviving trees and BA of new trees that recruited into a plot); and (iii) stand-level BA mortality rates (sum of the cross-sectional areas of all trees that died in a plot). Similar patterns were observed on a per-stem basis: (i) stem density (number of stems per hectare; 1 hectare is 10(4) m2) increased significantly over time (0.94 +/- 0.63 stems ha(-1) yr(-1)); as did both (ii) stem recruitment rates; and (iii) stem mortality rates. In relative terms, the pools of BA and stem density increased by 0.38 +/- 0.15% and 0.18 +/- 0.12% yr(-1), respectively. The fluxes into and out of these pools-stand-level BA growth, stand-level BA mortality, stem recruitment and stem mortality rates-increased, in relative terms, by an order of magnitude more. The gain terms (BA growth, stem recruitment) consistently exceeded the loss terms (BA loss, stem mortality) throughout the period, suggesting that whatever process is driving these changes was already acting before the plot network was established. Large long-term increases in stand-level BA growth and simultaneous increases in stand BA and stem density imply a continent-wide increase in resource availability which is increasing net primary productivity and altering forest dynamics. Continent-wide changes in incoming solar radiation, and increases in atmospheric concentrations of CO2 and air temperatures may have increased resource supply over recent decades, thus causing accelerated growth and increased dynamism across the world's largest tract of tropical forest.
PMCID: PMC1693337  PMID: 15212094
21.  Forests Regenerating after Clear-Cutting Function as Habitat for Bryophyte and Lichen Species of Conservation Concern 
PLoS ONE  2011;6(4):e18639.
The majority of managed forests in Fennoscandia are younger than 70 years old but yet little is known about their potential to host rare and threatened species. In this study, we examined red-listed bryophytes and lichens in 19 young stands originating from clear-cutting (30–70 years old) in the boreal region, finding 19 red-listed species (six bryophytes and 13 lichens). We used adjoining old stands, which most likely never had been clear-cut, as reference. The old stands contained significantly more species, but when taking the amount of biological legacies (i.e., remaining deciduous trees and dead wood) from the previous forest generation into account, bryophyte species number did not differ between old and young stands, and lichen number was even higher in young stands. No dispersal effect could be detected from the old to the young stands. The amount of wetlands in the surroundings was important for bryophytes, as was the area of old forest for both lichens and bryophytes. A cardinal position of young stands to the north of old stands was beneficial to red-listed bryophytes as well as lichens. We conclude that young forest plantations may function as habitat for red-listed species, but that this depends on presence of structures from the previous forest generation, and also on qualities in the surrounding landscape. Nevertheless, at repeated clear-cuttings, a successive decrease in species populations in young production stands is likely, due to increased fragmentation and reduced substrate amounts. Retention of dead wood and deciduous trees might be efficient conservation measures. Although priority needs to be given to preservation of remnant old-growth forests, we argue that young forests rich in biological legacies and located in landscapes with high amounts of old forests may have a conservation value.
doi:10.1371/journal.pone.0018639
PMCID: PMC3072405  PMID: 21490926
22.  Patterns and Drivers of Tree Mortality in Iberian Forests: Climatic Effects Are Modified by Competition 
PLoS ONE  2013;8(2):e56843.
Tree mortality is a key process underlying forest dynamics and community assembly. Understanding how tree mortality is driven by simultaneous drivers is needed to evaluate potential effects of climate change on forest composition. Using repeat-measure information from c. 400,000 trees from the Spanish Forest Inventory, we quantified the relative importance of tree size, competition, climate and edaphic conditions on tree mortality of 11 species, and explored the combined effect of climate and competition. Tree mortality was affected by all of these multiple drivers, especially tree size and asymmetric competition, and strong interactions between climate and competition were found. All species showed L-shaped mortality patterns (i.e. showed decreasing mortality with tree size), but pines were more sensitive to asymmetric competition than broadleaved species. Among climatic variables, the negative effect of temperature on tree mortality was much larger than the effect of precipitation. Moreover, the effect of climate (mean annual temperature and annual precipitation) on tree mortality was aggravated at high competition levels for all species, but especially for broadleaved species. The significant interaction between climate and competition on tree mortality indicated that global change in Mediterranean regions, causing hotter and drier conditions and denser stands, could lead to profound effects on forest structure and composition. Therefore, to evaluate the potential effects of climatic change on tree mortality, forest structure must be considered, since two systems of similar composition but different structure could radically differ in their response to climatic conditions.
doi:10.1371/journal.pone.0056843
PMCID: PMC3581527  PMID: 23451096
23.  Assessing the long-term species composition predicted by PrognAus 
Forest Ecology and Management  2010;259(3):614-623.
Tree growth models are supposed to contain stand growth laws as so called “emergent properties” which derive from interactions of individual-tree growth and mortality functions. This study investigates whether the evolving tree species composition in a long term simulation by the distance-independent tree growth model PrognAus matches the species composition of the potential natural vegetation type which is expected to occur if one refrains from further management interventions and major disturbances, climate change, and changes in site conditions can be excluded. For this purpose the development of 6933 sample plots of the Austrian National Forest Inventory was predicted for 2500 years. The resulting species proportions, derived from volume per hectare of 15 tree species or species groups, were used to classify every sample plot according to potential natural forest types, following a classification scheme based on expert knowledge. These simulated potential natural vegetation types were compared with expert reconstructions of the sample plots of the Austrian National Forest Inventory. A total of 5789 plots were actually classified with the scheme; in 33% of the cases the classification on the basis of the PrognAus-simulations was identical with the classification by the Austrian National Forest Inventory. A predominantly correct classification was achieved for the subalpine Picea abies-type and the Fagus sylvatica-type although PrognAus showed a tendency to overestimate the proportion of F. sylvatica and P. abies. Weaknesses in the ability to simulate forest types dominated by Quercus spp., Acer spp., and Pinus sylvestris were identified. This shortcoming might be caused by the mortality model which allows a larger diameter at breast height for F. sylvatica or by the ingrowth model whose terms for the consideration of inter-specific competition may lead to a disadvantage of Quercus spp., P. sylvestris, and Abies alba. Moreover, the ingrowth model might be influenced by management effects and the effect of selective browsing.
doi:10.1016/j.foreco.2009.11.020
PMCID: PMC2982682  PMID: 21151325
Individual-tree growth model; Species distribution; Potential natural vegetation type; Forest stand development
24.  Stimulating seedling growth in early stages of secondary forest succession: a modeling approach to guide tree liberation 
Excessive growth of non-woody plants and shrubs on degraded lands can strongly hamper tree growth and thus secondary forest succession. A common method to accelerate succession, called liberation, involves opening up the vegetation canopy around young target trees. This can increase growth of target trees by reducing competition for light with neighboring plants. However, liberation has not always had the desired effect, likely due to differences in light requirement between tree species. Here we present a 3D-model, which calculates photosynthetic rate of individual trees in a vegetation stand. It enables us to examine how stature, crown structure, and physiological traits of target trees and characteristics of the surrounding vegetation together determine effects of light on tree growth. The model was applied to a liberation experiment conducted with three pioneer species in a young secondary forest in Vietnam. Species responded differently to the treatment depending on their height, crown structure and their shade-tolerance level. Model simulations revealed practical thresholds over which the tree growth response is heavily influenced by the height and density of surrounding vegetation and gap radius. There were strong correlations between calculated photosynthetic rates and observed growth: the model was well able to predict growth of trees in young forests and the effects of liberation there upon. Thus, our model serves as a useful tool to analyze light competition between young trees and surrounding vegetation and may help assess the potential effect of tree liberation.
doi:10.3389/fpls.2014.00345
PMCID: PMC4102908  PMID: 25101100
forest restoration; gap creation; photosynthesis model; light competition; Vietnam
25.  Light and competition gradients fail to explain the coexistence of shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings 
Annals of Botany  2013;112(7):1421-1430.
Background and Aims
The coexistence of forest tree species has often been linked to differences among species in terms of their response to light availability during the regeneration stage. From this perspective, species coexistence results from growth–growth or mortality–growth trade-offs along spatial light gradients. Experimental evidence of growth–growth trade-offs in natural conditions is sparse due to various confounding factors that potentially hinder the relationship. This study examined growth hierarchies along light gradients between two tree species with contrasting shade tolerance by controlling potential confounding factors such as seedling size, seedling status, seedling density and species composition.
Methods
Natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings were used, and growth rankings over a 4-year period were compared in 8- to 10-year-old tree seedlings.
Key results
No rank reversal occurs between the two species along the light gradient, or along the density, mixture or seedling size gradients. The shade-tolerant species was always the more competitive of the two. Pronounced effects of initial size on seedling growth were observed, whereas the effects of light and competition by neighbours were of secondary importance. The paramount effect of size, which results from the asymmetric nature of interseedling competition, gives a strong advantage to tall seedlings over the long term.
Conclusions
This study extends previous efforts to identify potential drivers of rank reversals in young tree mixtures. It does not support the classical assumption that spatial heterogeneity in canopy opening explains the coexistence of the two species studied. It suggests that spatial variation in local size hierarchies among seedlings that may be caused by seedling emergence time or seedling initial performance is the main driver of the dynamics of these mixed stands.
doi:10.1093/aob/mct200
PMCID: PMC3806531  PMID: 24036670
Rank reversal; seedling growth; density; mixture; niche differentiation; forest dynamics; succession; competition gradient; plant–plant interactions; Fagus sylvatica; Quercus petraea

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