Here, we describe successional vegetation dynamics based on case studies in northeastern Costa Rica and Chiapas, Mexico. These studies encompass different land-use histories, different stem size classes and different temporal scales. As we illustrate below, some of the community-level patterns observed in these dynamics studies are remarkably consistent with chronosequence predictions. However, these vegetation dynamics studies also reveal idiosyncratic patterns driven by initial species composition, site factors, land-use history and landscape composition. These studies also provide insight into the mechanisms and drivers of successional change following the abandonment of agricultural lands.
(a) Case study: Sarapiquí region, Costa Rica
In wet lowlands of northeastern Costa Rica, Chazdon and collaborators have monitored vegetation dynamics annually for 8 years (1997–2004) in four 1
ha plots of secondary forest on abandoned pastures in the Caribbean lowlands (Capers et al. 2005
; Chazdon et al. 2005
), and Finegan and collaborators have monitored forest dynamics in four plots over 16 years (1987–2003); three plots are 1
ha and one plot (initially 1-year old) is 0.3
ha. These four plots studied by Finegan were not used for pasture, but were cleared and prepared for planting and then abandoned or were used for one cycle of cultivation. Although these sites had lighter land use than the sites studied by Chazdon, results from both the sets of plots are combined in some of the analyses presented here.
Lowland forests of the Sarapiquí region of northeastern Costa Rica are classified as tropical wet forest (sensu Holdridge et al. 1975
), receiving ca
mm of precipitation annually (Sanford et al. 1994
). The driest months, February–April, still receive more than 100
mm of rain in most years (but see Chazdon et al. 2005
). The average monthly temperature is 25.8°C, with little annual variation. The region is a patchwork of cattle pastures, agricultural areas (bananas, heart of palm and pineapple), residential areas, forest fragments and second-growth forests. Soil fertility varies throughout this region due to erosion of old and more recent lava flows, nutrient-enriched alluvium associated with flood zones and phosphorus-enriched zones associated with geothermal waters at La Selva Biological Station (Pringle 1991
; Sollins et al. 1994
). These factors, as well as variation in land-use history (logging versus pasture versus crops), affect species composition and regeneration in both secondary and mature forests of the region (Herrera & Finegan 1997
; Clark et al. 1999
; Finegan & Delgado 2000
In the time-series analyses shown here, we combine data for seven 1
ha plots and one 0.3
ha plot for basal area and stem density. Analysing the plots as a chronosequence across the initial age range from 1 to 25 years, the tree basal area in the first 25 years of regrowth increased linearly with time since abandonment (R2
). Resampling these plots over time revealed few deviations from the initial chronosequence projection (a
). These results, along with findings from many chronosequence studies, support the view that the basal area is a predictable emergent feature of regenerating forest communities on sites with low to intermediate intensities of agricultural use. At least for sites less than 40 years following abandonment, dominated by long-lived pioneers like Goethalsia meiantha
, Laetia procera
, Simarouba amara
and Vochysia ferruginea
, the basal area increases linearly with time. Basal area in secondary forests over 25 years is similar to or higher than values from mature forests of this area (Guariguata et al. 1997
). The only site where basal area decreased over time was the youngest site (a
); in this case, decreases in basal area from 6 to 9 years were due to the decline of the initially dominant pioneer species, Ochroma pyramidale
Figure 1 Chronosequence data for trees greater than or equal to 10cm DBH in eight secondary forest monitoring plots in northeastern Costa Rica (black dots) and successional trajectories within the same set of plots. (a) Basal area (m2ha−1 (more ...)
Tree density also showed a clear chronosequence trend, but the increase with stand age was nonlinear (b). Within sites, however, tree density showed highly variable dynamics, and did not conform well to chronosequence trends. The youngest site showed a rapid increase for the first 4 years, followed by a rapid decline in density, due to the dynamics of the dominant species, O. pyramidale. Density changes in the older sites varied widely over time; some plots showed no changes in tree density, whereas others showed decreases or increases (b).
The remarkably consistent patterns in basal area despite the inconsistent temporal patterns of density suggest that at the stand level, basal area increments are driven by overall tree growth increments rather than by changes in stem density, a pattern also observed in even-aged temperate forests (Peet & Christensen 1987
). Indeed, diameter growth rates in young forests are significantly higher than rates in older stands, even when comparing the same species and size classes (R. L. Chazdon 2006, unpublished work). In the Lindero Sur site (initially 12 years old), the per capita
basal area increment for trees (greater than or equal to 10
cm DBH) was more than six times higher than in the Cuatro Rios site (initially 25 years old). Accumulation of basal area and biomass is driven by growth rates of large trees and can proceed rapidly even if stem density is declining (as occurs in even-aged stands during self-thinning; White & Harper 1970
). Tree mortality in these secondary forests is concentrated in the smaller stem size classes (less than 10
cm; Chazdon et al. 2005
) and has relatively little impact on basal area.
Species density rises during forest regeneration in this chronosequence, although the relationship is not as consistent as the pattern observed for basal area (R2=0.47; c). Although there is a significant increase in species density with age since abandonment, the former-pasture sites showed little or no change in species density over time. In part, this stabilization of species density reflects species turnover within plots and replacement of early colonizing tree species with later recruiting species, with little or no net change in the number of species. These results show, however, that changes in species density over time cannot explain initial differences among plots in the chronosequence. Thus, chronosequence patterns in species density that appear to be based on plot age may actually reflect intrinsic differences in levels of tree diversity among plots that are predominantly unrelated to age.
Similar conclusions apply to comparisons of tree species richness greater than 10
cm DBH for four former-pasture sites studied by Chazdon et al. (2005)
, based on rarefaction analyses. In both 1997 and 2004, species richness was significantly higher in the oldest site (25 years, Cuatro Rios; 66 species/420 stems) compared with the youngest site (12 years, Lindero Sur; 43 species/420 stems; p
<0.05), but the two sites of intermediate age did not differ from the others (). Interpreting these trends as a chronosequence suggests that tree species richness increases slowly during succession compared with other forest structural characteristics. Within each plot, species richness (number of trees/420 stems) for trees greater than 10
cm DBH was also compared between 1997 and 2004. Tree species richness did not change significantly within any of these plots over the 7-year study period, in parallel with observations on species density. Thus, for species richness, plot-level changes did not conform to the overall chronosequence trend.
Figure 2 Sample-based rarefaction curves for trees greater than 10cm DBH in 1997 (initial census) for four second-growth forest 1ha plots in northeastern Costa Rica on former pasture. Site ages are given in parentheses next to each solid line. (more ...)
Species richness remained stable or increased despite substantial turnover rates for stems and species (a
). Younger forests (less than 15 years since abandonment) are certainly more dynamic in overall stem density and population sizes than older sites (Chazdon et al. 2005
), but this dynamism at the level of stems and species does not necessarily translate into significant changes in species richness. For the former-pasture sites, rates of stem gain were highest (31.6%) in the youngest site and decreased with forest age, whereas rates of stem loss remained relatively constant (13.5–16.5%) across sites (a
). Rates of species turnover were lower than rates of stem turnover in all the four former-pasture plots (a
). In the two younger plots (Lindero Sur and Tirimbina), rates of species gain exceeded rates of species loss, whereas the reverse was found for the two older plots (Lindero El Peje and Cuatro Rios; a
Figure 3 Percentage of stems greater than or equal to 10cm DBH lost through mortality and gained through recruitment and percentage of species lost and gained in secondary forest plots in northeastern Costa Rica from (a) 1997 to 2004 in four plots on (more ...)
A slightly different pattern of stem and species turnover was observed in the three 1
ha light-use sites studied by Finegan from 1989 to 2003, a 14-year period (b
). Over this extended time period, rates of stem gain were higher than rates of stem loss in two plots (initially 15 and 25 years old). Rates of species gain exceeded species loss in all the three plots; in plot 1 (initially 25 years old), species gains were substantial despite no net change in stem density. In plot 2 (initially 25 years old), rates of stem gain exceeded rates of species gain, because most species of new recruits were already present in the plot. Higher rates of stem loss than species loss in all the three plots indicate cohort decline in species that were initially abundant. Half of the total species lost from the three sites belonged to pioneer genera (Cecropia
) and the family Melastomataceae. Plot differences in stem and species turnover rates may also reflect variation in soil fertility and proximity to seed sources. We lack a clear understanding of how these factors interact with stand demography to bring about changes in species richness over time in tropical secondary forests. We do know, however, that these changes are slow and may take centuries (Finegan 1996
; Chazdon 2003
For trees greater than or equal to 10
cm DBH, the four former-pasture plots differed significantly in species composition in 2004 (p
<0.05; ). Similarity values (Chao Jaccard Abundance Estimator; Chao et al. 2005
) ranged from 0.44 to 0.80. The 15-year-old plot (Tirimbina), which has the longest history of disturbance and isolation, was the most divergent. The same pairwise similarities were also computed within each site between 1997 and 2004 to determine whether species composition changed significantly over time (). None of the four sites showed a significant change in species composition over time, however, suggesting that age differences are not the major factor contributing to differences in species composition. Each plot appears to follow an idiosyncratic pathway of species accumulation, likely driven by edaphic factors, land-use history and landscape matrix.
Table 1 Pairwise similarity for trees greater than 10cm DBH in four second-growth forest plots in Costa Rica in 2004, based on the Chao Jaccard Abundance Estimator ±95% confidence limits (Chao et al. 2005). (Plot age since abandonment is given (more ...)
Table 2 Pairwise similarity for trees greater than 1cm DBH in 10 second-growth forest plots in Chiapas, Mexico, based on the Chao Jaccard Abundance Estimator ±95% confidence limits (Chao et al. 2005) (Values along the diagonal are similarities (more ...)
(b) Case study: Chiapas, Mexico
The second case study took place in the Marquéz de Comillas region, Chiapas, Mexico, where Breugel, Martínez-Ramos, Bongers and collaborators have monitored secondary forest succession in ten 500
secondary forest plots since 2000. The climate of this region is cooler, drier and more seasonal than that of the Sarapiquí region. The average annual rainfall is ca
mm, with less than 100
mm per month falling in the dry season (February–April). The mean annual temperature is ca
24°C. The original vegetation consists mainly of lowland tropical rainforests and semi-deciduous forests (Ibarra-Manríquez & Martínez-Ramos 2002
). Today, the region is a mosaic of small-scale agriculture, pastures, mostly young (less than 10 years) secondary forests and remnants of old-growth forests. In the area, three morphological units are found, of which the Low Hills topographic unit with sandy and limestone-based soils with low pH (less than 5.5; Siebe et al. 1996
) is most common. Geomorphology and former land use, more specifically abandoned pastures versus abandoned cornfields, have been shown to affect successional patterns in this region (Méndez-Bahena 1999
). Sites were selected in Low Hills on former cornfields (milpas), with fallows ranging from 1.5 to 17 years post-abandonment and mostly with only one cycle of cultivation. In these plots, growth, mortality and recruitment of trees greater than or equal to 1
cm DBH are being monitored annually; here we present data for the first 3 years examined. The vegetation in these sites is expected to exhibit faster dynamics than the larger size classes and older sites described in the case study of northeastern Costa Rica.
Both among-plot comparisons and within-plot dynamics showed high variability for all the three parameters (basal area, stem density and species density; ). In contrast to the Sarapiquí study, fallow age was not a significant predictor of initial basal area when the plots were analysed as a chronosequence (a
). Initial basal area varied widely among plots with one of the highest values for the 1-year-old plot (due to extremely high density and fast growth of early recruits). Basal area increased in all but one plot by an average of 126% during the first 3 years of the study. The only exception to this trend was in one of the intermediate-aged plots (8 years at the beginning of the study), where, as observed in the youngest plot in the Sarapiquí study, massive mortality of the pioneer species, Ochroma pyramidale
, drove an overall decline in basal area (Breugel et al. 2006
). In every other case, there was an increase in basal area ranging from 33 to 304% in the 3-year period, resulting in several plots with basal area over 25
. These are surprisingly high values, higher than the average basal area in neighbouring old-growth forest plots on comparable sites (M. Martínez-Ramos et al
. 2006, unpublished work). We have to realize however that this does not directly translate into a relatively high biomass, mainly owing to lower plant stature and low average specific wood density of the dominant species in our plots (Balvanera et al. 2005
; M. van Breugel et al
. 2006, unpublished work).
Figure 4 Chronosequence data for trees greater than or equal to 1cm DBH in 10 secondary forest monitoring plots in Chiapas, Mexico (black dots) and successional trajectories within the same set of plots. (a) Basal area (m2ha−1); (b) stem (more ...)
Net changes in basal area sometimes obscured faster dynamics in the constituent processes: growth, mortality and recruitment. Breugel et al. (2006)
analysed rates of change of these parameters as a function of age, and all three declined significantly with stand age. However, diameter increment growth rates exceeded mortality and recruitment in all stands although the margin was widest in the youngest sites. In these sites, large losses in basal area from mortality were offset by larger gains from growth and recruitment.
Initial stem density was not related to stand age, and stem density within plots followed unpredictable trajectories (b
). Although density generally decreased within plots over the 3 years, rates of change varied widely among plots. The most dramatic decline in density was shown in the two plots with densities over 1000 trees per plot, suggesting strong density-dependent effects on mortality. Rates of change in the other plots were not related to initial density. An important factor contributing to variation in initial density and the subsequent changes might be variation in species composition, at least in young secondary forests. Variation in stem density of a few very dominant species such as Trema micrantha
, Trichospermum mexicanum
and Cecropia peltata
reflects to a large extent initial differences in stem density in most plots. Variation in rates of change in stem density is therefore largely explained by the interspecific differences in demographic rates of a few dominant species (Breugel et al. 2006
, Breugel et al. in preparation
Species density showed a nonlinear chronosequence trend across the initial age range (R2=0.40; c). However, this trend is rather dependent on which census is used, as there was no significant relationship between age and density for the last census. In most individual plots, species density increased dramatically, in most cases much faster than suggested by the chronosequence trend. Especially in the youngest plots (up to 3 years old), changes were very rapid and several plots had a higher species density than in the older plots at the end of the 3-year interval. These trends also applied to species richness. When stands were compared after rarefaction to account for effects of stem density, species richness did not differ significantly with age for any of the censuses (). Within plots, however, species richness (number of trees/276 stems) increased over the 3-year period on average by 81.6% (±18.4 s.e.) and this increase was significant in all but the oldest site.
Species richness in secondary forest plots in Chiapas, Mexico in year 3 of the study. Species richness was rarefied to the lowest stem density among plots and was not related to plot age for any of the 3 years. Bars give the 95% confidence intervals.
The fact that species richness increases significantly within these stands, but not within the Costa Rican abandoned pasture sites, may reflect the smaller diameter cut-off as well as the younger age of the sites. The probability that a recruit belongs to a new species may be higher in early succession simply because species density is lower, i.e. with succession an increasing proportion of the local species pool would already have arrived at the site. In the first years of succession, fast growing pioneer species still constitute a considerable fraction of new recruits, apparently because high mortality in the early phases of secondary forests opens up new recruitment possibilities (Breugel et al. 2006
; in preparation
). The lower diameter limit implies that newly established individuals will be included in the tree community more rapidly.
Rates of stem turnover (greater than 1
cm DBH) were very high, with values of stem gains from 13 to 90% and stem losses between 15 and 93% (). Stem loss was negatively related to age (R2
=0.52), but there was no relationship between stem gain and age (a
). Species turnover was very high as well, with species gains up to 75% and species losses up to 50%. Both species loss and gain (R2
=0.48 and 0.65, respectively; b
) were negatively related to age. In contrast to the results presented for an 8-year period for larger stems in the Sarapiquí plots (a
), dynamics at the level of stems did translate in species dynamics, as species loss was significantly related to stem loss and species gain to stem gain (R2
=0.58 and 0.65, respectively).
Figure 6 Turnover of stems and species in secondary forest plots in Chiapas, Mexico. (a) Percentage of stems greater than or equal to 1cm DBH lost through mortality and gained through recruitment. Stem gain was not significantly related to age, but stem (more ...)
The rate of successional dynamics, expressed in species and stem turnover rates, thus decreases with plot age in this series of plots. Although dynamics in the older Mexican plots do not seem to be very different from the Sarapiquí light-use plots (b), a robust comparison is difficult owing to the differences in diameter limits, plot size and census period.
Similarity in species composition (Chao Jaccard Abundance Estimator; Chao et al. 2005
) between pairs of plots ranged from almost total dissimilarity (0.02) to complete similarity (1.00; ). Across plots, only seven species dominated the sites, of which T. mexicanum
, T. micrantha
and C. peltata
were by far the most abundant. Since only a few species were dominant in these plots, variation in their abundance strongly influenced overall similarity between plots. For example, the plots F2 and H2, both dominated by T. mexicanum
, were highly similar. H17 on the other hand was dominated by Vochysia guatemalensis
, a species that was not found in the first two plots and consequently similarity was very low. We also compared pairwise similarities between initial species composition and species composition after 3 years in each plot (). Only in the three 2-year-old plots did species composition change significantly over the 3-year period. As was observed in the Costa Rican case study, among-plot differences in species composition are not reflected in compositional changes over time within plots; therefore, they seem to be the result of other factors, such as the interaction between site factors (soil, land-use history) and factors related to species colonization (e.g. distance to seed sources, regional species pool), rather than being strongly determined by time since agricultural abandonment.