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1.  Increased water use efficiency does not prevent growth decline of Pinus canariensis in a semi-arid treeline ecotone in Tenerife, Canary Islands (Spain) 
Annals of forest science  2016;73(3):741-749.
Key message
Intrinsic water-use efficiency of Pinus canariensis (Sweet ex Spreng.) growing at a semi-arid treeline has increased during the past 37 years. Tree-ring width by contrast has declined, likely caused by reduced stomatal conductance due to increasing aridity.
Context
Rising atmospheric CO2 concentration (Ca) has been related to tree growth enhancement accompanied by increasing intrinsic water-use-efficiency (iWUE). Nevertheless, the extent of rising Ca on long-term changes in iWUE and growth has remained poorly understood to date in Mediterranean treeline ecosystems.
Aims
This study aimed to examine radial growth and physiological responses of P. canariensis in relation to rising Ca and increasing aridity at treeline in Tenerife, Canary Islands, Spain.
Methods
We evaluated temporal changes in secondary growth (tree-ring width; TRW) and tree ring stable C isotope signature for assessing iWUE from 1975 through 2011.
Results
Precipitation was the main factor controlling secondary growth. Over the last 36 years P. canariensis showed a decline in TRW at enhanced iWUE, likely caused by reduced stomatal conductance due to increasing aridity.
Conclusion
Our results indicate that increasing aridity has overridden the potential CO2 fertilization on tree growth of P. canariensis at its upper distribution limit.
doi:10.1007/s13595-016-0562-5
PMCID: PMC4961253  PMID: 27482149
Climate change; Mediterranean climate; drought; stable carbon isotope; Canary Island Pine; treeline
2.  The response of tropical rainforests to drought—lessons from recent research and future prospects 
Annals of Forest Science  2015;73:27-44.
Key message
We review the recent findings on the influence of drought on tree mortality, growth or ecosystem functioning in tropical rainforests. Drought plays a major role in shaping tropical rainforests and the response mechanisms are highly diverse and complex. The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical rainforests on the three continents. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance.
Context
Tropical rainforest ecosystems are characterized by high annual rainfall. Nevertheless, rainfall regularly fluctuates during the year and seasonal soil droughts do occur. Over the past decades, a number of extreme droughts have hit tropical rainforests, not only in Amazonia but also in Asia and Africa. The influence of drought events on tree mortality and growth or on ecosystem functioning (carbon and water fluxes) in tropical rainforest ecosystems has been studied intensively, but the response mechanisms are complex.
Aims
Herein, we review the recent findings related to the response of tropical forest ecosystems to seasonal and extreme droughts and the current knowledge about the future of these ecosystems.
Results
This review emphasizes the progress made over recent years and the importance of the studies conducted under extreme drought conditions or in through-fall exclusion experiments in understanding the response of these ecosystems. It also points to the great diversity and complexity of the response of tropical rainforest ecosystems to drought.
Conclusion
The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical forest regions. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance.
doi:10.1007/s13595-015-0522-5
PMCID: PMC4810888  PMID: 27069374
Carbon; Climate; Drought; Global change; Growth; Mortality; Soil; Tropical; Water
3.  Effects of climate variables on intra-annual stem radial increment in Pinus cembra (L.) along the alpine treeline ecotone 
Annals of forest science  2009;66(5):503.
Within the alpine treeline ecotone tree growth is increasingly restricted by extreme climate conditions. Although intra-annual stem growth recorded by dendrometers can be linked to climate, stem diameter increments in slow-growing subalpine trees are masked by changes in tree water status.We tested the hypothesis that intra-annual radial stem growth in Pinus cembra is influenced by different climate variables along the treeline ecotone in the Austrian Alps. Dendrometer traces were compared with dynamics of xylem cell development to date onset of cambial activity and radial stem growth in spring.Daily fluctuations in stem radius reflected changes in tree water status throughout the treeline ecotone. Extracted daily radial increments were significantly correlated with air temperature at the timberline and treeline only, where budburst, cambial activity and enlargement of first tracheids also occurred quite similarly. A close relationship was detected between radial increment and number of enlarging tracheids throughout the treeline ecotone.We conclude that (i) the relationship between climate and radial stem growth within the treeline ecotone is dependent on a close coupling to atmospheric climate conditions and (ii) initiation of cambial activity and radial growth in spring can be distinguished from stem re-hydration by histological analysis.
doi:10.1051/forest/2009038
PMCID: PMC3059571  PMID: 21423861
dendrometer; Pinus cembra; radial increment; treeline ecotone; xylem formation
4.  Photosynthetic temperature adaptation of Pinus cembra within the timberline ecotone of the Central Austrian Alps 
Annals of forest science  2010;67(2):201.
Temperature is suggested to determine the upper limit of tree life. Therefore, future climate warming may be of importance for tree distribution within the European Alps, where low temperatures limit carbon metabolism.
We focused on the effects of air and soil temperature on net photosynthesis (Pn) of Pinus cembra an evergreen climax species of the timberline ecotone of the Central Austrian Alps. Light response and temperature response curves were estimated along an altitudinal gradient ranging from the forest limit up to the krummholz limit in both summer and fall.
In general, Pn was significantly lower in fall as compared to summer. Nevertheless, independent from season mean Pn values tended to increase with elevation and were positively correlated with root zone temperatures. The specific leaf area by contrast declined with increasing elevation. Furthermore, the temperature optimum of net photosynthesis declined with increasing elevation and was positively correlated with the mean maximum air temperature of the 10 days prior the date of measurement.
Thus, our findings appear to reflect a long-term adaptation of the photosynthetic apparatus of Pinus cembra to the general temperature conditions with respect to elevation combined with a short term acclimation to the prevailing temperature regime.
doi:10.1051/forest/2009094
PMCID: PMC3047779  PMID: 21379394
net photosynthesis; temperature; cembran pine; timberline ecotone; global warming
5.  Effects of atmospheric and climate change at the timberline of the Central European Alps 
Annals of forest science  2009;66(4):402.
This review considers potential effects of atmospheric change and climate warming within the timberline ecotone of the Central European Alps. After focusing on the impacts of ozone (O3) and rising atmospheric CO2 concentration, effects of climate warming on the carbon and water balance of timberline trees and forests will be outlined towards conclusions about changes in tree growth and treeline dynamics.Presently, ambient ground-level O3 concentrations do not exert crucial stress on adult conifers at the timberline of the Central European Alps. In response to elevated atmospheric CO2 Larix decidua showed growth increase, whereas no such response was found in Pinus uncinata. Overall climate warming appears as the factor responsible for the observed growth stimulation of timberline trees.Increased seedling re-establishment in the Central European Alps however, resulted from invasion into potential habitats rather than upward migration due to climate change, although seedlings will only reach tree size upon successful coupling with the atmosphere and thus loosing the beneficial microclimate of low stature vegetation.In conclusion, future climate extremes are more likely than the gradual temperature increase to control treeline dynamics in the Central European Alps.
doi:10.1051/forest/2009023
PMCID: PMC3047780  PMID: 21379395
Alpine timberline; treeline; global warming; CO2; ozone; water balance; carbon gain

Results 1-5 (5)