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Global warming poses great challenges for forest managers regarding adaptation strategies and species choices. More frequent drought events and heat spells are expected to reduce growth and increase mortality. Extended growing seasons, warming and elevated CO2 (eCO2) can also positively affect forest productivity. We studied the growth, productivity and mortality of Fagus sylvatica L. and Abies alba Mill. in the Black Forest (Germany) under three climate change scenarios (representative concentration pathways (RCP): RCP2.6, RCP4.5, RCP8.5) using the detailed biogeochemical forest growth model GOTILWA+. Averaged over the entire simulation period, both species showed productivity losses in RCP2.6 (16&ndash, 20%) and in RCP4.5 (6%), but productivity gains in RCP8.5 (11&ndash, 17%). However, all three scenarios had a tipping point (between 2035&ndash, 2060) when initial gains in net primary productivity (NPP) (6&ndash, 29%) eventually turned into losses (1&ndash, 26%). With eCO2 switched off, the losses in NPP were 26&ndash, 51% in RCP2.6, 36&ndash, 45% in RCP4.5 and 33&ndash, 71% in RCP8.5. Improved water-use efficiency dampened drought effects on NPP by between 4 and 5%. Tree mortality increased, but without notably affecting forest productivity. Concluding, cultivation of beech and fir may still be possible in the study region, although severe productivity losses can be expected in the coming decades, which will strongly depend on the dampening CO2 fertilization effect.

The anticipated climate change during the next decades is posing crucial challenges to ecosystems. In order to decrease the vulnerability of forests, introducing tree species&rsquo, mixtures are a viable strategy, with deep-rooting native Silver fir (Abies alba) being a primary candidate for admixture into current pure stands of European beech (Fagus sylvatica) especially in mountainous areas. Such a change in forest structure also has effects on the regional scale, which, however, have been seldomly quantified. Therefore, we measured and modeled radiative balance and air chemistry impacts of admixing Silver fir to European beech stands, including changes in biogenic volatile organic compound emissions. An increased fraction of Silver fir caused a smaller albedo and a (simulated) larger evapotranspiration, leading to a dryer and warmer forest. While isoprene emission was negligible for both species, sesquiterpene and monoterpene emissions were larger for fir than for beech. From these differences, we derived that ozone concentration as well as secondary organic aerosols and cloud condensation nuclei would increase regionally. Overall, we demonstrated that even a relatively mild scenario of tree species change will alter the energy balance and air quality in a way that could potentially influence the climate on a landscape scale.

Global warming is raising concerns about the acclimatory capacity of trees and forests, especially in Mediterranean-type ecosystems. The sensitivity of photosynthesis to temperature is a key uncertainty for projecting the magnitude of terrestrial feedbacks on future climate change. While boreal, temperate and tropical species have been comparatively well investigated less attention has been paid to the Mediterranean trees. We quantified seasonal changes in the responses of net photosynthesis (Anet), stomatal conductance (gs), mesophyllic conductance (gm), and electron-transport rate (Jcf) and investigated their sensitivity to drought and thermal stress in sunlit and shaded leaves of Quercus ilex, Q. pubescens, Pinus halepensis, and Arbutus unedo. All four species showed a remarkably dynamic acclimation of Topt characterised by high thermal tolerance. Seasonal changes in Topt were consistent in all four species, whereas the shape of the response curves was highly species-specific. Surprisingly, severe drought decreased Topt (similar to winter) accompanied by narrower response curves above all in P. halepensis, reducing the optimal range for photosynthesis to the cooler morning/evening periods. In contrast, less strict stomatal control under severe drought led to leaf shedding in Q. ilex and Q. pubescens and additionally branch dieback in A. unedo. Water availability was thus a key factor, in addition to growth temperature to explain acclimation of photosynthetic responses to temperature. More frequent and severe drought periods as predicted by climate change may disrupt the acclimatory capacity of Mediterranean trees, which could lead to reduced growth and, for some species such as A. unedo, increased mortality risk.

[eng] The Mediterranean Basin is a climate-change hotspot of the world. Predicted reductions in annual precipitation, increases in mean temperature, and increases in the variability and occurrence of extreme droughts and heat waves are likely to affect species abundance and distribution. The existence of sympatric plants with different morphological and phenological strategies raises the question how they will respond to novel climate conditions. There is a strong need to improve the mechanistic understanding of key foliar ecophysiological parameters in response to abiotic stressors on a small scale if we are to predict the carbon budget of plant ecosystems in larger scales. We first reviewed contrasting growth responses to temperature of angio- and gymnosperms in the Iberian Peninsula. Secondly, we studied the seasonal acclimation of different foliar ecophysiological traits in two leaf positions of four Mediterranean tree species in extensive field experiments. We aimed to shed light on the mechanistic understanding of the foliar respiratory and photosynthetic responses to abiotic stress such as drought and temperature. We found contrasting demographic responses in Mediterranean conifer and angiosperm trees. Widespread forest successional advance of angiosperms and negative growth responses of gymnosperms to temperature are currently occurring in the Iberian Peninsula. Trait-based differences in these two groups contribute to explain their different responses to temperature and their different role during successional processes. The acclimation behaviour of photosynthetic and morphological traits to seasonal variable growth conditions was strongly pronounced in all tree species. Photosynthetic machineries were resilient to moderate drought, whereas severe drought induced acclimation of morphological traits, photosynthetic downregulation and leaf abscission. The lack of replenishment of soil-water reserves during the early growing season critically enforced the summer drought. We also observed a notable seasonal acclimation of the thermal optima and of the curvature of temperature responses of photosynthetic assimilation. The photosynthetic system was better acclimated to lower temperatures in winter and to heat stress in the drier and hotter year. Mild winter temperatures provided a period of growth and recovery that resulted in biochemical recovery, new shoot growth, and moderate transpiration across all evergreen species. High radiation and sudden low temperatures had a combinatory negative effect on the photosynthetic apparatus leading to photoinhibitory stress -especially in sunlit leaves. Species-specific acclimation partly offset these overall trends in responses to drought and temperature stress. Quercus ilex L. and to a lesser extent Q. pubescens Wild. showed the highest plasticity in photosynthetic traits whereas Pinus halepensis Mill. was most tolerant across the seasons with the most stable temperature response pattern. Arbutus. unedo L. was the most vulnerable to drought and photoinhibitory stress in winter. A. unedo and Q. pubescens had a less sclerophyllic leaf habit and invested the least in acclimation of the morphological structure being most vulnerable to drought-induced leaf abscission. Shaded leaves showed generally a lower photosynthetic potential, but cushioned negative impacts under stress periods. A long-term rainfall-exclusion experiment in a Q. ilex forest increased the foliar carbon-use efficiency and the plasticity of foliar respiratory and photosynthetic traits, but did not affect the biochemical photosynthetic potential. A favourable growth period was thus exploited more efficiently. Overall, our results indicate that Mediterranean climax-species exhibit a strong acclimatory capacity to warmer and drier conditions, but can be sensitive to extreme drought and extreme temperature stress. The performance of the plants during winter might give important insights in the dynamics of Mediterranean forest communities under novel environmental conditions. Leaf position is an indispensable factor when estimating the canopy carbon balance. Angiosperms and gymnosperms had fundamental different photosynthetic strategies of stress-avoidance versus stress-tolerance, respectively., [spa] El cambio climático aumentará la sequía en la Cuenca Mediterránea y posiblemente afectará a la abundancia y la distribución de especies. Revisamos las respuestas contrastadas del crecimiento a la temperatura de angio- y gimnospermas en la Península Ibérica. Estudiamos la variación de los efectos del estrés térmico y por sequía en rasgos morfológicos, fotosintéticos y de la respiración foliar según la especie y la posición en el dosel. Además, evaluamos el efecto de una sequía crónica sobre la respiración foliar y la fotosíntesis de Quercus ilex L. La maquinaria fotosintética se mostró resiliente frente a la sequía moderada, mientras que la sequía extrema, agravada por las bajas reservas de agua en el suelo, indujo la aclimatación de la morfología foliar, la inhibición de la bioquímica fotosintética y la abscisión foliar. El sistema fotosintético se aclimató mejor a las temperaturas bajas que al estrés por calor. Las temperaturas suaves en invierno derivaron en la recuperación bioquímica, un nuevo crecimiento de los brotes y una transpiración moderada. La elevada radiación y el frío repentino mostraron un efecto combinado negativo, causando estrés fotoinhibitorio. El estrés térmico y por sequía fue más pronunciado en hojas de sol y aminorado en hojas de sombra. Q.ilex y, en menor grado, Q. pubescens Wild. mostraron la plasticidad más elevada de los rasgos fotosintéticos, mientras que Pinus halepensis Mill. fue más tolerante, mostrando la respuesta más estable a la temperatura. Arbutus unedo L. fue la especie más vulnerable a la sequía y al estrés fotoinhibitorio. En respuesta a la sequía crónica, Q. ilex incrementó la eficiencia en el uso del carbono y la plasticidad de los atributos fotosintéticos y de respiración foliar, pero no afectó al potencial fotosintético. En resumen, las especies climácicas mediterráneas se aclimatan frente a condiciones más cálidas y secas, pero pueden ser sensibles ante sequías extremas. El funcionamiento durante el invierno es vital para entender la dinámica de los bosques mediterráneos. La posición de las hojas en la copa es indispensable para estimar el balance de carbono del dosel. Angiospermas y gimnospermas presentan estrategias fotosintéticas contrastadas, de evitación y tolerancia del estrés, respectivamente.

handed in by Dominik Sperlich Zsfassung in dt. Sprache Wien, Univ. für Bodenkultur, Masterarb., 2010