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  • Publication
    Métadonnées seulement
    Convergence of leaf-out timing towards minimum risk of freezing damage in temperate trees
    (2016-6-7)
    Lenz, Armando
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    Hoch, Günter
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    Körner, Christian
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    1. Within the same forest stand, temperate deciduous trees generally exhibit a distinct pattern in leaf-out timing, with some species flushing earlier than other species. This study aimed to explain the timing of leaf-out of various temperate tree species in relation to the risk of freezing damage to leaves. 2. We combined long-term series of leaf-out date (14–32 years) of five temperate tree species located in both low and high elevations in Switzerland, daily minimum temperatures recorded at the same sites and species-specific freezing resistance (LT50) of emerging leaves. We calculated temperature safety margins (the temperature difference between absolute minimum temperature during leaf-out and species-specific LT50 values), and date safety margins (time lag between the last day when temperature falls below species-specific LT50 values and the date of leaf-out). 3. Leaf-out occurred when the probability to encounter freezing damage approaches zero, irre- spective of climatic conditions (low vs. high elevation) and species (early- and late-flushing spe- cies). In other words, trees leaf out precisely at the beginning of the probabilistically safe period. Interestingly, the temperature safety margins did not differ significantly between low and high elevation. Yet, the date safety margin was smaller at high elevation, presumably due to a faster increase in temperature during the leaf-out period at high elevation. 4. When species-specific freezing resistance is taken into account, the time of leaf-out con- verges among species towards a marginal risk of freezing damage. Thus, leaf-out time has likely evolved in a way that the risk of freezing damage is minimized over a large spectrum of climatic conditions. Species with a small safety margin against freezing temperature, like Fagus sylvatica, appear to employ photoperiod co-control of spring phenology, whereas species with a large safety margin depend largely on temperature for the right timing of leaf-out. 5. Our results offer a new avenue to explain the differences in leaf-out timing among co-occur- ring tree species. They further suggest that in a warming climate, tree species can expand their distribution range to the extent their phenology matches the stochasticity of freezing tempera- tures in spring.
  • Publication
    Métadonnées seulement
    Coordination between growth, phenology, and carbon storage in three coexisting deciduous tree species in a temperate forest
    (2016-6-1)
    Klein, Tamir
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    Hoch, Günter
    n deciduous trees growing in temperate forests, bud break and growth in spring must rely on intrinsic carbon (C) reserves. Yet it is unclear whether growth and C storage occur simultaneously, and whether starch C in branches is suf cient for refoliation. To test in situ the relationships between growth, phenology and C utilization, we monitored stem growth, leaf phenology and stem and branch nonstructural carbohydrate (NSC) dynamics in three deciduous species: Carpinus betulus L., Fagus sylvatica L. and Quercus petraea (Matt.) Liebl. To quantify the role of NSC in C investment into growth, a C balance approach was applied. Across the three species, >95% of branchlet starch was consumed during bud break, con rming the importance of C reserves for refoliation in spring. The C balance calculation showed that 90% of the C investment in foliage (7.0–10.5 kg tree−1 and 5–17 times the C needed for annual stem growth) was explained by simultaneous branchlet starch degradation. Carbon reserves were recovered sooner than expected, after leaf expansion, in parallel with stem growth. Carpinus had earlier leaf phenology (by ∼25 days) but delayed cambial growth (by ∼15 days) than Fagus and Quercus, the result of a competitive strategy to ush early, while having lower NSC levels.
  • Publication
    Métadonnées seulement
    Where, why and how? Explaining the low-temperature range limits of temperate tree species
    (2016-6-1)
    Körner, Christian
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    Basler, David
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    Hoch, Günter
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    Kollas, Chris
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    Lenz, Armando
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    Randin, Christophe F.
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    Zimmerman, Niklaus E.
    1. Attempts at explaining range limits of temperate tree species still rest on correlations with climatic data that lack a physiological justification. Here, we present a synthesis of a multidisci- plinary project that offers mechanistic explanations. Employing climatology, biogeography, dendrol- ogy, population and reproduction biology, stress physiology and phenology, we combine results from in situ elevational (Swiss Alps) and latitudinal (Alps vs. Scandinavia) comparisons, from recip- rocal common garden and phytotron studies for eight European broadleaf tree species. 2. We show that unlike for low-stature plants, tree canopy temperatures can be predicted from weather station data, and that low-temperature extremes in winter do not explain range limits. At the current low-temperature range limit, all species recruit well. Transplants revealed that the local envi- ronment rather than elevation of seed origin dominates growth and phenology. Tree ring width at the range limit is not related to season length, but to growing season temperature, with no evidence of carbon shortage. Bud break and leaf emergence in adults trees are timed in such a way that the probability of freezing damage is almost zero, with a uniform safety margin across elevations and taxa. More freezing-resistant species flush earlier than less resistant species. 3. Synthesis: we conclude that the range limits of the examined tree species are set by the interactive influence of freezing resistance in spring, phenology settings, and the time required to mature tissue. Microevolution of spring phenology compromises between demands set by freezing resistance of young, immature tissue and season length requirements related to autumnal tissue maturation.
  • Publication
    Métadonnées seulement
    Temperate and boreal forest tree phenology: from organ-scale processes to terrestrial ecosystem models
    (2016-1-1)
    Delpierre, Nicolas
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    Chuine, Isabelle
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    Joannès, Guillemot
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    Bazot, Stephane
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    Ruthishauser, This
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    Rathgeber, Cyrille B,K.
    -Key message We demonstrate that, beyond leaf phenol- ogy, the phenological cycles of wood and fine roots present clear responses to environmental drivers in temperate and boreal trees. These drivers should be included in terrestri- al ecosystem models. -Context In temperate and boreal trees, a dormancy period prevents organ development during adverse climatic condi- tions. Whereas the phenology of leaves and flowers has re- ceived considerable attention, to date, little is known regard- ing the phenology of other tree organs such as wood, fine roots, fruits, and reserve compounds. -Aims Here, we review both the role of environmental drivers in determining the phenology of tree organs and the models used to predict the phenology of tree organs in tem- perate and boreal forest trees. -Results Temperature is a key driver of the resumption of tree activity in spring, although its specific effects vary among organs. There is no such clear dominant environmental cue involved in the cessation of tree activity in autumn and in the onset of dor- mancy, but temperature, photoperiod, and water stress appear as prominent factors. The phenology of a given organ is, to a certain extent, influenced by processes in distant organs. -Conclusion Inferring past trends and predicting future trends of tree phenology in a changing climate requires spe- cific phenological models developed for each organ to consid- er the phenological cycle as an ensemble in which the envi- ronmental cues that trigger each phase are also indirectly in- volved in the subsequent phases. Incorporating such models into terrestrial ecosystem models (TEMs) would likely im- prove the accuracy of their predictions. The extent to which the coordination of the phenologies of tree organs will be affected in a changing climate deserves further research.
  • Publication
    Métadonnées seulement
    Declining global warming effects on the phenology of spring leaf unfolding
    (2015-10-1)
    Fu, Yongshuo
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    Zhao, Hongfang
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    Piao, Shilong
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    Peaucelle, Marc
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    Peng, Shushi
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    Zhou, Guiyun
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    Ciais, Philippe
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    Huang, Mengtian
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    Menzel, Annette
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    Peñuelas, Josep
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    Song, Yang
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    Zeng, Zhenzhong
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    Janssens, Ivan A.
    Earlier spring leaf unfolding is a frequently observed response of plants to climate warming1–4. Many deciduous tree species require chilling for dormancy release, and warming-related reductions in chilling may counteract the advance of leaf unfolding in response to warming5,6. Empirical evidence for this, however, is limited to sap- lings or twigs in climate-controlled chambers7,8. Using long-term in situ observations of leaf unfolding for seven dominant European tree species at 1,245 sites, here we show that the apparent response of leaf unfolding to climate warming (ST, expressed in days advance of leaf unfolding per 6C warming) has significantly decreased from 1980 to 2013 in all monitored tree species. Averaged across all spe- cies and sites, ST decreased by 40% from 4.0 6 1.8 days 6C21 during 1980–1994 to 2.3 6 1.6 days 6C21 during 1999–2013. The declining ST was also simulated by chilling-based phenology models, albeit with a weaker decline (24–30%) than observed in situ. The reduction in ST is likely to be partly attributable to reduced chilling. Nonetheless, other mechanisms may also have a role, such as ‘photo- period limitation’ mechanisms that may become ultimately limiting when leaf unfolding dates occur too early in the season. Our results provide empirical evidence for a declining ST, but also suggest that the predicted strong winter warming in the future may further reduce ST and therefore result in a slowdown in the advance of tree spring phenology
  • Publication
    Métadonnées seulement
    Increased heat requirement for leaf flushing in temperate woody species over 1980-2012: effects of chilling, precipitation and insolation
    (2015-7-1)
    Fu, Yongshuo
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    Piao, Shilong
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    Zhao, Hongfang
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    De Boeck, Hans
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    Liu, Qiang
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    Yang, Hui
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    Weber, Ulrich
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    Hänninen, Heikki
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    Ivan A., Janssens
    Recent studies have revealed large unexplained variation in heat requirement-based phenology models, resulting in large uncertainty when predicting ecosystem carbon and water balance responses to climate variability. Improving our understanding of the heat requirement for spring phenology is thus urgently needed. In this study, we estimated the species-specific heat requirement for leaf flushing of 13 temperate woody species using long-term phenological observations from Europe and North America. The species were defined as early and late flushing species according to the mean date of leaf flushing across all sites. Partial correlation analyses were applied to determine the temporal correlations between heat requirement and chilling accumulation, precipitation and insolation sum during dormancy. We found that the heat requirement for leaf flushing increased by almost 50% over the study period 1980–2012, with an average of 30 heat units per decade. This temporal increase in heat requirement was observed in all species, but was much larger for late than for early flushing species. Consistent with previous studies, we found that the heat requirement negatively correlates with chilling accumulation. Interestingly, after removing the variation induced by chilling accumulation, a predominantly positive partial correlation exists between heat requirement and precipitation sum, and a predominantly negative correlation between heat requirement and insolation sum. This suggests that besides the well-known effect of chilling, the heat requirement for leaf flushing is also influenced by precipitation and insolation sum during dormancy. However, we hypothesize that the observed precipitation and insolation effects might be artefacts attributable to the inappropriate use of air temperature in the heat requirement quantification. Rather than air temperature, meristem temperature is probably the prominent driver of the leaf flushing process, but these data are not available. Further experimental research is thus needed to verify whether insolation and precipita- tion sums directly affect the heat requirement for leaf flushing.