Vitasse, Yann

2016-6-7, Lenz, Armando, Hoch, Günter, Körner, Christian, Vitasse, Yann

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.

2016-6-1, Körner, Christian, Basler, David, Hoch, Günter, Kollas, Chris, Lenz, Armando, Randin, Christophe F., Vitasse, Yann, 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.

2016-6-1, Klein, Tamir, Vitasse, Yann, 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.