European deciduous trees exhibit similar safety margins against damage by spring freezing along elevational gradients

Background/Question/Methods

Freezing tolerance presumably is the single most important factor that can explain the known range limits of deciduous broadleaved tree taxa along latitudinal and elevational gradients. Within the ERC-project ‘TREELIM’ we investigated the effect of low temperature extremes on climatic boundaries of eight major European tree species. Particularly, we investigated the freezing resistance of buds and leaves before, during, and after the period of leaf emergence at the upper elevational limits of these taxa in the Swiss Alps. By reconstructing the spring phenology of these species over the last eight decades using a thermal sum model, we linked freezing resistance with long-term minimum temperature data along elevational gradients. We addressed the following questions: (i) What is the seasonal variation in the freezing resistance of deciduous broadleaved trees at their elevational limit? (ii) Does the risk of freezing damage during leaf unfolding increase to a critical level towards the tree species' limits?

Results/Conclusions

In all investigated species, freezing resistance of buds was tightly correlated with the advancement of spring phenology. Interestingly, this correlation leads to very similar mean safety margins (5 to 8.5 K) between low temperature extremes and actual empirical freezing damage across elevations. Absolute minimum temperatures in winter are only critical for species with relatively low elevational range limits. The elevational time-shift of bud burst allows to minimise the risk of freezing damage in spring at the cost of a shorter duration of the growing season at higher elevations. We suggest that freezing resistance, the associated timing of spring phenology, and the resulting length of the growing season jointly reflect species-specific life history requirements and thus, tree species range limits. The question of whether freezing resistance or minimum season length exerts ultimate evolutionary control over the species specific range limit remains open.