Northern latitude/high-elevation distribution limits have long been associated with the cold temperatures that occur in these environments. Ecological niche models have been employed countless times and consistently show that temperature is a determining factor when it comes to cold range limits in temperate tree species. The issue with using ecological niche models is that they only establish correlative relationships between predictors and responses, which can mislead researchers when trying to understand the drivers of distributions. It is difficult, if not impossible, in ecological niche models to tell whether a predictor actually mechanistically affects a species or if it is simply a proxy for a causal mechanism. In their article 'Where, why and how? Explaining the low-temperature range limits of temperate tree species' Körner et al. (2018) searched for the mechanism(s) that cause cold range limits in trees.
Körner et al. took an exhaustive approach to establishing causal mechanisms by using a combination of latitudinal- and elevation-based field work, lab experiments, and common garden provenance studies. A remarkable finding, counter to the hypotheses of the authors, was that reproductive potential was not reduced at the cold edge of populations. Along with recruitment issues, soils and limited carbon were likewise ruled out as causal mechanisms of cold range limits. Summer temperatures are clearly not the limiting factor for trees, as this is when the majority of growth and production are taking place. Autumn weather was also eliminated as the potential mechanism, since tissues are produced by late summer and fall temperatures only control tissue maturation and entry into dormancy. The authors also show that winter temperatures do not limit distributions, as the dormant state of temperate trees largely prevents temperature-caused mortality during this period. Spring temperatures however, were the determining factor for cold-range limits. Specifically, freezing resistance in early spring was the suspected mechanism, as determined by the timing of leaf flushing. An evolutionary interplay at the cold range limit of temperate trees thereby becomes the decisive factor, whereby a balance must be stricken between the risk of leafing out early and the loss of growing season duration caused by leafing out late.
While this study confirms the broad assumption that cold temperatures are important limiting factors at northern/high-elevation range limits, having the specifics of the mechanisms involved and the timing of effective cold temperatures is monumental. Spatial predictions, like those made in ecological niche models, can now be made more accurate by using cold extremes of the spring as a predictor as opposed to mean temperatures. Further, the results from this study can be implemented in process-based phenological models to improve their accuracy. In a time of rapid climate change, information like that presented in this study greatly improves our ability to predict species reactions to the aforementioned changes.
Paper reference:
Körner C., Basler D., Hoch G., Kollas C., Lenz A., Randin C.F., Vitasse Y., Zimmerman N.E. (2016). Where, why and how? Explaining the low-temperature range limits of temperate tree species. Journal of Ecology. 104: 1076 - 1088.
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