Where, Why and How? Explaining the Low-Temperature Range Limits of Temperate Tree Species. Körner, C.; Basler, D.; Hoch, G.; Kollas, C.; Lenz, A.; Randin, C. F.; Vitasse, Y.; and Zimmermann, N. E. 104(4):1076–1088.
Where, Why and How? Explaining the Low-Temperature Range Limits of Temperate Tree Species [link]Paper  doi  abstract   bibtex   
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 multidisciplinary project that offers mechanistic explanations. Employing climatology, biogeography, dendrology, population and reproduction biology, stress physiology and phenology, we combine results from in situ elevational (Swiss Alps) and latitudinal (Alps vs. Scandinavia) comparisons, from reciprocal common garden and phytotron studies for eight European broadleaf tree species. [] 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 environment 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. [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. [Excerpt] [] [...] In summary, our results allow, for the first time, to establish a theory of low temperature range limit controls for non-treeline, temperate zone tree species. The overarching influence of thermal extremes during spring is obvious, but these do not act in isolation. As we have shown, species sensitive to freezing during early spring flush later, and thus, are at a similar risk of being damaged by late freezing than more tolerant, earlier flushing species. This risk mitigation has a trade-off in terms of the remaining length of the growing season. Hence, the evolutionary selection for later flushing in those species cuts the growing period short. This may not matter over most of the species' range, but it matters at the species' range limit, where the climatic conditions and plant internal developmental controls constrain the duration of the season. Some species may tolerate shorter seasons than others, related to their specific seasonal life cycle. Our data do not suggest that this limitation is tied to reduced carbon acquisition, because we found no evidence of carbon shortage at the range limit of any species. [] One important conclusion for modelling species range limits is that neither any temperature means, nor extremes of winter temperature are directly involved in the biological mechanisms responsible for the formation of range limits at the scales explored here. In cases where correlations between upper range limits and winter temperatures were found, it may be due to the proxy nature for other facets of temperature. Deciduous tree range limits are most likely tied to events during the early growing season, no matter how cold the winter might be, provided the remaining season is long enough to permit tissue maturation, that is, ripening seeds, completing the sapwood, late wood and bark formation, mature winter buds (including flower primordia) and current year shoots. Should these developmental processes remain incomplete by the time trees are forced into autumnal dormancy, they are not going to survive the following winter, or they will regularly fail to reproduce, or lose newly grown shoots. [...] [] We thus arrive at a triangular interaction of inherent freezing tolerance of foliage in spring, that selects for a certain phenological control of spring flushing, which in turn truncates the length of the growing season, and thus, the time available for tissue maturation [...] [] [...] [] Future studies aiming at predicting species range limits will have to account for this threefold interaction of drivers. This also calls for accounting for temperature extremes rather than means in projecting tree responses to future climates. [...] [] Once we arrive at a mechanistic understanding of tree phenology, and species-specific freezing tolerance in spring is known, the probability of tree failure due to low-temperature extremes can be distilled from climate data bases that report extremes (absolute minima [...]). [...]. Similar to the results presented here, any attempt at isolating one single factor (e.g. heat sums, temperature means) is prone to failure, because native species most likely respond to the interactive, nonlinear influences of several climatic factors such as chilling signals (telling the plant whether it is autumn or spring [...]), photoperiod signals (the weather-independent astronomic calendar) and actual temperature ('thermal forcing'), with the last gaining influence, once the first two permit [...].
@article{kornerWhereWhyHow2016,
  title = {Where, Why and How? {{Explaining}} the Low-Temperature Range Limits of Temperate Tree Species},
  author = {Körner, Christian and Basler, David and Hoch, Günter and Kollas, Chris and Lenz, Armando and Randin, Christophe F. and Vitasse, Yann and Zimmermann, Niklaus E.},
  date = {2016-07},
  journaltitle = {Journal of Ecology},
  volume = {104},
  pages = {1076--1088},
  issn = {0022-0477},
  doi = {10.1111/1365-2745.12574},
  url = {https://doi.org/10.1111/1365-2745.12574},
  abstract = {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 multidisciplinary project that offers mechanistic explanations. Employing climatology, biogeography, dendrology, population and reproduction biology, stress physiology and phenology, we combine results from in situ elevational (Swiss Alps) and latitudinal (Alps vs. Scandinavia) comparisons, from reciprocal common garden and phytotron studies for eight European broadleaf tree species. [] 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 environment 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.

[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.

[Excerpt] 

[] [...] In summary, our results allow, for the first time, to establish a theory of low temperature range limit controls for non-treeline, temperate zone tree species. The overarching influence of thermal extremes during spring is obvious, but these do not act in isolation. As we have shown, species sensitive to freezing during early spring flush later, and thus, are at a similar risk of being damaged by late freezing than more tolerant, earlier flushing species. This risk mitigation has a trade-off in terms of the remaining length of the growing season. Hence, the evolutionary selection for later flushing in those species cuts the growing period short. This may not matter over most of the species' range, but it matters at the species' range limit, where the climatic conditions and plant internal developmental controls constrain the duration of the season. Some species may tolerate shorter seasons than others, related to their specific seasonal life cycle. Our data do not suggest that this limitation is tied to reduced carbon acquisition, because we found no evidence of carbon shortage at the range limit of any species.

[] One important conclusion for modelling species range limits is that neither any temperature means, nor extremes of winter temperature are directly involved in the biological mechanisms responsible for the formation of range limits at the scales explored here. In cases where correlations between upper range limits and winter temperatures were found, it may be due to the proxy nature for other facets of temperature. Deciduous tree range limits are most likely tied to events during the early growing season, no matter how cold the winter might be, provided the remaining season is long enough to permit tissue maturation, that is, ripening seeds, completing the sapwood, late wood and bark formation, mature winter buds (including flower primordia) and current year shoots. Should these developmental processes remain incomplete by the time trees are forced into autumnal dormancy, they are not going to survive the following winter, or they will regularly fail to reproduce, or lose newly grown shoots. [...]

[] We thus arrive at a triangular interaction of inherent freezing tolerance of foliage in spring, that selects for a certain phenological control of spring flushing, which in turn truncates the length of the growing season, and thus, the time available for tissue maturation [...]

[] [...]

[] Future studies aiming at predicting species range limits will have to account for this threefold interaction of drivers. This also calls for accounting for temperature extremes rather than means in projecting tree responses to future climates. [...]

[] Once we arrive at a mechanistic understanding of tree phenology, and species-specific freezing tolerance in spring is known, the probability of tree failure due to low-temperature extremes can be distilled from climate data bases that report extremes (absolute minima [...]). [...]. Similar to the results presented here, any attempt at isolating one single factor (e.g. heat sums, temperature means) is prone to failure, because native species most likely respond to the interactive, nonlinear influences of several climatic factors such as chilling signals (telling the plant whether it is autumn or spring [...]), photoperiod signals (the weather-independent astronomic calendar) and actual temperature ('thermal forcing'), with the last gaining influence, once the first two permit [...].},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-14127744,climate-extremes,complexity,feedback,forest-resources,habitat-suitability,limiting-factor,niche-modelling,non-linearity,temperate-forests,temperature},
  number = {4}
}
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