Where, why and how? Explaining the low‐temperature range limits of temperate tree species

Abstract

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 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. 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 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. 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.

Plant adaptation to cold climates

“…Phenology controls also set the termination of growth activity in autumn before damaging freezing temperatures come into action. Plants adapted to cold climates must be able to complete their seasonal life cycle within the time frame set by phenology controls 10 . During the dormant period, long-lived plants native to cold climates have gone through evolutionary selection and thus are commonly safe from fatal freezing damage; some even survive dipping in liquid nitrogen during the coldest part of the year.…”

“…As a result, trees are able to develop mature, winter-hardy tissues. The required minimum season length is determined by life history traits such as fruit size and wood anatomy 10 .…”

Frost hardening and dehardening potential in temperate trees from winter to budburst

“…A recent study has shown the phylogenetic basis of the vulnerability to late frost events using 170 woody species growing in an arboretum in Germany, and suggests a tight link between the natural distribution range of a given species and its winter cold hardiness (Muffler et al ., ). However, the influence of spring phenology on the growing season length has been reconsidered recently as a key potential factor limiting the upper elevation limit of temperate tree species growing below the treeline (Körner et al ., ). The authors demonstrated by an array of experiments and in situ monitoring that, at the cold edge of species distribution, avoiding frost damage in spring by delaying phenology occurs at the cost of a dramatic reduction in the growing season, leaving too short a time for the complete maturation of cold hardy tissues (Körner et al ., ).…”

Tree diversity patterns along the latitudinal gradient in the northwestern Russia

“…forest fires, wind throw, topography, soil characteristics, interspecific competition and successional stage) mostly overrule the importance of regional climate (Pearson and Dawson 2003;Körner et al 2016;Kuosmanen et al 2016).…”

“…Just as everywhere else on our planet, Russian forests are generally shaped by a combination of various factors, which can be divided into four principal groups: historic, current abiotic, biotic and anthropogenic (Huston 1994). It is recognized that the modern differentiation of the tree species composition on a latitudinal gradient is determined primarily by climate (Sykes et al 1996;Körner et al 2016;Vetluzhskikh 2016), and for boreal and northern temperate European forests the most important factor is temperature (Morozova 2009). However, other factors acting on different spatial scales may have a significant impact on the local patterns of tree species richness (Caley and Schluter 1997;Svenning and Skov 2005;Svenning et al 2010).…”