Take a tree to the limit: the stress line.

A change in climate would be expected to shift plant distribution as species expand in newly favourable areas and decline in increasingly hostile locations (Kelly and Goulden 2008). Even under conservative scenarios, future climate changes are likely to include further increases in mean temperature with significant drying in some regions, as well as increases in the frequency and severity of extreme droughts, and heat waves (Allen et al. 2010). As climate changes rapidly with height over relatively short horizontal distances, so does vegetation and ecohydrology (Beniston 2003), and alpine regions have been suggested to be particularly vulnerable to climate change (Elkin et al. 2013). If the treeline moves up in elevation, the increased carbon storage and evapotranspiration from this effect may provide a partial negative feedback to climate change. Nevertheless, vegetation influences the absorption of energy by the surface via modification of the surface albedo. A shift to evergreen forest at the mountain treeline would amplify the alteration of the physical characteristics of the land surface induced by forest expansion, decreasing surface albedo, while increasing net evaporation and surface roughness. In the present issue of Tree Physiology, Charrier et al. (2013) provide a valuable study on the physiology of frost resistance at the altitudinal limit of 11 common European tree species (Betula pendula, Fagus sylvatica, Pinus sylvestris, Quercus robur, Corylus avellana, Juglans regia × nigra, Acer pseudoplatanus, Alnus cordata, Carpinus betulus, Prunus cerasifera and Robinia pseudoacacia). The central question of this study was to model potential altitudinal limits of these trees by monitoring ecophysiological and physiological parameters during the cold season. These authors applied an original multi-parametric analysis focused on frost resistance of living xylem cells (rays and the paratracheal parenchyma), embolism vulnerability of non-living water conduits (tracheids and vessel elements) and avoidance of spring frosts depending on timing of budburst. While wood anatomy appeared to be the main driver of altitudinal limit (narrow vessels being an important adaptive trait in frost exposed environments), osmotic-related parameters were also highly significant. Another important outcome of this study was that the maximal percentage of loss of conductivity was directly related to vessel and living-cell resistance and indirectly related to physiological or anatomical parameters, explaining these resistances. Once established, a tree growing at high elevation must reinforce mechanically (against wind and snow accumulation), and this process involves the capability of the species to accumulate carbon, providing the structural strength of cells, which will be diverted towards reinforcement and away from building an efficient hydraulic system. Nevertheless, the evidence for a trade-off between mechanical strength and hydraulic efficiency along climatic gradients is still ambiguous, and a trade-off between efficiency and strength may not occur. Frost resistance to winter embolism was addressed by Charrier et al. (2013) by examining the physical resistance to xylem cavitation (percentage of loss of conductivity after one freeze/thaw event) and the active refilling of embolized elements (maximal frost hardiness). They confirmed that hydraulic architecture integrity, considered the key physiological process in plant survival, is not only critical for trees under drought conditions (Choat et al. 2012) but also during winter freezing conditions. Survival under extreme climatic conditions and the potential altitudinal limit of trees would depend on the resilience of both living cells and non-living water transport systems to freezing events. In particular, the interaction between embolism and refilling might rely on the contribution of living cells undamaged by frost for the development of pressurization mechanisms for refilling. The frequency of freeze/thaw events (and Commentary