Dynamics of alpine treelines: positive feedbacks and global, regional and local controls

Abstract

Whilst it is clear that increasing temperatures from global environmental change will impact the positions of alpine treelines, it is likely that a range of regional and local scaled factors will mediate the overall impact of global scale climate drivers. We summarized 12 categories of abiotic and biotic factors as 4 groups determining treeline positions. First, there are global factors related to climate-induced growth limitation and carbon limitation. Second, there are seven regional and local factors related to treeline dynamics including frost stress, topography, water stress, snow, wind, fire and nonfire disturbance. Third, species-specific factors can control treeline dynamics through their influence on reproduction and life history traits. Fourth, there are positive feedbacks in structuring the dynamics of treelines. Globally, the commonly accepted growth limitation hypothesis is that growth at a treeline is limited by temperature. Meanwhile, positive feedbacks between canopy cover and tree establishment are likely to control the spatial pattern and temporal dynamics of many treelines. The presence of non-linear dynamics at treelines has implications for the use of treelines as barometers of climate change because the lagged responses and abrupt shifts inherent in non-equilibrium systems may combine to mask the overall climate trend.

Frequent wildfires erode tree persistence and alter stand structure and initial composition of a fire‐tolerant sub‐alpine forest

“…Indeed, E. pauciflora forests are known to expand into non-tree alpine areas under long fire-free intervals (ca > 40 yr) in favourable environmental conditions (Wearne & Morgan 2001;McDougall 2003). Our study illustrates the possibility that climate change-associated changes in large-scale disturbance regimes (such as fire) could override the putative effects of increased CO 2 and temperature on vegetation composition in alpine and sub-alpine environments (Jarrad et al 2008;Camac et al 2017), leading to 'ecological surprises' (Noble 1993;Paine et al 1998;Kim & Lee 2015) such as a relative increase in the proportion of receding tree lines. There is, therefore, a need to acknowledge and incorporate the effects of stochastic wildfires with short return intervals into predictive models of future vegetation structure and composition.…”

“…), leading to ‘ecological surprises’ (Noble ; Paine et al. ; Kim & Lee ) such as a relative increase in the proportion of receding tree lines. There is, therefore, a need to acknowledge and incorporate the effects of stochastic wildfires with short return intervals into predictive models of future vegetation structure and composition.…”

Global distribution and climatic controls of natural mountain treelines

“…In other regions, there are also some treelines that have shifted much more than expected (>10 m/year; Figure 4b): In boreal regions, these expectations are mainly in Russia and Canada; in temperate regions, they are geographically concentrated in East Asia (North Korea, Japan, and China). On the contrary, there are also cases of treelines receding at a high rate, possibly driven by fire in some areas, either through the physical destruction of trees that acts to lower the existing treelines, or through the destruction of seedlings established upslope that acts to prevent treeline advances (Kim & Lee, 2015). For example, treelines have significantly receded in the western United States where climate and vegetation are favorable for fire (Seven Devils Mountains, Swan Range, etc.…”