Global drivers of tree seedling establishment at alpine treelines in a changing climate

Abstract

Alpine and Arctic treeline expansion depends on establishment of tree seedlings beyond the current treeline, which is expected to occur with climate warming. However, treelines often fail to respond to higher temperatures, and it is therefore likely that other environmental factors are important for seedling establishment. We aimed to analyse our current understanding of how temperature and a range of other environmental drivers affect tree seedling establishment at the alpine and Arctic treelines world‐wide and to assess the relative importance of temperature compared with other factors and how they interact. We collected 366 observations from 76 experimental and observational papers for a qualitative analysis of the role of a wide range of environmental factors on tree seed germination, tree seedling growth, survival and natural occurrence. For a subset of these studies, where the experimental design allowed, we conducted formal meta‐analyses to reveal if there were global drivers for different seedling life traits. The analyses showed that a wide range of abiotic and biotic factors affected tree seedling establishment besides from temperature, including water, snow, nutrients, light and surrounding vegetation. The meta‐analyses showed that different seedling life stages do not respond similarly to environmental factors. For example, temperature had positive effects on growth, while tree seedling survival and germination showed mixed responses to warming. Further, warming was as often as not the strongest factor controlling tree seedling establishment, when compared to with one of five other environmental factors. Moreover, warming effects often depended on other factors such as moisture or the presence of surrounding vegetation. Our results suggest that population dynamics of trees at the alpine and Arctic treeline is responsive to environmental changes and show that there is a clear need for multifactorial studies if we want to fully understand and predict the interplay between warming and other environmental factors and their effect on tree seedling establishment across current treelines. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13137/suppinfo is available for this article.

Past the climate optimum: Recruitment is declining at the world's highest juniper shrublines on the Tibetan Plateau

“…, Elliott and Cowell , Liang et al. , Lett and Dorrepaal ). If shrub recruitment at shrubline decreases in the future, such biotic interactions would greatly affect the biodiversity and composition of alpine biomes and the capacity of these communities to regulate carbon and water cycles at high elevations.…”

“…Together with the continuous warming effect, we conclude that the decline in juniper shrub recruitment has resulted from increasing drought stress in our study area. Future studies should test whether such changing relationships between shrub regeneration and rising temperatures depend on threshold-type responses to climate (Elliott 2012), or if they are also affected by local-scale biotic interactions (Garc ıa-Cervig on et al , Elliott and Cowell 2015, Lett and Dorrepaal 2018. If shrub recruitment at shrubline decreases in the future, such biotic interactions would greatly affect the biodiversity and composition of alpine biomes and the capacity of these communities to regulate carbon and water cycles at high elevations.…”

Is subarctic forest advance able to keep pace with climate change?

“…Neither ENMs nor process‐based models are currently able to incorporate the range of abiotic and biotic factors associated with FTE advance (Dyderski, Paź, Frelich, & Jagodziński, 2017; Settele et al., 2014). Although experiments designed to analyse the role of the main ecological or site‐specific factors governing boundary response provide valuable knowledge about the short‐term significance of some FTE drivers (Lett & Dorrepaal, 2018; Løkken, Hofgaard, Dalen, & Hytteborn, 2019; Olofsson et al., 2009; Speed, Austrheim, Hester, & Mysterud, 2010), responses to climate change depend on a multitude of abiotic and biotic factors whose effective roles are highly variable through time and space (Hofgaard et al., 2013; Holtmeier & Broll, 2005; Martin, Jeffers, Petrokofsky, Myers‐Smith, & Macias‐Fauria, 2017). Behaviours and rates based on driver‐restricted short‐term data could thus be of limited value if used in long‐term models (Callaghan et al., 2002; Harsch, Hulme, McGlone, & Duncan, 2009; Hofgaard et al., 2013; Kullman & Öberg, 2009).…”

Towards a mechanistic understanding of global change ecology

“…Other environmental factors likewise had variable effects across species, with interactions common. Together, the studies reviewed by Lett and Dorrepaal () demonstrate that temperature, although an important contributor to seedling establishment at tree lines, is just one of many important variables influencing tree line location.…”

“…Although tree lines do often shift as predicted, there are many cases in which tree lines do not shift as predicted, suggesting that other variables mediate seedling establishment and limit tree line shifts (Harsch, Hulme, McGlone, & Duncan, ). In this issue, Lett and Dorrepaal () review our understanding of how environmental factors influence alpine and arctic tree lines, and perform a meta‐analysis exploring the relative influences of temperature, water availability, nutrients, light and surrounding vegetation (among other variables) on tree seedling establishment at the tree line. They find that temperature can limit seed germination and seedling growth, but such effects vary widely across species and stages of development.…”