Winter conditions – not summer temperature – influence establishment of seedlings at white spruce alpine treeline in Eastern Quebec

Abstract

Aims While treeline positions are globally correlated to growing season temperatures, seedling establishment, an important process of alpine treeline dynamics, is additionally controlled by regional‐scale factors such as snow cover duration, desiccating winds and biotic interactions. Knowing that alpine treelines have shown contrasting responses to climate change, we determined the relative importance of key abiotic and biotic factors involved in seedling survival and growth. Location McGerrigle Mountains, Parc National de la Gaspésie, Appalachian Range, eastern Quebec, Canada. Methods In two white spruce (Picea glauca) treeline sites, we used the microclimate in the vicinity of tree islands, densely packed clusters of trees isolated from each other by alpine tundra vegetation, to assess the effects of abiotic variables (sum of degree days [DD], snowpack duration and a wind exposure index) as well as the effects of biotic interactions with neighbouring vegetation on the survival and growth of transplanted white spruce seedlings. For 3 yr, we surveyed seedling survival twice a year to discriminate between winter and summer survival, and measured seedling growth at the end of each growing season. We used Bayesian hierarchical models to estimate the relative effects of covariates on survival and growth. Results Survival probability decreased in microsites where winter DD was high, and increased in microsites with longer snowpack duration. In wind‐exposed microsites, seedling survival increased when neighbouring vegetation was present, indicating facilitative mechanisms. Seedling growth was positively affected by the duration of snow cover and tended to increase with higher DD during the previous year. In wind‐sheltered microsites, seedling growth tended to be negatively affected by neighbouring vegetation, indicating competitive mechanisms. Conclusions Our study demonstrates that seedling establishment is more sensitive to winter conditions, notably to the length of snow cover (which protects seedlings from frost and desiccation), than to summer temperature. Biotic interactions increased seedling establishment when environmental stresses were higher. We suggest that regional‐scale factors such as winter climate and biotic interactions should be included in modelling exercises to improve future treeline location forecasts.

The role of bryophytes for tree seedling responses to winter climate change: Implications for the stress gradient hypothesis

“…As we transplanted young seedlings, we cannot infer about the impact of winter conditions on earlier phases of seedling establishment (germination). However, our study supports growing evidence that winter processes are important for tree seedling establishment at the alpine tree line (Batllori et al., ; Renard et al., ). Furthermore, the effect of snow on seedling survival observed here was smaller than or of similar magnitude as the effect of presence vs. absence, or of species identity of bryophytes (Table ).…”

“…For instance, higher air temperatures may reduce the insulating snow cover, which can lead to colder soils or enhanced freeze–thaw regimes and therefore greater climatic stress during winter (Groffman et al., ; Wipf, Rixen, & Mulder, ). Snow conditions may thus affect plant establishment in alpine and high‐latitude ecosystems (Gornish et al., ; Renard, McIntire & Fajardo, ) through direct effects on soil temperatures, and also by shifting the balance between competition and facilitation in plant interactions (Wipf et al., ). How interactions of existing vegetation with plants establishing beyond their current range are affected by changing winter snow conditions is, however, poorly understood.…”

It takes a few to tango: changing climate and fire regimes can cause regeneration failure of two subalpine conifers

“…, Renard et al. ). Reductions in winter snowpack are projected to be greatest at mid elevations in Yellowstone vs. high elevations, where snowpack is projected to remain consistent (Tercek and Rodman ), or low elevations, where snow pack is already less.…”

Feedback effects of clonal groups and tree clusters on site conditions at the treeline: implications for treeline dynamics

“…It has been argued that at the alpine treeline of white spruce Picea glauca, winter conditions rather than summer temperatures are critical for seedling establishment (Renard et al 2016). Moisture supply from melting snow might facilitate germination of seeds and support tree seedlings as well as ground vegetation.…”