Microclimate shapes intraspecific trait patterns in sub-Arctic plants

1. Within-species trait variation is a substantial part of plant functional diversity. However, this intraspecific trait variation (ITV) is rarely investigated in relation to the key characteristic of the Arctic and alpine ecosystems: fine-scale microclimatic heterogeneity. Here, we quantified the influence of microclimate (namely, soil moisture, snow, and local temperatures) on plant functional traits, specifically on intraspecific trait variation. 

2. We focused on six wide-spread northern vascular plant species, and measured four traits, namely plant height, leaf area, leaf dry matter content (LDMC), and specific leaf area (SLA). We related intraspecific trait variation with field and remotely sensed microclimate data collected for 150 study plots within six distinct study grids. The grids were located within a 76-metre altitudinal belt in three contrasting environments: the tundra, tundra-forest ecotone, and mountain birch forest in Kilpisjärvi, northwestern Finland.

3. We compared the range of the observations in this local trait dataset (n = 5493) to observations in global trait databases (n = 10383). We found that the information in the local dataset covers a relatively large portion of the global databases. The proportion varied from trait and species to another, and the largest portion was 74% for variation in leaf area of Vaccinium uliginosum, and the lowest 19% for LDMC of Betula nana.

4. We found that intraspecific variation in height was mostly related to local temperatures, and leaf area showed less clear patterns along any of the microclimatic gradients. Whereas, SLA and LDMC were more related to soil moisture and snow conditions. However, species also showed contrasting relationships with the microclimate drivers.

5. We conclude that microclimate profoundly shapes the within-species variation in northern plants and that even a very compact geographic area can contain a large amount of ITV. The influence of the microclimatic conditions varies from functional trait and species to another, which highlights the adaptive but complex capability of tundra plants to cope with the mosaic of microclimatic conditions.