Life at the edge: plant responses to extreme alpine environments

Abstract

Snow is an essential element determining plant composition and performance in alpine environments. Snowbank communities are an ideal model community to identify plant species distribution and plant strategies along environmental gradients. Small-scale changes require plant species-specific adaptations to sustain a population in changing environmental conditions in space and time. Plant strategies (expressed by plant functional traits), without reference to the taxonomy of individual species, can assist to identify vegetation responses to historical and current environmental conditions. In this study, plant traits were quantified along snowmelt gradients in New Zealand and Australian snowbanks, both individually and in a biogeographic context.

In summer 2009/2010 the timing of snow release and plant community patterns (by using a transect approach) were measured in six snowbanks in Central Otago, New Zealand, and six snowbanks in the Snowy Mountains, Australia. Selected plant traits (specific leaf area (SLA), leaf dry matter content (LDMC), seed mass, height at maturity, life form) and elevation range (only for New Zealand) were collated for the most abundant vascular plant species at each snowbank site to determine the relationships between their distributions and the snow melt gradient. Plant species trait patterns were analysed for each country, both at the quadrat and species level, and between countries.

In New Zealand, measured plant species traits at the quadrat level altered along the snowmelt gradient with seed mass (0.00001 - 0.0028 g), LDMC (120 – 580 mg/g) and height (2 – 290 mm) decreasing and SLA (2 – 34 mm2/g) increasing the later the exposure date. The elevation range of plant species changed across the whole gradient from 1700 to 600m. Species level analysis suggested that plant species at the earliest exposed sites ensure stress resistance through high structural investment (high LDMC), while the latest exposed sites particularly select for strategies associated with reproductive traits (i.e. seed mass). At the species level, at the latest exposed sites shrubs and forbs dominated the earlier exposed sites while graminoids were more frequent at the later exposed areas.

In Australia a subset of traits showed strong variations along the snowmelt gradient. At the quadrat level height at maturity (4 – 250 mm) increased the later the exposure date, though the trend was weak, with high variation in the data. Seed mass (0.00001 – 0.04 g) consistently decreased the later the exposure date. At the species level, pant species exhibiting tough, high fibrous leaves (low SLA) were more prominent at the earliest exposed sites (1 – 77 mm2/g). LDMC also displayed an increase (35 -890 mg/g) the later the exposure date within the earliest exposed sites, though the trend was highly variable and weak. Both at earliest and latest exposed sites shrubs dominated the early exposed sites while graminoids and forbs were restricted to later exposed snowbank areas.

The comparison between both countries displayed similar trends for height at maturity (decrease) and SLA (increase with later snowmelt). However, LDMC and seed mass showed contrasting trends, with steeper decrease of both seed mass and LDMC in New Zealand. The steeper trait shifts in New Zealand suggest that snowbank habitats are more distinct compared to the apparently more uniform habitats in Australia. In New Zealand, plant species exhibit distinct strategies at the early and late exposed sites to withstand site specific environmental constraints. A trade-off of between resource conservation at the early exposed (long growing season, high LDMC) sites and fast resource acquisition at the late exposed (short growing season, low LDMC) sites is suggested. In contrast, the environmental conditions from early to late exposed sites appear less pronounced in Australia probably because the environmental conditions are more homogenous in the snowbanks habitat along the observed snowmelt gradient. This study provides an understanding of general principles along snowmelt gradients in New Zealand and Australian snowbanks. Further experimental research with a focus on biotic interactions and the distribution of nutrients in the snowbanks is required to complement the findings of this work.