Snow is an important control of plant community functional composition in oroarctic tundra

Abstract

The functional composition of plant communities is a critical modulator of climate change impacts on ecosystems, but it is not a simple function of regional climate. In the Arctic tundra, where climate change is proceeding the most rapidly, communities have not shifted their trait composition as predicted by spatial temperature–trait relationships. Important causal pathways are thus missing from models of trait composition change. Here, we study causes of plant community functional variation in an oroarctic tundra landscape in Kilpisjärvi, Finland. We consider the community-weighted means of plant vegetative height, as well as two traits related to the leaf economic spectrum. Specifically, we model their responses to locally measured summer air temperature, snow conditions, and soil resource levels. For each of the traits, we also quantify the importance of intraspecific trait variation (ITV) for between-community functional differences and trait–environment matching. Our study shows that in a tundra landscape (1) snow is the most influential abiotic variable affecting functional composition, (2) vegetation height is under weak local environmental control, whereas leaf economics is under strong local environmental control, (3) the relative magnitude of ITV differs between traits, and (4) ITV is not very consequential for community-level trait–environment relationships. Our analyses highlight the importance of winter conditions for community functional composition in seasonal areas. We show that winter climate change can either amplify or counter the effects summer warming, depending on the trait.Electronic supplementary materialThe online version of this article (10.1007/s00442-019-04508-8) contains supplementary material, which is available to authorized users.

Relationships between aboveground plant traits and carbon cycling in tundra plant communities

“…These effects manifested through different mechanisms: summer warming increased photosynthesis by increasing plant size, while winter warming both increased plant size and accelerated leaf economics. This demonstrates how environmental conditions outside the growing season are important for understanding changes in high-latitude ecosystem functioning (Happonen et al 2019) .…”

“…At the study area, we measured Photosynthetically Active Radiation (PAR) as photosynthetic photon flux density (PPFD, µmol m 2 s -1 ) continuously every 10 minutes from July 8th to August 17th 2017 (Methods S1 and Supplementary Table 2). Soil moisture was measured during the growing seasons of 2016-2018 3-6 times per year as volumetric water content (%) ( (Happonen et al 2019;Kemppinen et al 2018) , Supplementary Tables 1-2 ) . In each soil moisture plot, three measurements were taken, and the average was used in further analyses.…”

“…We quantified vascular plant community species composition in the majority of the plots where CO 2 flux measurements were conducted using the point-intercept method during the peak growing season in 2017 (Supplementary Table 1). These data have previously been used to study the relationship between the environment and functional composition (Happonen et al 2019) . A circular frame (20 cm diameter) with 20 evenly spaced pinholes was placed over the soil collar (see section CO 2 flux data) after CO 2 flux measurements.…”

“…We measured plot-specific plant height to indicate plant size, and LDMC as well as SLA to represent the leaf economics spectrum for all species (Happonen et al 2019) . Due to time constraints, we did not measure other traits.…”

“…Moreover, we simulate how summer and winter warming could cause cascading effects on the carbon uptake capacity of the ecosystem, measured as the peak-season CO 2 budget. We explain the functional properties of communities and carbon cycling with environmental variables describing soil resources, and summer and winter temperatures which have been shown to explain functional composition in this system (Happonen et al 2019) . To understand carbon cycling comprehensively, we focus not only on CO 2 fluxes (gross primary productivity, ecosystem respiration, soil respiration), but also on carbon stocks (above-ground and soil organic carbon stocks; for abbreviations used in Materials and Methods, and Results, see Table 1).…”