Drivers of plant community composition and species richness in Western Greenland

The Arctic experiences rapid climate change, but our ability to predict how this will influence plant communities is hampered by a lack of data on the extent to which different species are associated with particular environmental conditions, how these conditions are interlinked, and how they will change in coming years. Increasing temperatures may negatively affect plants associated with cold areas due to increased competition with warm-adapted species, but less so if local temperature variability is larger than the expected increase. Here we studied the potential drivers of vegetation composition and species richness along coast to inland and altitudinal gradients by the Nuuk fjord in western Greenland using Hierarchical Modelling of Species Communities (HMSC) and linear mixed models. Community composition was more strongly associated with random variability at intermediate spatial scales (among plot groups 500 m apart) than with large-scale variability in summer temperature, altitude or soil moisture, and the variation in community composition along the fjord was small. Species richness was related to plant cover, altitude and slope steepness, which explained 42% of the variation, but not to summer temperature. Jointly, this suggests that the direct effect of climate change will be weak, and that many species are associated with microhabitat variability. However, species richness peaked at intermediate cover, suggesting that an increase in plant cover under warming climatic conditions may lead to decreasing plant diversity.

Methods
The study was conducted at five different sites along the Nuuk fjord (Godthåbsfjorden) in western Greenland. 

In 2011–2013, 414 permanent plots were established across the five study sites. The plots were circular, with a diameter of 2 m. Three groups of six plots were selected for every 100 m increase in altitude. The position of the first plot at each altitude was selected by walking uphill straight towards a pre-selected point until the isocline was reached (measured using a hand-held GPS). Plot centres were placed exactly 10 m apart along the isoclines, or slightly more if needed to prevent plots from being entirely in water or having an average slope >45°. Plot groups were placed exactly 500 m apart. The lowest plots were moved to 20 m a.s.l. to avoid exposure to salt. The highest altitudes used in sites 1–5 were 200, 200, 400, 500 and 500 m a.s.l., respectively, reflecting that the mountains are higher further east. The study design was the exact same as previously used in Young Sund (Nabe-Nielsen et al. 2017).

The vegetation survey included a complete inventory of all species of vascular plants in each plot. This inventory was conducted in 2011–2013. Maximum height and diameter were measured for the woody species in 2021–2022. The cover of woody species, graminoids, and herbaceous plants was assessed for each plot by two independent observers as percent cover <2 m from plot centres. Subsequently the two estimates were averaged. Plants were named following Böcher et al.(1978).