Climate-growth: site level
The climate-growth model comparison calculated at the site level over the entire period (1893-2013) indicated that summer temperatures were the main limiting climate factor for annual shoot length growth ofCassiope tetragona at the research site on Disko Island, Greenland (Table 1, Fig. 4). Higher summer temperatures led to longer shoot length increments (slope: 0.25 ± 0.03). The winter temperature model (slope: 0.12 ± 0.04) was the best non-summer model at the site level over this period, suggesting a positive influence of winter temperatures on growth. The site-level climate-growth model comparison computed over 1964-2013 gave similar results (Table S3). The mean summer temperatures plotted together with mean site level shoot length (Fig. 4a) show that summer temperature and shoot length generally fluctuated in the same direction from year-to-year. However, growth did not keep up with extreme summer temperatures, especially during the warm summers of the late 1940s, early 1960s and in recent decades. The site level model comparison over 1984-2013 showed that summer temperatures were still the most limiting climate factor for growth during this period, although mean late summer (July-August) temperatures became more important than mean summer (June-August) temperatures. Notably, the slopes of the summer temperature models calculated over the recent period (1984-2013) are lower than those calculated over the complete period (1893-2013). The early summer precipitation model was the only non-summer temperature model with ∆AIC>2, suggesting that June-July precipitation sums have been a limiting factor for shoot length growth at the site level during 1984-2013.
Table 1 Statistics of the selected climate-growth models (∆AIC>2) at the site level calculated over the periods 1893-2013 (a) and 1984-2013 (b). AIC, Akaike Information Criterion; SE, standard error; mR2, marginal coefficient of determination (proportion of variance explained by fixed effect); cR2, conditional coefficient of determination (proportion of variance explained by fixed and random effects); T, mean temperature; P, precipitation sum; prev., previous.