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.