Climate-growth: shrub level
A similar picture emerged from the climate-growth model analyses on the
individual shrub level (Table S4, Figs S4 and 5). Computed over the
complete period (1893-2013), all shrubs had a summer climate model as
best model. The summer temperature model was the best model for 12/25,
the late summer temperature model for 8/25, and the early summer
temperature model for 4/25 shrubs. For one shrub, the previous summer
precipitation model was the best model. All best models had a positive
slope, which ranged between 0.15 and 0.37 (Table S4a). For 14/25 shrubs
sampled, non-summer climate models had a ∆AIC>2, 11 of
which were winter temperature models: growth of five shrubs responded to
winter (November - April), three to early winter (November - December),
and three to late winter (March - April) temperatures. All winter
temperature models had a positive slope (between 0.18 and 0.33; Table
S4a), suggesting a beneficial effect of higher winter temperatures
preceding the growing season on shoot length development at the study
site for 11 shrubs. Climate in spring (May) was found to be the best
non-summer climate model for two of the shrubs: May temperature for one
and May precipitation for the other. Both these models had a positive
slope. Winter precipitation had a negative impact on shoot length of one
of the shrubs (DI25), which otherwise pre-dominantly responded to late
summer temperature. The model comparison calculated over 1964-2013 at
the individual shrub level produced comparable results (Table S5). As
seen on the site level, annual shoot length growth of many individual
shrubs tracked year-to-year variability in summer temperatures during
most years with divergence during warm extremes in recent decades, as
well as earlier periods (Figures S4a and S5). There was a spatial
pattern visible in the distribution of shrub response to summer climate,
with shrubs responding to early summer temperatures confined to up-slope
positions, a late summer temperature response found mostly downslope,
and a summer temperature response in between and at overlapping
positions with either (Fig. 5a).
Summer temperature models best explained growth for 12/25, i.e. less
than half of the shrubs, when the analysis was performed over the period
1984-2013 (Table S4b, Fig. S4b). Shoot length growth of seven shrubs
responded to late summer (July-August) temperature and that of five to
summer (June-August) temperature. These summer temperature models had a
positive slope, which ranged between 0.17 and 0.34 (Table S4b). For 5/25
shrubs, the early summer precipitation model was the best model (slope
between 0.25 and 0.38). For 4/25 shrubs, a non-summer climate model was
the best model. For two shrubs, early or late winter temperature models
best explained shoot length growth. Both models had a positive slope.
Two other shrubs showed a negative response to previous fall
temperatures (Table S4b). Four of the 25 shrubs did not respond to
climate over this period. Over 1984-2013, non-summer climate models had
a ∆AIC>2 for 11/25 shrubs, 7 of which were winter
temperature models: one early (prev. November- prev. December), three
late (March-April) and three winter (prev. November-April) temperature
models, all with a positive slope (Table S4b). Three shrubs showed a
negative response to previous fall temperatures. In contrast to the
calculation over 1893-2013, in the recent period (1984-2013) late winter
precipitation had a positive impact on shoot length of one of the shrubs
(DI24), which pre-dominantly responded to late summer temperature.
Shrubs with a non-summer model as best model and non-responsive shrubs
were generally positioned further upslope (Fig. 5c), as were shrubs with
previous fall temperature as best non-summer model (Fig. 5d).