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).