Growth patterns
The number of leaves per shoot remained relatively stable through time and retained an average of 4.8 ± 0.13 leaves in cool-cool treatments, 4.3 ± 0.75 in centre-centre treatments and 4.3 ± 0.16 leaves per shoot in warm-warm treatments by the end of the experiment. Centre-warm treatments were the exception and declined from 4.0 ± 0.29 leaves shoot-1 at the beginning, to 2.5 ± 0.79 leaves shoot-1 by the end of the experiment. Underlying this pattern of stability, however, were marked differences in leaf production and turnover between treatments. New leaf production displayed a positive relationship with maximum temperature and significant differences between source populations, characterised by a higher rate of new leaf production in treatments sourced from cool-edge population, than centre or warm-edge populations (Table S2, Fig. 4A). In relation to heat stress anomalies, new leaf production displayed a strong positive relationship and no difference between source populations (Table S2, Fig. 4B), suggesting local thermal adaptation or acclimatization within source populations. P. oceanica growth rates (shoot elongation) and total leaf length per shoot displayed non-significant relationships with maximum temperatures and heat stress anomalies (p> 0.05, Fig. 4D, E, G, H). Rather, source populations were the best predictor of growth and total leaf length over the course of the experiment (Table S2). At the end of the thermal stress period, growth rates were significantly different among transplant treatments (ANOVA, F(5,112) = 3.8, p = 0.003, Fig. 4F). Cool-cool and warm-warm treatments displayed the highest growth rates despite experiencing up to 6ºC difference in temperature, and centre-centre and centre-warm treatments displayed significantly lower growth rates (Fig. 4F). By the end of the recovery period, growth rates had increased in all treatments except centre-warm shoots which remained significantly lower than cool-edge (Tukeys-HSD, p = 0.005) and warm-edge controls (Tukeys-HSD, p = 0.018). For total leaf length, shoots recorded their shortest length at the end of the heat stress period and recovered to be significantly longer by end of the recovery period (Fig. 4I). Moreover, at the end of the thermal stress period, cool-edge procedural controls were significantly longer than all other treatments and warm-edge procedural controls were significantly longer than cool-warm or central-warm treatments (Table S2). These relative differences held to the end of the recovery period (Fig. 4I).
Growth rates in natural meadows displayed similar patterns to transplants, albeit with higher rates over the same heat stress and recovery periods (Fig. S2). During the thermal stress period, growth rates were significantly higher in cool-edge meadows than warm-edge meadows and both cool-edge and warm-edge meadows displayed significantly higher growth rates than central meadows (ANOVA, F(2,66)= 17.12, p<0.001). New leaf production was highest in central meadows and significantly lower in cool-edge and warm-edge meadows during the thermal stress period (ANOVA, F(2,66) = 13.56, p<0.001).