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).