Native Australian seedlings exhibit novel strategies to acclimate to repeated heatwave events

Heatwaves are becoming more intense and frequent. Plant photosystem
thermal thresholds can vary with species, but also shift in response to
environmental triggers. Both upper and lower thresholds can acclimate to
repeated heatwaves through ecological stress memory, where prior exposure
primes them for subsequent events. The extent to which acclimation to
repeated heat stress events varies among environmental origin and/or
species is unknown. Different acclimation strategies might reflect biome
of origin, or may be species-specific. For 12 species from two contrasting
biomes – extreme desert and benign coastal temperate – we investigated
responses to two simulated heatwaves, via shifts in upper and lower
critical temperatures of photosystem II, and the difference between these
thresholds, thermal tolerance breadth (TTB).  Biome of origin had
no effect on thermal tolerance. Observed differences among species
following heat events suggested two possible acclimatory strategies. In
some cases, species increased thermal thresholds during the first
heatwave, but at the cost of reduced thermal tolerance during the second
heatwave, a sprinter strategy. Other species acclimated to the first
heatwave and further increased thermal tolerance to a second heatwave,
indicative of ecological stress memory, a marathoner strategy. 
Synthesis: These among-species responses to heatwaves could suggest
distinct vulnerabilities and resilience to repeat heat stress events, with
some species having limited capacity to tolerate consecutive heatwaves,
possibly as the cost of acclimation is too great, with other species
having the advantage of increased tolerance via stress memory, helping
them survive future stress, at least in the short-term.