Fire-adapted plants in a changing climate

Fire is an inherent disturbance in Mediterranean ecosystems, and plant species have evolved two main mechanisms by which to persist in these fire-prone ecosystems; post-fire recruitment from long-lived seedbanks in which seeds germinate profusely in response to fire-related cues such as heat or smoke (seeders), and post-fire resprouting, in which plants generate new shoots from surviving dormant buds (resprouters). Plant strategies for fire persistence interact with strategies for drought response. Thus, in this thesis, we hypothesize that climate can modify fire responses, and fire responses can modify climate responses. The capacity of species to adjust regeneration traits in response to rapid environmental change is mostly unknown. We hypothesized that local climate has generated among-population variability in regeneration mechanisms across climate gradients, an indicator of resilience to global change (Chapter 2 and 3). We first explored if post-fire seeders with heat-released seed dormancy, whose persistence depends on the maintenance of the soil seed bank until the arrival of fire, have variability in the minimum heat capable of releasing dormancy related to summer temperatures (Chapter 2). Our results show that seeds from populations with historically warmer summers required more heat to release dormancy, in order to avoid germinations in the inter-fire period. Resprouting species depend on stored carbohydrate reserves and vigorous regrowth for survival after disturbance, thus we investigated intraspecific variability in their resprouting response after cutting along a gradient of historical aridity and fire activity (Chapter 3). Our results show that resprouting was primarily determined by pre-disturbance plant condition, and that the majority of variability was found within populations, rather than among them, suggesting a high degree of independence from climate. We also found a secondary positive effect of aridity on plant condition and resprouting response, suggesting resilience to disturbance under low levels of water availability; and a negative effect of fire activity and of a second cutting, suggesting sensitivity to increased disturbance frequency. The contrast of post-fire regeneration niches drives differences in plant traits that determine drought resistance, thus, we also hypothesized that plant growth and survival in novel climate conditions will differ for seeders and resprouters (Chapter 4). The first summer after fire has a particularly high risk of drought-related mortality for post-fire germinations, but future increases in atmospheric CO2 concentrations may ameliorate plant stress via reduced stomatal conductance. Thus, we tested the effect of elevated CO2 (eCO2) on plant resource allocation, water-use, and drought stress for post-fire seeders, including both obligate and facultative (resprouting) species, under increasingly severe drought (Chapter 4). Our results demonstrated that eCO2 did not ameliorate drought stress in our studied species, and further, that the ability to resprout was a significant determinant of plant drought response. Non-resprouters were more tolerant of low-moderate drought, but tended to be more vulnerable to intense drought than resprouting species, even under eCO2. Overall, this thesis provides important insights into the complex interactions between fire, climate, and plant traits. We propose that examining global change effects through the lens of post-fire regeneration syndromes can provide a framework to understand diverging trajectories of Mediterranean biodiversity.