Background/Question/Methods
Background. Climate change is projected to increase the frequency and intensity of summer drought in fire-prone Mediterranean areas. Plant regeneration after fire may occur from seed germination (seeders) or vegetative regrowth (resprouters). Whereas seeders usually have a shallow root system, resprouters have a deeper one. Rooting depth is related to drought survival. Therefore, the strategies to survive fire and drought may be related. Understanding this nexus may be important under ongoing changes in climate.
Question. What is the effect of different drought intensities on plant water status in species with differing fire-regeneration strategies? How does this affect photosynthesis and growth?
Methods. We modified the amount and timing of rainfall with an automatic rainout-shelter and irrigation system in sixteen 6x6m plots, in a shrubland dominated by a seeder (Cistus ladanifer) and three resprouter (Erica arborea, Erica scoparia and Phyllirea angustifolia) species in Central Spain. Four treatments were implemented: a) environmental control (no rainfall manipulation); b) mean historical rainfall (imitation of historical patterns); c) 25% reduction (5 months drought); d) 50% reduction (7 months drought). Soil moisture and micrometeorological variables were continuously monitored in the study plots. Plant water status (Ψpd), photosynthetic gas exchange and shoot growth were measured monthly during one growing season.
Results/Conclusions
Results. Drought treatments effectively reduced soil moisture and caused significant differences in water availability to plants in late summer. Minimum plant water potentials (Ψpd) were highest in P. angustifolia (-3.5MPa) and lowest in E. arborea (-7.5MPa). There were no significant differences on gas exchange among drought treatments, but yes among species. C. ladanifer showed highest initial photosynthetic rates, decreasing drastically until the end of the dry period (23 to 2.5 μmol CO2·m-2·s-1). The other species showed relatively constant photosynthetic rates during drought (10 to 2 μmol CO2·m-2·s-1). C. ladanifer had the greatest increase in biomass during the growing season (~30mg·g-1·day-1), also showing differences among treatments.
Conclusions. The designed system for rainfall exclusion was successful to reduce soil moisture to target levels. Rooting depth did not seem to be related to plant water availability, as two resprouter species (E. arborea and P. angustifolia) were the most and least sensitive to drought, respectively. C. ladanifer (seeder) showed a high resistance to drought, synchronizing its growth and gas exchange rates in relation to water availability in the soil. This didn’t happen with E. arborea, the species that could suffer more in a future scenario of increasing frequency and intensity of summer drought.