1st-year seedlings function as a demographic bottleneck in conifers, with mortality rates often greater than 90%. Increased drought episodes are anticipated to increase mortality rates of 1st-year seedlings, inhibiting future forest regeneration and changing the range limits of species. Within the genus Pinus, 1st-year seedlings of coexisting arid species display contrasting responses to drought, with species altering varying morphological and physiological traits. Currently, it is unclear as to which responses are associated with increased survival during prolonged droughts, and how these responses differ between yellow and white pines, as well as between small and large seeded species. In this greenhouse study, four montane pine species (Pinus jeffreyi, P. longaeva, P. strobiformis, P. teocote) found in arid climates were exposed to a gradual dry-down until mortality was reached. Throughout the study, gas exchange, allometry, tracheid anatomy, and leaf hydraulics were quantified to determine the plasticity of these parameters to drought, and how this plasticity was related to drought tolerance. We hypothesized that yellow pines and small seed species would show greater plasticity in physiological traits while white pines and large seed species would show greater plasticity morphological traits.
Results/Conclusions
Species-specific responses to drought were highlighted by their capacity to alter physiological characteristics. The small seeded yellow pine (P. teocote) showed little ability to acclimate to drought, showing no plasticity in morphological traits and a delayed physiological response. Both the large seeded yellow (P. jeffreyi) and white (P. strobiformis) pines inhibited gas exchange early on in the drought and increased their leaf mass per area throughout. The small seeded white pine (P. longaeva) also inhibited gas exchange early on in the drought, but showed no changes in morphological characteristics. Leaf hydraulic traits showed little plasticity in any of the four species, with each species maintaining narrow hydraulic safety margins throughout the dry-down. Together, it appears that 1st-year pine seedlings have species-specific drought tolerant strategies, based primarily around the species’ ability to inhibit gas exchange. Hydraulic traits appear to be relatively canalized, potentially highlighting the inability of such carbon limited seedlings to alter such carbon expensive traits.