With rising air temperatures and the rates of anthropogenic nitrogen (N) emissions, temperate forests are expected to experience more frequent and severe droughts in the future while N availability increases. Because N is one of the major nutrients that limit tree growth, the shift in N availability may influence the ways trees respond to and recover from droughts. We examined physiological responses and resource allocations of ponderosa pine (PIPO), Douglas-fir (PSME), and lodgepole pine (PICO) seedlings to drought and post-drought conditions in a greenhouse study. We hypothesized higher water and N availability prior to drought lead to greater photosynthetic capacities and storage of resources, which in turn leads to greater resilience to and faster recovery from drought. In the first half of the experiment, we exposed the seedlings to four different treatments: well-watered with high-N (WN) or low-N (LWN), low water with high-N (LWN) or low-N (LWLN). Following this initial phase, the seedlings underwent a severe drought (no water), followed by a recovery period in which all seedlings received sufficient water. To evaluate physiological responses and resource allocation of the seedlings, we measured photosynthesis, Vcmax, photosynthetic N-use efficiency, and shifts in biomass and N allocations among needles, stems, and roots.
Results/Conclusions
Our results show that the seedlings responded differently to the combinations of water and N availability. We observed the highest mortality for PSME during the severe drought (25-64%), occurring in all the water-N treatments. PICO showed 27% mortality only for LWN treatment, whereas no mortality was observed for PIPO. Furthermore, for PIPO, the rate of photosynthesis (A) and Vcmax increased in all the treatments before the drought and declined toward the end of the experiment with the earliest decline occurring in WN treatment. In contrast, A and Vcmax of PSME declined in all the water-N treatments over the course of the experiment, while the rates were relatively unchanged for PICO. We also found that PIPO responded best to the post-drought conditions with the greatest gain in total biomass observed in WLN treatment. Finally, the N content of PIPO needles was lowest in WN treatment and highest in the two low-water treatments, suggesting potential declines in N-use efficiency when water availability is high. Our findings imply that the responses of conifers to drought and post-drought conditions differ by species and by the availability of water and N prior to a drought.