Contrasting foliar responses to experimental drought and rainfall addition in co-occurring piñon and juniper trees

Background/Question/Methods

Piñon pines and junipers are the two dominant woody species of the semi-arid woodlands of the southwestern United States and they differ markedly in their drought-tolerance and stomatal behaviour. The severe drought that occurred in 2000-2003 led to extensive piñon mortality while juniper mortality was much more limited. As predicted regional climate changes progress, lower precipitation and warmer temperatures are likely to reduce piñon abundance compared to juniper hence affecting ecosystem composition and function. We compared the performance of the two species under different precipitation regimes using an ecosystem scale rainfall manipulation experiment. Beginning in 2007, field grown piñons and junipers were subjected to three different precipitation regimes, an irrigated, an ambient and a partial rainfall exclusion treatment. The impact of opposite precipitation changes on leaf traits and functions was addressed by measuring leaf properties and gas-exchange rates over the course of two growing seasons. Based on these measurements, we calculated leaf-scale water use efficiency (WUE), nitrogen use efficiency (NUE) and carbon use efficiency (CUE) to compare species within each precipitation treatment.

Results/Conclusions

Precipitation treatment significantly affected leaf water potential, stomatal conductance and photosynthesis of the two species, with all three variables increasing in proportion to water availability in the treatments. Juniper exhibited higher gas exchange rates on a leaf area basis in every treatment, but rates were higher for piñon on a leaf mass basis. Despite differences in leaf mass per area (LMA) and photosynthesis per unit leaf mass, the two species had a similar NUE (photosynthetic assimilation per unit nitrogen). The intrinsic WUE, assessed from the 13C composition of recently assimilated sugars, was higher for piñon pines under dry or ambient precipitation regimes but similar for the two species in the irrigated treatment. The ratio of leaf photosynthesis to respiration, a surrogate of CUE, was higher for juniper than for piñon in every treatment, but decreased with drought for the two species. We conclude that the increased drought resistance of juniper compared to piñon was not due to a better WUE or NUE, but rather to the ability to perform photosynthesis at lower water potentials and a better CUE. Junipers are thus more likely than piñons to keep a positive carbon balance under future dryer conditions.