Print PageSemi-arid piñon–juniper woodlands of the southwestern United States are water-limited ecosystems that have recently experienced extensive drought-induced mortality. Mortality has been more common in piñon pine (Pinus edulis) than for co-occurring juniper (Juniperus monosperma), reflecting differences between the two species in drought-tolerance and stomatal regulation. As predicted regional climate changes progress, lower precipitation and warmer temperatures are likely to increase water stress and drought-induced mortality. We studied the impact of drought on the response of field grown piñon pines and junipers subjected to three different precipitation regimes, an irrigated, an ambient and a partial rainfall exclusion treatment. Over six months during the 2010 growing season, we measured diurnal patterns of leaf gas-exchange rates for the two species in the different precipitation treatments. These data were used in conjunction with measurements of leaf water potential and carbon isotope discrimination to address the effect of drought on stomatal conductance and photosynthetic assimilation, and to compare the performance of the two species under varying water availabilities.
Results/Conclusions
The precipitation regimes significantly impacted leaf water potentials in both species but differences between treatments were more pronounced for the anisohydric juniper than for the isohydric piñon pine. Leaf water potential and precipitation treatments had significant effects on stomatal conductance and photosynthesis for the two species. But in every treatment and across all leaf water potentials, juniper always exhibited higher stomatal conductance and photosynthetic assimilation than piñon pine. The drought sensitivity of leaf gas-exchange, the change in stomatal conductance or photosynthesis with decreasing water potential, was higher for piñon pine than for juniper but was conserved among treatments. Irrigation pulses produced a similar enhancement effect in both species. The intrinsic water use efficiency, assessed from the 13C composition of recently assimilated sugars, was higher for piñon pines under dry or ambient precipitation regimes but higher for junipers in the irrigated treatment. Junipers outperformed piñon pines across every precipitation regime and drought intensity by exhibiting higher gas-exchange rates under well-watered conditions and a lower sensitivity to drought stress. They maintained higher photosynthetic carbon gain than piñon pines despite experiencing lower water potentials and are thus more likely to keep a positive carbon balance under future dryer conditions.
