A study was conducted within a northern Arizona forest / woodland ecotone on three commonly occurring woody species (Pinus ponderosa, ponderosa pine; P. edulis, pinyon pine; and Juniperus osteosperma, Utah juniper) to determine the relative differences in stomatal regulation during drought.  We hypothesized that, unlike J. osteosperma, P. ponderosa and P. edulis would strongly regulate stomatal conductance (g_S) during drought.  We selected two mature individuals of each species growing at the same site.  To assess differences in soil water content, we used both a portable time-domain reflectometry (TDR) soil probe, and a pressure chamber to measure predawn water potential (Ψ_PD).  We compared diurnal changes in predawn to midday water potential (ΔΨ_PD-MD), and measured g_S concurrently with leaf Ψ and vapor pressure deficit (VPD) throughout the day. Measurements were taken during non-drought- (May) and drought-stressed (late June) periods within a growing season.

Results/Conclusions
Both TDR and Ψ_PD indicated the soil moisture in late June was significantly drier than in early May.  The ΔΨ_PD-MD was approximately 1 MPa for each species in early May.  By late June only J. osteosperma maintained that difference, while ΔΨ_PD-MD was nearly zero for both pine species.  In spring, g_S steadily dropped throughout the day for all species, with P. ponderosa exhibiting the highest correlation with time at r²=0.87.  As drought stress increased by late June, the g_S of both pine species dropped rapidly in the morning to nearly zero, while juniper g_S continued throughout the day.  For both seasons and all three species, VPD was significantly correlated with g_S (p<0.05).  Only in late June for J. osteosperma was the Ψ significantly and negatively correlated with VPD when the sample trees were analyzed separately (p=0.03, r²=0.65; p<0.0001, r²=0.99).  Our results confirm strong stomatal closure in response to high evaporative demand for all species, but especially for P. ponderosa and P. edulis.  However, during the drought-stressed period in late June, stomatal closure rapidly occurred in both pine species by mid-morning, while J. osteosperma was able to maintain g_S and thus carbon uptake throughout the day.  Unlike the isohydric behavior of the pine species, J. osteosperma exhibited an anisohydric behavior by tolerating increasing xylem tensions, i.e. by lowering its Ψ levels likely due to its low vulnerability to drought-induced xylem cavitation, which allowed g_S to continue throughout the day in late June.