River valleys are important corridors of conservation within northern semiarid grasslands. The valley hillsides are often unaffected by agricultural and industrial development because of their steep slopes and variable soil quality. Topographic heterogeneity is high in hillside ‘coulee’ environments adjacent to river valleys, resulting in a high degree of microclimatic variability. In this study, we measure photosynthetic gas-exchange, leaf reflectance, chlorophyll fluorescence, sap flow and stable carbon isotope composition, to assess the impact of slope aspect on seasonal patterns of physiological acclimation to drought stress in grass and shrub species of the southern Alberta coulees.

Results/Conclusions

During summer 2007, drought stress resulted from below average precipitation (24 % of the 1971-2000 normal) and higher than average mean daily temperature (2.8°C above the normal) during June-August. Soil moisture levels were relatively high before the drought, because of ample precipitation in May (84 mm). Response to these conditions varied by functional type (grasses vs. shrubs) and slope aspect (N-facing vs. S-facing). In the grass species Stipa viridula and Agropyron cristatum, high net photosynthesis rates (A_max), in the 19 to 32 μmol m^-2 s^-1 range, were observed in May, but rates fell to zero in late June, except for N-facing specimens of S. viridula, which continued to photosynthesize during the drought, at rates of 3 to 7 μmol m^-2 s^-1. The leaf reflectance indices PRI (photochemical reflectance index), NDVI and CI (chlorophyll index) decreased sharply in June, except in N-facing specimens of S. viridula. In the shrub species Artemisia cana and Rhus trilobata, A_max decreased with day of year (DOY) in close association with vapour pressure deficit (VPD) and leaf temperature, from peaks of 11 to 23 μmol m^-2 s^-1 in June, to zero on S-facing slopes by mid-summer. However, A_max remained positive on N-facing specimens of both species (4 to 7 μmol m^-2 s^-1) and rebounded in S-facing plants after September rains. Higher leaf stable carbon isotope composition (δ¹³C) was observed in S-facing shrubs from DOY 192 to 264. NDVI and CI varied little with DOY in A. cana, due to its reflective leaf hairs, but PRI decreased. By contrast, PRI and CI exhibited a parabolic seasonal pattern in R. trilobata of both slopes, but NDVI decreased slightly after peaking in June. The seasonal pattern of sapflow was consistent with decreasing transpiration and stomatal conductance in response to declining soil moisture and vapour pressure deficit.