Browsing Journal of Range Management, Volume 55, Number 1 (January 2002) by Subjects
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Drought and grazing: IV. Blue grama and western wheatgrassAn understanding of the impacts of grazing during and following drought on rangeland ecosystems is critical for developing effective drought management strategies. This study was designed to examine the effects of drought and grazing on blue grama [Bouteloua gracilis (H.B.K) Lag. ex Griffiths] and western wheatgrass [Pascopyrum smithii Rydb. (Love)] tiller growth dynamics. Research was conducted from 1993 to 1996 at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. An automated rainout shelter was used during 1994 to impose a severe late spring to early fall (May to October) drought on 6 of twelve, 5- x 10-m non-weighing lysimeters. Twice replicated grazing treatments were: 1) grazed both the year of (1994) and the year after (1995) drought; 2) grazed the year of and rested the year after drought; and 3) no grazing either year. Drought had minimal impact on tiller relative growth rates of plants grazed twice, although it reduced (P less than or equal to 0.01) rates of axillary tiller emergence for blue grama (79%) and western wheatgrass (91%), respectively. Defoliation periodically increased relative growth rates (P less than or equal to 0.05) and tiller emergence (P less than or equal to 0.01) of both species. Neither drought nor grazing affected tiller densities or tiller replacement rates of either species nor did they affect productivity of blue grama. Drought, however, reduced (P less than or equal to 0.01) productivity of western wheatgrass 50% in 1994 whereas grazing reduced productivity (P less than or equal to 0.01) by 46% in 1994 and 69% in 1995. Moderate stocking levels (40-50% utilization) during and after drought did not adversely affect the sustainability of these dominant native grasses.
Effects of top-soil drying on saltcedar photosynthesis and stomatal conductancePhreatophytes are trees and shrubs with deep roots tapping the water tables. As such they are presumed to be able to tolerate a water deficit in the top soil. Growth of some phreatophytes is decoupled from environmental factors such as incident precipitation. This study examined the effects of surface soil drying on gas exchange and stomatal conductance of a riparian phreatophyte Tamarix gallica L. (saltcedar) during 2 consecutive growing seasons in which summer precipitation varied substantially. Daily average gas exchange (A) was 13.5 micromol m(-2) sec(-1) in June and 13.4 micromol m(-2) sec(-1) in September, 1991 when surface soil was wet as compared to the same periods of 1990 in which very little rain occurred (6.44 and 8.08 micromol m(-2) sec(-1), respectively, P < 0.0001). Stomatal conductance (g) or maximal conductance showed a similar trend of photosynthesis. Both average gas exchange and stomatal conductance were correlated with water content in the upper portion of the soil (r = 0.83 to 0.88 for A, P < 0.05 and r = 0.65 to 0.70 for g, P < 0.05) in 1990 (a dry year). The variations in gas exchange or stomatac conductance of saltcedar were mainly caused by water availability in the upper soil layers, not by depth to the water table (0.65 vs 2.74 m). The responses of gas exchange and stomatal conductance to surface soil drying in the phreatophyte saltcedar were similar to that of several crop species [lupin (Lupinus cosentinii Guss. cv. Eregulla), wheat (Triticum aestivum L. cv. Cadensa) and sunflower (Helianthus annuus L.)]. Our data suggest that upon soil re-wetting, when water availability to shallow lateral roots increased, the entire root system of saltcedar was actively involved in water uptake, leading to higher stomatal conductance and photosynthesis.