Methods

Between 3 June and 1 July, 2005 I collected water infiltration rate data inside and outside the following long-term livestock exclosures in southern Arizona: National Audubon Society Research Ranch near Elgin, Buenos Aires National Wildlife Refuge near Sasabee, Chiricahua National Monument near Wilcox, Fort Huachuca Army Base near Sierra Vista and San

Bernardino National Wildlife Refuge near Douglas. These sites span approximately 200 km. All sites are dominated by Thermic Semiarid soils (Tubac-Sonoita-Grabe association at both Chiricauha National Monument and San Bernardino Wildlife Refuge; White House-Bernardino-Hathaway association at the other three sites [Hendricks 1985]). Livestock grazing occurs at present outside each grazing fence (personal observation) and likely has occurred in the vicinity of each site over most of the past century (Bahre 1991).

These sites exhibit great differences in vegetation ranging from dominance by perennial grasses (e.g., Audubon Research Ranch [Bock and Bock 2000]) to dominance by mesquite shrubs (Prosopis glandulosa) (e.g., San Bernardino Wildlife Refuge) (Table 1). P. glandulosa is found at all sites and is commonly associated with the desertification of arid grassland habitats in southwestern North America [Van Auken 2000]. Differences in vegetation across the sites likely result from historic differences in grazing intensity. All sites are at an elevation that historically supported arid grassland habitat [Brown and Lowe 1974; Bahre 1991].

At each of these sites, I recorded water infiltration rate over a 10 minute period using a Turf-Tech double ring infiltrometer [Castellano and Valone 2007]. The infiltrometer was placed on level patches of bare soil at paired locations on opposite sides of, and equidistant to, the grazing fence in areas that exhibited similar vegetation composition on both sides of the grazing fence. This yielded 9 - 37 grazed-ungrazed paired locations at each of the five sites (National Audubon Research Ranch: n = 37 locations across the North, East and South boundary fences; Buenos Aires National Wildlife Refuge: n = 9 locations across the West boundary fence; Chiricahua National Monument: n = 19 locations across the West boundary fence; Fort Huachuca Army Base: n = 22 locations across the West boundary fence; San Bernardino National Wildlife Refuge: n = 24 locations across the East boundary fence).

Table 1. A description of the study sites indicating the year the grazing fence was established and the dominant vegetation at present. Grass refers to perennial grass. Shrubs refers to mesquite (P. glandulosa).

Name

Year

Vegetation

Elevation (m)

Audubon Society Research Ranch 1968

Grass

1500

Buenos Aires National Wildlife Refuge

1985

Grass and Shrubs

1200

Chiricahua National Monument

1977

Grass and Shrubs

1400

Fort Huachuca Army Base

1950

Grass and Shrubs

1500

San Bernardino

National Wildlife Refuge 1980

Shrubs

1200

All locations were more than 10 m from the fence and adjacent locations were separated by at least 10 m. At each site, all data were collected on the same day within a 3 hour period to minimize differences in abiotic conditions during data collection (e.g., soil moisture and temperature). All data were collected during the dry season when soil was dry to a depth of greater than 10 cm (personal observation). At each site, I compared water infiltration rate values inside (ungrazed) versus outside (grazed) the grazing fence using a paired t-test.

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