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Jackrabbit (Lepus Californicus) Herbivory Changes Dominance in Desertified Chihuahuan Desert Ecosystems

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Jackrabbit (Lepus Californicus) Herbivory Changes Dominance in Desertified Chihuahuan Desert Ecosystems ARTICLE IN PRESS Journal of Arid Environments Journal of Arid Environments 70 (2007) 418–426 www.elsevier.com/locate/jaridenv Jackrabbit (Lepus californicus) herbivory changes dominance in desertified Chihuahuan Desert ecosystems G.A. Rotha,b, W.G. Whitfordb,Ã, Y. Steinbergera aFaculty of Life Sciences, Bar-Ilan University, Israel bUSDA-ARS Jornada Experimental Range (JER), MSC 3JER New Mexico State University, Las Cruces, N.M. 88003, USA Received 21 August 2006; received in revised form 2 January 2007; accepted 29 January 2007 Available online 23 March 2007 Abstract This study addressed the question: can herbivory by a medium size herbivore, black-tail jackrabbits (Lepus californicus), change dominance in desertified ecosystems dominated by two species of shrubs. Shrubs that were pruned by jackrabbits in plant communities dominated by creosotebush (Larrea tridentata) and tarbush (Flourensia cernua) were compared to shrubs not browsed by the rabbits. In the mixed shrub area, herbivory on F. cernua resulted in death of 46.6% of the shrubs, compared to only 4.8% of L. tridentata shrubs. There was no evidence of jackrabbit browsing of dead F. cernua in a tarbush monoculture area. The canopy volumes of F. cernua plants that survived repeated browsing were significantly smaller than predicted based on unbrowsed plants with the same basal stem areas. Jackrabbit browsing resulted in increased canopy volume of creosotebush shrubs. Creosotebush average canopy volume significantly exceeded predicted values because of compensatory growth of stems from nodes below the severed point. Close spatial association of lightly browsed creosotebushes with heavily browsed tarbush may be a factor contributing to low utilization of creosotebush stems by jackrabbits. Differential browsing by the rabbits is shifting these ecosystems toward an L. tridentata monoculture. r 2007 Elsevier Ltd. All rights reserved. Keywords: Browsing; Compensatory growth; Creosotebush; Flourensia cernua; Larrea tridentata; Preferential feeding; Tarbush ÃCorresponding author. Tel.: +1 505 521 1358. E-mail address: [email protected] (W.G. Whitford). 0140-1963/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2007.01.009 ARTICLE IN PRESS G.A. Roth et al. / Journal of Arid Environments 70 (2007) 418–426 419 1. Introduction The most widespread result of desertification worldwide during the past two centuries has been the loss of grasslands and the establishment of shrubs and small trees in the former grasslands (Whitford, 2002). Changes in life form and species composition of the vegetation plus increased erosion by wind and water has been attributed to heavy grazing by livestock plus variation in rainfall resulting in drought (Archer, 1994; Buffington and Herbel, 1965; Grover and Musick, 1990). The Chihuahuan Desert in North America was primarily grassland in the 1850s and was rapidly converted to ecosystems dominated by creosotebush (Larrea tridentata), honey mesquite (Prosopis glandulosa) and tarbush (Flourensia cernua)(Buffington and Herbel, 1965). When grasslands transition to shrublands, the resulting shrub dominated ecosystems are considered to be alternate stable states that resist attempts to re-establish grassland (Gibbens et al., 2005; Whitford, 2002). In some desertified ecosystems, the vegetation may still be changing albeit at a much slower rate. Such changes in plant community structure may result from activities of animals, which is an unappreciated role of animals in arid ecosystems. In addition to livestock grazing, vegetation dynamics are directly and indirectly affected by animal activities (Chew, 1974; Chew and Whitford, 1992; Curtis et al., 2000; Steinberger and Whitford, 1983; Whitford and Kay, 1999). In the Chihuahuan Desert, the black-tailed jackrabbit (Lepus californicus) is the only mammalian herbivore that has been documented to feed on creosotebush (Dabo, 1980; Jaeger, 1948). Jackrabbits feed on shrubs primarily during the winter when there is little or no green vegetation available (Currie and Goodwin, 1966; Steinberger and Whitford, 1983; Westoby, 1980). Although availability of desirable forage largely determines jackrabbit diets, they do feed preferentially and utilize plants in different proportions from their relative abundance (Johnson and Anderson, 1984). Desertified landscapes in the Chihuahuan Desert include areas where creosotebush and tarbush are co-dominants and areas where these shrubs are essentially monotypic dominants. In areas where creosotebush and tarbush were co-dominant, we observed what we thought to be differential pruning of stems by jackrabbits on these species. Therefore, we designed a study to answer the following questions: (1) Does jackrabbit pruning affect the long-term growth characteristics of the shrubs? (2) Does differential pruning of one species potentially affect the species composition of the area? (3) Is there evidence that jackrabbit pruning of stems contributes to the mortality of either shrub species. Following stem browsing by jackrabbits, shrubs often exhibit compensatory growth, producing multiple new shoots from nodes below the cut (Steinberger and Whitford, 1983). In many cases, heavy browsing changes the plant morphology, creating a shrub volume smaller than expected, with much denser foliage in comparison to unbrowsed shrubs. It has been suggested that heavy browsing enhances the plant’s vigor and survivorship due to compensatory growth. We compared compensatory growth responses of L. tridentata and F. cernua in different topographic locations with different proportions of shrubs. 2. Methods Two study sites were on the piedmont slopes of the Dona Ana Mountains in the northern Chihuahuan Desert of southern New Mexico. Elevation varies from 1200 to ARTICLE IN PRESS 420 G.A. Roth et al. / Journal of Arid Environments 70 (2007) 418–426 1385 m. One study site was located at the lower piedmont slope approximately 500 m from the edge of the ephemeral lake at the terminus of the watershed. The second site was located on the upper piedmont slope where the surface was dissected by numerous small drainage channels. The study was conducted on landscape units with creosotebush (L. tridentata) as the dominant shrub mixed with tarbush (F. cernua) as subdominant. These sites were compared with an area dominated by F. cernua in essentially a monoculture on a run-on area with clay-loam soils. On the lower slope we established 20 plots of 6 Â 10 m in which we measured the height and diameter of each shrub recording dead stems and stems that had been browsed by jackrabbits in the past. Canopy volume of L. tridentata was estimated from height and diameter using the equation for an inverted cone (Ludwig et al., 1975). Canopy volume of F. cernua was calculated as the volume of a cylinder using canopy height and diameter. In order to determine the effect of browsing on the shrub, we measured the diameters of all stems of each shrub at ground level or at the junction of the stem and root crown. The sum of the diameters of basal stems of a shrub provided the best predictor of canopy volume (Figs. 1 and 2). We used the stem diameter data with the canopy volume data of unbrowsed or lightly browsed shrubs to develop regressions to predict the canopy volume and biomass of the shrubs in the absence of jackrabbit browsing. On all shrubs that had been browsed by jackrabbits, we estimated the percent of stems that had been browsed. Stems that had been browsed by jackrabbits were characterized by an angular cut on stems greater than 2.0 mm diameter. Regressions of plant volume of unbrowsed plants against the sum of basal stem diameters yielded r2 of 0.90 for F. cernua and 0.86 for L. tridentata. Maximum values for predicted plant volume were set according to the largest individual measured in the field, 3.59 m3 for tarbush and 3.62 m3 for creosotebush (Figs. 1 and 2). The regression equation for calculating F. cernua volume in the absence of herbivory is: y ¼ 0.0131e18.298x; Herbivory effect on plant volume - Tarbush 1.2 1 ] 3 0.8 0.6 0.4 Plant Volume [m 0.2 0 00.1 0.2 0.3 0.4 Sum of Basal stem Diameters [m] Fig. 1. The effect of different browsing levels on the actual canopy volume in comparison to the predicted canopy volume of tarbush (Flourensia cernua). ~/ Browsing level 0o10% of stems browsed; r2 ¼ 0.67 (Diamonds), ’/ Browsing level 1; 10–50% of stems browsed; r2 ¼ 0.70 (Squares), m/ Browsing level 2; 450% of stems browsed; r2 ¼ 0.73 (Triangles). ARTICLE IN PRESS G.A. Roth et al. / Journal of Arid Environments 70 (2007) 418–426 421 Herbivory effect on plant volume - Creosotebush 4 3.5 ] 3 3 2.5 2 1.5 1 Plant Volume [m 0.5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Sum of Basal stem Diameters [m] Fig. 2. The effect of herbivory on creosotebush (Larrea tridentata) canopy volume in comparison to the predicted canopy volume based on plants with no evidence of prior browsing by jackrabbits. ~/ Browsing level 0;o10% of stems browsed; r2 ¼ 0.65 (Diamonds), ’/ Browsing level 1; 10–50% of stems browsed; r2 ¼ 0.61 (Squares), m/ Browsing level 2; 450% of stems browsed; r2 ¼ 0.64 (Triangles). R2 ¼ 0.9004 where y is the plant volume and x is the sum of the diameters of the basal stems. The regression equation for calculating plant volume of L. tridentata that exhibit no evidence of previous browsing is: y ¼ 0.0353e9.787x; R2 ¼ 0.8638 where y is the volume and x is the sum of diameters of basal stems. Browsed shrubs were assigned to one of three categories: (1) negligible signs of previous browsing to less than 10% clipped branches, (2) moderate signs of browsing, between 10% and 50% clipped branches, (3) heavy browsing, 450% clipped branches and/or a morphologic change in plant structure (morphologic change is a common outcome of heavy browsing in both species).
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