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a Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: Bouteloua • defoliation resistance • genetic variation • grazing • phenotypic plasticity • Poaceae • sideoats grama
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Grazing by livestock, especially during the period 1870–1930, is associated with significant changes in the composition of vegetation in the desert grasslands of North America (Bahre, 1995). (Herein, grazing is assumed to include defoliation of aboveground herbaceous plant tissues by an animal as well as fouling, trampling, uprooting, and root feeding [Valentine, 1990].) Historical evidence suggests that the sizes of native grass populations were drastically reduced by intensive livestock grazing, and this was often exacerbated by drought (Hastings and Turner, 1965). Local extinction of perennial grasses (O'Connor, 1991) may have been common as this occurred (Humphrey, 1958). Historical photographs (Hastings and Turner, 1965) show that recolonization by native plants occurred at many grassland sites following reductions in grazing intensity. Little is known of the possible genetic consequences of the period of overgrazing on populations of recolonizing plants. The sequence of events described could have resulted in significant genetic changes. For example, extreme declines in effective population size could have resulted in inbreeding or reductions in levels of genetic variation due to random genetic drift in recolonizing populations (Polans and Allard, 1989; Barrett and Kohn, 1991). Genetic variation may have been further affected by selection for colonizing ability (Barrett and Husband, 1990) or resistance to the stresses of herbivory or the grazing-modified environment (Aarssen and Turkington, 1985). Alternatively, there may have been little lasting genetic effect of extended exposure to frequent defoliation or changes in local population size if plants responded plastically to defoliation and the postgrazing environment.
Comparison of the responses to defoliation in conspecific populations with different grazing histories represents a common experimental strategy for studying defoliation resistance. Numerous studies of this sort have taken place in the Great Plains of North America (Detling and Painter, 1983; Aarssen and Turkington, 1985; Carmen and Briske, 1985; Painter, Detling, and Steingraeber, 1989, 1993). Fewer comparable studies have been done with species from the more arid grasslands of North America where large native ungulates were less common than in the relatively mesic Great Plains (Parmenter and Van Devender, 1995). Trlica and Orodho (1989) compared heavily grazed and protected populations of Oryzopis hymenoides [Roem. & Schult] Ricker from a Great Basin grassland biotic community (Brown, 1994) in northwestern New Mexico. When grown in a common garden, no differences were observed in height or aboveground biomass between plants from a population that was heavily grazed and one that had been protected from livestock for >50 yr. These authors found no evidence for differential selection for defoliation resistance in the protected and grazed populations.
The research described here involved a comparison of the responses of two populations of a perennial nonrhizomatous grass Bouteloua curtipendula var. caespitosa (sideoats grama) (Gould and Kapadia, 1964) from southern Arizona to increasing frequencies of defoliation. The populations exist on desert grassland sites that differ in their history of exposure to domestic grazers. One site has natural physical barriers that effectively prevent access by these grazers. The other site has been regularly grazed by cattle for nearly 100 yr. Traits thought to be associated with defoliation resistance (Briske, 1996) were evaluated in ramets from both populations. The experimental design used permitted distinguishing that proportion of the total phenotypic variation over different frequencies of defoliation that was attributable to genetic differences within and between the populations. In addition to differentiating the two populations, this approach permitted estimates of the relative magnitudes of genetic variation and phenotypic plasticity for traits related to response to defoliation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Fifteen randomly selected mature B. curtipendula plants (basal area >15 cm) were dug from both populations in 1994. Plants from both populations are high polyploids with 2n = 75–92 (D. D. Showalter, University of Arizona, unpublished data). The plants dug were separated by at least 3 m along a 400-m transect at each site. Since B. curtipendula var. caespitosa is strongly caespitose (Gould and Kapadia, 1964), each of these plants was assumed to represent a unique genet. These genets were grown in 4-L pots in 1:1 (by volume) soil:potting mix in a shadehouse in Tucson, Arizona, for 6 mo. A minimum of 12 ramets was then established for each genet from single rooted tillers. These ramets were grown in 4-L pots in a greenhouse and treated uniformly for an additional 7.5 mo until differential defoliation treatments were imposed. This was done to reduce the possibility that any differences between the populations reflected transient effects associated with the original site of collection (Quinn and Miller, 1967). To insure equal opportunity for induction of any defoliation-induced responses in both populations, ramets were clipped 5 cm above the soil surface (removing all leaf blades) every 6 wk during the 13.5-mo acclimatization period, and the herbage was discarded. This clipping height, which represents ~70% removal of aboveground biomass (Schmutz, 1978), was used because it corresponds to the average defoliation observed on grazed plants on the cattle-impacted site when plants were dug.
Experimental design and treatments
Fourteen days following the final clipping of the acclimatization period, three defoliation treatments (frequencies) were imposed on the 15 genets from each population using a split-plot arrangement with four replications. Defoliation treatment (every 14, 28, or 56 d) was the main plot factor and population was the sub plot factor. Defoliation treatments continued for 168 d (14 d, N = 12; 28 d, N = 6; 56 d, N = 3). Preliminary experiments (S. E. Smith, unpublished data) showed that fresh and dry shoot biomasses were closely correlated across the range of developmental stages of tillers produced in the three defoliation treatments. Fresh biomass was found to be ~16% dry matter in these experiments and this value was used to calculate dry biomass, which is reported throughout.
Mean daily temperature in the greenhouse during the experiment was 25.7°C (mean low of 13.2°C, mean high of 38.0°C). Ramets were irrigated individually when leaf curling and color indicated drought stress. Soil moisture content was ~20–30% of field capacity at this point. Additional ramets grown in 4-L pots bordered the experimental ramets. Border ramets were defoliated on the same schedule as the experimental ramets they adjoined.
I measured traits associated with defoliation resistance (Briske and Richards, 1994) rather than focusing on traits likely to be selectively neutral, such as most molecular genetic markers (Eckert, Manicacci, and Barrett, 1996). This approach permitted evaluation of developmental plasticity and the likelihood and effects of selection on ecologically relevant genetic variation. Only vegetative tillers were produced by ramets in all treatments of this experiment. The number of newly produced or regrowing tillers not previously defoliated and the amount of fresh biomass removed was recorded after each defoliation. Morphological measurements were taken on each ramet before defoliation on day 112. Traits measured included the length, maximum width, specific mass of a 3-cm section, and angle of divergence from the vertical of a fully expanded leaf blade, length of the leaf sheath on the longest single tiller, and height (measured as soil surface to tip of tallest extended leaf since plants defoliated most frequently produced exclusively leaf blades). Relative tiller number was evaluated as the number of live tillers per ramet at the final defoliation of the experiment as a percentage of that in the defoliation immediately before treatments were imposed. Increases in this trait may reflect increased production of tillers and/or reduced tiller mortality. Ramets that failed to regrow 3 wk after the final defoliation were considered dead. The validity of this mortality assessment is supported by the observation that all border ramets that had failed to regrow 3 wk after the final defoliation also had not regrown >8 mo later (data not shown) under environmental conditions that would favor rapid growth in B. curtipendula. Three weeks after the final defoliation, soil was washed from roots, which were then dried at 60°C and weighed. Analyses for all traits other than ramet mortality were conducted only on ramets that survived until the final defoliation.
Statistical analyses and their interpretation
A mixed-model analysis of variance was used to examine the sources of variation in response to defoliation treatments (Miller and Fowler, 1993). In analyses containing data from both populations (combined analyses), populations and defoliation treatments were considered fixed effects and approximate significance of these effects and their interactions was determined by likelihood ratio Type III F tests using restricted maximum-likelihood estimation in the procedure MIXED of SAS (SAS, 1992). Using this procedure, variation within populations was also estimated and its significance evaluated by determining whether the component divided by the standard error (Z value) was different from zero. Three-way interactions were not significant and are not reported. A mixed-model analysis of variance was conducted for each population separately for traits when a significant population effect or population x treatment interaction was observed in the combined analysis, or when a significant variance component for genets was observed for at least one population in the separate analysis. Before analysis, normality was evaluated using the method of Shapiro and Wilk (1965) and homogeneity of variances tested using the Fmax test (Sokal and Rohlf, 1981). Accordingly, total aboveground biomass and plant height were log transformed, and the number of tillers per ramet was square-root transformed. Least-squares means and their associated SEs are reported. Significance of differences in ramet mortality between populations for each treatment and between treatments within a population were determined by constructing 2 x 2 contingency tables (mortality/survival vs. populations or treatments). Significance of any differences in mortality was determined by log likelihood ratio tests (Sokal and Rohlf, 1981). In all cases, statistical significance was assigned at P 
0.05.
With this analysis, genetic differentiation between the populations for a given trait is indicated by a significant population effect. Total genetic variation is estimated by the variance component for genets within populations. Because the genets examined could not be considered a random sample of the genetic variation within B. curtipendula var. caespitosa, estimates of genetic variance based on this analysis should be viewed conservatively. Evidence for phenotypic plasticity is provided by significant effects for defoliation treatment or the interaction between populations or genets and defoliation treatment. Significant variation in phenotypic plasticity (i.e., differences in slope of the reaction norm) is manifested in a significant population x defoliation treatment interaction (Miller and Fowler, 1993).
Variation in tiller number per ramet was observed among single-tiller-derived ramets at the time defoliation treatments were imposed. However, this trait was not significantly correlated with any other trait measured. Variation in this trait was also insignificant when included as a covariate in analysis of variance with each of the other traits analyzed. Therefore, no adjustment for initial tiller number per ramet was done.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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It has been demonstrated that frequently defoliated plants will be more productive if additional available carbohydrates are invested in rebuilding photosynthetic area, or if such plants exhibit higher photosynthetic rates following defoliation (Briske, 1996). Duncan et al. (1967) proposed that at leaf area indices less than ~3.5, photosynthesis across a community will be maximized when leaves are more horizontal. Therefore, producing more horizontally displayed leaves may be one way to increase carbon fixation following defoliation. Additionally, increased leaf angles may be associated with defoliation avoidance in grasses by placing more leaf biomass low to the ground where it would be less accessible to large herbivores (Painter, Detling, and Steingraeber, 1989). Differences in leaf blade angle of this sort were observed among populations of Andropogon gerardii Vitman, Pascopyrum smithii (Rydb.) Löve, Schizachyrium scoparium (Michx.) Nash, and Bouteloua gracilis (H.B.K.) Griffiths that had either been exposed to or protected from defoliation by prairie dogs and associated ungulates (Detling and Painter, 1983; Painter, Detling, and Steingraeber, 1989, 1993). In my experiment, the highest leaf angles (closest to horizontal) were exhibited with defoliation every 14 d in the cattle-free population (Fig. 1). These angles were uniformly high in this population in all treatments. The possible importance of this trait in affecting response to defoliation is also indicated by the apparent lack of genetic variation in this trait within the cattle-impacted population (Table 4). This would imply that strong selection toward more horizontal leaf blade display has occurred in this population. However, this result could also be due to selection acting on other traits that are strongly genetically correlated with the expression of leaf blade angle. Since high leaf blade angles would increase access to light by leaves in a closed canopy, differences in this trait between populations could also have been affected by differences in interplant competition at the two sites (Caldwell and Richards, 1986). This seems unlikely, however, given the low vegetative cover at the two sites and the associated reduced probability for aboveground competition among genets.
In addition to leaf blade angle, my results suggest that exposure to cattle may have resulted in reduced plasticity in plant height and relative tiller number in response to variation in defoliation frequency in the cattle-impacted population (Table 4, Fig. 1). Little difference in mean height between the populations indicates that reductions in variability in this trait within the cattle-impacted population may simply reflect the effects of selection for traits more closely related to defoliation resistance. Given the generally greater differences in relative tiller number between the populations and the association of relative tiller number with defoliation resistance (Briske and Richards, 1994), it appears reasonable to assume that selection for defoliation resistance is responsible for the reduction in genetic variation in this trait within the cattle-impacted population. Nevertheless, reductions in genetic variation within this population cannot be attributed entirely to the effects of selection. Random genetic drift could be at least partially responsible, although it is not possible to separate the individual effects of selection and random genetic drift with these data. The cattle-impacted population did exhibit significant genetic variation in tiller mass while the cattle-free population did not. Comparable findings were reported by Polley and Detling (1988) in studies with Pascopyrum smithii, although any adaptive significance of variation in this trait has not been explained.
My research shows that plants from two Bouteloua curtipendula populations from southern Arizona show remarkable developmental plasticity in many traits in response to different frequencies of defoliation. It also suggests that long-term exposure to cattle or other environmental differences between the sites the two populations occupy may have resulted in selection for traits associated with defoliation resistance. However, inadequate sampling when collecting genets for this study, random genetic drift, or selection for traits genetically correlated with those associated with defoliation resistance cannot be entirely excluded as factors leading to reduced genetic variation. It is also possible that uncharacterized environmental differences between the two sites are at least partially responsible for the differences between the two populations.
Reductions in genetic variation in the cattle-impacted population could negatively affect its evolutionary potential both in the presence and absence of domestic herbivores. The exact consequences of these reductions in genetic variation are difficult to predict, although it is likely that the potential for rapid adaptation to changing environments has been reduced as a result of exposure to domestic herbivores (Bazzaz, 1996). Moreover, my findings agree with Painter, Detling, and Steingraeber (1989) who suggested that caution should be used when interpreting research using artificial exclosure areas where domestic herbivores have been excluded for varying lengths of time. While the species composition within these areas may resemble the assumed pre-exclosure conditions, the genetic constitution of the populations may differ considerably between ones impacted by an exotic environmental force, such as domestic livestock, from those of native, naturally disturbed environments.
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FOOTNOTES |
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