drought and ecological site interaction on plant composition of a semi-arid rangeland

Drought and Ecological Site Interaction on PlantComposition of a Semi-Arid Rangeland

J. D. Scasta1 and B. S. Rector2

1Oklahoma State University, Natural Resource Ecology and Management,Stillwater, Oklahoma, USA2Texas A&M University, Ecosystem Science and Management,College Station, Texas, USA

Fluctuating climatic patterns are increasing the frequency and severity of drought,a concern for native plant communities on grazed semi-arid rangelands. Vegetationsuccessional models have focused on the impact of management and have failed toquantify the effects of extreme drought. From 2001 to 2011, plant communitycomposition was sampled on ecological sites in a semi-arid rangeland managed withconservative grazing and frequent fire since 1937. Ordination and classification wereused to assess the interactive effects of ecological site and extreme drought on plantspecies composition, holding all other external drivers constant. Deeper soil clayloam sites had 4x greater beta diversity than shallower and rockier low stony hillsites, an indication of greater species turnover and instability in response to extremedrought. Cumulative effects of drought years explained similarity between sites andspecies composition. Response to extreme drought varied by species; no response(Bouteloua curtipendula), decreased (Nassella leucotricha), and increased(Bouteloua rigidiseta and Eriochloa sericea). Annual C3 plant responses wereexplained by short-term drought and perennial C3 and C4 plant responses wereexplained by long-term drought. Clay loam sites had maximum species richnessand diversity values during neutral periods with quadratic declines associatedwith climatic extremes (dry or wet) compared to the more xeric sites which hadminimum species richness and diversity during neutral periods with quadratic increasesduring climatic extremes. The interaction between site and drought, holding all otherexternal drivers constant, can enhance our understanding of plant community dynamicsand secondary plant succession of degraded semi-arid rangelands.

Keywords climate, degraded, ecological site descriptions, multivariate, savanna,species diversity

Climatic variation and global warming trends continue to be an issue of concernworldwide as it relates to biodiversity and the stability of plant communities(Thomas et al., 2003), especially on grazed arid and semi-arid rangelands. It is

Received 28 February 2013; accepted 8 July 2013.This paper is dedicated to Dr. Jake Landers and to the TSSRM YRW Directors for their

commitment to understanding rangeland plant communities of the Texas Edwards Plateau.Address correspondence to J. D. Scasta, Oklahoma State University, Natural Resource

Ecology and Management, 008C Agricultural Hall, Stillwater, Oklahoma 74075, USA.E-mail: [email protected]

Arid Land Research and Management, 28:197–215, 2014Copyright # Taylor & Francis Group, LLCISSN: 1532-4982 print=1532-4990 onlineDOI: 10.1080/15324982.2013.824046

197

expected that these trends will result in a higher frequency and severity of drought inthe Northern Hemisphere (Meehl & Tebaldi, 2004; Burke & Brown, 2008). Concernregarding the sensitivity of arid and semi-arid rangeland plant communities to esca-lating drought is valid based on the long-term vegetation shifts and slow recoverythat has been documented globally (Mworia et al., 1997; Gamoun et al., 2011; Sasakiet al., 2013). Plant community stability in response to drought has been shown to bepositively correlated with species diversity (H’) (Frank & McNaughton, 1991).Variability from year to year in total aboveground plant biomass is also lowerin communities with greater plant species richness due to interspecific competition(Tilman, 1996). Plant species respond variably to extreme drought as some may growmore, some may not be impacted, and some may grow less (Penuelas et al., 2007).Furthermore, the interaction between drought and grazing can lead to long-termshifts in plant species composition (Danckwerts & Stuart-Hill, 1988; Milton et al.,1994; Mworia et al., 1997). Arid and semi-arid rangelands, dominated by perennialwarm-season grasses are subject to highly variable rainfall and particularly vulner-able to long-term degradation (Knapp & Smith, 2001; Fay et al., 2003; Sasakiet al., 2013). Ecological sites have been recommended as the basic land unit thatshould be used for assessing the condition and trends of rangeland plant communi-ties (Creque et al., 1999). Given the highly variable precipitation of arid andsemi-arid rangelands and concerns about global warming, we propose this must alsoinclude escalating drought in addition to management (Burke & Brown, 2008; Meehl& Tebaldi, 2004). Additionally, on arid and semi-arid rangelands it can be quitedifficult to untangle the interactive effects of climatic variation, fire, and grazing(Fuhlendorf et al., 2001). Our focal hypothesis is that over the long-term, plantcommunities vary in stability and response to extreme drought, holding all externaldrivers constant (specifically fire and grazing). The primary questions addressed inthis project are: 1) Do plant species composition, diversity, and richness of ecologicalsites differ in response to extreme droughts, holding all other external drivers con-stant? 2) Is similarity between ecological sites influenced by drought severity? 3) Dospecies response curves differ along the drought severity gradient?

Methods

Study Location

The study sites were on semi-arid rangeland in Menard County, TX (30� 460 0100 N,99� 460 4000 W) at 660 meters above sea level. Menard County is located within theEdwards Plateau vegetation area south-east of San Angelo, Texas, USA. Precipi-tation typically occurs in a bimodal pattern averaging 559mm annually with drywinters and hot summers (USDA, 2013). Soils formed over a substrate of Cretaceouslimestone with a significant gradient of deeper to shallower soils (Fowler & Dunlap,1986; Fuhlendorf et al., 2001).

The 1212 ha of rangeland was heavily grazed until 1937 when it was purchasedby the family that has maintained ownership and management since that time. Graz-ing from 2001 through 2011 (the timeframe of this study) used mixed species grazingwith beef cattle and meat goats. Stocking rate was light consisting of 45 animal unitsannually (27 ha per animal unit) of which �45% were meat goats. From 1987 to2007, all research locations had been burned four times with the most effective firein 1997 and the most recent fire in 2005 (5 year fire return interval). Fire and grazing

198 J. D. Scasta & B. S. Rector

were similar across ecological sites (Landers, 2012). Low Stony Hill (LSH) is themajor (comprising >80 percent of the study area) ecological site and is characterizedby shallow soils (6 to 20 inches), moderate slopes and more exposed substrate (stonyand cobbly surface texture). The horizon is comprised of 50%, by volume, of frag-ments of limestone over a petrocalcic horizon (USDA, 2013). Clay Loam (CL) isthe minor (comprising <20% of the study area) ecological site and is characterizedby deep (26 to 56 inches), calcareous alluvium soils and is more level than LSH(USDA, 2013). LSH is the more xeric of the sites and typically the less productive.

Vegetation Sampling

Vegetation sampling was conducted annually in late June from 2001 to 2011 (elevenyear period). Plant species composition was determined at each ecological site usinga modified step-point pace transect method (Owensby, 1973; Mentis, 1981; Smith &Knapp, 2001). Within the ecological sites, five transects were randomly placed witha semi-permanent starting point and oriented in a predetermined permanent direction.Transects were 100m in length and every 2m along the transect, observers recordedthe nearest plant species, plant functional group (graminoid, forb, cacti, or woodyplant) or the dominant soil cover (bare soil, rock, or litter). Nomenclature adheresto Diggs et al. (1999), Gould (1975), Hatch et al. (1990), and Hatch and Pluhar(1993). As an observational, non-experimental study assessing uncontrolled eventslike drought, we have made efforts to avoid issues of pseudoreplication. Primarystrategies include the strip-transect type design, annual randomization, multipleobservers, and pooling data which is described in the following sections (Hurlbert,1984; Schwarz, 1998; Jona�ssova & Prach, 2008; Gordo & Sanz, 2009).

Climate Data

During the sampling period (2001 to 2011) three extreme droughts occurred (2006,2009, and 2011) that have been characterized as some of the worst in recordedhistory (mean precipitation in 2011 was 255mm). We chose to use two assessmentsof drought conditions based on Palmer’s indices (Palmer, 1965) by using Palmer’s ZIndex (Z Index) which measures short-term drought on a monthly scale and thePalmer Drought Severity Index (PDSI), a measure of the length and intensity ofcumulative effects of drought patterns (Jensen, 2003). We used mid-June values fromthe southwest sub-regional location in San Angelo, TX (NCDC, 2013).

Statistical Analysis

In order to avoid pseudoreplication as much as possible, we pooled all plant speciesrichness, diversity, cover class, and functional group data across transects for eachsite and used the resulting mean as the input for analysis of variance (Jona�ssova &Prach, 2008; Gordo & Sanz, 2009). The species level plant composition data wasused to develop the following plant diversity and richness metrics: N2 Diversity,N1 Richness, N2=N1 Evenness, Number of Species, and Shannon’s Diversity (H’)(Hill, 1973; Magurran, 2004). One-way analysis of variance was used to compareeleven year means between sites and t-tests were used to compare cover classesand plant functional groups (SAS Institute, 2011). Ordination techniques wereapplied using CANOCO 4.5 and CANODraw to account for the complex species

Drought-Site Interaction on Semi-Arid Rangeland 199

data set and multiple variables (ter Braak & Smilauer, 2002). Detrended correspon-dence analysis (DCA) is a unimodal response technique of indirect gradient analysis.Using species data for all years and sites, DCA was first conducted as a preliminaryanalysis to test the magnitude of species composition change along the firstordination axis (Pellerin et al., 2008; Auestad et al., 2009; Verstaeten et al., 2012).Canonical correspondence analysis (CCA) was then applied using species data andexplanatory variables. As a constrained approach, CCA allows for the identificationof ordinal axes based on known explanatory variables in explaining plantcommunity variation (Ugurlu et al., 2012). Based on the a priori decision to assessthe interactive effects between site and drought the explanatory variables used wereecological site, PDSI and Z Index. Interactions between drought indices and ecolo-gical site were (PDSI�Site and Z Index�Site) assessed. Monte Carlo permutation testswith 999 permutations were used to assess the significance of canonical axes (terBraak & Smilauer, 2002). A cluster analysis using Ward’s minimum variance methodby site, relative drought situation and year was conducted using PROC CLUSTERand PROC TREE (SAS Institute, 2011). This statistical method groups into clustersby maximizing within-cluster homogeneity, and a semi-partial R-squared (SPR)measures the loss of homogeneity attributed to each subsequent grouping step(Sharma & Kumar, 2006; SAS Institute, 2011). Linear and non-linear least squaresregression was used to assess the response of vegetation diversity and richnessmetrics and species response curves of the dominant graminoids for each site alongthe drought severity gradient (SysStat, 2012).

Results

During the survey period, 109 plant species from 35 families and 89 genera wererecorded at all sampling locations and ecological sites combined. Low Stony Hill(LSH) ecological site had 31 families, 74 genera, and 89 total plant species present,and Clay Loam (CL) ecological site had 28 families, 69 genera, and 81 total plantspecies present. Appendix 1 provides a list of all vascular plant species identifiedduring the survey period and includes plant origin, season of growth, life cycle, growthhabit, and mean percentage composition by ecological site. LSH had higher plantspecies diversity (N2 Diversity, N1 richness, and Shannon’s Diversity), higher rockcover, and lower litter cover (Table 1 and Table 2). CL had lower plant species diver-sity and variability, lower rock cover, and higher litter cover. Species richness and

Table 1. Eleven year mean� standard error for plant speciesrichness and diversity metrics by ecological site (LSH¼Low StonyHill and CL¼Clay Loam)

Metric LSH CL p-value

Number of Species 31.7� 2.6 27.6� 0.9 0.158N2 Diversity 10.2� 0.6 7.7� 0.6 0.006�

N1 Richness 15.3� 1.1 11.8� 0.7 0.016�

N2=N1 Evenness 0.7� 0.02 0.6� 0.02 0.278Shannon’s Diversity (H’) 2.7� 0.1 2.5� 0.1 0.015�

�Significant a 0.05.


evenness was similar between ecological sites. Detrended Correspondence Analysis(DCA) of the species data revealed a gradient length of 2.422 standard deviationunits of species response and thus moderate beta diversity of species composition.Total inertia for the species data was 1.868 indicating the spread of species in ordinalspace. The first DCA axis had an eigenvalue of 0.266 and explained 14.3% of thevariance of the species data and the second axis had an eigenvalue of 0.132. The firsttwo axes combined explain 21.3% of the variance of species data. The first axisappears to be based on ecological site (Figure 1). The second axis appears to be

Figure 1. Detrended correspondence analysis (DCA) biplot of plant species composition datawith site scores labeled by ecological site (LSH¼Low Stony Hill and CL¼Clay Loam) andyear. Envelopes are drawn around ecological sites that do not overlap even during years ofextreme drought. The first axis has a gradient length of 2.422 standard deviation units (anindication of beta diversity) and total inertia was 1.868 (sum of all eigenvalues) indicatingspread in ordinal space.

Table 2. Eleven year mean� standard error percentage cover classand plant functional group by ecological site (LSH¼Low StonyHill and CL¼Clay Loam)

Cover class LSH CL p-value

Bare Soil 20.3� 2.2 17.3� 2.7 0.268Rock 8.2� 0.7 2.7� 0.8 <0.001�

Litter 47.1� 3.6 54.9� 3.7 0.015�

Grass 20.0� 2.4 21.6� 3.0 0.675Forb 2.9� 1.1 1.9� 0.9 0.452Cacti 0.6� 0.2 0.6� 0.1 0.933Brush 1.0� 0.2 1.0� 0.3 0.989

�Significant a 0.05.


explained by some combination of site and year (possibly influenced by drought fluc-tuation). Canonical Correspondence Analysis (CCA) revealed an eigenvalue of 0.174for the first axis, explaining 31.7% of the species-environment relation, which is wellcorrelated with the explanatory data (r¼ 0.928). The second axis had an eigenvalueof 0.146 and the first two axes explain 58.2% of the species environment relationwhereas the first four axes cumulatively explain 90.3%. The test of significance forthe first canonical axis and all canonical axes under the full model was significant(F¼ 1.87; P¼ 0.001). The first axis is best explained by ecological site and the inter-action between site and long-term drought (PDSI�Site). The second axis is bestexplained by short-term drought (Z Index) and the interaction with ecological site(Z Index�Site) (Figure 2). Total inertia is 1.868, an indication of the variation of spe-cies scores. Along the first axis, CL has 4x the beta diversity compared to LSH (1.2and 0.3 standard deviation units respectively), an indication of species turnover orinstability as driven by extreme drought.

Displaying the dominant 30 species in an ordination biplot based on speciesscores and explanatory variables reveals additional insight about the species compo-sition that defines each ecological site (Figure 3). Dominant C4 perennial graminoidspecies for each site are spread along the first axis which is best explained by site.Woody plants and cacti that are also considered to be closely associated with site

Figure 2. Canonical correspondence analysis (CCA) biplot with site scores and explanatoryvariables as vectors [Site, PDSI (long-term drought), Z Index (short-term drought), and inter-actions between site and drought (PDSI�Site, Z Index�Site)]. The first axis had an eigenvalueof 0.174 and is explained by ecological site and long-term drought and their interaction. Thisexplains 31.7% of the species-environment relation and is well correlated with explanatorydata (r¼ 0.928). Monte Carlo test for significance (999 permutations) is significant(p¼ 0.001). Along the first axis, CL has 4x the beta diversity compared to LSH (1.2 and 0.3standard deviation units, respectively), an indication of species turnover or stability as drivenby extreme drought.


are also displayed along the first axis site gradient. Several forbs display site associ-ation, including: Siphonoglossa pilosella (hairy tubetongue, present only on LSH) onthe left and S. elaeagnifolium and Cirsium texanum (Texas thistle, present only onCL) to the right. Several species seem to be displayed along the second axis andwould be more susceptible to long term drought. These include: Vulpia octoflora (six-weeks fescue, an annual C3 season grass) and Vicia ludoviciana (Lousiana vetch, anannual C3 legume).

Classification using Ward’s minimum variance method of cluster analysis revealssimilarity based on heterogeneity between potential clusters (Figure 4). Ecological sitesin 2001 and 2002 were similar and had the highest SPR value (0.23). The next clusterseparated CL sites in moderate to extreme drought years (2006, 2008, 2009, 2010, and2011) from all other sites and years (SPR¼ 0.20). An interesting exception to this is theinclusion of CL site in 2007, an extremely wet year. However, this year occurred afteran extremely dry year of 2006 indicating a lack of recovery from the extreme droughtin 2006 (considered as cumulative drought effects). The next cluster separated allremaining CL sites from all LSH sites, regardless of drought extent for the LSH sites(SPR¼ 0.11) with the exception of LSH in 2005, a relatively wet year. The next clusterseparated CL in 2011 from the other CL sites (SPR¼ 0.06), which we interpret as thecumulative effects of numerous dry or very dry years. The next cluster separated LSH2010 and 2011 from the remaining LSH sites (SPR¼ 0.05), which we interpretsimilarly, as the cumulative effects of numerous dry or very dry years.

Figure 3. Canonical correspondence analysis (CCA) biplot of explanatory variables as vectors[Site, PDSI (long-term drought), Z Index (short-term drought), and interactions between siteand drought (PDSI�Site, Z Index�Site)]. Thirty dominant vascular plants are displayed. Filledcircle symbols (.) represent cool season forbs, empty circle symbols (�) represent warm seasonforbs, plus sign symbols (þ) represent warm season grasses, filled triangle symbols (~) rep-resent cool season grasses or sedges and empty square symbols (&) represent cacti, vines,shrubs, or trees. Refer to Appendix 1 for additional details for plant species.


Responses to the drought severity gradient differed by site and vegetation metricalthough no relationships were significant (a 0.05) (Figure 5). As drought increases inseverity (increasingly negative PDSI values) responses by ecological site vary. Thecharacteristic response for CL sites is maximizing all values during neutral periodswith declines associated with climatic extremes (dry or wet) (Figure 5a–e). Thecharacteristic response of LSH sites minimizes number of species, N1 richness andH’ during neutral periods with increases associated with climatic extremes (Figure 5a,5c, and 5e), a poorly correlated N2 diversity linear trend (Figure 5b) and a trendsimilar to CL sites for evenness (Figure 5d).

Species response curves for the dominant graminoids for the ecological sitesstudied in this project were analyzed using linear and non-linear regression in res-ponse to the Palmer Drought Severity Index (PDSI). Only two species had significantresponses. B. rigidiseta in the LSH site, increased linearly in relative abundance asdrought escalated in severity. E. sericea in the CL site, increased in a quadratic(non-linear) fashion as drought escalated in severity (Figure 6).

Figure 4. Dendrogram from cluster analysis using Ward’s minimum variance method by site(LSH¼Low Stony Hill, CL¼Clay Loam), relative drought situation (ExtD¼Extremely Dry,D¼Dry, N¼Neutral, W¼Wet, ExtW¼Extremely Wet) and year. Using PROC CLUSTERand PROC TREE in SAS. The semi-partial R-Squared (SPR) indicates the loss of homogen-eity associated with merging two clusters. Small values are indicative of homogenous clustersand larger SPR values are indicative of the merging of heterogeneous clusters.


Discussion

Characteristic plant succession of rangeland in the Edwards Plateau includeswoody plant encroachment, particularly Juniperus ashei J. Buchholz (ashe juniper)(Fowler & Simmons, 2008). Interestingly, J.ashei was not found in the vegetationsurvey period on any of the sites which is attributed to the five year fire return inter-val on the research site (Taylor, 2003; Allred et al., 2012). Comparing ecologicalsites, the shallower more xeric site, LSH, has higher plant diversity and a unique

Figure 5. Diversity and richness response to the drought severity gradient (PDSI) by site for(a) number of species, (b) N2 diversity, (c) N1 richness, (d) Evenness, and (e) Shannon’sdiversity index (H’). Regression lines and r2 values are presented for ecological sites.


Figure 6. Species response curves for dominant perennial graminoids for LSH and CLEcological Sites. Scatter plots with Palmer Drought Severity Index (PDSI) on the x-axisand a standardized index on the y-axes. Linear and nonlinear regression between PDSI andrelative abundance for the species by site. Dashed lines indicate the confidence intervals onlyfrom significant analyses (a¼ 0.1).


plant community composition and lower beta diversity or species turnover inresponse to drought (Figure 2 and Figure 3) (Frank & McNaughton, 1991; Gamounet al., 2011).

Consecutive years of drought have been shown to have a greater negative effect onthe plant community than a single year (Ganskopp & Bedell, 1981; Herbel & Gibbens,1996) which was shown in both our ordination and classification analyses. In semi-aridlandscapes with persistent bare ground and interspaces, species turnover in response toepisodic precipitation events is attributed to annual forb recruitment and establish-ment in the interspaces between perennial grasses. This concept drives our interpret-ation of frequency data as species compositional changes and species dominance.Ganskopp and Bedell (1981) indicated that grasses that were light or moderatelygrazed persisted under drought conditions better than ungrazed plants, which is sup-ported by the species response curves (Figure 1). Species responses are also influencedby photosynthetic pathway and life cycle (annual or perennial). Annual C3 plants(V. octoflora and V. ludoviciana) responded to short-term drought while perennialC3 plants (N. leucotricha) responded to long-term drought (Figure 4 and Figure 6).Dominant grasses either showed no relationship to drought severity, or in the instanceof two mid-grasses (B. curtipendula on LSH and E. sericea on CL), actually increasedin relative abundance as drought severity increased. E. sericea, a perennial C4midgrass characteristic of CL sites (along with Nassella leucotricha), increased quad-ratically in response to escalating drought which is attributed to species turnover,especially C4 response relative to C3 (decline of Nassella leucotricha) and the co-dominance of these two species. E. sericea did not appear to be responsive in neutralto wet years but very responsive to the cumulative effects of drought. Extreme wet anddry periods did seem to influence the similarity between the ecological sites we studiedconsidering the cluster analysis (Figure 4). Both sites in 2001 and 2002 were differentthan all other sites, regardless of site or drought severity. This is attributed to thecumulative effects of long-term drought as these sites were coming out of the 1990’swhich was a neutral to wet decade, as opposed to the drier 2000’s (dry years occurredin 2006, 2008, 2009, 2010, and 2011). This response is also evident in the greater varia-bility of response seen in dry years compared to wet years (Figure 6).

Conclusions

Ecological sites appear to be a relevant concept even in anticipation of escalatingdrought conditions (Thomas et al., 2003; Meehl & Tebaldi, 2004; Burke & Brown,2008) especially for managers and technical advisors. However, degradation fromover-grazing and abuse may have long-lasting impacts in these semi-arid rangelands.In our study area, the historical climax plant community consisted of mid and tallgrass species [Andropogon gerardii Vitman, Sorghastrum nutans (L.) Nash, andTripsacum dactyloides (L.) L.] (USDA, 2013), none of which were found duringthe eleven year period of sampling. Based on peak historical stocking densities inthe Edwards Plateau of Texas in the early 1900s (Wilcox et al., 2012) and the priorknowledge of overgrazing at our research location, these grasses were likely grazedout. Given the conservative management of our research site since 1937 (includingincreasingly regular fire and conservative grazing), and the current dominance ofshort and midgrasses, supports other research that indicates recovery of arid andsemi-arid native plant communities is a very long-term process after severe dis-turbance (Milton et al., 1994; Sasaki et al., 2013). Maintaining the native plant


community is a critical component of ecosystem stability and resiliency in the face ofanticipated warming conditions and will serve to sustain ecosystem goods andservices on arid and semi-arid rangelands.

Our study supports experimental studies that have documented variable res-ponses to extreme drought among species: some grow more, some grow less andsome are not impacted at all (Penuelas et al., 2007) and that grazing impacts recoveryof semi-arid plant communities (Danckwerts & Stuart-Hill, 1988; Mworia et al.,1997). The interaction between site and drought, holding all other external driversconstant, can enhance our understanding of plant community dynamics and second-ary plant succession. The exposed substrate, slope, and topography of the LSH siteoffer a greater range of micro-sites and niches for a broader range of plants, impact-ing diversity measures. Specifically, the soil physical and hydrological propertiescharacteristic of each ecological site influence plant community composition drivenby the necessary water-efficient strategies of dominant species and changes to cli-matic fluctuations, as demonstrated by differential responses of the ecological sitesin this study (De Boeck et al., 2006). This concept is especially valid in arid andsemi-arid environments that developed under the historical disturbance regime offire and grazing and has a history of abuse from over-grazing (Fuhlendorf et al.,2001; Taylor, 2003; Allred et al., 2012).

References

Allred, B. W., S. D. Fuhlendorf, F. E. Smeins, and C. A. Taylor. 2012. Herbivore species andgrazing intensity regulate community composition and an encroaching woody plant insemi-arid rangeland. Basic and Applied Ecology 13: 149–158.

Auestad, I., K. Rydgren, and R. H. Økland. 2009. Scale-dependence of vegetation-environment relationships in semi-natural grasslands. Journal of Vegetation Science 19:139–148.

Burke, E. J., and S. J. Brown. 2008. Evaluating uncertainties in the projection of futuredrought. Journal of Hydrometeorology 9: 292–299.

Creque, J. A., D. B. Scot, and N. E. West. 1999. Viewpoint: Delineating ecological sites.Journal of Range Management 52: 546–549.

Danckwerts, J. E., and G. C. Stuart-Hill. 1988. The effect of severe drought and managementafter drought on the mortality and recovery of semi-arid grassveld. Journal of theGrassland Society of Southern Africa 5: 218–222.

De Boeck, H. J., C. M. H. M. Lemmens, H. Bossuyt, S. Malchair, M. Carnol, R. Merckx, I.Nijs, and R. Ceulemans. 2006. How do climate warming and plant species richness affectwater use in experimental grasslands? Plant and Soil 288: 249–261.

Diggs, G. M., B. L. Lipscomb, and R. J. O’Kennon. 1999. Shinners & Mahler’s IllustratedFlora of North Central Texas. Botanical Research Institute of Texas, Fort Worth, Texas.

Fay, P. A., J. D. Carlisle, A. K. Knapp, J. M. Blair, and S. L. Collins. Productivity responsesto altered rainfall patterns in a C4-dominated grassland. Oecologia 137: 245–251.

Fowler, N. L., and Dunlap, D. W. 1986. Grassland vegetation of the eastern Edwards Plateau.American Midland Naturalist 115: 146–155.

Fowler, N. L., and M. T. Simmons. 2008. Savanna dynamics in central Texas: Just succession?Applied Vegetation Science 12: 23–31.

Frank, D. A., and S. J. McNaughton. 1991. Stability increases with diversity in plantcommunities: Empirical evidence from the 1988 Yellowstone drought. Oikos 62: 360–362.

Fuhlendorf, S. D., D. D. Briske, and F. E. Smeins. 2001. Herbaceous vegetation changein variable rangeland environments: The relative contribution of grazing and climaticvariability. Applied Vegetation Science 4: 177–188.


Gamoun, M., M. Tarhouni, A. O. Belgacem, M. Neffati, and B. Hanchi. 2011. Response ofdifferent arid rangelands to protection and drought. Arid Land Research and Management25: 372–378.

Ganskopp, D. C., and T. E. Bedell. 1981. An assessment of vigor and production of rangegrasses following drought. Journal of Range Management 34: 137–141.

Gordo, O., and J. J. Sanz. 2009. Impact of climate change on plant phenology in Mediterra-nean ecosystems. Global Change Biology 16: 1082–1106.

Gould, F. W. 1975. The Grasses of Texas. Texas A&M University Press, College Station,Texas.

Hatch, S. L., K. N. Gandhi, and L. E. Brown. 1990. Checklist of the Vascular Plants of Texas.Texas Agricultural Experiment Station, Texas A&M University System, College Station,Texas.

Hatch, S. L., and J. Pluhar. 1993. Texas Range Plants. Texas A&M University Press, CollegeStation, Texas.

Herbel, C. H., and R. P. Gibbens. 1996. Post-drought vegetation dynamics on arid rangelandsin southern New Mexico, New Mexico Agricultural Experiment Station Bulletin 776.New Mexico State University, Las Cruces, New Mexico.

Hill, M. O. 1973. Diversity and evenness: A unifying notation and its consequences. Ecology54: 427–431.

Hurlbert, S. H. 1984. Pseudoreplication and the design of ecological field experiments.Ecological Monographs 54: 187–211.

Jensen, D. T. 2003. Monitoring drought in the West. Arid Land Research and Management 17:449–454.

Jona�ssova, M., and K. Prach. 2008. The influence of bark beetles outbreak vs. salvage loggingon ground layer vegetation in Central European mountain spruce forests. BiologicalConservation 141: 1525–1535.

Knapp, A. K., and M. D. Smith. 2001. Variation among biomes in temporal dynamics ofaboveground primary production. Science 291: 481–484.

Landers, J. Q. 2012. Personal communication with Dr. Jake Q. Landers, Ph.D., retired range-land plant ecologist and co-owner=manager of the research sites.

Magurran, A. E. 2004. Measuring biological diversity, pp. 72–130. Blackwell Science Ltd.,Malden, Massachusetts.

Meehl, G. A., and C. Tebaldi. 2004. More intense, more frequent and longer lasting heatwaves in the 21st century. Science 305: 994–997.

Mentis, M. T. 1981. Evaluation of the wheel-point and step-point methods of veld conditionassessment. Proceedings of the Annual Congresses of the Grassland Society of SouthernAfrica 16: 89–94.

Milton, S. J., W. R. J. Dean, M. A. du Plessis, and W. R. Siegfried. 1994. A conceptual modelof arid rangeland degradation. BioScience 44: 70–76.

Mworia, J. K., W. N. Mnene, D. K. Musembi, and R. S. Reid. 1997. Resilience of soils andvegetation subjected to different grazing intensities in a semi-arid rangeland of Kenya.African Journal of Range & Forage Science 14: 26–31.

NCDC. 2013. Historical Palmer Drought Indices. National Oceanic and AtmosphericAdministration. http://www.ncdc.noaa.gov/

Owensby, C. E. 1973. Modified step-point system for botanical composition and basal coverestimates. Journal of Range Management 26: 302–303.

Palmer, W. C. 1965. Meteorolgical drought. Research Paper No. 45. U.S. WeatherBureau, Asheville, NC.

Pellerin, S., M. Mercure, A. S. Desaulniers, and C. Lavoie. 2008. Changes in plant communi-ties over three decades on two disturbed bogs in southeastern Quebec. Applied VegetationScience 12: 107–118.

Penuelas, J., P. Prieto, C. Beier, C. Cesaraccio, P. De Angelis, G. De Dato, B. A. Emmett,M. Estiarte, J. Garadnai, A. Gorissen, E. K. Lang, G. Kroel-Dulay, L. Llorens,


G. Pellizzaro, T. Riss-Nielsen, I. K. Schmidt, C. Sirca, A. Sowerby, D. Spano, andA. Tietema. 2007. Response of plant species richness and primary productivity along anorth-south gradient in Europe to seven years of experimental warming anddrought: reductions in primary productivity in the heat and drought year of 2003. GlobalChange Biology 13: 2563–2581.

SAS Institute. 2011. SAS user’s guide. SAS Institute Cary, NC.Sasaki, T., T. Ohkuro, K. Kakinuma, T. Okayasu, U. Jamsran, and K. Takeuchi. 2013.

Vegetation in a post-ecological threshold state may not recover after short-term livestockexclusion in mongolian rangelands. Arid Land Research and Management 27: 101–110.

Schwarz, C. J. 1998. Chapter 3: Studies of uncontrolled events, pp. 19–39, in V. Sit and B.Taylor, eds., Statistical methods for adaptive management studies, Land ManagementHandbook No. 42. British Columbia Ministry of Forests Research Program, Victoria,British Columbia, Canada.

Sharma, S., and A. Kumar. 2006. Cluster analysis and factor analysis, pp. 365–393, in R.Grover and M. Vriens, eds., The handbook of marketing research. Sage, Thousand Oaks,California.

Smith, M. D., and A. K. Knapp. 2001. Size of the local species pool determines invisibility of aC4-dominated grassland. Oikos 92: 55–61.

SysStat. 2012. SigmaPlot for Windows, Version 12.0. SysStat Software, Chicago, Illinois.Taylor, C. A. 2003. Rangeland monitoring and fire: Wildfires and prescribed burning, nutrient

cycling, and plant succession. Arid Land Research and Management 17: 429–438.Thomas, C. D., A. Cameron, R. E. Green, M. Bakkenes, L. J. Beaumont, Y. C. Collingham,

F. F. N. Erasmus, M. Ferreira de Siqueira, A. Grainger, L. Hannah, L. Hughes, B.Huntly, A. S. van Jaarsveld, G. F. Midgley, L. Miles, M. A. Ortega-Huerta, A. T.Peterson, O. L. Phillips, and S. E. Williams. 2004. Extinction risk from climate change.Nature 427: 145–148.

ter Braak, C. J. F., and P. Smilauer. 2002. CANOCO reference manual and CanoDraw 487for windows user’s guide: Software for canonical community ordination (version 4.5).Microcomputer Power, Ithaca, New York.

Tilman, D. 1996. Biodiversity: Population versus ecosystem stability. Ecology 77: 350–363.Ugurlu, E., J. Rolecek, and E. Bergmeier. 2012. Oak woodland vegetation of Turkey – a first

overview based on multivariate statistics. Applied Vegetation Science 15: 590–608.USDA. 2013. Ecological site characteristics MLRA 081B-Edwards Plateau, Central Part.

United States Department of Agriculture, Natural Resource Conservation Service,Ecological Site Description. http://esis.sc.egov.usda.gov/ESDReport/

Verstaeten, G., L. Baeten, T. den Broeck, P. Frenne, A. Demey, W. Tack, B. Muys, and K.Verheyen. Temporal changes in forest plant communities at different site types. AppliedVegetation Science 16: 237–247.

Wilcox, B. P., M. G. Sorice, J. Angerer, and C. L. Wright. 2012. Historical changes in stockingdensities on Texas rangelands. Rangeland Ecology & Management 65: 313–317.


Appendix

1.Plantspeciesrecorded

inthestudyareaduringthe11year(2001to

2011)samplingperiod.Origin

(NativeorExotic),Season

(Warm

,CoolorBoth);LifeCycle(A

nnual,Perennial,Biennial);growth

habit(G

R¼graminoid,FB¼forb=herb,SS¼subshrub<1.5m,

SH¼shrub<4m

including

cacti,

TR¼tree

>4m,VI¼Vine).Eleven

yearmean

percentcomposition

ofeach

speciesateach

site

(LSH¼Low

StonyHillandCL¼ClayLoam)where‘–’indicatesspeciesisnotpresent

Family

Species

Origin

Season

Life

Type

LSH

CL

Acanthaceae

RuelliahumilisNutt.

Native

WP

FB

0.03

–Siphonoglossapilosella(N

ees)

Torr.

Native

WP

FB

0.03

–Agavaceae

YuccaconstrictaBuckley

Native

CP

SR=FB

1.57

0.91

Apiaceae

DaucuspusillusMichx.

Exotic

CP

FB

0.33

0.61

Torilisarvensis(H

uds.)Link

Exotic

CA

FB

0.26

0.11

Asclepiadaceae

Asclepiasasperula

(Decne.)Woodson

Native

WP

FB

0.09

–Asteraceae

Amblyolepis

setigeraDC.

Native

WA

FB

0.23

–AmbrosiapsilostachyaDC.

Native

WP

FB

0.15

0.22

Aphanostephusspp.Shinners

Native

WP

FB

0.15

–Chaetopappaasteroides

Nutt.ex

DC.

Native

WA

FB

0.47

0.09

Cirsium

texanum

Buckley

Native

WB=P

FB

–0.09

Evaxprolifera

Nutt.ex

DC.

Native

CA

FB

1.77

1.82

Gutierreziadracunculoides

(DC.)S.F.Blake

Native

WA

FB

–0.04

Melampodium

leucanthum

Torr.&

A.Gray

Native

WP

FB

0.03

–Ratibidacolumnifera(N

utt.)Wooton&

Standl.

Native

WP

FB

0.11

0.38

Tetraneurislinearifolia(H

ook.)Greene

Native

WP

FB

0.47

0.21

Berberidaceae

Berberistrifoliolata

Moric.

Native

BP

SR

1.41

1.06

Boraginaceae

Heliotropium

indicum

L.

Exotic

WA

FB

0.04

–Tiquilia

canescens(D

C.)A.T.Richardson

Native

WP

FB

–0.04

Brassicaceae

Lepidium

virginicum

L.

Native

CA

FB

0.03

0.04

Cactaceae

EchinocactustexensisHoppfer

Native

CP

SR

0.03

–OpuntiakleiniaeDC.

Native

CP

SR

0.07

–OpuntialindheimeriEngelm.

Native

CP

SR

0.28

0.62

(Continued

)

211

Appendix

1.Continued

Family

Species

Origin

Season

Life

Type

LSH

CL

Campanulaceae

Triodanisperfoliata

(L.)Nieuwl.

Native

CA

FB

0.09

0.10

Cyperaceae

CarexplanostachysKunze

Native

CP

GR

1.40

–Ephedraceae

Ephedra

antisyphiliticaBerland.ex

C.A

.Mey.

Native

WP

SR=VI

–0.04

Euphorbiaceae

Chamaesyce

serpens(K

unth)Small

Native

WA=P

FB

0.06

0.04

CrotondioicusCav.

Native

WP

FB

0.07

0.19

CrotonmonanthogynusMichx.

Native

WA

FB

0.05

–Phyllanthuspolygonoides

Nutt.ex

Spreng.

Native

WP

FB

–0.06

Tragia

ramosa

Torr.

Native

WP

FB

0.82

0.46

Fabaceae

Acaciagreggiivar.gregiiA.Gray

Native

WP

TR=SR

0.05

–MedicagopolymorphaL.

Exotic

CA

FB

–0.04

Prosopisglandulosa

Torr.

Native

WP

TR=SR

0.23

0.16

Sennaroem

eriana(Scheele)H.S.Irwin

&Barneby

Native

WP

FB

0.28

0.04

Sophora

affinisTorr.&

A.Gray

Native

CP

SR=TR

–0.05

Sophora

secundiflora

(Ortega)Lag.ex

DC.

Native

CP

TR=SR

–0.05

Vicia

ludovicianaNutt.

Native

CA

FB

0.10

0.06

Fagaceae

QuercusvirginianaMill.

Native

BP

TR

0.04

0.31

Geraniaceae

Erodium

cicutarium

(L.)L’H

er.ex

Aiton

Exotic

CA=B

FB

0.14

–Lamiaceae

HedeomadrummondiiBenth.

Native

WP

FB

0.07

–Salvia

farinaceaBenth.

Native

WP

FB

1.54

1.56

ScutellariadrummondiiBenth.

Native

CP

FB

0.51

0.04

Liliaceae

Allium

drummondiiRegel

Native

CP

FB

0.03

0.04

Malvaceae

Abutilonfruticosum

Guill.&

Perr.

Native

WP

FB

0.25

–Rhynchosidaphysocalyx(A

.Gray)Fryxell

Native

WP

FB

0.23

0.30

SidaabutifoliaMill.

Native

WP

FB

0.14

0.03

Sphaeralcea

coccinea

(Nutt.)Rydb.

Native

WP

FB

0.03

–Nyctaginaceae

Acleisanthes

longiflora

A.Gray

Native

WP

FB

0.03

–

212

Oxalidaceae

OxalisdilleniiJacq.

Native

WP

FB

0.17

0.44

Papaveraceae

Argem

onealbiflora

Hornem

.ssp.texanaG.B

Ownbey

Native

CA=B

FB

0.04

0.05

Plantaginaceae

PlantagorhodospermaDecne.

Native

CA

FB

3.09

2.46

Poaceae

AristidaoliganthaMichx.

Native

WA

GR

0.17

0.55

AristidapurpureaNutt.

Native

WP

GR

4.95

3.21

AristidapurpureaNutt.var.wrightii(N

ash)Allred

Native

WP

GR

6.22

2.86

Bothriochloaedwardsiana(G

ould)Parodi

Native

WP

GR

0.04

0.11

Bothriochloaischaem

um

(L.)Kengvar.song.(R

upr.ex

Fisch.&

C.A

.Mey.)Celerier&

Harlan

Exotic

WP

GR

0.04

–

Bothriochloalaguroides

(DC.)Hertersubsp.torr.(Steud.)

Allred&

Gould

Native

WP

GR

0.68

0.12

Boutelouacurtipendula

(Michx.)Torr.

Native

WP

GR

10.81

7.69

Boutelouahirsuta

Lag.

Native

WP

GR

0.10

0.13

Boutelouarigidiseta(Steud.)Hitchc.

Native

WP

GR

9.64

5.39

BoutelouatrifidaThurb.

Native

WP

GR

2.07

0.14

BromusarvensisL.

Exotic

CA

GR

1.15

1.59

BromuscatharticusVahl

Exotic

CA

GR

0.20

0.07

Buchloedactyloides

(Nutt.)Engelm.

Native

WP

GR

6.73

9.02

Chlorisverticillata

Nutt.

Native

WP

GR

0.03

–Dichanthelium

oligosanthes

(Schult.)Gould

var.scrib.

(Nash)Gould

Native

CP

GR

–0.06

Digitariacalifornica(Benth.)Henrard

Native

WP

GR

–0.63

Digitariacognate

(Schult.)Pilg.

Native

WP

GR

1.33

0.35

ElymuscanadensisL.

Native

CP

GR

0.14

0.12

Eragrostis

interm

edia

Hitchc.

Native

WP

GR

0.11

0.30

Eriochloasericea(Scheele)Munro

exVasey

Native

WP

GR

0.17

6.77

Erioneuronpilosum

(Buckley)Nash

Native

WP

GR

3.68

1.06

Hilariabelangeri(Steud)Nash

Native

WP

GR

15.62

14.84

(Continued

)

213

Appendix

1.Continued

Family

Species

Origin

Season

Life

Type

LSH

CL

Poaceaecont.

Hordeum

pusillum

Nutt.

Native

CA

GR

–0.15

Lim

nodea

arkansana(N

utt.)L.H

.Dew

eyNative

CA

GR

0.37

0.03

Muhlenbergia

reverchoniiVasey&

Scribn.

Native

WP

GR

0.03

–Nassella

leucotricha(Trin.&

Rupr.)Barkworth

Native

CP

GR

10.39

23.97

Panicum

halliiVasey

Native

WP

GR

1.52

0.53

Panicum

obtusum

Kunth

Native

WP

GR

0.69

1.15

PoaarachniferaTorr.

Native

WP

GR

–0.04

Schedonnarduspaniculatus(N

utt.)Trel.

Native

WP

GR

0.82

0.07

Schizachyrium

scoparium

(Michx.)Nash

Native

WP

GR

–0.09

Setariareverchonii(V

asey)Pilg.

Native

WP

GR

0.10

–SetariatexanaW.H

.P.Emery

Native

WP

GR

–0.93

Sorghum

halepense

(L.)Pers.

Exotic

WP

GR

0.04

–Sporoboluscompositusvar.comp.(Poir.)Merr.

Native

WP

GR

0.09

0.06

Sporoboluscompositus(Poir.)Merr.var.drum.(Trin.)

Kartesz&

Ghandi

Native

WP

GR

0.12

2.24

Sporoboluscryptandrus(Torr.)A.Gray

Native

WP

GR

0.11

0.62

Tridensalbescens(V

asey)Wooton&

Standl.

Native

WP

GR

–0.15

(Vasey&

Scribn.)Nash

Native

WP

GR

–0.06

214

Poaceaecont.

Tridensmuticus(Torr.)Nash

var.elongates(Buckley)

Shinners

Native

WP

GR

0.18

–

Tridenstexanus(W

ats.)Nash

Native

WP

GR

0.73

–Vulpia

octoflora

(Walter)Rydb.

Native

CA

GR

0.84

0.05

Polygalaceae

Polygala

lindheimeriA.Gray

Native

WP

FB

0.13

0.16

Ranunculaceae

ClematisdrummondiiTorr.&

A.Gray

Native

WP

FB

0.34

–Rhamnaceae

ZiziphusobtusifoliaHook.ex

Torr.&

A.Gray

Native

CP

TR=SR

–0.04

Rubiaceae

Hedyotisnigricans(Lam.)Fosberg

Native

WP

FB

0.07

–Rutaceae

Zanthoxylum

hirsutum

Buckley

Native

CP

TR=SR

0.14

0.03

Scrophulariaceae

Verbascum

thapsusL.

Exotic

WB

FB

0.12

0.11

Smilacaceae

Smilaxbona-noxL.

Native

CP

SR=VI

0.60

0.44

Solanaceae

Chamaesarachaconiodes

(Morich.ex

Dunal)Britton

Native

WP

FB

–0.06

PhysalislongifoliaNutt.

Native

WP

FB

0.05

–Solanum

elaeagnifolium

Cav.

Native

WP

FB

0.29

0.70

Ulm

aceae

Celtislaevigata

Willd.var.texana(Scheele)Sarg.

Native

CP

TR=SR

–0.04

Urticaceae

ParietariapensylvanicaMuhl.ex

Willd.

Native

WA

FB

–0.13

Verbenaceae

Glandulariabipinnatifida(N

utt.)Nutt.

Native

WP

FB

0.35

0.04

Verbenaneomexicana(A

.Gray)Small

Native

WP

FB

0.38

0.11

VerbenaofficinalisL.ssp.haleiSmall

Native

WP

FB

0.20

–

215

drought and ecological site interaction on plant composition of a semi-arid rangeland

Documents