Interannual Herbaceous Biomass Response to Increasing Honey Mesquite Cover on Two Soils

Transcription

1 Rangeland Ecol Manage 61: September 2008 Interannual Herbaceous Bomass Response to Increasng Honey Mesqute Cover on Two Sols W. Rcard Teague, 1 R. Jm Ansley, 1 Wllam E. Pncak, 1 Steven L. Dowower, 2 Sannon A. Gerrard, 3 and J. Alan Waggoner 3 Autors are 1 Professor, 2 Senor Researc Assocate, and 3 Researc Assocate, Texas AgrLfe Researc, PO Box 1658, Vernon, TX , USA, and Department of Ecosystem Scence and Management, Texas A&M Unversty System, College Staton, TX 77843, USA. Abstract Ts study quantfed erbaceous bomass responses to ncreases n oney mesqute (Prosops glandulosa Torr.) cover on two sols from 1995 to 2001 n nort central Texas. Vegetaton was sampled randomly wt levels of mesqute rangng from 0% to 100%. Wt no mesqute coverng te slt loam sols of bottomland stes, peak erbaceous bomass averaged (6SE) kg? a 21 vs kg? a 21 on clay loam sols of upland stes (P ). A lnear declne of kg? a 21 n erbaceous bomass occurred for eac percent ncrease n mesqute cover (P ). Te slope of ts declne was smlar between sols (P ). Herbaceous bomass wt ncreasng mesqute cover vared between years (P ) as dd te slope of declne (P ). Warm-season erbaceous bomass decreased lnearly wt ncreasng mesqute cover averagng a % reducton at 100% mesqute cover (P ) compared to 0% mesqute cover. Coolseason erbaceous bomass was smlar between sols wt no mesqute, kg? a 21 for slt loam vs kg? a 21 for clay loam sols, but averaged kg? a 21 more on slt loam tan on clay loam sols at 100% mesqute cover (P ). Multple regresson analyss ndcated tat eac centmeter of precptaton receved from te prevous October troug te current September produced erbaceous bomass of 51 kg? a 21 on slt loam and 41 kg? a 21 on clay loam sols. Herbaceous bomass decreased proportonally wt ncreasng mesqute cover up to 29 kg? a 21 at 100% mesqute cover for eac centmeter of precptaton receved from January troug September. Increasng mesqute cover reduces lvestock forage productvty and ntensfes drougt effects by ncreasng annual erbaceous bomass varablty. From a forage producton perspectve tere s lttle advantage to avng mesqute present. Resumen Este estudo cuantfcó las respuestas en la bomasa de erbáceas debdo a los ncrementos en la cobertura de mezqute (Prosops glandulosa Torr.) en dos tpos de suelo de 1995 a 2001 en el Norte Centro de Texas. La vegetacón fue muestreada al azar, con nveles de mezqute dentro del rango de 0 a 100%. En ausenca de mezqute en suelos franco-arcllosos de stos bajos, la máxma bomasa de erbáceas promedó , comparada con kg? a 21 en los suelos arcllosos de los stos de altplano (P50.001). Se encontró una dsmnucón lneal de kg? a 21 en la bomasa de erbáceas por cada punto porcentual en el aumento de la cobertura de mezqute (P ). La pendente de esta dsmnucón fue smlar entre tpos de suelo (P50.135). La bomasa de erbáceas con el aumento en la cobertura de mezqute varó entre años (P50.001) así como la pendente dsmnuyó (P ). La bomasa de erbáceas de verano dsmnuyó lnealmente con el aumento de la cobertura del mezqute, promedando una reduccón de % al 100% de cobertura de mezqute (P50.001), comparado con el 0% de cobertura de mezqute. La bomasa de erbáceas nvernales fue smlar entre suelos donde no abía mezqute, kg? a 21 para suelos francos, contra kg? a 21 para suelos arcllosos, pero promedaron kg? a 21 más en los suelos francos que en los suelos arcllosos con una cobertura de mezqute del 100% (P ). El análss de regresón múltple ndcó que por cada cm de precptacón que se tuvo del Octubre prevo asta el actual Septembre se produjo una bomasa de erbáceas de 51 kg? a 21 en el suelo franco y 41 kg? a 21 en el suelo arclloso. La bomasa de erbáceas dsmnuyó proporconalmente con el aumento en la cobertura de mezqute asta 29 kg? a 21 al 100% de cobertura de mezqute por cada cm de lluva recbdo desde Enero asta Septembre. Incrementando la cobertura de mezqute se reduce la productvdad de forraje para el ganado e ntensfca los efectos de la sequía, ncrementando la varabldad en la bomasa de erbáceas. Desde una perspectva de produccón de forraje, ay pocas ventajas de tener la presenca de mezqute. Key Words: competton, drougt, precptaton varablty, Prosops glandulosa, savanna, speces composton, woody encroacment, woody plant Fundng was provded by te E. Paul and Helen Buck Waggoner Foundaton, Inc., te US Dept of Agrculture Natonal Researc Intatve Compettve Grants Program Agrcultural Systems Grants and , and te Texas Agrcultural Experment Staton under project H Correspondence: Rcard Teague, Texas AgrLfe Researc, PO Box 1658, Vernon, TX , USA. Emal: r-teague@tamu.edu Manuscrpt receved 10 October 2007; manuscrpt accepted 19 June INTRODUCTION Increased woody plant abundance n grasslands and savannas over te past century as been wdely documented (Arcer et al. 2001). Ts wdespread nvason and ncrease of woody speces as resulted n a declne n bodversty (West 1993; Knopf 1994), a reducton n ecosystem reslence (Peterson et 496 RANGELAND ECOLOGY & MANAGEMENT 61(5) September 2008

2 al. 1998), and a greater lkelood of rreversble canges n plant speces composton (Westoby et al. 1989) and as consequences for land surface atmosperc nteractons (Arcer et al. 2001). In te soutwestern Unted States, mesqute (oney mesqute [Prosops glandulosa Torr.]) s te prmary encroacng woody plant speces n many rangeland ecosystems. Uncecked, mesqute encroacment progresses to form closed-canopy woodlands, wc sgnfcantly reduce erbaceous productvty (Beduna and Sosebee 1984; Arcer 1995; Ansley et al. 2001, 2004), treatenng te sustanablty of lvestock rancng (Hamlton and Ueckert 2004) as well as wldlfe abtat (Rollns and Cearley 2004) and grassland brds (Knopf 1994). A number of factors ncludng clmate, sols, and dsturbance regmes determne te abundance and composton of trees and srubs and te nfluence tey ave on erbaceous composton and bomass n dry ecosystems (Walker 1987; Scoles and Arcer 1997). Woody plant grass relatons are strongly nfluenced by sol. Tese ntegrated nteractons need to be quantfed to determne te producton (Brock et al. 1978; McDanel et al. 1982; Scfres and Hamlton 1993; Andales et al. 2006), envronmental (West 1993; Knopf 1994; Peterson et al. 1998; Arcer et al. 2001), and economc consequences (Vantassell and Conner 1986; Teague et al. 2001) of dfferent land management optons tat are desgned to manage woody plants. Te nterannual varablty n precptaton and resultng varablty of productvty on semard rangeland ecosystems s proportonally greater tan tat of more mesc ecosystems n te Nort Amercan Great Plans (Knapp and Smt 2001). Ts makes development of effectve management strateges dffcult and complex. Smulaton models ave great potental as researc tools and decson ads for natural resource managers by collatng results from experments n dfferent felds or locatons wtn te context of a more encompassng systems management framework tat treats te ranc busness as a complete bo-economc unt (Andales et al. 2006; Teague et al. 2008). Suc models requre essental bologcal data under realstc management settngs to perform adequately. Te feld researc reported n ts paper provdes data on te compettve effects of mesqute on erbaceous bomass for use n buldng models to assess te relatve merts of dfferent strategc and tactcal land management optons. Publsed researc on erbaceous bomass and composton response to ncreasng mesqute as documented sort-term measurements (2 yr to 3 yr) mmedately followng mesqute removal wt few ntermedate levels of mesqute canopy cover evaluated (Scfres and Polk 1974; Scfres et al. 1977; Dal et al. 1978; Beduna and Sosebee 1984; Hetscmdt and Dowower 1991; Warren et al. 1996), or long-term dscontnuous measurements (10 yr to 20 yr) from a sngle sol type (Ansley et al. 2004). Few of tese studes ave evaluated erbaceous bomass responses over te range of varablty of precptaton, te full range from no mesqute to 100% mesqute canopy cover, and dfferent sol types experenced n tese semard ecosystems. Te goal of ts study was to examne te nfluence of ncreasng levels of mesqute cover (0% to 100%) on two sol types under a wde varaton of annual and nterannual precptaton on total erbaceous plant bomass and te relatve bomass of warm- and cool-season erbaceous plant functonal groups. Te study was conducted from 1995 to Te two sols cosen for te study are bot assocated wt g levels of mesqute, occur wdely troug te regon, and support dfferent levels of erbaceous bomass (US Department of Agrculture Sol Conservaton Servce 1969; Teague et al. 2004). We also measured responses under a ranc-scale management scenaro tat ncluded lvestock grazng. We requred an ntegrated erbaceous and lvestock response to ncreasng mesqute cover under realstc grazng condtons to parameterze a smulaton model we used to furter explore wole-ranc system responses to dfferent grazng and burnng nterventons (Teague et al. 2008). As a separately publsed part of ts study (Ansley et al. 2004) we ave reported te response of erbaceous plants to ncreasng mesqute n grazng exclosures on one sol type. A unque feature of ts regon s tat tere s codomnance of warm-season and cool-season erbaceous mdgrasses tat results from sgnfcant precptaton receved n bot warm and cool seasons (Hetscmdt et al. 1982). METHODS Te nvestgaton was conducted n nort-central Texas on te a Waggoner Expermental Ranc (lat 33u509N, long 99u59W) near Vernon. Elevaton ranges from 335 m to 396 m. Te clmate s contnental wt an average of 220 frost-free growng days. Mean annual precptaton s 648 mm (range: 490 mm to mm), wc s bmodally dstrbuted wt peaks n May (95 mm) and September (76 mm), toug sgnfcant precptaton can be expected n any mont. Mean montly temperature vares from 3.9uC n January to 36.4uCnJuly. Te woody vegetaton conssts of mesqute wt some lotebus (Zzpus obtusfola [Hook. ex. Torr. & A. Gray] A. Gray). Succulents represented by Opunta spp. are common. Te erbaceous vegetaton was domnated by a cool-season perennal, Texas wntergrass (Nassella leucotrca Trn. & Rupr.); warm-season perennals slver bluestem (Botrocloa lagurodes DC.), sdeoats grama (Bouteloua curtpendula [Mcx.] Torr.), meadow dropseed (Sporobolus compostus [Por.] Merr.), buffalograss (Bucloe dactylodes [Nutt.] Engelm.); te cool-season annual Japanese brome grass (Bromus japoncus Tunb. ex Murray); and te warm-season forbs western ragweed (Ambrosa pslostacya DC.), annual broomweed (Guterreza texana [DC.] Torr. & A. Gray), and eat aster (Aster ercodes L.). Nomenclature follows Dggs et al. (1999). Measurements were made on egt landscape-scale replcates rangng n sze from a to a. Equvalent stockng rates of commercal beef cattle n a cow calf enterprse were used on all replcates pror to and trougout ts study and cows and calves grazed on te pastures all year. Stockng rate was ntally set at 7.5 a? anmal unt yr 21 n 1995 based on Natural Resources Conservaton Servce recommendatons. Rates were reduced to 11 a? cow 21 n 1996 and 14 a? cow 21 by 2001 followng several drougts. No part of ts study was mpacted by fre. Te study was conducted on two domnant sol types (slt loam bottomland and upland clay loam flats) tat represent dfferent ecologcal stes (Table 1). 61(5) September

3 Table 1. Summary of sol caracterstcs from US Department of Agrculture Sol Conservaton Servce (1969) for domnant sols wt sgnfcant mesqute populatons on te Waggoner Expermental Ranc n Wlbarger County, Texas. Parameter Clay loam Slt loam Catenal Poston Summt Footslope Permeablty Slow, subject to crustng Moderate, well draned Sol seres Tllman Weatwood Hydrology Run-off Run-on Slope (%) Ecologcal ste Clay loam Loamy bottomland Ste dentfcaton no. R0 78 CY096 TX R0 78CY103 TX Taxonomc class Fne, mxed, superactve, termc Typc Paleustoll Fne-slty, mxed, actve, termc Fluventc Haplustept Measurements Herbaceous standng bomass and woody plant cover (all speces) were montored on eac sol type n Marc, July, and November from 1995 to 2001 n all egt pasture replcates to nclude sprng-, summer-, and fall-aspect plants tat occur n ts regon. In eac replcate, samplng was conducted along four separate m-long transects randomly placed on eac of te two sol types. Along eac transect lne, 20 sample ponts were located approxmately every 50 m. Eac of tese ponts represents one of seven possble mesqute cover levels wt assocated erbaceous vegetaton. Sample ponts on eac samplng date were located randomly and dfferent ponts were sampled at eac date. All sample areas were exposed to lgt-to-moderate lvestock grazng. Te assessment of te effect of mesqute on vegetaton depended on an estmate of mesqute cover and erbaceous bomass at eac samplng pont (quadrat). Te center of eac quadrat was te samplng pont used to estmate woody plant cover. Herbaceous bomass and mesqute cover were ten calculated for eac sol and pasture, for analyss by year. To estmate erbaceous bomass assocated wt dfferent levels of mesqute cover we determned peak erbaceous standng bomass n summer or fall wtn eac sol type and pasture, resultng n 720 total sample ponts. Te peak standng bomass occurred n eter late summer or fall and was varable for dfferent speces and n dfferent years. We used te gest (peak) standng bomass n every case to estmate te level of mesqute cover on erbaceous bomass. To smplfy nterpretaton we report erbaceous bomass n two functonal groups: warm-season or cool-season erbaceous plants (.e., grasses or forbs). Bot functonal groups ncluded annual and perennal speces. Te predomnant speces comprsng tese functonal groups are appended (Appendx 1). Peak erbaceous bomass estmates were based on aboveground standng bomass along tese transects, usng te drywegt rank metod of t Mannetje and Haydock (1963) as modfed by Jones and Hargreaves (1979). At eac sample pont we ranked (by standng bomass) te top tree grass and top tree forb speces separately n eac 0.05-m 2 ( cm) quadrat. Vsual estmates of standng (lve + dead) erbaceous bomass (g? m 22 ) by grass and forb were made before te quadrat was clpped for dryng and wegng as outlned by Dowower et al. (2001). All 20 quadrats were arvested for eac m transect and were combned to yeld a total transect dry wegt by grass and forb. Vsually estmated speces standng bomass from te 20 quadrats was ten corrected usng total clpped vs. total estmated grass and forb bomass to gve speces lve and dead bomass estmates per quadrat. Woody speces ave a dscontnuous nfluence on ter mmedate vcnty and beyond (Belsky et al. 1989, 1993; McPerson and Wrgt 1990; Belsky 1992; Vrgna et al. 1992; Owens et al. 1995; Dalgren et al. 1997; Laxson et al. 1997). Most samplng metods record mean values over te area beng sampled, not accountng for nformaton at te scale of pattern created by ndvdual woody plants, wc s of prme mportance n ts study. To overcome ts sortcomng we use a samplng tecnque developed by Dowower et al. (2007) to assess woody plant nfluence at a sngle pont or erbaceous vegetaton sample unt. At eac samplng pont four quarters were establsed as for te pont-centered quarter metod of Cottam and Curts (1956). Woody plant cover was estmated at eac erbaceous samplng pont on a scale of 0 2 n eac of te four quarters. If egt of woody plants wtn eac quarter was less tan 45u vertcally from te sample pont, woody cover was consdered to be mnmal and was assgned a score of 0. Oterwse, f te woody cover partally flled te quarter wt an angle of 45u to 75u vertcally from te sample pont, a score of 1 was assgned. A score of 2 was assgned f te egt of woody plants was. 75u vertcally from te sample pont. Summng of te values for all four quarters provded a score of 0 8, wt eac rank approxmatng 12.5% cover. Tus, a score of 8 equaled 100% woody plant cover. Comparson of scorng on 22 areas of mesqute cover measured usng ts metod wt te lne-ntercept metod of Canfeld (1941) provded regresson coeffcents (R 2 ) of 0.96 (Dowower et al. 2007). In order to examne te effects of precptaton amount on erbaceous response to dfferent levels of mesqute cover we analyzed annual precptaton (January troug December), warm growng season precptaton (January troug October), offset annual precptaton (October troug September te followng year), and cool growng season precptaton (October troug June). Statstcal Analyses Analyses of covarance of mesqute effects was a comparatve experment wt mesqute cover levels assocated wt random erbaceous samplng locatons. Results are assocatve rater tan causal. We tested to determne f a curvlnear ft would be a sgnfcant mprovement compared to a lnear regresson ft over te full range of mesqute cover levels. Te two-stage concept of te Lard Ware model was employed were a partcular class of models s appled to dfferent groups or treatments (Scabenberger and Perce 2001, p ; S. Duke, personal communcaton, June 2006). In our case te followng regresson model s appled between two sols over 7 yr: bomass 5 ntercept + B 3 mesqute + error. Percentage of mesqute cover was used as a dscrete covarate and sol and year were classfcaton varables. Solutons were estmated ndcatng te mpact of mesqute (slope) on erbaceous bomass per functonal group for te two sols and 7 yr 498 Rangeland Ecology & Management

4 (ntercepts; PROC MIXED; SAS 2003). Homogenety of slopes was tested for mesqute-wtn-sol and mesqute-wtn-year. Compound symmetry covarance structure was used. Year was ndcated as a repeated measure and random portons were dentfed as sol wtn pasture and sol 3 year wtn pasture (Scabenberger and Perce 2001). Dfferences n bomass occurred among years and between sols wtn years as well as te rate of decrease of erbage wt ncreasng mesqute between years. Tese dfferences were expected to be strongly assocated wt te yearly and seasonal dfferences n precptaton. Multple regressons usng precptaton perods and precptaton perods 3 mesqute and sols were conducted to arrve at a smple dynamc model tat would pertan to erbaceous bomass wtn ts range of observed condtons. Use of no-ntercept regressons caused precptaton to account for all erbaceous bomass varaton wtn te model varyng by year. Seasonal precptaton was averaged at 3-mo ntervals: October troug December, January troug Marc, Aprl troug June, and July troug September. Factors were dropped wen nonsgnfcant and lke factors were combned wen smlar. Multple regresson models were compared to te model used n te Mxed Procedure usng GLM type 1 sum of squares to ceck te relatve ft to te data (SAS 2003). RESULTS Clmatc Condtons and Herbaceous Dynamcs Precptaton (Fg. 1) and erbage dynamcs vared markedly over te study perod of 1995 to Annual totals for te 7 yr ranged from 480 mm to 914 mm. Te dry summers and/or wet wnters experenced n four of te seven expermental years (1996, 1998, 2000, and 2001) ad a major mpact resultng n a 2.5-fold dfference n erbaceous bomass among years. Interactve Effects of Mesqute Cover, Sol, and Year on Herbaceous Bomass Peak Total Herbaceous Bomass. Mesqute cover (M) ad a sgnfcant effect on peak total erbaceous bomass as dd te year-to-year varaton (Y) and sol (S; Table 2). Tere was a sgnfcant nteracton between M and Y for total erbaceous bomass (M 3 Y) and warm-season erbaceous bomass and for te M 3 S nteracton for warm-season erbaceous bomass, ndcatng te nfluence of ncreasng mesqute cover on erbaceous bomass decreased more sarply wen peak erbaceous bomass was greater. However, ts nteracton was small relatve to man effects of M, Y, and S (Table 2). Peak total erbaceous bomass canged sgnfcantly eac year n response to dfferences n annual precptaton (Y; Table 2). Slt loams were more productve, averagng kg? a 21 vs kg? a 21 for clay loam sols (S; Table 2) but te decrease of bomass wt an ncrease n mesqute from 0% to 100% was smlar for bot sols, averagng kg? a 21 (M 3 S; Table 2). Tests for curvlnear ft were not sgnfcantly dfferent from te lnear regressons presented (P ). Precptaton from te prevous October troug September of te current year vared from approxmately 400 mm to mm over te 7-yr expermental perod. As precptaton ncreased above 400 mm, erbaceous bomass n response to ncreasng annual precptaton cover ncreased sarply untl approxmately 600 mm, after wc relatvely small gans were measured wt addtonal precptaton (Fgs. 2a and 2b). Bomass on slt loam sols was approxmately 700 kg? a 21 ger tan on clay loam sols but overall response to ncreasng mesqute and ncreasng annual precptaton was oterwse very smlar (cf. Fgs. 2a and 2b). Altoug total erbaceous bomass was consstently ger on slt loam tan on clay loam sols t was not sgnfcantly dfferent between te two sols n 1999 (P ) and 2000 (P ; Fg. 3). In eac year te slope of te declne of total erbaceous bomass wt ncreasng mesqute cover was almost dentcal on te two sols. In contrast, slopes of te regressons for eac year n response to ncreasng mesqute cover dffered wt drer years avng flatter slopes (Fg. 3). Peak Warm-Season Herbaceous Bomass. Warm-season erbaceous bomass was nfluenced strongly by mesqute cover (M; Table 2; Fg. 4) and was dfferent eac year (M 3 Y; Table 2). Tests for curvlnear ft were not sgnfcantly dfferent from te lnear regressons presented (P ). Dfferences occurred between sols, wt slt loam sols beng more productve, averagng kg? a 21 vs kg? a 21 for clay loam sols (Table 2). Te slope of te declne n erbaceous bomass between te two sols wt ncreasng mesqute cover was not parallel but proportonal to bomass wtn any one year wt erbaceous bomass decreasng an average of kg? a 21 on slt loam vs kg? a 21 for clay loam sols (M 3 S; Table 2). Slopes of te regressons dffered eac year n response to dfferng amounts of summer ranfall (Fg. 4). Peak Cool-Season Herbaceous Bomass. Cool-season plants produced consderably less bomass tan te warm-season plants (compare Fgs. 4 and 5). Te response to ncreasng mesqute cover dd dffer between years (M 3 Y; Table 2; Fg. 5) but ts effect was weak compared to te response of warm-season plants. In bot 1995 and 2000 cool-season plant bomass ncreased margnally wt ncreasng mesqute cover (P # 0.09; Fg. 5). An examnaton of Fgures 3, 4, and 5 llustrates tat bomass ad only 1 yr eac (1996 for total and 1999 for cool-season) were te M 3 S nteracton was sgnfcant. In contrast, te warm-season bomass ad 4 yr (1996, 1997, 1998, and 2001) were te M 3 S nteracton was sgnfcant. Wt no mesqute cover, dfferences n standng cool-season bomass due to sols were margnal, wt slt loam sols averagng kg? a 21 vs. 930 kg? a 21 for clay loam sols (S; Table 2). Wt ger levels of mesqute cover (90% to 100%) cool-season erbaceous plants were more productve on slt loams wt kg? a 21 compared to 870 kg? a 21 on clay loam sols. Regresson Models Incorporatng Precptaton. Best models ndcated tat addng precptaton from te prevous year s October troug December perod to te current year s January troug September perod mproved te current year s peak total erbage estmate. Usng a 12-mo perod begnnng n September or November was nferor. 61(5) September

5 Fgure 1. Montly precptaton (&) relatve to te long-term mean montly precptaton ) for te Waggoner Expermental Ranc n nortcentral Texas from 1994 to Long-term mean annual ranfall ( mm. Best precptaton-based regresson estmates of peak standng crop (kg? a 21 ) were as follows: Total erbaceous plants ~ 30 ppt October{December z July{September z 51 ppt January{June { 29 ppt January{September mesqute z 10 ppt October{January{September slt loam sol [R ; P, ] Warm season erbaceous plants ~ 40 ppt January{June z 27 ppt July{September { 26 ppt January{September mesqute z 13 ppt January{September slt loam sol { 11 ppt January{September mesqute slt loam sol [R ; P, ] 500 Rangeland Ecology & Management

6 Table 2. Mxed model testng effect of mesqute cover (M), sol (S), and year (Y) on total, warm-season, and cool-season erbaceous bomass n on slt loam and clay loam sols on te Waggoner Expermental Ranc, Wlbarger County, Texas. Numerator Denomnator Parameter Source df df F P. F Total erb M M 3 S M 3 Y S Y S 3 Y Warm season M M 3 S M 3 Y S Y S 3 Y Cool season M M 3 S M 3 Y S Y S 3 Y Cool season erbaceous plants ~ 32½ppt October{December Š z 15 ppt January{June { 3ppt October{January{June mesqute z9 ppt October{January{June slt loam sol [R ; P, ] were ppt 5 precptaton for noted perod (cm) and mesqute 5 mesqute cover percentage. For clay loam sol te slt loam sol multpler s zero. Te precptaton based regresson models (n 5 720) accounted for all but 2 3% varaton of te covarable models. Sgnfcant dfferences for te varable Y stll occurred but F values wt precptaton regressons were reduced 92% for total and 85% for warm-season erbaceous plants compared to te covarable model. Includng precptaton from te prevous fall mproved model R 2 values. Regressons usng annual precptaton of January troug December of te current year ndcated lower model R 2 values of 0.90 for total, 0.81 for warm-season, and 0.76 for cool-season erbaceous plants. F- values remanng for te varable year were reduced from 0 70% of covarable model F values. Oter effects and covarables were sgnfcant as well. Te precptaton-drven model ndcated tat eac centmeter of precptaton produced 41 kg? a 21 of total erbaceous plants on clay loam sols n open areas free of mesqute nfluence. Slt loams were 24% more productve tan clay loam sols for total erbaceous plants and 36% more for warm-season erbaceous plants. Mesqute reduced erbaceous bomass for precptaton receved from January troug September. Full (90% to 100%) mesqute cover reduced warm-season erbaceous plants by 75%. However, full mesqute cover ncreased Fgure 2. Regressons of total erbaceous bomass (kg? a 21 ) n response to ranfall from October troug September for and dfferent levels of mesqute cover (%) on a, slt loam and b, clay loam sols on te Waggoner Expermental Ranc n nort-central Texas. cool-season erbaceous plants on slt loams by 30%. Based on average precptaton, full mesqute cover reduced total erbaceous bomass 45% on slt loams and 56% on clay loam sols. DISCUSSION Tese data were collected over a seres of years tat ad a wde range n total precptaton and provde a broader understandng of erbaceous bomass response to woody cover compared to studes tat typcally are only conducted over 2 yr or 3 yr. Ts study llustrates te strong effect of varable seasonalty and amount of precptaton on te relatve productvty of warm- and cool-season erbaceous vegetaton n response to ncreases n mesqute cover and sol type. It also llustrates te strong carryover effect of condtons n one year to te next tat can become addtve over years. 61(5) September

7 Fgure 3. Regressons of total erbaceous bomass (kg? a 21 ) n response to dfferent levels of mesqute cover for te calendar years on slt loam (SL) and clay loam (CL) sols on te Waggoner Expermental Ranc n nort central Texas. 502 Rangeland Ecology & Management

8 Fgure 4. Regressons of total warm-season erbaceous bomass (kg? a 21 ) n response to dfferent levels of mesqute cover for te calendar years on slt loam (SL) and clay loam (CL) sols on te Waggoner Expermental Ranc n nort-central Texas. 61(5) September

9 Fgure 5. Regressons of total cool-season erbaceous bomass (kg? a 21 ) n response to dfferent levels of mesqute cover for te calendar years on slt loam (SL) and clay loam (CL) sols on te Waggoner Expermental Ranc n nort-central Texas. 504 Rangeland Ecology & Management

10 Increasng mesqute cover ad a domnant effect on erbaceous bomass and te relatve contrbuton of warmand cool-season speces to total erbaceous bomass, reducng total peak erbaceous standng crop 50% on average. Total erbaceous bomass was correlated best wt precptaton from te prevous October troug September of te followng year. Warm-season erbage bomass was most correlated wt January troug September precptaton. Cool-season erbage, wc was less affected by mesqute cover, was most correlated wt October troug June precptaton. Our data are comparable to predctons of erbaceous bomass levels of publsed data from te Great Plans. Sala et al. (1988) developed a smple predctve model based on stes trougout te Central Grassland regon of te Unted States tat estmated annual net prmary producton of 60 kg? a 21 for eac centmeter of precptaton. Ts compares to 51 kg? a 21 predcted for slt loam and 41 kg? a 21 for clay loam sols per centmeter of precptaton estmated by our model wt no mesqute cover. Our model compares favorably consderng our estmates were based on summer or fall peak standng crop wle under moderate grazng by lvestock. Te dfference n peak standng crop between our two sols would be expected because te slt loam sol occurred on bottomland stes subject to recevng runoff and were lkely sub-rrgated wereas clay loam sols occurred on flat expanses of 1 5% slopes tat receve no net runoff. Regardng reductons n erbaceous bomass wt ncreasng mesqute cover, our results are comparable to oter results from ts ecoregon. Scfres and Polk (1974) measured a 3kg? a 21 per centmeter precptaton gan n grass producton wt cemcally top-kllng a mesqute stand wt 12% cover. Our model predcts 3.4 kg? a 21 per centmeter of precptaton gan n grass producton for ter ste. Hetscmdt and Dowower (1991) report a reducton of net erbaceous producton rangng from kg? a 21 to kg? a 21 (mean kg? a 21 ) wt 80% mesqute cover n a 3-yr study. In comparson, our study over 7 yr ad a range n reducton of erbaceous bomass of kg? a 21 (mean kg? a 21 ) wt 100% mesqute cover. Total and warm-season erbaceous producton was ger on te slt loam tan te clay loam sols, and eac year te slope of te declne of total erbaceous bomass wt ncreasng mesqute cover was almost dentcal on tese two sols. However, total erbaceous bomass was not sgnfcantly dfferent between te two sols n 1999 and Ts resulted from drougt-related mortalty of perennals from 1997 to te A declne n perennal basal cover from 6 6% to 6 1% took place over ts perod (Teague et al. 2004) due to successve dry summer and fall perods (Fg. 1). Smlar results were found by Albertson and Weaver (1944) and Box (1967) n response to drougt condtons. Te effect of tese successve dry warm seasons resulted n an addtve effect from one year to te next wt muc lower bomass on bot sols and reduced te dfference n bomass between te plant communtes growng on tese sols n 1999 and Te slopes of te regressons for eac year n response to ncreasng mesqute cover dffered wt drer years avng flatter slopes. Because warm-season plants normally provde most of te erbaceous bomass on eter sol, te dry summers and varablty n tmng of precptaton canged bot te slope and y-axs ntercepts for te relaton between sols eac year. In te absence of mesqute cover, warm-season erbaceous plants normally are more productve tan cool-season erbs on eter sol n ts ecosystem. Warm-season plants were reduced about 75% under te gest levels of mesqute cover for any year on eter sol. Mesqute s decduous and grows n summer and s, terefore, more compettve wt warm-season tan wt cool-season erbaceous plants. In addton, te number of dry summers (1996, 1998, 2000, 2001) and wet wnters experenced over te expermental perod ad a major negatve mpact on warm-season plants. Ansley et al. (2004) report very smlar results for clay loam sols at mesqute cover levels of 10% to 20% and 50% to 60% n a 3-yr study. However, tey ndcate a more precptous warm-season declne at ntermedate mesqute cover levels tan n ts study. Our data ndcate a lnear decrease n warm-season erbaceous bomass n response to mesqute cover ncreasng from 0% to 100%. We speculate tat ts s probably because te current study ad a greater number of data ponts at ntermedate levels of mesqute cover (20% to 50%) and more years of data collecton, and because grazng was excluded n te Ansley et al. (2004) study. Dal et al. (1978) and McDanel et al. (1982) obtaned results n te same range as ts study at ntermedate levels of mesqute cover followng clearng mesqute of 30% and 25% cover, respectvely. Cool-season erbaceous bomass was not nfluenced muc by ncreasng mesqute cover because muc of te growt occurs wen mesqute s dormant. Cool-season erbaceous response to mesqute was varable between years wt slt loams avng about 30% ncreased erbaceous bomass under te gest levels of mesqute cover. Te ncrease of cool-season erbaceous plants on slt loams under full mesqute cover would account for te more parallel decrease observed for total erbaceous plants on slt loam and clay loam sols wt ncreasng mesqute cover. Te slgtly erratc response from year to year for total erbaceous plants was probably caused by cool-season plants sowng lttle response to ncreasng mesqute cover as reported by Scfres et al. (1982) and Ansley et al. (2004). In addton, oter factors suc as declnng bomass of competng grasses over te study perod and cangng seasonalty of precptaton may ave contrbuted to te erratc response. Increased cool-season erbaceous growt reduced te negatve mpacts of ncreased mesqute cover on total erbaceous producton. MANAGEMENT IMPLICATIONS Tese fndngs ave a number of management mplcatons. Te decrease n erbaceous bomass wt ncreasng mesqute s one of te man reasons tat ranc managers try to lmt te abundance of ts woody speces. Tese data ndcate an < 50% reducton of total erbaceous bomass wt 100% mesqute cover. Consequently, durng a severe drougt, woody plant competton can sarply reduce forage for lvestock. Te lower warm-season erbaceous bomass wt ncreasng mesqute also ncreases te annual varablty of erbaceous bomass and exacerbates te effects of summer drougt. Altoug te decrease n warm-season erbaceous bomass s 61(5) September

11 often partally offset by an ncreasng proporton of cool-season erbaceous plants, as we found n some of te years, ts does not compensate for te reducton n warm-season erbaceous bomass (Scfres et al. 1982) and leaves sgnfcant forage defcts n te summer. Ts s due to a combnaton of a lower amount of forage as well as lower forage qualty of cool-season grasses durng te summer monts n ts ecosystem (Hetscmdt et al. 1989). Te parallel slope response of erbaceous bomass to ncreasng mesqute cover on te two sols suggests tat tere wll be a smlar ncrease n erbaceous bomass followng woody plant removal. However, because slt loam sols are more productve, te response to mesqute removal wll result n a greater economc beneft to clearng mesqute on slt loam tan on clay loam sol (Teague et al. 2001). Te slt loam sols produced < 28% more erbaceous bomass tan te clay loam sols, so for te same cost of treatment an ncrease n ts amount of forage could be expected. Cool-season forages are very mportant to rancers because tey provde g proten forage wen warm-season forages are dormant and, terefore, reduce wnter feed costs consderably (Teague et al. 2001). However, cool-season perennals are found n adequate quanttes at very low mesqute cover f grazng levels are lgt or moderate (Hetscmdt et al. 1989), so tere s no advantage for lvestock producton to ncreasng te proporton of cool-season perennals. Because cool-season perennal grasses are muc less productve tan warm-season grasses tat tey replace, tere s no economc advantage to avng mesqute cover greater tan 15% to 20% as dscussed by Hetscmdt et al. (1989). In addton, te response of warmseason grasses followng mesqute removal treatments may be delayed for a few growng seasons, especally on degraded areas tat ave low presence of warm-season mdgrasses (Ansley et al. 2004). Terefore, from a strctly erbaceous bomass perspectve tere s lttle advantage to avng presence of mesqute. Altoug woody plant reducton as tradtonally focused on enancng erbaceous bomass for domestc lvestock (Scfres and Hamlton 1993; Scoles and Arcer 1997), conservaton goals and ncome from wldlfe are mportant and are ncreasngly becomng te prmary management focus on many rances (Bernardo et al. 1994; Rollns and Cearley 2004). Snce many wldlfe speces requre some level of woody plant cover, te data presented n ts study wll be useful for determnng te consequences of managng for dfferent levels of mesqute cover for bot lvestock and wldlfe goals. ACKNOWLEDGMENTS Te autors gratefully acknowledge te facltes and lvestock made avalable by te W. T. Waggoner estate. We tank Rob Borcardt, Cody Bradley, Mcel Bryant, Justn Clarey, Bob Cross, Dane Conover, Davd Ecels, Joelyn Foy, Russell Frtsc, Tracy Gwaltney, Jason Holt, El Ornelas, Gerral Sculz, Rley Soaf, Gna Spratt, Dustn Stacey, Jon Todd, Tm Tunnel, Dan Vasquez, and Jan Worrall for scentfc and tecncal assstance. Te project s also ndebted to te competent nputs of te Waggoner Ranc staff, partcularly Weldon Hawley, Dee Robnson, and Anne Plcer. We also tank Drs Tm Fulbrgt, Urs Kreuter, and Sam Fulendorf for constructve crtcsm of earler drafts and Dr Sara Duke of te US Department of Agrculture Agrcultural Researc Servce for statstcal advce. LITERATURE CITED ALBERTSON, F. W., AND J. E. WEAVER Effects of drougt, dust, and ntensty of grazng on cover and yeld of sort-grass pastures. Ecologcal Monograps 14:1 29. ANDALES, A. A., J. D. DERNER,L.R.AHUJA, AND R. H. HART Strategc and tactcal predcton of forage producton n nortern mxed-grass prare. Rangeland Ecology and Management 59: ANSLEY, R. J., W. E. PINCHAK,W.R.TEAGUE,B.A.KRAMP,D.L.JONES, AND P. W. JACOBY Long-term grass yelds followng cemcal control of oney mesqute. Journal of Range Management 57: ANSLEY, R. J., B. X. WU, AND B. A. KRAMP Long-term ncreases n mesqute canopy cover n nort Texas. Journal of Range Management 54: ARCHER, S Herbvore medaton of grass woody nteractons. Tropcal Grasslands 29: ARCHER, S., T. W. BOUTTON, AND K. A. HIBBARD Trees n grasslands: bogeocemcal consequences of woody plant expanson. In: E. D. Sculze, S. Harrson, M. Hemann, E. Holland, J. Lloyd, I., and D. Scmel [EDS.]. Global bogeocemcal cycles n te clmate system. San Dego, CA, USA: Prentce.p BEDUNAH, D. J., AND R. E. SOSEBEE Forage response of a mesqute buffalograss communty followng range reabltaton. Journal of Range Management 37: BELSKY, A. J Effects of trees on nutrtonal qualty of understory gramneous forage n tropcal savannas. Tropcal Grasslands 26: BELSKY, A. J., R. G. AMUNDSON,J.M.DUXBURY,S.J.RIHA,A.R.ALI, AND S. M. MWONGA Te effects of trees on ter pyscal, cemcal, and bologcal envronments n a sem-ard savanna n Kenya. Journal of Appled Ecology 26: BELSKY, A. J., S. M. MWONGA, R. G. AMUNDSON, J. M. DUXBURY, AND A. R. ALI Comparatve effects of solated trees on ter undercanopy envronments n g- and low-ranfall savannas. Journal of Appled Ecology 30: BERNARDO, D. J., G. W. BOUDREAU, AND T. C. BIDWELL Economc tradeoffs between lvestock grazng and wldlfe abtat: a ranc-level analyss. Wldlfe Socety Bulletn 22: BOX, T. W Influence of drougt and grazng on mortalty of fve west Texas grasses. Ecology 48: BROCK, J., R. H. HAAS, AND J. C. SHAVER Zonaton of erbaceous vegetaton assocated wt oney mesqute n nort central Texas. In: 1st Internatonal Rangeland Congress; August 1978; Denver, CO, USA. Denver, CO, USA: Socety for Range Management. p CANFIELD, R. H Applcaton of te lne ntercept metod n samplng range vegetaton. Journal of Forestry 39: COTTAM, G., AND J. T. CURTIS Te use of dstance measures n pytosocologcal samplng. Ecology 37: DAHL, B. E., R. E. SOSEBEE,J.P.GOEN, AND C. S. BRUMLEY Wll mesqute control wt 2, 4, 5-T enance grass producton? Journal of Range Management 31: DAHLGREN, R. A., M. J. SINGER, AND X. HUANG Oak tree and grazng mpacts on sol propertes and nutrents n a Calforna oak woodland. Bogeocemstry 39: DIGGS, G. M., B. L. LIPSCOMB, AND R. J. O KENNON Illustrated flora of nort central Texas. Fort Wort, TX, USA: Botancal Researc Insttute of Texas p. DOWHOWER, S. L., W. R. TEAGUE, R. J. ANSLEY, AND W. E. PINCHAK Dry-wegtrank metod assessment n eterogeneous communtes. Journal of Range Management 54: DOWHOWER, S. L., W. R. TEAGUE,S.A.GERRARD, AND D. M. CONOVER Angle cover class: a varable plot tecnque for estmatng srub quanttes n rangeland areas. Ard Land Researc and Management 21: HAMILTON, W. T., AND D. N. UECKERT Rangeland woody plant and weed management-past, present, and future. In: W. T. Hamlton, A. McGnty, D. N. Ueckert, C. W. Hanselka, and M. R. Lee [EDS.]. Brus management: past, present and future. College Staton, TX, USA: Texas A&M Unversty Press. p Rangeland Ecology & Management

12 HEITSCHMIDT, R. K., AND S. L. DOWHOWER Herbage response followng control of oney mesqute wtn sngle tree lysmeters. Journal of Range Management 44: HEITSCHMIDT, R. K., S. L. DOWHOWER, W. E. PINCHAK, AND S. K. CANON Effect of stockng rate on quantty and qualty of avalable forage n a soutern mxed grass prare. Journal of Range Management 42: HEITSCHMIDT, R. K., D. L. PRICE,R.A.GORDON, AND J. R. FRASURE Sort duraton grazng at te Texas Expermental Ranc: effects on aboveground net prmary producton and seasonal growt dynamcs. Journal of Range Management 35: JONES, R. M., AND J. N. G. HARGRAVES Improvements to te dry-wegt-rank metod for measurng botancal composton. Grass and Forage Scence 34: KNAPP, A. K., AND M. D. SMITH Varaton among bomes n temporal dynamcs of aboveground prmary producton. Scence 19: KNOPF, F. L Avan assemblages on altered grasslands. Studes n Avan Bology 15: LAXSON, J. D., W. H. SCHACHT, AND M. K. OWENS Aboveground bomass yelds at dfferent denstes of oney mesqute. Journal of Range Management 50: MANNETJE, L. T., AND K. P. HAYDOCK Te dry-wegt rank metod for botancal analyss of pasture. Journal of te Brts Grassland Socety 18: MCDANIEL, K. C., J. H. BROCK, AND R. H. HAAS Canges n vegetaton and grazng capacty followng oney mesqute control. Journal of Range Management 35: MCPHERSON, G. R., AND H. A. WRIGHT Effects of cattle grazng and Junperus pncot canopy cover on erb cover and producton n western Texas. Amercan Mdland Naturalst 123: OWENS, M. K., R. B. WALLACE, AND S. R. ARCHER Landscape and mcroste nfluences on srub recrutment n a dsturbed sem-ard Quercus Junperus woodland. Okos 74: PETERSON, G., G. R. ALLEN, AND C. S. HOLLING Ecologcal reslence, bodversty and scale. Ecosystems 1:6 18. ROLLINS, D., AND K. CEARLEY Integratng wldlfe concerns nto brus management. In: W. T. Hamlton, A. McGnty, D. N. Ueckert, C. W. Hanselka, and M. R. Lee [EDS.]. Brus management: past, present and future. College Staton, TX, USA: Texas A&M Unversty Press. p SALA, O. E., W. J. PARTON, L. A. JOYCE, AND W. K. LAUENROTH Prmary producton of te central grassland regon of te Unted States. Ecology 69: SAS [computer program] SAS/STAT Verson 9.1 for Wndows. Cary, NC, USA: SAS Insttute Inc. SCHABENBERGER, O., AND F. J. PIERCE Contemporary statstcal models for te plant and sol scences. New York, NY, USA: CRC Press. 738 p. SCHOLES, R. J., AND S. R. ARCHER Tree grass nteractons n savannas. Annual Revew of Ecology and Systematcs 28: SCIFRES, C. J., G. P. DURHAM, AND J. L. MUTZ Range forage producton and consumpton followng aeral sprayng of mxed brus. Weed Scence 25: SCIFRES, C. J., AND W. T. HAMILTON Prescrbed burnng for brusland management: te sout Texas example. College Staton, TX, USA: Texas A&M Unversty Press. 246 p. SCIFRES, C. J., J. L. MUTZ,R.E.WHITSON, AND D. L. DRAWE Interrelatonsps of Husace canopy cover wt range forage on te Coastal Prare. Journal of Range Management 35: SCIFRES, C. J., AND D. B. POLK, JR Vegetaton response followng sprayng of lgt nfestaton of oney mesqute. Journal of Range Management 27: TEAGUE, W. R., R. J. ANSLEY, U. P. KREUTER, W. E. PINCHAK, AND J. M. MCGRANN Economcs of managng mesqute n nort Texas: a senstvty analyss. Journal of Range Management 54: TEAGUE, W. R., S. L. DOWHOWER, AND J. A. WAGGONER Drougt and grazng patc dynamcs under dfferent grazng management. Journal of Ard Envronments 58: TEAGUE, W. R., W. E. GRANT, U. P. KREUTER, H. DIAZ-SOLIS, S. DUBE, M. M. KOTHMANN, W. E. PINCHAK, AND R. J. ANSLEY An ecologcal economc smulaton model for assessng fre and grazng management effects on mesqute rangelands n Texas. Ecologcal Economcs 64: US DEPARTMENT OF AGRICULTURE SOIL CONSERVATION SERVICE Sol survey of Wlbarger County, Texas. Natonal Cooperatve Sol Survey. Fort Wort, TX, USA: US Department of Agrculture. 76 p. VANTASSELL, C. W., AND J. R. CONNER An economc analyss of brus control practces and grazng systems n te Rollng Plans of Texas, MP College Staton, TX, USA: Texas Agrcultural Experment Staton, Texas A&M Unversty. VIRGINIA, R. A., W. M. JARRELL, W. G. WHITFORD, AND D. W. FRECKMAN Sol bota and sol propertes n te surface rootng zone of mesqute (Prosops glandulosa) n storcal and recently desertfed Cuauan desert abtats. Bology and Fertllty of Sols 14: WALKER, B. H Determnants of tropcal savannas. Oxford, Unted Kngdom: IRL Press Lmted. 156 p. WARREN, A., J. HOLECHEK, AND M. CARDENAS Honey mesqute nfluences on Cuauan desert vegetaton. Journal of Range Management 49: WEST, N. E Bodversty on rangelands. Journal of Range Management 46:2 13. WESTOBY, M. B., B. WALKER, AND I. NOY-MEIR Opportunstc management for rangelands not at equlbrum. Journal of Range Management 42: (5) September

13 Appendx 1. Speces domnatng eac erbaceous functonal group on te Waggoner Expermental Ranc, Wlbarger County, Texas. Speces n eac functonal group are lsted n order of abundance. Fgures n parenteses ndcate te number of speces n tat functonal group encountered. Only speces averagng. 1kg? a 21 are lsted. Warm-season mdgrass (22) Bouteloua curtpendula Sporobolus compostus Botrocloa lagurodes Arstda longseta Erocloa sercea Trdens albescens Hlara mutca Sporobolus cryptandrus Sporobolus arodes Pancum obtusum Bracara clatssma Dgtara calfornca Perennal cool-season mdgrass (4) Nassella leucotrca Agropyron smt Poa aracnfera Elymus canadenss Annual cool-season grass (12) Bromus japoncus Hordeum pusllum Perennal warm-season forbs (50) Ambrosa pslostacya Aster ercodes Solanum elaeagnfolum Artmsa ludovcana Vernona margnata Teucrum lacnatum Warm-season sortgrass (11) Bucloe dactylodes Bouteloua rgdseta Clors vertcllata Bouteloua gracls Scedonnardus panculatus Pancum all Warm-season tall grass (5) Andropogon gerard Sczacyrum scoparum Pancum vrgatum Sorgastrum nutans Appendx 1. Contnued. Annual warm-season forbs (16) Guterreza texana Conza canadenss Gaura parvfola Iva angustfola Croton monantogynus Dracops amplexcauls Helenum mcrocepalum Xantum strumarum Annual cool season forbs (36) Plantago rodosperma Lepdum densflorum Gaura trangulata Dacus pusllus Centaurea amercana Gallarda pulcella 508 Rangeland Ecology & Management