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GRAZING AND PLANT PERFORMANCE'

M. J. TRLleA AND L. R . RITTENHOUSE Range SCIence [}(>partmem. Colorado SlOle Uml"('I'sfly. ForI COl/illS. Colorado 80S}) USA

Abs/rac/. Grazing is more than just defoliation of plants. The impact of herbivory affects ecosystem structure and function. both above and below ground. Ultimately. effects of herbivory are expressed to varying degrees at many levels of the ecosystem. Herbivory has been shown to affect plant physiology, morphology, and genetics. Plants have evolved many ways to avoid or tolerate herbivory. Whether plants overcompensate, equally compensate. or undercompensate to herbivory depends on pre- and post-harvest conditions of the plants and their environment. To be imponant to the manager, the magnitudeofcompensation must begreaterthan the inherent "noise" in the system. Natural resources managers use scientific information about herbivory to reduce ambiguity in decision-making in an environment of uncenainty. Ifan ecological response like compen­ sation is to have practical application for the manager, then meaningful effects must occur on time and spatial scales that the manager can respond to with available resources.

K('y"'o rtis; compensatory gro"'th; de/oliation rt'sponses; gra:illg al"()idmlC(' I·S. /olerance: grazing resis/mree: hierarchical C'Cological/('.·('ls; in/ormOlionnl't'ds 0/namral r('sOllre('s managers: planl fil liess: plulII ph,'si%gical SIOIIIS: rang(' managemelll: s('mi·arid rungt'lollds.

I NTRODUCTION efit the plant that is utilized. Whereas a number of Grazing is more than just defoliation of pi ants. Large studies from agriculture systems and semiarid and arid grazing herbivores change energy balance at the soi l rangelands have indicated that plants are often detri­ surface, create different levels of system disturbance, mentally affected by defoliation. and respond by un­ impact colonization of pi ants, remove and redistribute dercompensation (Trlica 1977. lacey and Van Poollen nutrients. and influence interactions wi th other kinds 1981. Whitham and Mopper 1985. 8elski 1986. 1987. of animals. ECOlogists have been primarily concemed Painter and Belsky 1993). still other studies have shown with the defoliation aspect of grazing and with indi­ that herbivores may have little effect on the plants on vidual plant species responses to defoliation. which they feed (lee and Bazzaz 1980. McNaughton Most studies have used clipping or mechanical har­ and Chapin 1985. Maschinski and Whitham 1989). vest ing. rather than the grazing animal. to apply treat­ These apparent contradictions have puzzled scientists men ts. We have often assumed that defoliated plants and resource managers for years. Where does the truth respond in the same way as grazed plants (Trlica 1977). lie? We propose that individual plants have quite plas­ Rarely is this the case. as competitive background of tic responses to herbivory, and that whether overcom­ the studied plant is usuall y also altered by the animal. pensation, equal compensation. or undercompensation We know from many empirical studies that clipping responses arc measured after herbivory depends on a rarely ca n be used to si mulate grazing. By clipping an number of conditions experienced by the plant in the individual plant and not neighbors. or vice versa, we post-harvest period. All too often only aboveground make simplifying assumptions about intra- and inter­ components of plants have been studied. and interest­ plant competition. Mueggler (1972) long ago showed ing belowground phenomena ignored. Ifplants exhibit how individual plants responded differentl y to d ifferent overcompensation in fruit or aboveground biomass alterations in competitive background. production after being grazed at the expense of root growth, increased cltudation. or reduction in depth of COMPENSATORY GROWTH AND root penetration. what should this response be called. OTHER R ESPONSES and what mechanisms are involved? We need to better understand whole-plant responses to herbivory and to­ Two ex tremes eltist on whether plants respond pos­ tal plant carbon balance (Mooney 1972. Coyne et al. itively or negatively to herbivory. McNaughton (1976, 1993). 1979, 1983, 1984. 1986), Owen and Wiegen (1976). Belsky (t 986, 1987) suggested that an individual plant Dyer. Turner and Seastedt (1991). and others have might exhibit overcompensation for herbivory under pointed out that some le vel of defoliation or herbivory most favorable environmental conditions, but under may result in overcompensation and may actually ben- Icss-than-favorable conditions this same individual might exhibit either equal compensation or undercom­

I Manuscript received 4 May 1992. pensation. This hypothesis was recently field tested by 22 M.}. TRUCA AND L. R. RIITEN HOUSE EcoIorical Aw/icaliOOlt \\>I. 1. No. I

Maschinski and Whitham (1989) utilizing the mono­ ity and the probability of a plant being grazed. The carpic biennial species. Ipomopsis arr~onica . where capability for rapid regrowth is affected by the avail­ plants were subjected 10 varyi ng levels of soil waler ability of meristems and by physiological status. In­ and nutrient availability, interspecific competition, and teractions among plant structure and function deter­ defoliation. Their study illustrated a continuum of mine a plant's ability to respond to grazing. compensation responses in plant fitness (fruit sel), de­ Plant mechanisms to cope with grazing vary greatly pending on what phenological siage plants were de­ among species, and are more important to resource foliated. on nutrient and water regime. and on com­ managers than is compensation after foliage removal. petitive background. They concluded that a plant Avoidance reduces the probability and severity of de­ probably will not compensate for herbivory 35 com­ fo liation. whereas tolerance mechanisms facilitate petition with neighbors increases. as nUlrienl avail­ growth after defoliation (Bri ske 1991). One or both ability decreases, and as the timingofherbivoryccmes mechanisms may be employed by a plant. Grazing later in the growing season. Overcompensation oc­ tolerance may be e}[hibi ted by compensatory photo­ curred only when grazed plants were supplemented synthesis. by rapid leaf replacement, or through alter­ with nutrients and were not competing with neighbors. ation in carbon allocation patterns. Avoidance results More studies like this one are needed to test other from physical (e.g .. spines or growth form) or chemical organisms. such as grasses, where filness may be more (e.g .. secondary compounds that deter herbivores or associated with biomass production. longevity, and interfere with their metabolism) deterrents. However, ase}[ual reproduction. rather than on a one-time seed the carbon costs associated with grazing avoidance are production event. considerable. More attention muSt be given to a plant's pheno­ A trade-offproabably e}[ists between grazing avoid­ logical and physiological status when treatments are ance and competitive ability. Those plants that in vest imposed and responses measured. to determine level heavily in defenses may not be as competitive as plants ofcompensation achieved. For compensation to occur. that invest liule in such mechanisms. So, which plant physiological and morphological constraints must be is more fit? Obviously, it will depend on future envi­ overcome, and adequate abiotic and biotic conditions ronmental conditions that the plant must endure. and must e}[ist (Maschinski and Whitham 1989). Obvi­ ifgrazing or competition will be more prevalent in the ously. plant responses to herbivory are conditioned by future. past history, current environmental conditions, and Belsky (1986, 1987) did a good job of elarifying the interactions among biotic and abiotic components. A ecological hierarchy in which plants e}[ist. What might better understanding of these interactions is what is be viewed to be detrimental at some lower level in the presently nceded. hierarchy. may actually be neutral or beneficial at some The linka ge between compensation in plants and its higher level (e.g .. an individual plant may be detri­ effect on animal populations may be weak at best. As mentally affected by grazing, but community-level pro­ pointed out by May (1973) and reiterated in Cra wley duction might be increased). Plants do not grow as (\ 983), fluctuations in plant and animal populations isolated individuals, but rather as members of a pop­ may occur quite independent of each other. This is ulation, community. and ecosystem. Grazing is also a especially important on the time and spatial scales of hierarchical process, Interactions of plants and animals concern to managers. among hierarchical levels are governed by specific an­ Plants respond in a multitude of ways to grazing. imal decision rules. Therefore, foragingbehaviorvaries Their individual responses vary depending on growth with the ecological scale (Senfl et al. 1987). form. species genetic capabilities, morphology, phys­ Grazing management is aimed at altering in tra- and iology, and phenology. The three most important, interspecific competitive interactions. Whether an in­ manageable variables that inftuence plant response to dividual plant overcompensates. undercompensates, grazing are (I) the timingofthe grazing event in relation or equall y compensates afler being grazed is of lillie to the opportunity to grow or regrow, (2) the frequency concern to the resource manager. Grazing-induced of defoliation of an individual plant and its neighbors. modifications in competit ive interactions are eventu­ and (3) the intensity of use (i.e. , the level of defoliation). ally e}[pressed in changes at the population level (Briske Plants cope with grazing by minimizing the proba­ 1991). A decrease in basal area or density ofa species bility of being grazed or rapidly replacing leaf area results in a reduction in rcsource acquisition within the removed by herbivores (Trlica and Orodho 1989, Uriske community by that species. A change in species com­ 1991). Individua ll y, the ri sk to a plant. or its immediate position then alters the quantity and quality of pro­ neighbors. of being defoliated under moderate stocking duction and. ultimately. the allocation and flow of en­ levels is relatively low. However, as the incidence of ergy in the ecosystem. Thus, studiesofindividual plant patch-grazing increases, so does the probability of an responses to defoliation may lead to identification of individual bei ng grazed. Morphological features and vital mechanisms involved in conferring f\lness. but biochemical compounds can also inftuence accessibil- these studies may be of little value to someone re- February 1993 GRAZING AND RANGELAND MANAGEMENT 2J sponsible for management at the ecosystem or land­ Crawley. M. J. 1983. Herbivory: the dynamics of plant­ scape level of organization. animal interactions. University of California Press. Berke­ ley, California. USA. MANAGERIAL NEEDS AND E COLOGICAL Dyer. M. I., C. L. Turner. and T. R. Seastedt. 1991. Mowing RESEARCH and fertilizingeffe(:tson productivity and sp«"ial reflectance ill Bromus illermis plots. Ecological Applications 1:559- Resource managers function in an environment of 564. uncertainty. and natural resource problems are open­ Lacey. 1, R .. and H. w , Van Poollen, 1981. Co mparison of ended. These decision-makers use many sources of in­ herooge production on moderately grazed and ungrazed western ranges. Journal of Range Management 34:210-212. formation; some are based on science, some on other Lee. T. D .. and F. A. lIazzaz. 1980. Effects of defoliation values. T his has. in some instances, resulted in aduality and competition 011 growth and reproduction in the annual between science and management. T he major reason plant Ablililolllheophrasli. Journal of Ecology 68:813-821 . managers use research information is to reduce the risk Maschinski. J .. and T. G. Whitham. 1989. The continuum of plallt responses to herbivory: the influence of plant as­ of making incorrect decisions. If a duality ellists be­ $()ciation, nutrient availability. and liming. American Nat­ tween ecological research and application, a concerted uralist 134:1-19. effort should be made to eliminate the gap. May, R. M. 1973. Slability and complexity ill model eco­ Although theories provide important constructs for systems. PrincelOn University Press. Princeton, New Jer­ decision-making at strategic levels, they are not veT)' sey. USA. McNaughton. S. J. 1976. Serengeti migratory wildebeest: helpful to the manager at the tactical and operational facilitation of energy flow by grazing. Science 191 :92-94. level. Which is the hierarchical level of interest to the 1979. Grazing as an optimization process: grass­ manager? What the manager needs is a potpourri of ungulate relationships in the Serengeli. American Naturalist principles or fundamen tals that are applicable to spe­ 11 3:691-703. --. 1983, Co mpensatory plant growth as a response to cific conditions. Principles or fundamentals can be or­ herbivory. Oikos 40:329-336. ganized and applied at different hierarchical levels in ~~-. 1984. Grazing lawns: animals in herds. plant form. the system. The information that is relevant is only and coevolution. American Naturalist 124:863-886. one hierarchical level more complell than the question. 1986. On plants and herbivores. American NalU­ Knowledge of the mechanisms of organ isma I and pop_ rali5t I28:765-770. Mc Naughton, S. J.. and F. S. Chapin 1!1. 1985. Effects of ulation response makes decision-making more effi­ phosphorus nutrition and defoliation on C, graminoids from cient. but problems can be solved without knowing the Serengeti plains. Ecology 66: 1617-1629. specific biological mechanisms. Sometimes empirical Mooney, H. A. 1972. The carbon oolance of plants. Annual relationships are as valuable as mechanisms to a man­ Review of Ecology and Systematics 3:315-346. Mueggler. w . F. 1972. Influence of competition on the reo ager. What may be scientifically interesting to an ecol­ sponseofbluebunch wheatgrass toclipping. Journal of Range ogist, may be of little consequence to the manager, and Management 25:88-92. vice versa. If an ecological response. like compensa­ Owen. D. F.. and R. G. Wiegert. 1976. Do consumers mall­ tion. is to have practical application. the magnitude of imize plant fitness? Oikos 27:488--492. the effect must be greater than the intrinsic noise in Painter. E. L.. and A. J. Belsky. 1993. Application of her_ bivore optimization theory to rangelands of the western the system. and the effect must occur on time scales to United States. Ecological Applications 3:2-9. which a manager can respond with resources and labor Senft. R. L . M. B. Coughenhour. O. W. Bailey, L R. Ritten­ that is available. house, O. E. Sala, and O. M. Swift. 1987. Large herbivore foraging and ecological hierarchies. BioScience 37:789-799. LITERATURE CiTED Trlica. M. J. 1977. Effects of frequency and intensity of Belsky. A. J. 1986. Does herbivory benefit plants? A review defoliation on primary producers of arid and semiarid of the evidence. American NalUralist I27:870-892. rangelands. Pages 25-55 ill Proceedings of the Second Unit­ --. 1987. The effects of grazing: confounding ofeco­ ed States-Australian Range Panel. 1972. Adelaide. AustrJ­ systems. community, and organism scales. American Nat­ lia. Australian Rangeland Society. Perth. Western Austra· uralist 129:777-783. lia. Briske. D. D. 1991. Developmental morphology and phys­ Trlica. M. J.. and A. B.Orodho. 1989. EffIXts of protection iology of grasses. Pages 85-108 in R. K. Heitschmidt and from grazing on morphological and chemical characteristics J. Stuth, editors. Grazing management: an ecological per­ of Indian ricegrass. Oryzopsis hymenoides. Oikos 56:299- spective. Timber Press. Inc .. Portland. Oregon. USA. 308. Coyne, P. L. M. J. Trlica. and C. E. Owensby. 1993. Carbon Whi tham . T. G .. and S. Mopper. 1985. Chronic herbivory: and nitrogen dynamics in range plants. /11 D. Bedunah. impacts on architecture and sex expression of pinyon pine. editor. Range plant physiology. Society for Range Man­ Science 228: 1089-1 091. agement. Denver. Colorado. USA. in press.