Long-term effects of fire on cactus in the southern mixed prairie of Texas.

Few brownspine pricklypear were immediately killed by the direct effects of fire. Most plants resprouted after burning in the spring, but mortality averaged 70% by the end of the fourth year after burning. Interactions of fire with insects and rodents caused most of the brownspine pricklypear mortality. V>alkingstick cholla and tasajillo were more directly affected by fire than brownspine pricklypear. First-year mortality was 40 to 65%, respectively; and fourth year mortality was 57 to 80%. Mortalities of other minor species of cactus varied from 49 to 100%. The increase of cactus on grassland ranges, particularly in the Southwest, has been a problem for many years. During a 17-year period (1932-1949) in southern Arizona, jumping cholla (Opuntia fulgida) increased from 5 to 368 plants/ha, cane cholla (0. spinosior) increased from 14 to 785 plants/ha, and Engelmann pricklypear (0. engelmnannii) increased from 0 to 74 plants/ha in a grazed area (Glendening 1952). On an ungrazed area, percentage increase for cane cholla and Engelmann pricklypear were similar to the increases on a grazed area over the same period of time (Glendening 1952). These data indicate that neither reduced grazing pressure nor increasd grass competition will prevent the increase of cacti density. However, Cook (1942), working in the Central Great Plains with pricklypear (0. humifusa), observed that ranges with dense stands of grass also supported high populations of Melitera dle,tcitca, a cactus feeding moth, during years with aboveaverage precipitation. The cactus feeding moth was a major factor in reducing cactus density. Ranges in poor condition did not have high enough populations of moths to cause cactus mortality. Thus, reduced grazing pressure in combination with above-normal moisture can reduce the density of cacti, at least on the eastem edge of cacti ranges. Drouths favor cacti throughout their range (Cook 1942; Houston 1963), and fire and animal use are left as the primary factors to control their density (Humphrey 1958). High densities of cactus plants can reduce forage production and utilization. Rea and Pieper (1973) found that controlling walkingstick cholla (0. imbricata) on blue grama (Bouteloua grtacilis) range increased herbage production by 20% and utilization by 9%. Bement (1968), however, was unable to show an increase in herbage production after control of plains pricklypear (0. polyacantha). Authors are assistant professor of range management, College of Forestry, Wildlife, and Range Sciences, University of Idaho, Moscow 83843 and Horn professor of range management, Texas Tech University, Lubbock, Texas 79409; and assistant professor of Forestry, College of Forestry, Wildlife, and Range Sciences, University of Idaho, Moscow 83843. This research is a contribution of the College of Agricultural Sciences, Texas Tech University, Publication Number T-9-184. Manuscript received February 16, 1978. Cactus plants vary in their response to fire, depending on species, size, and how soon they are observed after a fire. In Arizona, Humphrey and Everson (1951) found that 1 year after a fire the densities of Englemann pricklypear, jumping cholla, and cane cholla had decreased 9, 29, and 7%, respectively. However, Reynolds and Bohning (1956) reported that a prescribed fire reduced densities of barrel cactus (Feroca(ctus JtiSlizeni) 67%, jumping cholla 44%, cane cholla 45% and Engelmann pricklypear 28% after 2 years. Similarly, Cable (1967) found that fire reduced densities of jumping cholla 63%, cane cholla 45%, and Engelmann pricklypear 32% by the second year after a fire. In Texas, mortality of brownspine pricklypear (0. phaeicanitha) was 20% the year after burning and 68% after the second year (Heirman and Wright 1973). Thus, first year mortality is not an adequate indication of the effect of fire on cacti mortality. Small cacti are more susceptible to fire damage than the taller plants (Dwyer and Pieper 1967; Heirman and Wright 1973). Density of walkingstick cholla less than 0.3 m tall was reduced 50%, but larger plants were not severely damaged by fire (Dwyer and Pieper 1967). Hierman and Wright (1973) found that the mortality of walkingstick cholla less than 0.3 m tall was 81%, and for those greater than 0.3 m tall, mortality was 45%. They observed little difference in the mortality of large (>10 pads) and small ( 25) and cholla and tasajillo by height (? 0.6 m and >0.6 m tall). Mortality was recorded during July and December each year until December, 1976. Mortality was determined by the absence of living tissue above ground on permanently marked plants on each sampling date. Subsequent mortalities were accumulative for every species on each prescribed burn. An individual cactus plant might be recorded as "dead" on one date and then resprout and be recorded as "living" on a following date. This resulted in a decline in mortality for tasajillo during some years. Chi-square analysis was used to test for differences in mortality between burned and unburned treatments and between size classes of cacti plants. Results and Discussion Brownspine Pricklypear Fire caused little immediate mortality of brownspine pricklypear. The plants were usually top-killed, but within a few weeks after the burn all had resprouted. Average mortality was only 20% at 6 months after the fire. By the end of the third year, however, mortality exceeded 70% (Fig. 1). After the third year, mortality of pricklypear increased about the same rate as it did for the unburned plants. Mortality during the first 3 years apparently was caused primarily by fire weakening the plant and making it more susceptible to damage by insects, rodents, and rabbits. Insects entered the plant through fire-scarred tissue and spread to the young living pads, opening the plant to bacterial and fungal infections. Pads sprouting from burned plant tissue often lost their vascular connection to the root system because of insect damage and decay. Plants sprouting from the root crown survived for a longer period but many of these also died. Most of the plants that survived after 3 years, however, had sprouted from the root crown. Young pads require several weeks before the spines are fully developed. During this time they are frequently browsed by lagomorphs and rodents. This is particularly important during the winter and dry summers when other green material is scarce. The mortality of both burned and control (unburned) plants increased during years when precipitation was below average. Over 50% of one set of control pricklypear plants located on a shortgrass site died during one drought year, probably because they were easily visible and eaten by lagomorphs and rodents. Only 8% of the control plants on tobosagrass sites died during this period. The taller grasses on these sites seem to provide 100 OPUNT IA PHAEACANTHA