oslfire Rodent Succession Following scribed Fire In Southern Chaparral1

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1 oslfire Rodent Succession Following scribed Fire In Southern Chaparral1 William 0. Wirtz, II,? David Hoekrn~n,~ John R. M~hrn,~ and Sherrie L. Souza5 Abstract.-This paper describes species composition and density changes in rodent populations during postfire succession following prescribed fire in the chaparral community of the San Gabriel Mountains. Conclusions are drawn from a 4-year, live-trap, mark and release study of postfire succession in two watersheds receiving "hot" burns and two receiving "normal" burns. The chaparral community of southern California is associated with nearly two million years of fire history (Hanes 1971 ). In recent centuries major fires have occurred at intervals of 20 to 40 years (Byrne et al. 1977; Philpot 1977). Postfire plant succession (Fatric and Hanes 1964, Hanes and Jones 1967, Hanes 1971) and the fire itself have varying short term effects on the birds and small mammals found in the chaparral (Lawrence 1966, Quinn 1979, Wirtz 1977, 1979). Wirtz (1977) summarized the work of earlier authors concerning conditions in small vertebrate microhabi ta ts during fire, vertebrate behavior during fire, and survival of small vertebrates exposed to fire. Both Lawrence (1966) and Quinn (1979) studied rodent populations before and after a burn, in addition to documenting microhabitat conditions during the fire. Wirtz (1977, 'Paper presented at symposium, Managemen t of Amphibians, Reptiles, and Small Mammals in North America. (Flagstaff, AZ, July , 1988.) William 0. Wirlz, I1 is Professor of Biology, Department of Biology, Pomona College, Claremont, CA David Hoekrnan is a research assistant, Department of Biology, Pomona College, Claremon t, CA John R. Muhm is a research assistant, Department of Biology, Pomona College, Claremont, CA ?Sherrie I. Souza is a research assistant, Departmen t of Siolog y, Pomona College, Claremont, CA ,1984) presents preliminary analyses of data collected on postfire rodent succession following wildfire in the chaparral community of southern California. Because of the recently recognized significance of the use of prescribed fire in the management of chaparral ecosystems, the Pacific Southwest Forest and Range Experiment Station, USDA Forest Service, began formulating plans in 1983 for a series of prescribed fires in the San Dimas Experimental Forest, located in the San Gabriel Mountains of southern California, that might be utilized for long range studies of the effects of prescribed fire in chaparral. In October, 1984, the Forest Service burned four chaparral watersheds of approximately 40 ha each in the San Dimas Experimental Forest. This paper describes the changes in rodent community structure for the 4-year period following prescribed burning. Methods In October, 1984, four chaparral watersheds of approximately 40 ha each were subjected to prescribed burns in the San Dimas Experimental Forest. The vegetation of the two of these watersheds (874 and 775) had been hand cut in the spring of 1984 to produce the dried fuel for an exceptionally hot fire. Two adjacent watersheds (804 and 776) burned normally for climatic conditions at the time. A fifth watershed (803), which has been extensively studied since 1976 (see Wirtz 1977,1979,1982,1984), serves as a control for studies on the prescribed burn areas. Rodent live-trap, mark and release, studies were conducted on all experimental areas prior to the burns to document the size and species composition of the prefire rodent community on all watersheds, and 175 individuals were permanently marked by toe-clipping to provide a prefire pool of marked rodents from which to determine survival rates following the burn. Following the fire grids of 50 stations at 15 m intervals were established in each of the four watersheds on the sites of the prefire censusing, and a live-trap, mark and release, program was initiated to determine fire survival and postfire rodent succession patterns. For this paper, population estimates were done by the Hayne (1949) equation. Area sampled, for each species, for each month, was estimated by determining the mean distance traveled for each species between captures, for each month, and then adding a zone equal to the mean distance travelled to the perimeter of the grid. Biomass was determined by the product of the estimated population times the mean weight for each species for the month, and these values are then summed for all species taken on the grid for the month.

2 Results Postfire trapping was initiated in February 1985, and both experimental and control plots were sampled bi-monthly. Hayne equation population estimates for rodent populations on each study plot are presented in figures 1-5. The absence of data points from February through April or May means that no rodents were trapped, except for watershed 803 in which trapping was not begun until June Mice of the genus Peromyscus (deer mouse, P. rnaniculatus; brush mouse, P. boylii; California mouse, P. californicus), and California pocket mice, Perognafhus californicus, constitute the bulk of the postfire rodent population. Pacific kangaroo rat, Dipodomys agilis, dusky-footed wood rat, Neotoma fuscipes, and California vole, Microtus californicus, are present in low numbers, and a few Botta's pocket gopher, Thomomys bottae, and Figure 2.-Hayne equation estimates of population size of rodent species in Bell 804, normal prescribed burn. Note that points at 0 on the x-axis against the y-axis are populations estimates prefire. Figure 4.-Hayne equation estimates of population size of rodent species in San Dimas 776, normal prescribed burn. Note that points at 0 on the x-axis against the y- axis are populations estimates prefire. Figure 1.-Hayne equation estimates of population size of rodent species in Bell 803, the 28-year-old chaparral control plot Figure 3.-Hayne equation estimates of populations size of rodent species in Bell 874, hot prescribed burn. Note that points at 0 on the x-axis against the y-axis are populations estimates prefire. Figure 5.-Hayne equation estimates of population size of rodent species in San Dimas 775, hot prescribed bum. Note that points at 0 on the x-axis against the y-axis are populations estimates prefire.

3 western harvest mouse, Reithrodontomys megalotis, have also been taken. Larger mammals observed in burned watersheds, for which no quantitative data are available, included Beechey ground squirrel, Spermophilus beecheyi, Audubonfs cottontail, Sylvilagus auduboni, brush rabbit, S. bachmani, coyote, Canis la trans, black bear, Ursus americanus, badger, Taxidea taxus, and mule deer, Odocoileus hemionus. Fire Survival No marked wood rats survived the fires. Nine (12.5%) Peromyscus survived normal fires, and one (1.4%) survived hot fires. Tow (12.5%) Pocket mice survived normal fires, and two (12.5%) Survived hot fires. These data support the currently held opinion that some rodents do survive fires, and help provide the nucleus, along with immigration from unburned areas, for rodent postfire succession. Larger mammals seen in the burned watersheds in the first month postfire included coyote, black bear, badger, and mule deer. Early Postfire Succession Pocket mice and all three Peromyscus species were present on one hot burn (874) by April 1985, six months postfire, but no rodents were present on the other hot burn (775). Pocket mice moved into this hot burn (775) by May, and two Peromyscus species, (P. californicus, P. maniculatus) were present by July. Pacific kangaroo rats appeared on some burned areas by June or July 1985 (they are rare in mature chaparral). Woodrats appeared on one normal bum (804) by June 1985, and another (776) by September 1985, and on one hot burn (874) by August Single pocket gophers and harvest mice have been taken on one hot burn (775). Demography Sampling was not begun on the control plot (803) until June The rodent population on this plot consists chiefly of wood rats, California mice, and pocket mice (fig. 1 ). The California mouse population peaked during the fall, winter, and spring of , and again in the winter and spring of Pocket mice were rare on the control until the fall of 1986 and remained common until the summer of 1987 (fig. 1). The wood rat population has peaked in each summer studied to date. the normal bum in Be11 (804) was composed primarily of woodrats, with smaller numbers of other species (fig. 2) (note that symbols at 0 on the x-axis against the y-axis represent prefire density estimates). The postfire rodent population on this grid has been composed primarily of brush mice and pocket mice, with population peaks of the latter in each winter (1985,1986, and 1987). Wood rat populations did not show significant increases on this grid until the spring of 1987, about 30 months after the burn, and they have yet (June 1988) to reach prefire densities (fig. 2). Pacific kangaroo rats have occurred on this burned area in numbers above prefire densities since the summer of Brush and California mouse populations have occurred in numbers above prefire densities since the winter of (fig. 2). the hot burn in Bell (874) was composed largely of wood rats, California mice, and pocket mice (fig. 3). All species, except kangaroo rats, were present again on this grid by August 1985, 10 months postfire. The postfire rodent community on this hot burn has been dominated by brush mice and pocket mice (fig. 31, with both species reaching, or exceeding, prefire densities by the winter of 1985, approximately a year after the burn. California mouse and wood rat populations have yet (June 1988) to reach prefire densities (fig. 3). the normal burn in San Dimas (776) was composed primarily of California mice and wood rats, with smaller numbers of pocket mice and no brush mice (fig. 4). The postfire rodent community has been dominated by California mice and pocket mice, with both species exceeding prefire densities by the winter of 1985, approximately one year postfire. Wood rats have yet (June 1988) to reach prefire densities, brush mice have not appeared on this grid, and California voles were common in the summer of 1987 and the spring of 1988 (fig. 4). the hot burn in San Dimas (775) was very similar to that on the normal burn here (fig. 5). And, like the normal bum, the postfire comnaunity has been dominated by California mice and pocket mice, with pocket mice exceeding prefire densities by the summer of 1985 and California mice exceeding prefire densities by the fall of 1986 (fig. 5). Pacific kangaroo rats also exceeded prefire densities within one year postfire on this grid. Comment should be made about the presence of deer mice (P. maniculatus) and California voles (Microtus) on these grids. Neither species was present on any grid prefire, and neither has been taken on the control (figs. 1-5). P. maniculatus has been taken on all burned grids, with peaks of abundance by the second year postfire and declining abundance by the fourth year postfire (figs. 4 and 5). Effects of Hot and Normal Fires The effects of hot and normal fires on rodent demography were examined by (1) comparing pre and post fire populations in areas exposed to these two fire regimes (figs. 6 and 7), (2) comparing the number of captures of each species postfire under each fire

4 regime (fig. 81, and (3) comparing total postfire biomass on areas exposed to different fire regimes (fig. 9) (note again that points at 0 on the x- axis against the y-axis are prefire populations estimates). Only species with relatively high abundances are considered in this paper. Prefire populations of brush mice were essentially the same on both areas to be burned in Bell, while densities of pocket mice and California mice were greater on the area to receive the hot burn, and deer mice were not present on either grid (fig. 6). All prefire populations were severely impacted by fire, dropping in most instances to near zero for several months postfire. Pocket mice increased to twice their prefire density on the hot bum and 25 times prefire density on the normal burn (fig. 6). Brush mice increased to 14 times their prefire density on the hot burn and six times prefire density on the normal bum (fig. 6). California mice returned to prefire density by one year postfire on the normal burn, and numbers have remained relatively constant since then. Deer mice were present on both burned areas postfire, but have been more abundant on the hot burn (fig. 6). Prefire populations of California mice and pocket mice were similar on both areas to be burned in San Dimas (fig. 7). Some individuals survived the normal burn. Pocket mouse populations exceeded prefire densities on both normal and hot burns by eight months postfire (fig. 7). California mouse populations exceeded prefire densities by one year postfire on the normal burn, but took two years to reach prefire densities on the hot bum (fig. 7). Two species not present prefire, Pacific kangaroo rats and deer mice, colonized both burned areas by eight months postfire; kangaroo rats have remained numerous on the hot burn, and deer mice are more numerous on the hot bum than on the normal burn (fig. 9). Captures of California mice postfire are greater on normal burns than on hot burns, and exceed captures on the control on one normal burn (776) (fig. 8). Captures of brush mice postfire are greater on both hot bums and one normal burn than on the control, and captures on hot bums are greater than on normal burns for each pair of watersheds burned (fig. 8). Deer mice have not been captured on the control; captures are greater postfire on hot burns than on normal burns for each pair of watersheds burned (fig. 8). Captures of wood rats are less on all burned areas than on the control, and they are less on hot bums than on normal burns for each pair of watersheds burned (fig. 8). California voles have not been taken postfire on the control nor on one normal burn, and are greater on the other normal burn than on either hot bum (fig. 8). Captures of Pacific kangaroo rats postfire are greater on Figure 6.-Comparison of rodent postfire population growth on normal (804) and hot (874 prescribed fire plots in Bell. Note that points at 0 on the x-axis against the y-axis are populations estimates prefire. both normal and one hot burn than on the control, while captures of pocket mice postfire are greater on all burned areas than on the control (fig. 8). Total biomass on the control, not 28 years old, has fluctuated during the period of study, but shows a slight increasing trend (fig. 9). Total biomass on both burned plots in Bell, the location of the control, has also fluctuated, with a slight increasing trend, in a fashion similar to that of the control (fig. 9). Total biomass on the burned plots in San Dimas has also fluctuated, with slight increasing trend, but with two dramatic biomass increases, one in the Spring of 1987 and the other in the spring of 1988 (fig. 9). The pattern of fluctuation, and increase, on the normal burn in San Dimas is similar to that observed for the control, and the pattern of fluctuation, and increase, if Figure 7.-Comparison of rodent postfire population growth on normal (776) and hot (775) prescribed fire plots in San Dirnas. Note that points at 0 on the x-axis against

5 the two sharp peaks are not considered, is also similar to the control (fig. 9). Discussion General patterns of rodent postfire succession following these prescribed burns are similar to those reported by Wirtz (1977,1982,1984) for succession following wildfire in the chaparral of the San Gabriel Mountains, but lack the dramatic increases in density, and therefore biomass, observed in these earlier studies. He notes (1984) that rodent succession following wildfire takes about four years before populations stabilize at essentially prefire conditions found in older chaparral stands. The response of species to these prescribed fires varied, with some species reaching prefire densities in less than four years and others having not yet reached prefire densities at essentially four years postfire. Only slight differences are noted between rodent postfire succession CONTROL m NORMAL 0 HOT - on normal and hot bums, and these may probably be attributed to differences in the biology of individual species. In Bell, both normal and hot burns were dominated postfire by pocket mice and brush mice, though pocket mice had the highest density on the normal burn (804) and brush mice had the highest density on the hot burn (874) (fig. 6). California mice recovered to prefire density on the normal bum, but have not yet (June 1988) recovered on the hot burn, and wood rats have not recovered to prefire densities on either burned area (fig. 6). Deer mice have been more prevalent on the hot burn than on the normal burn during the period of the study. By the second year postfire, populations of all species, except wood rats, exceeded prefire densities on the normal burn (fig. 2), and populations of brush mice and pocket mice had exceeded prefire densities on the hot bum (fig. 3). In San Dimas, where considerable brush was left alive on the normal burn (776), both normal and hot bums were dominated postfire by l o 0 U n, 0 0,-,- CNHNH CNHNH pocket mice and California mice (fig. 7). Both of these species recovered to prefire densities on the normal burn by one year postfire (fig. 41, as did pocket mice on the hot bum (fig. 5), but California mice did not reach prefire densities on the hot burn until the second year postfire (fig. 5). For reasons not immediately apparent, but probably because of the presence of some grass prefire, California voles were found only in these two watersheds postfire. The greater relative abundance of Pacific kangaroo rats on the hot burn is most likely due to the fact that more open space, necessary for kangaroo rat saltitorial locomotion, was left by the hot fire here. Pocket mice increase rapidly on burned areas, there being essentially no difference between normal and hot burns (figs. 6 and 7). Brush mice, if present prefire, recover more rapidly postfire than California mice, and the latter recover more rapidly on normal burns than on hot burns (figs. 6 and 7). Deer mice, virtually nonexistent in mature chaparral, colonize both normal and hot burns, and increase more rapidly on hot burns (figs. 6 and 7). Data on captures (fig. 8) indicate that increase of deer mice on hot burns. The species is known to colonize disturbed areas, whether they be caused by fire, logging, or over- Perom~scua Pmrwnyscur Peromyrcur caltlornicus boylii maniculrlu8 Figure 8.-Comparison of postfire captures of all rodent species on control and prescribed burn plots. Figure 9.-Total postfire biomass (grams) for control and burned plots.

6 gazing (Williams 1955). These data also illustrate the decline of California mice on hot burns and its increase in normal burns, and the increase of brush mice, where present prefire, on both normal and hot bums. Burning favors density increases of pocket mice, with essentially no difference between normal and hot burns. Kangaroo rats exhibit variable increases in response to fire, and wood rats are severely impacted by fire. Biomass increases in response to fire are variable, and in this study, were similar in variability to those occurring on the control (fig. 9). The sharp peaks in biomass observed on one hot burn (775) are due to large density increases in pocket mice during these periods. It is important to note, when comparing data for normal and hot bums, that in one normal burn (776) a lot of unburned brush remained, perhaps more accurately simulating an "island" in a burn rather than a burn per se. So, for this study, the data for 776 are somewhat atypical, and 804 represents more accurately the situation following a normal bum. But it is also important to note that "islands" of unburned vegetation are frequently left by wildfire, providing refugia for both plants and animals from fire. Several general conclusions may be drawn from the rodent data: (1) fire may impact rodent species severely, probably chiefly through loss of habitat resources, especially shelter and food; (2) some individuals survive fire; (3) colonization from adjacent habitats may be rapid; (4) postfire succession is somewhat dependent on prefire species composition of the area; (5) in southern California chaparral, at least two species, deer mouse and California vole, are fire specialists, entering the system only for relatively short periods of the postfire succession; (6) species requiring brush for cover and/or food, like wood rats and California mice, are most severely impacted by fire, and require the longest time to recover to prefire densities; (7) there is no clear-cut difference in rodent postfire succession following normal and hot fires; (8) rodent postfire succession is characterized by increases in successionally-adapted species, with declines in those species for which essential habitat features are lacking; and (9) recovery of the rodent community to its prefire condition probably takes four to six years, with the exact pattern of recovery being dependent on prefire species composition and features of the prefire plan community and postfire plant succession that have not been delineated. Acknowledgments This research was supported by USDA Forest Service, Pacific Southwest Forest and Range Experiment Station Grant Number PSW CA to WOW and a summer research assistantship from Pomona College to JRM. We are indebted to Susan Conard, Project Leader, Pacific Southwest Forest and Range Experiment Station, Forest Fire Laboratory, Riverside, CA, for her support and cooperation during this study. Many biology students at Pomona College have assisted with field work. The senior author wants to acknowledge 5 years of field work by Sherrie Souza and David Hoekman, all computer programming by David Hoekman, and all data analysis by John R. Muhm. We are grateful to Helen Wirtz for our figures. Preparation of this paper was greatly assisted by a summer research assistantship to JRM. Literature Cited Byrne, Roger, Joel Michaelsen and Andrew Soutar Fossil charcoal as measure of wildfire frequency in southern California: A preliminary analysis. p In Harold A. Mooney, Eugene C. Conrad, eds. Proceedings of the Symposium on the Environmental Consequences of Fire and Fuel Management in Mediterranean Ecosystems. USDA Forest Service, General Technical Report WO-3. Hanes, Ted L Succession after fire in the chaparral of southern California. Ecological Monographs 41(1): Hanes, Ted L. and Harold W. Jones Postfire chaparral succession in Southern California. Ecology 48: Hayne, Don W Two methods of estimating population from trapping records. Journal of Mammalogy 30(4): Lawrence, George E Ecology of vertebrate animals in relation to chaparral fire in the Sierra Nevada foothills. Ecology 47(2): Patric, James H. and Ted L. Hanes Chaparral succession in a San Gabriel Mountain Area of California. Ecology 45: Philpot, Charles W Vegetative features as determinants of fire frequency and intensity. p In Harold A. Mooney, Eugene C. Conrad, eds. Proceedings of the Symposium on the Environmental Consequences of Fire and Fuel Management in Mediterranean Ecosystems. USDA Forest Service, General Technical Report WO-3. Quinn, Ronald D Effects of fire on small mammals in the chaparral. Cal-Neva Wildlife Transactions, Williams, Olwen Distribution of mice and shrews in a Colorado montane forest. Journal of Mammalogy 36(2): Wirtz, William O., Vertebrate postfire succession. p In Harold A. Mooney, Eugene C. Conrad, eds., Proceedings of the Symposium on the Environmental Consequences of Fire and Fuel Management in Mediterranean Ecosystems. USDA Forest Service, General Technical Report WO-3, Washington, D.C., 498 p.

7 Wirtz, William O., Effects of fire on birds in chaparral. Cal- Neva Wildlife Transactions, Wirtz, William O., Postfire community structure of birds and rodents in southern California chaparral. p In Eugene C. Conrad, Walter C. Oechel, technical coordinators. Proceedings of the Symposium on Dynamics and Management of Mediterraneantype Ecosystems. USDA Forest Service, General Technical Report PSW-58. Wirtz, William O., Postfire rodent and bird communities in the chaparral of southern California. p In MEDECOS IV. Proceedings of the 4th International Conference on Mediterranean Ecosystems. University of Western Australia, Perth, Western Australia.