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Home , Ecosystem ecology, Extinction vortex, Lilium iridollae, Muhlenbergia torreyana, Solidago pulchra, Stylisma pickeringii

Extinction. Colonization, and Persistence of Rare Vascular Flora in the Longleaf Wiregrass : Responses to Fire Frequency and Size Janet Bracey Gray

RESEARCH ARTICLE ABSTRACT: We examined factors likely to be important in management of rare flora of the longleaf pine-wiregrass ecosystem on Fort Bragg and Camp Mackall Military Reservations in , USA. We documented 1,268 occurrences of thirty-six rare during two inventories • conducted from 1991 to 1993 and 1998 to 1999. Of these occurrences, 891 (70.3%) persisted, 258 (20.3%) went extinct, and 119 (9.4%) colonized between the first and second inventories; exceeded colonizations by 139 occurrences. We used analyses of contingency table frequencies and , logistic regressions to test hypotheses about temporal responses of local occurrences of rare flora. We found statistically significant effects of fire frequency on the colonization and extinction of rare flora; extinctions declined and colonizations increased with increasing fire frequency. There were statistically Colonization, and significant effects of both area occupied and stem number on the persistence and extinction of rare flora; extinctions declined and persistences increased with increasing area occupied and stem number. Our Persistence of Rare analyses emphasize the importance of fire and for the conservation of rare flora in this landscape. Vascular Flora in the Extinci6n, Colonizaci6n, y Persistencia de Flora Vascular Rara en los Longleaf Pi ne­ Ecosistemas de Longleaf Pine-Wiregrass: Respuestas a la Frecuencia del Fuego y el Tamafio Poblacional Wiregrass RESUMEN: Examinamos factores que podrian ser importantes en el manejo de flora rara en el ecosistema de longleaf pine-wiregrass en las reservas militares de Fort Bragg y en Camp Mackall en Carolina del Norte. Documentamos 1268 ocurrencias de 36 especies de plantas raras durante dos Ecosystem: inventarios realizados desde 1991 a 1993 y entre 1998 y 1999. De esas ocurrencias, 891 (70,3%) persistieron, 258 (20,3%) se extinguieron, y 119 (9,4%) fueron colonizadas entre el primer y el segundo Responses to Fire inventario; las extinciones superaron a las colonizaciones por 139 ocurrencias. Usamos aniilisis de contingencia, tablas de frecuencias, y regresiones logisticas para testear la hipotesis sobre la respuesta temporal de ocurrencias locales de flora rara. Encontramos efectos significativos de los efectos de la Frequency and frecuencia del fuego en la colonizaci6n y extincion de flora rara; las extinciones disminuyeron y las colonizaciones aumentaron con un aumento de frecuencia de fuego. Rubo efectos significativos del area ocupada y del mimero de troncos en la persistencia y la extinci6n de la flora rara; la extinci6n declino Population Size y la persistencia aumento con el aumento del area ocupada y el numero de troncos. Nuestro anruisis enfatiza la importancia del fuego y del tamafio poblacional para la conservaci6n de flora rara en el paisaje.

Janet Bracey Grayl Index terms: rare species, fire frequency, longleaf pine, extinction, colonization Department of the Army Public Works Business Center Endangered Species Branch HQ, Fort Bragg Garrison Command (Abn) Fort Bragg, NC 28310 USA INTRODUCTION (Hawkes and Menges 1995, Menges and Kimmich 1996, Kirkman et al. 1998, Quin­ Thomas R. Wentworth Fire is the primary tool used in the man­ tana-Ascencio et al. 1998, Lamont et al. Department of agement of rare flora associated with the 2000, Garnier and Dajoz 2001). Yet, there North Carolina State University longleaf pine-wiregrass ecosystem. The is an acute lack of knowledge of the tem­ Raleigh, NC 27695-7612 USA role of fIre in increasing poral responses of rare vascular flora to (Sparks et al. 1998, Kush et al. 2000), fIre, and, in particular, to fIre frequency. Cavell Brownie causing seasonal effects (Sparks et al. 1998, Further, we know of no -level Department of Statistics Engle et al. 2000), altering phenology (Platt analysis of the responses of rare flora to North Carolina State University et al. 1988, Brewer and Platt 1994), inHu­ fIre, where temporal responses are exam­ Raleigh, NC 27695-8203 USA encing demographics (Menges and Ko­ ined across suites of rare vascular species. hfeldt 1995, Brockway and Lewis 1997), Over time a rare plant occurrence will influencing open space (Hawkes and either persist or go extinct and new occur­ • Menges 1996), and perpetuating longleaf rences may become established, by way of pine itself (Heyward 1939, Glitzenstein et colonization. Knowing how fIre frequen­ al. 1995) is well documented at the com­ cy drives and influences colonization, ex­ 1 Corresponding author e-mail: munity level for the longleaf pine-wire­ tinction, and persistence is crucial to the [email protected] grass ecosystem. In addition, studies of effective management of rare flora associ­ Natural Areas Journal 23:210-219 individual taxon responses to fIre are be­ ated with the longleaf pine wiregrass eco­ coming more prevalent in the literature .

210 Natural Areas Journal Volume 23 (3),2003 Fire suppression and loss have di­ work. As it would not be feasible to devel­ compared to detecting responses of indi­ rectly contributed to the high incidence of op separate management plans for each of vidual species. rare flora associated with the longleaf pine­ these species, the Department of Defense wiregrass ecosystem. Hardin and White has developed and implemented an eco­ We specifically sought to quantify tempo­ (1989) identified 191 rare vascular species system management policy for its land ral responses of rare plant occurrences to associated with longleaf pine-wiregrass, management activities. Budget constraints fire frequency and popUlation size by test­ while Walker (1993) identified 389 rare and lack of personnel require managing ing the following hypotheses: (1) frequent vascular species associated with longleaf suites of species that inhabit an ecosystem fire increases colonization rates and de­ pine over the species' entire range (includ­ (Leslie et al. 1996). A multiple-species creases extinction rates of rare plant oc­ ing areas where wiregrass does not grow). approach may be particularly useful in the currences; (2) there are differences be­ Many of the rare vascular species associ­ absence of individual life history informa­ tween functional groups in response to fire ated with this system are either federally tion, a problem common to rare vascular frequency; (3) frequent fire increases both protected or state listed and are highly flora (Walker 1993). Thus, the focus of density and area occupied for persistent restricted geographically. Department of our analyses are directed across species rare plant occurrences over time; (4) local Defense installations harbor a dispropor­ and functional groups. Such broad-based extinctions decline and persistence of rare tionate number of rare species, as com­ analyses represent a conservative approach. plant occurrences increases with increas­ pared to lands managed by other federal Given the diversity of species involved, ing stem number and area occupied. agencies, due to the range of sought detecting responses to fire frequency and for diversified military training (Leslie et population size will be more difficult when al. 1996), and the protection and manage­ ment afforded these habitats.

Increasing the population size of rare is central to their conservation because North Carolina risk of extinction falls steeply with in­ creasing population size. Because of the confluence of negative genetic and demo­ graphic factors, small occurrences may be doomed by an extinction vortex (Gilpin and Soule 1986, Westemeier et al. 1998). Few studies in the plant literature have tested this largely theoretical hypothesis. The evaluation of classes (Fis­ cher and StOcklin 1997), outcrossing rates (Van treuren et al. 1993), reproductive success (Widen 1993), and percentage of seed germination (Menges 1991) support the expectation that extinctions decline with increasing population size. Ouborg (1993) demonstrated that occurrences that went locally extinct were smaller on aver­ age and that larger occurrences were more likely to persist through time.

This study examined the interaction be­ tween fire frequency, and the extinction, colonization, and persistence of rare plant occurrences, responses likely to be impor­ tant in the management of 36 rare species on Fort Bragg and Camp Mackall Military N Reservations, North Carolina, USA. It is clear that with so many rare plant species imperiled, a single species approach to t research and management is prohibitive and, in such a diverse system with one Figure 1. The rare plant occurrences found on Fort Bragg and Camp Mackall Military Reservations. dominant regime, will not Each dot represents a rare plant occurrence.

Volume 23 (3),2003 Natural Areas Journal 211 METHODS Surveys for Rare Flora RESULTS A comprehensive survey for rare flora was Study Area Survey Results for Rare Flora conducted on Fort Bragg and Camp Mack­ Our study areas, Fort Bragg and Camp all during 1991-93 by The Nature Conser­ Among 1,268 occurrences ofrare species, Mackall Military Reservations, occupy vancy (Table 1). A two-step process was there were 891 (70.3%) persistences, 258 approximately 65,000 ha of the longleaf used to identify rare plant occurrences. (20.3%) extinctions, and 119 (9.4%) colo­ pine-wiregrass ecosystem. The reserva­ The first consisted of habitat assessment nizations. Twenty-two (61.1 %) of the 36 tions are located within the sandhills re­ surveys conducted from October 1991 to species considered declined in the number gion of the inner coastal plain physio­ November 1992 using a parallel, zigzag of occurrences between the two surveys. graphic province, North Carolina, USA transect surveying technique. Habitat was Of the 36 rare species, some were more (Figure 1). assessed by each transect team as low, common than others. At one extreme, two medium, or high quality based on obser­ rare species, pulchra Small and The communities of the region are arrayed vations of degree of fire suppression, elliotti Chapman, represented by one along a moisture gradient from xeric enti­ amount of disturbance, level of spe­ occurrence each in 1991, were locally sols to mesic/wet ultisols. are de­ cies diversity, and the presence of rare extinct by 1999. At the other extreme, the rived from Cretaceous and/or Tertiary de­ species. state endangered sandhills pyxie moss posits of fluvial and marine sediments, (Pyxidanthera brevifolia Wells) contribut­ ranging from arkosic, feldspar-rich sands Step two of the inventory for rare flora ed 249 (25%) of the total number of occur­ to micaceous, arenaceous, compact clays. consisted of return visits to high and me­ rences extant in 1999, while the state threat­ The underlying sediments have a basal dium priority areas. Rare plant occurrenc­ ened Eupatorium resinosum Torr. ex DC conglomerate of pebbles and cobbles es encountered throughout the two assess­ and state candidate Tofieldia glabra Nutt. (Daniels et al. 1984). Mean annual tem­ ment periods were mapped on 1990 black each contributed 105 (10.4%) and 96 perature is 22.2°C and mean annual pre­ and white aerial photographs (scale (9.6%) occurrences, respectively (Table 1). cipitation for 30 y prior to and including 1:12,000) (Russo et al. 1993) and later 1999 was 1,186.2 mm y.l. transposed into a geographic information Of the 1,268 rare plant occurrences, 1,181 layer (GIS). Biological data recorded, per­ burned at least once, and these experi­ To date, 58 rare plant species have been tinent to this study, included the scientific enced an average of 2.38 fires during the identified on Fort Bragg and Camp Mack­ name of each occurrence, stem count, and study period. Fire frequency was highest all Military Reservations. These rare spe­ area occupied (Russo et al. 1993). Rare where rare plants colonized, lower where cies inhabit a number of communities con­ plant occurrences were defined as discrete they persisted, and lowest where they went tained in the longleaf pine-wiregrass entities separated by a minimum of 0.8 extinct (Figure 2). ecosystem that are burned on a 3- to 5-y km. rotation. These communities include xeric Effects of Fire Frequency on sandhill scrub, pine scrub/oak sandhill, The reservations were surveyed a second Temporal Responses mesic pine flatwoods, streamhead po­ time in 1998-99. Local occurrences docu­ cosins, and sandhill seeps (nomenclature mented during the initial survey were re­ There were statistically significant differ­ after Schafale and Weakley 1990). For our visited to determine presence or absence. ences in the proportions of persistence, study, we considered 36 of the 58 rare The area occupied by each persistent oc­ colonization, and of rare species that inhabit those longleaf pine currence was globally positioned to the plant occurrences in response to fire fre­ communities most influenced by fire. Of nearest meter and the number of stems quency when examined across all species these, 27 (75.0%) are herbaceous perenni­ was recorded. Suitable habitat was sys­ (P=0.0005, X2 0.05 (10 dO = 31.42, contin­ als, a proportion consistent with previous tematically surveyed for newly colonized gency table analysis). Deviations between findings that the vast majority of herba­ occurrences with an emphasis on areas expected and observed frequencies were ceous rare flora associated with longleaf surrounding existing occurrences. Suitable greatest for persistent occurrences. pine throughout its range are perennials habitats for potential colonizations were (Walker and Peet 1983, Hardin and White not identified, thus the proportion of col­ Using logistic regression, we examined 1989, Walker 1993). Twenty-three (64.0%) onizations to potential colonizable sites is extinction and colonization in response to of the 36 species inhabit mesic to wetland not known. The fire history, comprising fire frequency. We chose to contrast ex­ communities. This is also consistent with both dormant and growing season burns, tinction and colonization because these the finding that rare vascular flora associ­ was obtained for each occurrence for the are the most dynamic processes and dif­ ated with longleaf pine are more likely to period 1991 through 1999. fered the most with respect to fire frequen­ be found in wet habitats (Walker 1993) cy. Colonizations significantly increased and thus are now limited to pocosin/savan­ and extinctions declined as a of na (Frost 1995). fire frequency at P= 0.0009 (Figure 3). Between fire frequencies of four and five

212 Natural Areas Journal Volume 23 (3),2003 1- <:: 0 Table 1. List of rare flora, mean fires 1991-99, mean stem number 1998-99, total occurrences in 1991-93 and 1998-99, temporal responses, and state status. Botanical nomenclature follows C 3 Kartesz (1994). State statuses (Amoroso 1999) are abbreviated as follows: E=Endangered, T=Threatened, SR=SignificantIy Rare, SC=Special Concern, C=Candidate, and W1=Watch list. I n> N w Mean Fires Mean Stem No. Total Pop. Total Pop. State ~ Scientific Name 1991-99 1998-99 1991-93 Persistence Extinction Colonization 1998-99 Status N 0 0 Agalinis aphylla (Nutt.) Raf. 1.3 26.7 2 1 1 1 2 SR w Agalinis obtusifolia Raf. 1.5 5.0 2 1 1 0 WI Amorpha georgiana var. georgiana Wilbur 1.9 55.6 30 26 4 3 29 E Astragalus michauxii (Kuntze) F.J. Henn 2.1 10.7 88 69 19 18 87 T Carex tenax Chapman 1.0 200.0 1 1 0 0 1 C Cladium mariscoides (Muhl.) Torr. 2.0 535.0 5 5 0 0 5 SR Danthonia sericea Nutt. 2.6 161.8 18 16 2 1 17 SR Dionaea muscipula Ellis 3.8 89.2 9 9 0 1 10 C-SC Eriocaulon texense Koem. 2.8 30.0 4 4 0 1 5 C Eupatorium resinosum Torr. ex DC. 2.0 93.1 136 99 37 6 105 T-SC Gaillardia aestivalis (Walt.) H. Rock 3.3 16.9 5 2 3 2 4 C Gallactia mollis Michx. 3.0 4.8 7 4 3 0 4 C Gnaphalium helleri var. helleri Britt. 1.6 26.1 5 2 3 2 4 SR Kalmia cuneata Michx. 2.5 115.2 25 22 3 2 24 E-SC iridollae Henry 2.7 0.5 12 2 10 10 12 T Lindera subcoriacea B.E. Wofford 2.6 40.8 64 55 9 5 60 E torreyana (J.A. Schultes) A.S. Hitchc. 2.0 100.0 1 0 0 E Nestronia umbellula Raf. 1.5 541.9 25 23 2 2 25 WI Pamassia caroliniana Michx. 3.4 92.9 3 2 1 4 6 E Phaseolus polystachios (L.) B.S.P. 2.4 8.2 86 61 25 6 67 C Physalis lanceolata Michx. 1.8 15.5 11 5 6 2 7 WI Polygala grandiflora Walt. 2.1 48.5 7 5 2 6 SR Pyxidanthera brevifolia Wells 2.2 2.0 255 238 17 11 249 E Rhynchospora macra (C.B. Clarke) Small 2.5 125.2 10 8 2 0 8 E Rhynchospora oligantha Gray 2.1 36.7 12 11 1 0 11 C Ruellia ciliosa (Pursh) R.W. Long 3.0 12.5 1 0 1 1 1 C Solidago gracillima Torr. and Gray 1.8 50.8 6 6 0 0 6 SR Solidago pulchra Small 3.0 0.0 0 1 0 0 E Z Solidago vema M.A. Curtis 1.9 59.8 17 14 3 2 16 T ~...... c pickeringii var. pickeringii e!.. (Torr. ex M.A. Curtis) Gray 1.4 283.4 58 53 5 5 58 E ...;I> Tofieldia glabra Nutt. 2.5 8.3 145 79 66 17 96 C n> Tridens carolinianus (Steud.) Henr. 2.4 21.5 43 29 14 1 30 C rn~ '-- Warea cuneifolia (Muhl.ex Nutt.) Nutt. 2.0 2.5 4 1 3 0 1 C 0 c Xyris chapmanii Bridges and Orzell 2.1 31.2 26 21 5 5 26 C ...::s e!.. Xyris elliottii Chapman 3.0 0.0 1 0 1 0 0 C Xyris scabrifolia Harper 2.2 8.0 24 16 8 10 26 C N 1149 891 258 119 1010 ...w each increased with increasing flre fre­ 2.7 quency at P = < 0.0541 and P = 0.0360, respectively, while those of dicots declined. We found no statistically signiflcant rela­ 2.6 tionships between the responses of persis­ tence and colonization of legumes versus m2.5 c- nonlegumes, woody versus herbaceous ..... species, and annuals versus perennials to t;:: fIre frequency (Table 2, right side). '0..... 2.4 o r-- Q) J.. .c E Changes in Stem Number and Area :::J 2.3 ~ c:: Occupied as a Function of Fire c:: 1 Frequency ca 1 ~2.2 ~ ~ We hypothesized that increased flre fre­ quency increases both stem number and 2.1 r-- r-- area occupied for persistent rare plant oc­ n=25B n=B51 n=119 currences over time. There were statisti­ cally signiflcant effects of flre frequency 2 on changes in mean stem number during Extinction Persistence Colonization the period 1991-99 at P = 0.0023. Stem number increased by an average of 24.5 Figure 2. Mean number of fires incurred for occurrences of rare species representing each temporal state, during the period 1991-99. stems across all observations. Fifty per­ cent of the observations were at or near times during the 9-y period, the propor­ tions of colonizations and extinctions were 1.0 equal (Figure 3). en c: o Effects of Fire Frequency on ~ro N 0.8 Functional Groups 'c o We used logistic regression to test flre 8 frequency effects on functional groups by '0 0.6 classifying occurrences into one of two c:: mutually exclusive functional groups and o :eo assigning a 1 to the functional group of c.. interest and a 0 to the contrasting func­ e 0.4 tional group. Colonized and persistent c.. -- -' "0 occurrences were pooled and chosen for ...... Q) this analysis as these represent successful ro --- E 0.2 occurrences present in 1999. Proportions 17i of monocot occurrences increased while w proportions of dicot occurrences declined with increasing fIre frequency, signiflcant 0.0 at P = < 0.0001 (Table 2, left side). We found no statistically signiflcant relation­ o 1 2 3 4 5 ships between proportions of legumes ver­ Fire Frequency sus nonlegumes, woody versus herbaceous species, and annuals versus perennials as a Figure 3. Proportions of colonizations (n=119), assigned a value of 1, contrasted with extinctions function of fIre frequency (Table 2, left (n=258), assigned a value of 0, as a function of fire frequency and as estimated by logistic regression. side). We then evaluated separately func­ The broken lines represent the 95% confidence bands on each side of the fitted logistic relation. The arrow represents equilibrium of colonizations and extinctions at fire frequencies of four and five times tional group persistences and colonizations during the 9-y period. The relative numbers of colonizations and extinctions at each fire frequency are as functions of flre frequency. Proportions depicted as bars across the top and bottom of the graph, respectively. The relationship was significant of monocot colonizations and persistences at P = 0.0009, XZ 0.05(1) = 11.06.

214 Natural Areas Journal Volume 23 (3),2003 DISCUSSION Table 2. Tests for fire frequency as a predictor of functional group proportions and persistence and colonization proportions using logistic regression. We were able to assess with reasonable accuracy the extinction and persistence of Pairs of Per.·CoI. Combined Persistence Colonization rare plant occurrences. Return visits to Functional groups (n = 1010) (n = 891) (n = 119) occurrences recorded during the initial survey are likely to be accurate; either the Monocot VS. dicot P = < 0.0001 P =0.0036 P =0.0541 2 occurrence was present or not. Like Harri­ X = 15.32 X2= 8.50 X2= 3.71 son et al. (2000), we found that the major­ Legume vs. non-legume P = 0.7397 P = 0.8037 P = 0.5825 ity of the species we studied were perenni­ X2= 0.11 X2 = 0.06 X2= 0.30 als. This is an advantage when censuses Woody VS. herbaceous P =0.4656 P = 0.9827 P = 0.2370 are infrequent, because occurrences of X2= 0.53 X2= 0.00 X2= 1.40 longer-lived species have greater intrinsic Annual VS. perennial P = 0.3735 P = 0.3649 P = 0.3001 stability (Ricklefs 2001). Perhaps our least X2= 0.54 X2= 0.82 X2= 1.07 reliable data are those for colonizations. Psuedo-colonizations could exist if any occurrences were missed during the 1991- 93 survey and then discovered during the 1998-99 survey. However, given the length zero change, with high variability in chang­ tions. Of the rare plant occurrences that of the initial survey and the highly trained es in stem number at low fIre frequencies went extinct, 175 (71.1 %) had 1-10 stems, 2 botanists conducting the survey, missing and little variability in changes in stem and 164 (64.0%) occupied an area of 1 m • existing plant occurrences was minimized number at high fire frequencies. There were to the maximum extent possible. statistically signillcant effects of fIre fre­ quency on changes in average area occu­ pied at P = <0.0001. Area occupied exhib­ ited a mean decline of -0.24 ha across all observations during the 9-y period. As was the case with changes in stem number, 50% of the observations were at or near 1.0 zero change, with high variability in chang­ I es in area occupied at low fire frequencies (J) c and little variability in changes in area o occupied at high fire frequencies. t5 0.8 c ~ (I) Persistence versus Local Extinction as o a Function of Population Size -(J) 0.6 c o We hypothesized that extinctions of rare t: plant occurrences would decline and the o a. persistence of occurrences would increase e 0.4 with increasing stem number and area a. "0 occupied. We chose to contrast persistence (I) versus extinction as these occurrences dif­ 10 0.2 fered the most in population size. The pro­ E ~ portions of local extinctions declined and w -- the proportions of persistent occurrences - - increased significantly as a function of 0.0 ---- .. stem number (P = 0.0002, Figure 4) and area occupied (P = < 0.0001, Figure 5). 1-10 11-100 101-1000 >1000 Occurrences that went locally extinct had the smallest stem numbers and occupied Stem numbers (1991-1999) relatively small areas, while those of per­ sistent occurrences had the largest stem Figure 4. Proportions of extinctions (n=246), assigned a value of 1, contrasted with persistences (n=605), numbers and occupied the largest areas assigned a value of 0, as a function of stem number and as estimated by logistic regression. The broken lines represent the 95% confidence bands on each side of the fitted logistic relation. The relative (Figure 6). Small occurrences were dis­ numbers of extinctions and persistences are depicted as bars across the top and bottom of the graph, 2 proportionately represented among extinc- respectively. The relationship was significant at P = 0.0002, X 0.05(1) = 13.83.

Volume 23 (3),2003 Natural Areas Journal 215 ment. Persistent pappus-like hairs and awns aid in dispersal to new seedbeds created 1.0 I I by fire. Infrequent fire encourages en­ croaching woody vegetation, thus elilni­ (J) c nating open areas for seedling establish­ o ment (Keeley 1981). 15 0.8 c ~ There were declines in area occupied and '0 increases in stem number among rare plant (J) 0.6 occurrences in response to fIre frequency, c o but gains and losses were small. Fifty per­ 1:: o cent of the observations were at or near e0. 0.4 zero change in both stem number and area. 0. A case may be made for reduction in the "C fluctuation of stem number and area as a 0.2 means of stabilizing occurrences of rare E plants. Across the range of fIre frequen­ iii --l::""':-::""-~-:":-~ !, "*LU -- ~ies, occurrences exhibited the least change -- m stem number and area at the highest fire 0.0 frequencies, suggesting that burning every 2 y would be sufficient to lninilnize vola­ -5 to-4 -3.9 to-3 -2.9 to-2 -1.9 to -1 >-1 tility. Thus, we propose that 2 y be consid­ ered the lninimum fIre return interval for Log of area (1991-1993) rare vascular flora in this longleaf pine­ wiregrass ecosystem. This fIre return in­ Fi~re 5. Proportions of extinctions (n=258), assigned a value of 1, contrasted with persistences (n-885) terval is in the lnid-range of a presettle­ ~slgned a value of 0, as a function of the log of area and as estimated by logistic regression. The b~oke~ ment fIre regime of 1-3 y reported by hnes represent. the. 95% confid~nce bands on each side of the fitted logistic relation. The relative numbe~s of extinctions. and .perslste~ces are depicted as bars across the top and bottom of each gra h Frost (1998) for the southeastern coastal respectively. The relationship was highly significant at P = <00001• , X2 0.05(1) - 84 •10 • P , plain of the , where 50% to 90% of the landscape probably burned that frequently and in which many rare, fIre-dependent plant species appear to re­ among occurrences that burned between Fire frequency effects were apparent in quire a 1- to 3-year fIre return interval. the data for both colonization and extinc­ fo~r and fIve times during the 9-yr. period (FIg. 3). This indicates that increased fIre tion across species. Extinctions declined In contrast to the small variation in stem frequency is warranted in areas across the and colonizations increased with increas­ number and area at the highest fIre fre­ reservations with infrequent fIre and points ing fIre frequency. There were statistically quencies were large spikes of both stem to a fIre return interval of approximately 2 signifIcant effects of both area occupied number and area gained at zero fIre fre­ y for balancing rates of extinction and and stem number on the persistence and quency. These large gains in stem number colonization. extinction of rare flora; extinctions de­ and area were primarily attributable to clined and persistences increased with in­ Stylisma pickeringii var. pickeringii (Torr. Among the pairs of functional groups ex­ creasing area occupied and stem number. ex M.A. Curtis) Gray and Pyxidanthera amined, monocots and dicots varied sig­ brevi/alia. Large, densely populated areas nifIcantly in response to fIre. The propor­ Although most occurrences in this study of Stylisma pickeringii var. pickeringii are tions of monocots increased with were burned during the period 1991-99, common in physically disturbed areas, such increasing fIre frequency, as did monocot there were more than twice as many ex­ as roadsides and dolninated areas colonizations and persistences, when ex­ tinctions as colonizations. For long-term that are subject to frequent mechanical amined individually. The reverse was true stability of the number of occurrences of disturbance and that have low to no cano­ for dicots. It is plausible that monocots are rare species, extinctions must be offset by py cover. This appears to be indicative of more sensitive to fIre and more responsive colonizations. Our evidence suggests that the species' ability to proliferate under a to the open conditions created by frequent the rate of colonization balances the rate disturbance regime other than fIre, as long fIre than are other functional groups. This of extinction at fIre frequencies of between as for light is reduced. Small, may be particularly true for the four pe­ four and fIve times during the 9-y study widely scattered clumps of Pyxidanthera rennial grasses considered in our study. period (Figure 3). Occurrences that went brevi/alia are also contained within xeric Seeds of perennial grasses are light and locally extinct burned the fewest number sandhill communities that were not burned require openings for seedling establish- of times while extinctions declined to 50% during the time frame of this study. The

216 Natural Areas Journal Volume 23 (3),2003 to fIre frequency and few studies in the plant 100 1.2 literature that address extinction and persis­ tence as a function of popUlation size. Ext Col Per I~.A·.I Density 90 n=246 111 601 SId error 2.86 20.71 9.96 The relationships among the three tempo­ -Area n=258 117 879 1 ral states are complex and there are differ­ 80 SId error 0.05 0.03 0.15 ences among species in their responses to fue. Despite the fact that this study in­ CJ) 70 volved multiple species with a diversity of E 0.8 Q) life forms, life histories, and environmen­ +-' CJ) 60 tal relations, strong signals of effects of L!- fIre frequency and population size on tem­ .....0 CIS Q) ~ poral responses were apparent. .0' 50 0.6 « E c :::J CIS C Q) ACKNOWLEDGMENTS c 40 2 CIS This study was made possible with the Q) 0.4 2 30 support of the Department of the Army, Public Works Business Center, Endangered Species Branch, Fort Bragg. Amy Bivin 20 0.2 and Boroff provided valuable as­ sistance in the tedious task of data organi­ 10 zation. We also acknowledge Beth Evans and Brian Ball for their formatting skills. 0 ""--+ 0 Peter Hickman provided the study area map. This manuscript is much improved Extinction Colonization Persistence by the useful discussions and comments provided by Nick Haddad and Jon Stucky. Figure 6. Mean population size of temporal responses. Values for extinction and persistence are based on 1991-93 data and values for colonization are based on 1998-99 data. This paper is dedicated to Zachary Mills Shipley.

Janet Bracey Gray is an Endangered Spe­ xeric sandhill communities are character­ would be greatly reduced if a minimum cies Botanist for Fort Bragg and Camp ized by a sparse canopy and shrub layer, population size in excess of 100 stems Mackall Military Reservations in North minimal ground litter and large areas of could be achieved for occunences of rare Carolina, where she facilitates research open sand. In contrast to communities that plant species in this landscape. The fInd­ and land management projects within the contain Stylisma pickeringii var. picker­ ing that the average popUlation size for longleaf pine-wiregrass ecosystem. This ingii, these areas are not subject to fre­ persistent occurrences in 1991-93 was 74.0 research was conducted in partial fulfill­ quent mechanical disturbance. Persistence (Figure 6) stems and rose to 98 stems in ment ofher M.S. degree in Botany at North under these conditions is similar to that 1998-99 also supports our recommenda­ Carolina State University. seen in some rare scrub endemics tion for minimum population size. Al­ adapted to fIre cycles of 10 to 40 y. The though not addressed in this study, a spa­ Thomas R. Wentworth is Professor of Bot­ persistence of these endemics is highly tial analysis of the potential any at North Carolina State University, conelated with open sand conditions and structure of each rare species to include where he has taught courses in general is independent of fIre (Hawkes and Meng­ the variables of population size, location, and plant community ecology since es 1995). and the distances among them will further 1976. His research interests include de­ enlighten our understanding of the dynam­ scription, classification, and study of envi­ Extinctions declined and the persistence ics of extinction, colonization, and persis­ ronmental relationships of natural vegeta­ of rare plant occunences increased with tence and could serve to direct future man­ tion, with emphasis on the plant increasing population size (Figure 4 and agement efforts. communities of the Carolinas. 5). Is there a minimum population size This study is a fIrst step in elucidating the that best promotes the persistence of rare effects of prescribed fIre as a management Cavell Brownie is Professor ofStatistics at plant occunences? We found that 236 tool for rare vascular flora associated with North Carolina State Univ~rsity where her (96.0 %) of all extinctions occuned in the the longleaf pine-wiregrass ecosystem. teaching, consulting, and research efforts two smallest stem classes of 1-10 and 11- There are few studies in the literature that focus on the application of statistics in the 100 stems, suggesting that extinctions address the temporal responses of rare flora

Volume 23 (3),2003 Natural Areas Journal 217 biological . Her consulting and Gilpin, M.E., and M.E. Soule. 1986. Minimum Xanthorrhoea preissii. Austral Ecology research activities include a long-term, viable occurrences: processes of species 25:268-272. multidisciplinary project in sustainable extinctions. Pp. 19-34 in M.E. Soule, ed., Leslie, M., G.K. Meffe, lL. Hardesty, and , methods for estimating sur­ Conservation : The of Scar­ D.L. Adams. 1996. Conserving biodiversi­ city and Diversity. Sinauer, Sunderland, ty on military lands: a handbook for natural vival rates in Wildlife , and Mass. application of spatial analysis methods to managers. The Nature Conservan­ Glitzenstein, J.S., W,J. Plat,t and D.R Streng. cy, Arlington, Va. data from agricultural field trials. 1995. Effects of fire regime and habitat on Menges E. 1991. Seed germination percentage tree dynamics in north Florida longleaf pine increases with population size in a frag­ savannas. 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218 Natural Areas Journal Volume 23 (3),2003 crossing rates in locally endangered Salvia pratensis. 47:1094-1104. Walker, J. 1993. Rare taxa asso­ ciated with the longleaf pine : patterns in and ecology. Pp. 105- 125 in Sharon M. Hermann, ed., The Longleaf Pine Ecosystem: Ecology, Resto­ ration and Management. Tall Timbers Fire Ecology Conference, Proceedings No. 18. Tall Timbers Research Station, Tallahas­ see, Fla. Walker, J., and R.K. Peet. 1983. Composition and of pine-wiregrass sa­ vannas of the Green Swamp, North Caroli­ na. Vegetatio 55:163-179. Westemeier, R.L., J.D. Brown, and S.A. Simp­ son. 1998. Tracking the long-term decline and recovery of an isolated population. Science 282:1695-1698. Widen, B. 1993. Demographic and genetic ef­ fects on as related population size in a rare, perennial herb, Senecio inte­ grifolius (). Biological Journal of the Linnean Society 50:179-195.

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