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Regional frequencies of tree species associated with anthropogenic disturbances in three forest types
Forest Ecology and Management 117 (1999) 241±252
Regional frequencies of tree species associated with anthropogenic disturbances in three forest types Martin A. Stapaniana,*, David L. Cassellb a
Department of Evolution, Ecology and Organismal Biology, Ohio Cooperative Fish and Wildlife Research Unit, The Ohio State University 1735 Neil Ave., Columbus, OH 43210, USA b OAO Corporation, 200 SW 35th St., Corvallis, OR 97333, USA Received 5 May 1998; accepted 15 September 1998
Abstract We used a probability-based sampling scheme to survey the forested lands of Georgia and Alabama, two adjacent states in southeastern USA. Using a nationally consistent plot design and ®eld methods, we evaluated the presence/absence of tree species at 123 1/15 ha plots for individuals having a diameter at breast height >2.5 cm. Three forest cover types were considered: loblollyshortleaf pine, oak-hickory, and oak-pine. Visually evident anthropogenic disturbances (e.g. arti®cial regeneration, logging, and prescribed burning), if any, were recorded on each plot. We classi®ed plots with visually evident anthropogenic disturbance as `disturbed'. The remaining plots were classi®ed as `undisturbed'. For each of the three forest types considered, mean species richness of trees was signi®cantly greater in undisturbed plots. Of 61 species forest type combinations, eight (13.1%) had signi®cantly different frequencies in undisturbed than in disturbed areas. In the loblolly-shortleaf pine type, ®ve of the 18 species (27.8%) considered occurred signi®cantly more frequently on undisturbed plots. In the oak-hickory type, two of the 21 species considered (9.5%) occurred more frequently on undisturbed plots and one species, loblolly pine, occurred more frequently on disturbed plots. Species occurring more frequently on undisturbed areas in these forest types included occupants of both the mid and upper canopy. Loblolly pine is not a common associate in the undisturbed oak-hickory forest. These results and those from other studies strongly suggest that anthropogenic disturbance, particularly clearcutting and subsequent planting of pine plantations, is having profound effect on the structure and composition of the forested area of the region. Because the three forest cover types we considered account for approximately 71% of the total forested area of the region, these results should be useful for forest management and conservation planning in the states. Management implications of mixed pine-hardwood stands vs. pine plantations in the south-east are discussed. # 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Loblolly-shortleaf pine; Oak-hickory; Oak-pine; Georgia; Alabama; Anthropogenic disturbance; Monitoring; Trees; Species frequency
1. Introduction Millions of acres of hardwood forests in southeastern USA have been converted to agriculture and *Corresponding author. Tel.: +1-614-292-5166; e-mail: [email protected]
pine plantations since the 1930s, particularly in the oak-pine uplands and the mixed hardwood bottomlands in major river valleys (Smith and Linnartz, 1980 and references therein). Clearcutting and subsequent planting of pine plantations is expected to increase in the future (USDAFS, 1988; Alig et al., 1990; Boyce and Martin, 1993). Competing woody vegetation is
0378-1127/99/$ ± see front matter # 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00483-6
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typically suppressed in the established plantation by controlled burning (Richter et al., 1982; Binkley et al., 1992), application of synthetic hormones and herbicides (Walker, 1980), and other techniques (Swindel et al., 1984). As the planted pines become more dominant, diversity trends in the maturing plantations ``are expected to be less ecologically favorable'' (Swindel et al., 1984, p. 19). Other studies suggest that when stands of mixed species forests are cleared and replaced by pine plantations, the result is a locallyimpoverished ¯ora and fauna relative to the `original' stand (Atkeson and Johnson, 1979; Repenning and Labisky, 1985; Childers et al., 1986; Felix et al., 1986; Skeen et al., 1993). Since these management practices are so widespread in the Southeast, the above would suggest an overall loss of tree species richness at the regional level. However, this can not be assessed without statistically sound, quantitative estimates of trends of diversity at the regional or national level. Such estimates are lacking for most taxa, including trees, in the United States. There is a vast literature on the pattern and process of response of local (i.e. standlevel) plant diversity to anthropogenic disturbance in forests (e.g. Grime, 1979; Denslow, 1980; Oliver, 1981; Bazzaz, 1983; Pickett and White, 1985; Hunter, 1990). However, they cannot be used individually or collectively to estimate population parameters at a regional level (Wyant et al., 1991; Stapanian et al., 1997) for several reasons. Typically, such studies are based on non-probability samples, and they have insuf®cient sample sized from a regional perspective to allow for rigorous statistical analysis. Further, they are usually site- or landscape-speci®c, and/or do not use the same methodology throughout the region. In a previous work (Stapanian et al., 1997) from a large, probability-based synoptic survey of forests, we found that anthropogenically-disturbed forested lands in southeastern USA had signi®cantly lower tree species richness than in undisturbed forests. In particular, species richness of trees was signi®cantly lower in disturbed areas of the loblolly± (Pinus taeda) shortleaf pine (P. echinata), oak (Quercus spp.) -pine (Pinus spp.), and oak-hickory (Carya spp.) forest cover types. Further, we quanti®ed the proportion of the forested area that had visually evident anthropogenic disturbances (`disturbed').
These three forest cover types account for approximately 71% of the total forested area of Georgia and Alabama (Stapanian et al., 1997). Thus, changes detected in their species composition and structure have important rami®cations for conservation and forest management for the entire region. In this paper, we examine data for these three forest cover types from the same probability-based survey as Stapanian et al. (1997). For each forest type, we quantify differences in the frequencies of occurrence of individual tree species in undisturbed and disturbed plots. For a subset of the individual species, we test the null hypothesis that the frequency of occurrence in undisturbed plots is equal to that in anthropogenically disturbed plots. Tree species are only one component of forest biodiversity. However, trees are a logical starting point, given their ecological dominance (Packham and Harding, 1982), economic importance (Smith, 1986), and in¯uence on the forest environment and other organisms (Hunter, 1990). 2. Methods 2.1. Sampling design and plot layout The forest health monitoring (FHM) program, a large-scale synoptic forest survey system, is the source of our data. FHM is an inter-agency government project (Alexander and Barnard, 1993). FHM's objectives include the monitoring and statistical estimation of trends, changes, and current status in indicators of the condition of the country's forest resources, on a regional and national scale. Detecting changes in these indicators associated with anthropogenic disturbance is another major objective of FHM (Palmer et al., 1991). Species richness of trees is just one of those indicators. The basis of the FHM sampling design (Cassell, 1993; Schreuder and Czaplewski, 1993) is the Environmental Monitoring and Assessment Program (EMAP) design (Overton et al., 1991; Stevens, 1994; Stehman and Overton, 1994), which uses an equal-area triangular grid with points approximately 27 km apart. This design permits statistical estimation of population parameters using traditional sampling theory: Horvitz±Thompson estimation with Yates± Gundy estimates (Cochran, 1977). The target sam-
M.A. Stapanian, D.L. Cassell / Forest Ecology and Management 117 (1999) 241±252
pling density of the forested area of each state is achieved after 4 years of sampling, using a rotating panel design (Cassell, 1993). However, population parameters can be estimated after a minimum of one season of sampling. The FHM and EMAP sampling designs have been demonstrated to be effective in permitting the use of statistically-designed study sites in a probability-based network, for making regional estimates on ecological resources (Stevens, 1994; Stehman and Overton, 1994). Because the FHM design is probability based, each forest cover type is sampled in proportion to its area in the region. Similarly, attributes of stands, such as age and class, of each forest type are sampled in proportion to their area in the region. In a previous work, (Stapanian et al., 1997) we demonstrated the linkage between plot-level and regional diversity estimates from this design. Therefore, the data from the FHM design are appropriate for making regional inferences based on plotlevel information. Each forested plot located from the FHM sampling system is a circle of 1 ha (Tallent-Halsell, 1994). Within this circle are four ®xed-radius subplots, each of radius 7.3 m (24 ft). The combined area of the four subplots is 1/15 ha. Measurements for trees having diameter at breast height (dbh) are taken within these subplots. Within each subplot is a circular microplot of radius 2.1 m (6.8 ft), from which data for trees with dbh between 2.54 and 12.7 cm are taken. Only trees having dbh 2.54 cm are considered in this study. Statistical evaluations of the FHM plot design (Riitters et al., 1991; Lewis and Conkling, 1994) have demonstrated using sampling theory formulae (Cochran, 1977; Cassell, 1992) that the multi-tiered plot layout is well suited to the variety of current FHM indicators. 2.2. Field methods and descriptions of regions We examined data from 123 FHM plots in the states of Georgia and Alabama (Stapanian et al., 1997), which were predominantly in the Southeastern Plains and Southwestern Appalachian ecoregions (Omernik, 1987, 1995). Data were collected during the summers of 1990 through 1993. Plots were classi®ed as forested according to standard protocols used by the U.S. Forest Service (Tallent-Halsell, 1994). The list of species that are considered trees in FHM may also be found in Tallent-Halsell (1994). This list is similar
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to that of Little (1971) and Stapanian et al. (1997)). Forest cover type(s) (e.g., oak (Quercus spp.) -hickory (Carya spp.) forest) in each plot was (were) determined in the ®eld, based on tree species composition, according to standard protocols from the U.S.D.A. Forest Service (Tallent-Halsell, 1994 and see Eyre, 1980). Nomenclature of tree species was according to Harlow and Harrar (1969) and Harlow et al. (1991). In nearly all the cases, individual trees were identi®ed in the ®eld to species. In rare cases (<1%), a tree was identi®ed only to the genus level, because insuf®cient material was present to permit a precise identi®cation to species. Blackgum (Nyssa sylvatica) and swamp tupelo (N. s. var. bi¯ora) were treated as separate species in this analysis because they could be accurately identi®ed in the ®eld (Tallent-Halsell, 1994). We considered the data from only three forest cover types: loblolly±shortleaf pine, oak-pine, and oak-hickory. On each plot, up to three types of visually evident disturbances were recorded (Tallent-Halsell, 1994; Scott and Bechtold, 1995). This was done in accordance with the de®nition of White and Pickett (1985), p. 7) of disturbance as ``any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.'' Therefore, disturbance provides no a priori inference about the condition or health of ecosystems (Stapanian et al., 1997). Types of anthropogenic disturbances included planting stands, harvesting, cutting, thinning, prescribed burning, grazing by domestic livestock, and construction. Alterations by weather, disease, insects, and wild®re, as well as natural reversion to forest were not considered anthropogenic disturbances. These `natural' disturbances were recorded on too few plots to be analyzed separately (Stapanian et al., 1997). In order to be included in the analysis, a plot was considered only if the entire area of the plot was (1) forested and (2) composed of the same forest cover type. Further, anthropogenic disturbance was required to be recorded in either all (`disturbed') or none (`undisturbed') of the four subplots in order for a plot to be included in the analysis. These inclusion criteria are more rigorous than those in our previous study (Stapanian et al., 1997). Therefore, the number of plots considered in this study was slightly less for each forest cover type than in our previous paper. For
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each forest cover type, we tested the null hypothesis that the mean number of species per plot was the same in disturbed and undisturbed plots with an analysis of variance (ANOVA). For selected species we tested the null hypothesis of equal frequency of occurrence in the disturbed and undisturbed subsets of plots within a forest cover type. In order to be selected for testing, a species was required to occur on a minimum of two plots in either the disturbed or undisturbed subset within a forest cover type. Conventional 2 and tests based on it were likely to be poor choices for this analysis, since cell counts were often low. Therefore, we used likelihood ratio 2 (Fienberg, 1977), which accommodates low cell counts. 3. Results The loblolly±shortleaf pine forests clearly had the largest proportion (73.9%) of plots that had anthropogenic disturbance (Fig. 1). In particular, planted stands of loblolly pine accounted for more than one-half of that forest cover type. The disturbance types prescribed burning, cutting, site preparation, arti®cial regeneration, and harvesting also occurred more frequently in the loblolly-shortleaf pine forest type. Grazing by livestock was recorded only in the oak-pine type. As expected, planted stands and disturbance from arti®cial regeneration were not observed in the oak-hickory forest type. The mean number of species per plot was signi®cantly greater in the undisturbed plots than in the
Fig. 1. Proportion of plots in which anthropogenic disturbance was recorded (`disturbed') and the proportions in which specific types of disturbances occurred for three forest cover types. Up to three types of disturbance could be assigned to a plot. Therefore, the sum of the proportions of all disturbance types for a forest cover type >1. `Planted' refers to the plots in which humans had planted the trees. Abbreviations: site prep. site preparation, regen. artificial regeneration. Black loblolly±shortleaf pine type (LS), white oak-pine type (OP), striped oak-hickory type (OH).
disturbed plots for all the three forest cover types (ANOVA), values of p in Table 1). This result was similar to Stapanian et al. (1997), for which the inclusion criteria were much less rigorous. Further, the mean number of species per plot was least in the loblolly-shortleaf pine forests, both in undisturbed and disturbed plots. Although the oak-hickory forests had the fewest number of plots, it contained the greatest number of species when all the plots for that type were pooled (values of T in Table 1). The loblolly-shortleaf
Table 1 Summary statistics for the number of tree species found in plots that had visible anthropogenic disturbances (`disturbed') and those that did not (`undisturbed') in three forest types in southeastern USA Undisturbed plots Forest type Loblolly±shortleaf pine Oak-pine Oak-hickory
N 12 26 22
T 31 51 54
Disturbed plots Mean species per Plot (s) 7.25 (3.04) 9.15 (2.89) 9.23 (2.52)
N 34 19 10
T 36 43 24
Mean species per Plot (s) 4.32 (3.38) 6.58 (3.92) 6.50 (4.30)
ANOVA p>F 0.012 0.015 0.031
In each forest type, we tested the null hypothesis that the means species per plot was the same in disturbed and undisturbed plots with analysis of variance (ANOVA). Abbreviations: N number of plots; T the total number of species found when the data for all plots in a group were pooled; s standard deviation.
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pine forests had the lowest values of T of the three forest cover types investigated. These results agreed with the previous studies (e.g. Society of American Foresters, 1954; Crow, 1980; Mann, 1980; Smith and Linnartz, 1980). As expected, the subset (i.e. undisturbed or disturbed) within each forest cover type which contained more plots had the larger total number of species when the plots for that subset were pooled (values of T in Table 1). From the statistics in Table 1, we calculated a 95% con®dence interval about the difference between the mean number of species per plot in the disturbed and undisturbed subsets in each forest cover type. These three intervals were not signi®cantly different (t-test, p > 0.2 in all the cases) from one another. Thus, the results suggested that the decrease in mean species per plot associated with anthropogenic disturbance was equal in the three forest types. In this paper, we considered the frequencies for 48 `species,' including two varieties of N. sylvatica and three genera for which species sometimes (<1% of all individuals) could not be determined con®dently in the ®eld (Table 2). The study area is well within the geographic ranges of nearly all of the species listed in Table 2 (Little, 1971). Southern Georgia and Alabama lie along the northern periphery of the ranges of slash pine (P. elliotti) and live oak (Q. virginiana). The range of northern pin oak (Q. ellipsoidalias) is north of the study area. However, this species is sometimes planted in the study area. The study area represents the southern edge of the range of eastern white pine (P. strobus), Virginia pine (P. virginiana), sugar maple (Acer saccharum), and northern red oak (Q. rubra). Cucumbertree (M. acuminata) has a fragmented distribution in Alabama. Scarlet oak (Q. coccinea) is sparse south of central Georgia and Alabama. We compared the frequencies of these species in the three forest cover types in disturbed and undisturbed plots (Figs. 2 and 4). In order to be represented in Figs. 2±4, a species was required to occur on at least two plots in either the undisturbed or disturbed subset within a forest type. Of the 61 species forest type combinations shown in Figs. 2±4, eight (13.1%) had signi®cantly different frequencies (Table 3: likelihood ratio 2 test, p < 0.05). Seven of these eight combinations occurred more frequently on undisturbed plots than on disturbed. None of the selected species had signi®cant differences in frequency between undis-
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Table 2 Names and abbreviations of the tree species considered Common name
Scientific name
Abbrev.
Eastern red cedar Shortleaf pine Slash pine Spruce pine Longleaf pine Eastern white pine Loblolly pine Virginia pine Red maple Sugar maple American hornbeam Hickory sp. Pignut hickory Mockernut hickory Eastern redbud Flowering dogwood Common persimmon American beech Ash American holly Sweetgum Yellow-poplar Cucumbertree Sweetbay Red mulberry Water tupelo Blackgum Swamp tupelo Eastern hophornbeam Sourwood Cherry, plum spp. Black cherry White oak Scarlet oak Northern pin oak Southern red oak Laurel oak Swamp chestnut oak Water oak Willow oak Chestnut oak Northern red oak Shumard oak Post oak Black oak Live oak Sassafras Winged elm
Juniperus virginiana Pinus echinata Pinus elliottii Pinus glabra Pinus palustris Pinus strobus Pinus taeda Pinus virginiana Acer rubrum Acer saccharum Carpinus caroliniana Carya sp. Carya glabra Carya tomentosa Cercis canadensis Cornus florida Diospyros virginiana Fagus grandifolia Fraxinus sp. Ilex opaca Liquidambar styraciflua Liriodendron tulipifera Magnolia acuminata Magnolia virginiana Morus rubra Nyssa aquatica Nyssa sylvatica Nyssa sylvatica var. biflora Ostrya virginiana Oxydendrum arboreum Prunus sp. Prunus serotina Quercus alba Quercus coccinea Quercus ellipsoidalis Quercus falcata var. falcata Quercus laurifolia Quercus michauxii Quercus nigra Quercus phellos Quercus prinus Quercus rubra Quercus shumardii Quercus stellata Quercus velutina Quercus virginiana Sassafras albidum Ulmus alata
JUVI PIEC PIEL PIGL PIPA PIST PITA PIVI ACRU ACSA CACA CARYA CAGL CATO CECA COFL DIVI FAGR FRAXINUS ILOP LIST LITU MAAC MAVI MORU NYAQ NYSY NYSYB OSVI OXAR PRUNUS PRSE QUAL QUCO QUEL QUFA QULA QUMI QUNI QUPH QUPR QURU QUSH QUST QUVE QUVI SAAL ULAL
turbed and disturbed plots in the oak-pine forest type. In the loblolly-shortleaf pine type, however, ®ve of the 18 species (27.8%, including two varieties of N.
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ing dogwood (C. ¯orida) and chestnut oak (Q. prinus) occurred more frequently on undisturbed oak-hickory plots. Loblolly pine in the oak-hickory forest type was the only instance in which a species occurred signi®cantly more frequently on disturbed plots than on undisturbed plots. 4. Discussion
Fig. 2. Proportion of plots in which selected species (see text) occurred in loblolly-shortleaf pine forests in Georgia and Alabama. See Table 2 for species' abbreviations. Solid bars undisturbed plots. Striped bars disturbed plots.
sylvatica), considered occurred more frequently in undisturbed plots (Table 3). In the oak-hickory forest type, these differences were signi®cant for three of the 21 species (14.3%) of the species considered. Flower-
This study is the ®rst of which we are aware that quanti®es, with known con®dence, regional differences in frequency of occurrence for individual tree species in disturbed and undisturbed forests. Therefore, this study bridges an important gap between sitespeci®c forest studies and remote-sensing studies of the forests of a region. As mentioned, combining sitespeci®c studies for regional testing is not appropriate for rigorous testing at the regional level in most cases (Wyant et al., 1991; Stapanian et al., 1997). Conversely, many site-speci®c data (e.g. species presence, disturbance types, and plot-level species richness) are not available from most data sets collected by remote sensing. Concerns of widespread threats to biodiversity (e.g., USEPA, 1990; Raven and Wilson, 1992) underscore the importance of monitoring and quantifying changes in biodiversity and species composition at the regional level. The area in pine plantations continues to increase in southeastern USA. Between 1982 and 1989, the area in Georgia planted with loblolly pine plantations increased by 12% (Thompson, 1989). Loblolly pine stands account for approximately 24.3% of the forested area in Georgia alone (Thompson, 1989) and 29% of the forested area of Alabama and Georgia
Table 3 Species for which the frequency of occurrence in disturbed plots was significantly different than in undisturbed plots Likelihood ratio Species
Forest cover type
Pinus echinata Cornus florida Liriodendron tulipifera Nyssa sylvatica N. sylvatica var. biflora Pinus taeda Cornus florida Quercus prinus
Loblolly-shortleaf Loblolly-shortleaf Loblolly-shortleaf Loblolly-shortleaf Loblolly-shortleaf Oak-hickory Oak-hickory Oak-hickory
pine pine pine pine pine
2
p
11.58 4.68 4.48 4.55 5.64 4.60 4.46 8.26
0.001 0.030 0.034 0.033 0.018 0.032 0.035 0.004
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Fig. 3. Proportion of plots in which selected species (see text) occurred in oak-pine forests in Georgia and Alabama. See Table 2 for species' abbreviations. Solid bars undisturbed plots. Striped bars disturbed plots.
combined (Stapanian et al., 1997). In Florida, plantations of Pinus spp. account for approximately 32% of its forested area. This represents an increase of approximately 15% between 1987 and 1995 (Brown, 1996). The area in loblolly pine plantations increased by 49% in this time period. In South Carolina, pine plantations increased 33% between 1986 and
1993 (Conner, 1993) to an estimated 21.5% of the total forested lands in the state. Approximately 89% of the plantation area in South Carolina are loblolly pine. The abundance of individual tree species was not considered in this study. Objectively assessing the abundance of individual species would required a
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Fig. 4. Proportion of plots in which selected species (see text) occurred in oak-hickory forests in Georgia and Alabama. See Table 2 for species' abbreviations. Solid bars undisturbed plots. Striped bars disturbed plots.
sampling design that accurately samples the distribution of each species. Such a design was not practical, given the wide range of objectives in FHM. However, many biologists consider species richness, irrespective of abundance, to be the simplest, most practical, and most objective measure of species diversity (e.g.,
Poore, 1962; Whittaker, 1965, 1972; Greig-Smith, 1971; Hurlbert, 1971; Peet, 1974; Debinski and Brussard, 1991). Therefore, the frequency of occurrence for individual species should give an objective measure of their association with anthropogenic disturbance at a regional level.
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Anthropogenic disturbance is associated with signi®cantly lower tree species richness at the regional level in all the three forest cover types considered (Stapanian et al., 1997, present study). These results agree with those of Wyant et al. (1991) for forests in the North Carolina coastal plain. However, Wyant et al. (1991) did not consider planted stands as a disturbance type. The results from this study also suggest that the regional frequencies of a relatively large proportion of the tree species in two of the dominant forest cover types are signi®cantly less in managed plots. The only species for which there was a signi®cant increase in the regional frequency was loblolly pine in the oak-hickory forest. This species is not a common associate in the undisturbed oakhickory forest (Eyre, 1980), but may be planted for economic reasons. The species found to decrease in frequency are not restricted to the upper canopy. Flowering dogwood (C. ¯orida) typically occurs in the middle canopy when mature. Wigley et al. (1989) found that cover in midand upper canopy layers is greater in mixed pine hardwood stands than in pure pine stands in the southeast. Wildlife species such as fox squirrels (Sciurus niger), which depend on both layers, occur less frequently in pine plantations (Loeb and Lennartz, 1989). At least 3 of the 5 sub-species of fox squirrels known to occur in the southeast have experienced signi®cant decreases in the past century, and at least one more appears to be declining in the Carolinas (MacClintock, 1970; Loeb and Lennartz, 1989). This decline has been attributed to the decline of the mature mixed pine and hardwood forest, replaced by the pure pine forest (Loeb and Lennartz, 1989). S. niger is the primary seed disperser of many species of hardwood trees, particularly black walnuts (Juglans nigra), hickories, and many oaks (Stapanian and Smith, 1978, 1984, 1986). However, the effects of anthropogenic disturbance on tree species richness in the mid-canopy alone and frequencies of individuals mid-canopy species cannot be accurately quanti®ed with the data from this study. Several common mid-canopy species in the southeast, including mountain laurel (Kalmia latifolia) and rhododendron (Rhododendron spp.) are not considered trees in FHM (Tallent-Halsell, 1994) but are considered trees by other researchers (e.g. Little, 1971). However, anthropogenic disturbances are associated
249
with signi®cantly higher proportions of introduced plant species in the ground layer of vegetation (i. e. 0±1 m high) in Georgia and Alabama (Stapanian et al., in press). In particular, species originating in far eastern and sub-tropical Asia occur more frequently in disturbed forested areas than in undisturbed. Therefore, anthropogenic disturbance has a profound, region-wide effect on the structure and composition of at least two forest layers. Since most plots had more than one type of disturbance, determining which types are `causal' in decreasing a species' frequency is beyond the scope of this study. However, more species experienced signi®cant decreases in the loblolly-shortleaf pine forest than in any other cover type. The most widespread (>50% of the forested area of the region) disturbance in this forest type is planting monocultures of Pinus. From a purely economics and management standpoint, a mixture of pine and hardwoods in stands could mean a higher net rate of return because of reduced `start-up' and maintenance costs (Lentz et al., 1989) and other resource and economic bene®ts (Leopold et al., 1989), including wildlife diversity. This is especially important for private landowners in the region because mixed pine-hardwood stands also meet multiple-use objectives (Leopold et al., 1989). However, the volume of growth of pines per unit area would be lower in mixed pine-hardwood. Short-term rate of economic return may be less in mixed stands, because it takes most hardwoods longer to reach marketable volume. However, the lower volume may be offset by the higher values per board foot associated with selected hardwood species (Leopold et al., 1989). Such a management plan would require managers to assess value of a stand of timber based on both hardwood and pine. The results of this study strongly suggest that the long-term economics, including those from multiple use, of mixed stands warrant investigation. Acknowledgements We thank D. White and J. Tappiener for comments on an earlier draft of the manuscript. C. Liff and B. Cordova provided assistance with information management. The Forest Health Monitoring Program,
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an inter-agency program, funded this study. Participating agencies include the U.S.D.A. Forest Service, U.S.D.I. Bureau of Land management, and the U.S. Environmental Protection Agency. The research in this paper has been subjected to review by some of the participating agencies. It does not necessarily re¯ect the views of any of the agencies. No of®cial endorsement should be inferred. The USGS/BRD Ohio Cooperative Fish and Wildlife Research Unit provided support to work on this manuscript. References Alexander, S.A., Barnard, J.E., 1993. Forest health monitoring 1992 activities plan. EPA/620/R-93/002. US Environmental Protection Agency, Washington, DC. Alig, R.J., Hohenstein, W.G., Murphy, B.C., Haight, R.C., 1990. Changes in area of timberland in the United States 1952±2040 by ownership, forest type, and region and state. General Technical Report SE-64, USDA Forest Service, Southeastern Forest Experiment Station, Asheville, NC. Atkeson, T.D., Johnson, A.S., 1979. Succession of small mammals on pine plantations in the Georgia piedmont. A. Midland Naturalist 101, 385±392. Bazzaz, F.A., 1983. Characteristics of populations in relation to disturbance in natural and modified ecosystems. In: Mooney, H.A., Godron, M. (Eds.). Disturbance and Ecosystems. Springer, New York, pp. 259±277. Binkley, D., Richter, D., David, M.B., Caldwell, B., 1992. Soil chemistry in a loblolly/longleaf pine forest with interval burning. Ecol. Appl. 2, 157±164. Boyce, S.G., Martin, W.H., 1993. The future of terrestrial communities in the south-eastern coastal plain. In: Martin, W.H., Boyce, S.G., Echternacht, A.C. (Eds.), Biodiversity of the Southeastern United States: Upland Terrestrial Communities. Wiley, New York, pp. 339±366. Brown, M.J., 1996. Forest Statistics for Florida, 1995. USDA Forest Service Resource Bulletin. SE-109. Southeastern Forest Experiment Station, Asheville, NC. Cassell, D.L., 1992. Estimating optimal resource allocation for environmental monitoring networks. In: Proc. Section on Statistics and the Environment 1991. A. Statistical Assoc., Alexandria, VA, pp. 225±229. Cassell, D.L., 1993. Inclusion probabilities for environmental monitoring in FHM and EMAP. In: Proc. Section of Statistics and the Environment 1992. A. Statistical Assoc., Alexandria, VA, pp. 82±85. Childers, E.L., Sharik, T.L., Adkisson, C.S., 1986. Effects of loblolly pine plantations on songbird dynamics in the Virginia piedmont. J. Wildlife Manage. 50, 406±413. Cochran, W.G., 1977. Sampling Techniques, 3rd edn. Wiley, New York..
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M.A. Stapanian, D.L. Cassell / Forest Ecology and Management 117 (1999) 241±252 Loeb, S.C., Lennartz, M.R., 1989. The fox squirrel (Sciurus niger) in southeastern pine-hardwood forests. pp. 142±148 In: Waldrop, T.A. (Ed.). Proc. Pine-Hardwood Mixtures: A Symposium on Management and Ecology of the Type. Gen. Tech. Rep. SE-58, U.S.D.A. Forest Service, southeastern Forest Experiment Station, Asheville, NC, p. 271. MacClintock, D., 1970. Squirrels of North America. Van Nostrand Reinhold, New York, p. 184. Mann, W.F., 1980. Loblolly-shortleaf pine. In: Eyre, F.H. (Ed.). Forest Cover Types of the United States and Canada. Society of American Foresters, Washington, DC, p. 56. Oliver, C.D., 1981. Forest development in North American following major disturbances. Ecol. Manage. 3, 153±168. Omernik, J.M., 1987. Ecoregions of the coterminous United States. Ann. A. Geography 77, 118±125. Omernik, J.M., 1995. Ecoregions: a framework for environmental management. In: Simon, T., Davis, W. (Eds.). Tools for Water Resource Planning and Decision Making. Lewis, Chelsea, MI, pp. 49±62. Overton, W.S., White, D., Stevens, D.L. Jr., 1991. Design Report for EMAP, Environmental Monitoring and Assessment Program. EPA 600/3-91/053, U.S. Environmental Protection Agency, Environmental Research Laboratory Corvallis, Corvallis, Oregon. Packham, J.R., Harding, D.J.L., 1982. Ecology of Woodland Processes. Arnold, London, 262 pp. 262. Palmer, C.J., Riiters, K.K., Strickland, T., Cassell, D.L., Byers, G.E., Papp, M.L., Liff, C.I., 1991. Monitoring and Research Strategy for Forests-Environmental Monitoring and Assessment Program (EMAP). EPA/600/4-91/012. U.S. Environmental Protection Agency, Washington, DC. Peet, R.K., 1974. The measurement of species diversity. Ann. Rev. Ecolo. Systematics 5, 285±302. Pickett, S.T.A., White, P.S. (Eds.), 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic, New York, p. 472. Poore, M.E.D., 1962. The method of successive approximation in descriptive ecology. Adv. Ecol. Res. 1, 35±68. Raven, P.H., Wilson, E.O., 1992. A fifty-year plan for biodiversity surveys. Science 258, 1099±1100. Repenning, R.W., Labisky, R.F., 1985. Effects of even-aged timber management on bird communities of the longleaf pine forest in northern Florida. J. Wildlife Manage. 48, 895±911. Richter, D.D., Ralston, C.W., Harms, W.R., 1982. Prescribed fire: Effects on water quality and nutrient cycling. Science 258, 1099±1100. Riitters, K.K., Papp, M.L., Cassell, D.L., J. Hazard, J. (Eds.), 1991. Forest Health Monitoring Plot Design and Logistics Study. EPA/600/S3-91/051. U.S. Environmental Protection Agency, Research Triangle Park, NC. Schreuder, H.T., Czaplewski, R.L., 1993. Long-term strategy for the statistical design of a forest health monitoring system. Environ. Monitoring and Assessment 27, 81±93. Scott, C.T., Bechtold, W.A., 1995. Techniques for mapping plot clusters that straddle stand boundaries. For. Sci. 41, 46±61. Skeen, J.N., Doerr, P.D., van Lear, D.H., 1993. Oak-hickory-pine
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forests. In: Martin, W.H., Boyce, S.G., Echternacht, A.C. (Eds.), Biodiversity of the Southeastern United States: Upland Terresterial Communities. Wiley, New York, pp. 1±3. Smith, D.M., 1986. The Practice of Silviculture, 8th edn. Wiley, New York, p. 578. Smith, D.W., Linnartz, W.E., 1980. The southern hardwoods region. In: Barrett, J.W. (Ed.), Regional Silviculture of the United States. Wiley, New York, pp. 145±230. Society of American Foresters, 1954. Forest Cover Types of North America, 4th edn. Washington, DC. Stapanian, M.A., Sundberg, S.D., Baumgardner, G.A., Liston, A., in press. Exotic plant species composition and associations with anthropogenic disturbance in North American forests. Plant Ecol.. Stapanian, M.A., Cassell, D.L., Cline, S.P., 1997. Regional patterns of local diversity of trees: Associations with anthropogenic disturbance. For. Ecol. Manage. 93, 33±44. Stapanian, M.A., Smith, C.C., 1978. A model for seed scatterhoarding: Coevolution of fox squirrels and black walnuts. Ecology 59, 884±896. Stapanian, M.A., Smith, C.C., 1984. Density-dependent survival of scatterhoarded nuts: An experimental approach. Ecology 65, 1387±1396. Stapanian, M.A., Smith, C.C., 1986. How fox squirrels influence the invasion of prairies by nut-bearing trees. J. Mammalogy 67, 326±332. Stehman, S.V., Overton, W.S., 1994. Environmental sampling and monitoring. In: Patil, G.P., Rao G.R. (Eds.). Handbook of Statistics. Elsevier, Amsterdam, pp. 263±306. Stevens, D.L., 1994. Implementation of a national monitoring program. J. Environ. Manage. 42, 1±29. Swindel, B.F., Conde, L.F., Smith, J.E., 1984. Species diversity: Concept, measurement and response to clearcutting and site preparation. For. Ecol. Manage. 8, 11±22. Tallent-Halsell, N.G. (Ed.), 1994. Forest health monitoring 1994 field methods guide. U.S. Environmental Protection Agency EPA/620/R-94/027. EMSL-LV, Las Vegas, NV. Thompson, M.T., 1989. Forest statistics for Georgia, 1989. USDA Forest Service Resource Bulletin SE-109. Southeastern Forest Experiment Station, Asheville, NC. U.S. Environmental Protection Agency, 1990. Threats to biological diversity in the United States. Office of Policy, Planning and Evaluation. PM-223X, p. 57. U.S.D.A. Forest Service, 1988. The South's fourth forest: Alternative for the future. Forest Resource Report 24, USDA, Forest Service, Washington, DC. Walker, L.C., 1980. The southern pine region. In: Barrett, J.W. (Ed.), Regional Silviculture of the United States. WileyInterscience, New York, pp. 231±276. White, P.S., Pickett, S.T.A., 1985. Natural disturbance and patch dynamics: An introduction. In: Pickett, S.T.A., White, P.S. (Eds.), The Ecology of Natural Disturbance and Patch Dynamics. Academic, San Diego, pp. 3±13. Whittaker, R.H., 1965. Dominance and diversity in land plant communities. Science 147, 250±260. Whittaker, R.H., 1972. Evolution and measurement of species diversity. Taxon 21, 213±251.
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Regional frequencies of tree species associated with anthropogenic disturbances in three forest types Martin A. Stapaniana,*, David L. Cassellb a
Department of Evolution, Ecology and Organismal Biology, Ohio Cooperative Fish and Wildlife Research Unit, The Ohio State University 1735 Neil Ave., Columbus, OH 43210, USA b OAO Corporation, 200 SW 35th St., Corvallis, OR 97333, USA Received 5 May 1998; accepted 15 September 1998
Abstract We used a probability-based sampling scheme to survey the forested lands of Georgia and Alabama, two adjacent states in southeastern USA. Using a nationally consistent plot design and ®eld methods, we evaluated the presence/absence of tree species at 123 1/15 ha plots for individuals having a diameter at breast height >2.5 cm. Three forest cover types were considered: loblollyshortleaf pine, oak-hickory, and oak-pine. Visually evident anthropogenic disturbances (e.g. arti®cial regeneration, logging, and prescribed burning), if any, were recorded on each plot. We classi®ed plots with visually evident anthropogenic disturbance as `disturbed'. The remaining plots were classi®ed as `undisturbed'. For each of the three forest types considered, mean species richness of trees was signi®cantly greater in undisturbed plots. Of 61 species forest type combinations, eight (13.1%) had signi®cantly different frequencies in undisturbed than in disturbed areas. In the loblolly-shortleaf pine type, ®ve of the 18 species (27.8%) considered occurred signi®cantly more frequently on undisturbed plots. In the oak-hickory type, two of the 21 species considered (9.5%) occurred more frequently on undisturbed plots and one species, loblolly pine, occurred more frequently on disturbed plots. Species occurring more frequently on undisturbed areas in these forest types included occupants of both the mid and upper canopy. Loblolly pine is not a common associate in the undisturbed oak-hickory forest. These results and those from other studies strongly suggest that anthropogenic disturbance, particularly clearcutting and subsequent planting of pine plantations, is having profound effect on the structure and composition of the forested area of the region. Because the three forest cover types we considered account for approximately 71% of the total forested area of the region, these results should be useful for forest management and conservation planning in the states. Management implications of mixed pine-hardwood stands vs. pine plantations in the south-east are discussed. # 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Loblolly-shortleaf pine; Oak-hickory; Oak-pine; Georgia; Alabama; Anthropogenic disturbance; Monitoring; Trees; Species frequency
1. Introduction Millions of acres of hardwood forests in southeastern USA have been converted to agriculture and *Corresponding author. Tel.: +1-614-292-5166; e-mail: [email protected]
pine plantations since the 1930s, particularly in the oak-pine uplands and the mixed hardwood bottomlands in major river valleys (Smith and Linnartz, 1980 and references therein). Clearcutting and subsequent planting of pine plantations is expected to increase in the future (USDAFS, 1988; Alig et al., 1990; Boyce and Martin, 1993). Competing woody vegetation is
0378-1127/99/$ ± see front matter # 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00483-6
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typically suppressed in the established plantation by controlled burning (Richter et al., 1982; Binkley et al., 1992), application of synthetic hormones and herbicides (Walker, 1980), and other techniques (Swindel et al., 1984). As the planted pines become more dominant, diversity trends in the maturing plantations ``are expected to be less ecologically favorable'' (Swindel et al., 1984, p. 19). Other studies suggest that when stands of mixed species forests are cleared and replaced by pine plantations, the result is a locallyimpoverished ¯ora and fauna relative to the `original' stand (Atkeson and Johnson, 1979; Repenning and Labisky, 1985; Childers et al., 1986; Felix et al., 1986; Skeen et al., 1993). Since these management practices are so widespread in the Southeast, the above would suggest an overall loss of tree species richness at the regional level. However, this can not be assessed without statistically sound, quantitative estimates of trends of diversity at the regional or national level. Such estimates are lacking for most taxa, including trees, in the United States. There is a vast literature on the pattern and process of response of local (i.e. standlevel) plant diversity to anthropogenic disturbance in forests (e.g. Grime, 1979; Denslow, 1980; Oliver, 1981; Bazzaz, 1983; Pickett and White, 1985; Hunter, 1990). However, they cannot be used individually or collectively to estimate population parameters at a regional level (Wyant et al., 1991; Stapanian et al., 1997) for several reasons. Typically, such studies are based on non-probability samples, and they have insuf®cient sample sized from a regional perspective to allow for rigorous statistical analysis. Further, they are usually site- or landscape-speci®c, and/or do not use the same methodology throughout the region. In a previous work (Stapanian et al., 1997) from a large, probability-based synoptic survey of forests, we found that anthropogenically-disturbed forested lands in southeastern USA had signi®cantly lower tree species richness than in undisturbed forests. In particular, species richness of trees was signi®cantly lower in disturbed areas of the loblolly± (Pinus taeda) shortleaf pine (P. echinata), oak (Quercus spp.) -pine (Pinus spp.), and oak-hickory (Carya spp.) forest cover types. Further, we quanti®ed the proportion of the forested area that had visually evident anthropogenic disturbances (`disturbed').
These three forest cover types account for approximately 71% of the total forested area of Georgia and Alabama (Stapanian et al., 1997). Thus, changes detected in their species composition and structure have important rami®cations for conservation and forest management for the entire region. In this paper, we examine data for these three forest cover types from the same probability-based survey as Stapanian et al. (1997). For each forest type, we quantify differences in the frequencies of occurrence of individual tree species in undisturbed and disturbed plots. For a subset of the individual species, we test the null hypothesis that the frequency of occurrence in undisturbed plots is equal to that in anthropogenically disturbed plots. Tree species are only one component of forest biodiversity. However, trees are a logical starting point, given their ecological dominance (Packham and Harding, 1982), economic importance (Smith, 1986), and in¯uence on the forest environment and other organisms (Hunter, 1990). 2. Methods 2.1. Sampling design and plot layout The forest health monitoring (FHM) program, a large-scale synoptic forest survey system, is the source of our data. FHM is an inter-agency government project (Alexander and Barnard, 1993). FHM's objectives include the monitoring and statistical estimation of trends, changes, and current status in indicators of the condition of the country's forest resources, on a regional and national scale. Detecting changes in these indicators associated with anthropogenic disturbance is another major objective of FHM (Palmer et al., 1991). Species richness of trees is just one of those indicators. The basis of the FHM sampling design (Cassell, 1993; Schreuder and Czaplewski, 1993) is the Environmental Monitoring and Assessment Program (EMAP) design (Overton et al., 1991; Stevens, 1994; Stehman and Overton, 1994), which uses an equal-area triangular grid with points approximately 27 km apart. This design permits statistical estimation of population parameters using traditional sampling theory: Horvitz±Thompson estimation with Yates± Gundy estimates (Cochran, 1977). The target sam-
M.A. Stapanian, D.L. Cassell / Forest Ecology and Management 117 (1999) 241±252
pling density of the forested area of each state is achieved after 4 years of sampling, using a rotating panel design (Cassell, 1993). However, population parameters can be estimated after a minimum of one season of sampling. The FHM and EMAP sampling designs have been demonstrated to be effective in permitting the use of statistically-designed study sites in a probability-based network, for making regional estimates on ecological resources (Stevens, 1994; Stehman and Overton, 1994). Because the FHM design is probability based, each forest cover type is sampled in proportion to its area in the region. Similarly, attributes of stands, such as age and class, of each forest type are sampled in proportion to their area in the region. In a previous work, (Stapanian et al., 1997) we demonstrated the linkage between plot-level and regional diversity estimates from this design. Therefore, the data from the FHM design are appropriate for making regional inferences based on plotlevel information. Each forested plot located from the FHM sampling system is a circle of 1 ha (Tallent-Halsell, 1994). Within this circle are four ®xed-radius subplots, each of radius 7.3 m (24 ft). The combined area of the four subplots is 1/15 ha. Measurements for trees having diameter at breast height (dbh) are taken within these subplots. Within each subplot is a circular microplot of radius 2.1 m (6.8 ft), from which data for trees with dbh between 2.54 and 12.7 cm are taken. Only trees having dbh 2.54 cm are considered in this study. Statistical evaluations of the FHM plot design (Riitters et al., 1991; Lewis and Conkling, 1994) have demonstrated using sampling theory formulae (Cochran, 1977; Cassell, 1992) that the multi-tiered plot layout is well suited to the variety of current FHM indicators. 2.2. Field methods and descriptions of regions We examined data from 123 FHM plots in the states of Georgia and Alabama (Stapanian et al., 1997), which were predominantly in the Southeastern Plains and Southwestern Appalachian ecoregions (Omernik, 1987, 1995). Data were collected during the summers of 1990 through 1993. Plots were classi®ed as forested according to standard protocols used by the U.S. Forest Service (Tallent-Halsell, 1994). The list of species that are considered trees in FHM may also be found in Tallent-Halsell (1994). This list is similar
243
to that of Little (1971) and Stapanian et al. (1997)). Forest cover type(s) (e.g., oak (Quercus spp.) -hickory (Carya spp.) forest) in each plot was (were) determined in the ®eld, based on tree species composition, according to standard protocols from the U.S.D.A. Forest Service (Tallent-Halsell, 1994 and see Eyre, 1980). Nomenclature of tree species was according to Harlow and Harrar (1969) and Harlow et al. (1991). In nearly all the cases, individual trees were identi®ed in the ®eld to species. In rare cases (<1%), a tree was identi®ed only to the genus level, because insuf®cient material was present to permit a precise identi®cation to species. Blackgum (Nyssa sylvatica) and swamp tupelo (N. s. var. bi¯ora) were treated as separate species in this analysis because they could be accurately identi®ed in the ®eld (Tallent-Halsell, 1994). We considered the data from only three forest cover types: loblolly±shortleaf pine, oak-pine, and oak-hickory. On each plot, up to three types of visually evident disturbances were recorded (Tallent-Halsell, 1994; Scott and Bechtold, 1995). This was done in accordance with the de®nition of White and Pickett (1985), p. 7) of disturbance as ``any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.'' Therefore, disturbance provides no a priori inference about the condition or health of ecosystems (Stapanian et al., 1997). Types of anthropogenic disturbances included planting stands, harvesting, cutting, thinning, prescribed burning, grazing by domestic livestock, and construction. Alterations by weather, disease, insects, and wild®re, as well as natural reversion to forest were not considered anthropogenic disturbances. These `natural' disturbances were recorded on too few plots to be analyzed separately (Stapanian et al., 1997). In order to be included in the analysis, a plot was considered only if the entire area of the plot was (1) forested and (2) composed of the same forest cover type. Further, anthropogenic disturbance was required to be recorded in either all (`disturbed') or none (`undisturbed') of the four subplots in order for a plot to be included in the analysis. These inclusion criteria are more rigorous than those in our previous study (Stapanian et al., 1997). Therefore, the number of plots considered in this study was slightly less for each forest cover type than in our previous paper. For
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each forest cover type, we tested the null hypothesis that the mean number of species per plot was the same in disturbed and undisturbed plots with an analysis of variance (ANOVA). For selected species we tested the null hypothesis of equal frequency of occurrence in the disturbed and undisturbed subsets of plots within a forest cover type. In order to be selected for testing, a species was required to occur on a minimum of two plots in either the disturbed or undisturbed subset within a forest cover type. Conventional 2 and tests based on it were likely to be poor choices for this analysis, since cell counts were often low. Therefore, we used likelihood ratio 2 (Fienberg, 1977), which accommodates low cell counts. 3. Results The loblolly±shortleaf pine forests clearly had the largest proportion (73.9%) of plots that had anthropogenic disturbance (Fig. 1). In particular, planted stands of loblolly pine accounted for more than one-half of that forest cover type. The disturbance types prescribed burning, cutting, site preparation, arti®cial regeneration, and harvesting also occurred more frequently in the loblolly-shortleaf pine forest type. Grazing by livestock was recorded only in the oak-pine type. As expected, planted stands and disturbance from arti®cial regeneration were not observed in the oak-hickory forest type. The mean number of species per plot was signi®cantly greater in the undisturbed plots than in the
Fig. 1. Proportion of plots in which anthropogenic disturbance was recorded (`disturbed') and the proportions in which specific types of disturbances occurred for three forest cover types. Up to three types of disturbance could be assigned to a plot. Therefore, the sum of the proportions of all disturbance types for a forest cover type >1. `Planted' refers to the plots in which humans had planted the trees. Abbreviations: site prep. site preparation, regen. artificial regeneration. Black loblolly±shortleaf pine type (LS), white oak-pine type (OP), striped oak-hickory type (OH).
disturbed plots for all the three forest cover types (ANOVA), values of p in Table 1). This result was similar to Stapanian et al. (1997), for which the inclusion criteria were much less rigorous. Further, the mean number of species per plot was least in the loblolly-shortleaf pine forests, both in undisturbed and disturbed plots. Although the oak-hickory forests had the fewest number of plots, it contained the greatest number of species when all the plots for that type were pooled (values of T in Table 1). The loblolly-shortleaf
Table 1 Summary statistics for the number of tree species found in plots that had visible anthropogenic disturbances (`disturbed') and those that did not (`undisturbed') in three forest types in southeastern USA Undisturbed plots Forest type Loblolly±shortleaf pine Oak-pine Oak-hickory
N 12 26 22
T 31 51 54
Disturbed plots Mean species per Plot (s) 7.25 (3.04) 9.15 (2.89) 9.23 (2.52)
N 34 19 10
T 36 43 24
Mean species per Plot (s) 4.32 (3.38) 6.58 (3.92) 6.50 (4.30)
ANOVA p>F 0.012 0.015 0.031
In each forest type, we tested the null hypothesis that the means species per plot was the same in disturbed and undisturbed plots with analysis of variance (ANOVA). Abbreviations: N number of plots; T the total number of species found when the data for all plots in a group were pooled; s standard deviation.
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pine forests had the lowest values of T of the three forest cover types investigated. These results agreed with the previous studies (e.g. Society of American Foresters, 1954; Crow, 1980; Mann, 1980; Smith and Linnartz, 1980). As expected, the subset (i.e. undisturbed or disturbed) within each forest cover type which contained more plots had the larger total number of species when the plots for that subset were pooled (values of T in Table 1). From the statistics in Table 1, we calculated a 95% con®dence interval about the difference between the mean number of species per plot in the disturbed and undisturbed subsets in each forest cover type. These three intervals were not signi®cantly different (t-test, p > 0.2 in all the cases) from one another. Thus, the results suggested that the decrease in mean species per plot associated with anthropogenic disturbance was equal in the three forest types. In this paper, we considered the frequencies for 48 `species,' including two varieties of N. sylvatica and three genera for which species sometimes (<1% of all individuals) could not be determined con®dently in the ®eld (Table 2). The study area is well within the geographic ranges of nearly all of the species listed in Table 2 (Little, 1971). Southern Georgia and Alabama lie along the northern periphery of the ranges of slash pine (P. elliotti) and live oak (Q. virginiana). The range of northern pin oak (Q. ellipsoidalias) is north of the study area. However, this species is sometimes planted in the study area. The study area represents the southern edge of the range of eastern white pine (P. strobus), Virginia pine (P. virginiana), sugar maple (Acer saccharum), and northern red oak (Q. rubra). Cucumbertree (M. acuminata) has a fragmented distribution in Alabama. Scarlet oak (Q. coccinea) is sparse south of central Georgia and Alabama. We compared the frequencies of these species in the three forest cover types in disturbed and undisturbed plots (Figs. 2 and 4). In order to be represented in Figs. 2±4, a species was required to occur on at least two plots in either the undisturbed or disturbed subset within a forest type. Of the 61 species forest type combinations shown in Figs. 2±4, eight (13.1%) had signi®cantly different frequencies (Table 3: likelihood ratio 2 test, p < 0.05). Seven of these eight combinations occurred more frequently on undisturbed plots than on disturbed. None of the selected species had signi®cant differences in frequency between undis-
245
Table 2 Names and abbreviations of the tree species considered Common name
Scientific name
Abbrev.
Eastern red cedar Shortleaf pine Slash pine Spruce pine Longleaf pine Eastern white pine Loblolly pine Virginia pine Red maple Sugar maple American hornbeam Hickory sp. Pignut hickory Mockernut hickory Eastern redbud Flowering dogwood Common persimmon American beech Ash American holly Sweetgum Yellow-poplar Cucumbertree Sweetbay Red mulberry Water tupelo Blackgum Swamp tupelo Eastern hophornbeam Sourwood Cherry, plum spp. Black cherry White oak Scarlet oak Northern pin oak Southern red oak Laurel oak Swamp chestnut oak Water oak Willow oak Chestnut oak Northern red oak Shumard oak Post oak Black oak Live oak Sassafras Winged elm
Juniperus virginiana Pinus echinata Pinus elliottii Pinus glabra Pinus palustris Pinus strobus Pinus taeda Pinus virginiana Acer rubrum Acer saccharum Carpinus caroliniana Carya sp. Carya glabra Carya tomentosa Cercis canadensis Cornus florida Diospyros virginiana Fagus grandifolia Fraxinus sp. Ilex opaca Liquidambar styraciflua Liriodendron tulipifera Magnolia acuminata Magnolia virginiana Morus rubra Nyssa aquatica Nyssa sylvatica Nyssa sylvatica var. biflora Ostrya virginiana Oxydendrum arboreum Prunus sp. Prunus serotina Quercus alba Quercus coccinea Quercus ellipsoidalis Quercus falcata var. falcata Quercus laurifolia Quercus michauxii Quercus nigra Quercus phellos Quercus prinus Quercus rubra Quercus shumardii Quercus stellata Quercus velutina Quercus virginiana Sassafras albidum Ulmus alata
JUVI PIEC PIEL PIGL PIPA PIST PITA PIVI ACRU ACSA CACA CARYA CAGL CATO CECA COFL DIVI FAGR FRAXINUS ILOP LIST LITU MAAC MAVI MORU NYAQ NYSY NYSYB OSVI OXAR PRUNUS PRSE QUAL QUCO QUEL QUFA QULA QUMI QUNI QUPH QUPR QURU QUSH QUST QUVE QUVI SAAL ULAL
turbed and disturbed plots in the oak-pine forest type. In the loblolly-shortleaf pine type, however, ®ve of the 18 species (27.8%, including two varieties of N.
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ing dogwood (C. ¯orida) and chestnut oak (Q. prinus) occurred more frequently on undisturbed oak-hickory plots. Loblolly pine in the oak-hickory forest type was the only instance in which a species occurred signi®cantly more frequently on disturbed plots than on undisturbed plots. 4. Discussion
Fig. 2. Proportion of plots in which selected species (see text) occurred in loblolly-shortleaf pine forests in Georgia and Alabama. See Table 2 for species' abbreviations. Solid bars undisturbed plots. Striped bars disturbed plots.
sylvatica), considered occurred more frequently in undisturbed plots (Table 3). In the oak-hickory forest type, these differences were signi®cant for three of the 21 species (14.3%) of the species considered. Flower-
This study is the ®rst of which we are aware that quanti®es, with known con®dence, regional differences in frequency of occurrence for individual tree species in disturbed and undisturbed forests. Therefore, this study bridges an important gap between sitespeci®c forest studies and remote-sensing studies of the forests of a region. As mentioned, combining sitespeci®c studies for regional testing is not appropriate for rigorous testing at the regional level in most cases (Wyant et al., 1991; Stapanian et al., 1997). Conversely, many site-speci®c data (e.g. species presence, disturbance types, and plot-level species richness) are not available from most data sets collected by remote sensing. Concerns of widespread threats to biodiversity (e.g., USEPA, 1990; Raven and Wilson, 1992) underscore the importance of monitoring and quantifying changes in biodiversity and species composition at the regional level. The area in pine plantations continues to increase in southeastern USA. Between 1982 and 1989, the area in Georgia planted with loblolly pine plantations increased by 12% (Thompson, 1989). Loblolly pine stands account for approximately 24.3% of the forested area in Georgia alone (Thompson, 1989) and 29% of the forested area of Alabama and Georgia
Table 3 Species for which the frequency of occurrence in disturbed plots was significantly different than in undisturbed plots Likelihood ratio Species
Forest cover type
Pinus echinata Cornus florida Liriodendron tulipifera Nyssa sylvatica N. sylvatica var. biflora Pinus taeda Cornus florida Quercus prinus
Loblolly-shortleaf Loblolly-shortleaf Loblolly-shortleaf Loblolly-shortleaf Loblolly-shortleaf Oak-hickory Oak-hickory Oak-hickory
pine pine pine pine pine
2
p
11.58 4.68 4.48 4.55 5.64 4.60 4.46 8.26
0.001 0.030 0.034 0.033 0.018 0.032 0.035 0.004
M.A. Stapanian, D.L. Cassell / Forest Ecology and Management 117 (1999) 241±252
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Fig. 3. Proportion of plots in which selected species (see text) occurred in oak-pine forests in Georgia and Alabama. See Table 2 for species' abbreviations. Solid bars undisturbed plots. Striped bars disturbed plots.
combined (Stapanian et al., 1997). In Florida, plantations of Pinus spp. account for approximately 32% of its forested area. This represents an increase of approximately 15% between 1987 and 1995 (Brown, 1996). The area in loblolly pine plantations increased by 49% in this time period. In South Carolina, pine plantations increased 33% between 1986 and
1993 (Conner, 1993) to an estimated 21.5% of the total forested lands in the state. Approximately 89% of the plantation area in South Carolina are loblolly pine. The abundance of individual tree species was not considered in this study. Objectively assessing the abundance of individual species would required a
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M.A. Stapanian, D.L. Cassell / Forest Ecology and Management 117 (1999) 241±252
Fig. 4. Proportion of plots in which selected species (see text) occurred in oak-hickory forests in Georgia and Alabama. See Table 2 for species' abbreviations. Solid bars undisturbed plots. Striped bars disturbed plots.
sampling design that accurately samples the distribution of each species. Such a design was not practical, given the wide range of objectives in FHM. However, many biologists consider species richness, irrespective of abundance, to be the simplest, most practical, and most objective measure of species diversity (e.g.,
Poore, 1962; Whittaker, 1965, 1972; Greig-Smith, 1971; Hurlbert, 1971; Peet, 1974; Debinski and Brussard, 1991). Therefore, the frequency of occurrence for individual species should give an objective measure of their association with anthropogenic disturbance at a regional level.
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Anthropogenic disturbance is associated with signi®cantly lower tree species richness at the regional level in all the three forest cover types considered (Stapanian et al., 1997, present study). These results agree with those of Wyant et al. (1991) for forests in the North Carolina coastal plain. However, Wyant et al. (1991) did not consider planted stands as a disturbance type. The results from this study also suggest that the regional frequencies of a relatively large proportion of the tree species in two of the dominant forest cover types are signi®cantly less in managed plots. The only species for which there was a signi®cant increase in the regional frequency was loblolly pine in the oak-hickory forest. This species is not a common associate in the undisturbed oakhickory forest (Eyre, 1980), but may be planted for economic reasons. The species found to decrease in frequency are not restricted to the upper canopy. Flowering dogwood (C. ¯orida) typically occurs in the middle canopy when mature. Wigley et al. (1989) found that cover in midand upper canopy layers is greater in mixed pine hardwood stands than in pure pine stands in the southeast. Wildlife species such as fox squirrels (Sciurus niger), which depend on both layers, occur less frequently in pine plantations (Loeb and Lennartz, 1989). At least 3 of the 5 sub-species of fox squirrels known to occur in the southeast have experienced signi®cant decreases in the past century, and at least one more appears to be declining in the Carolinas (MacClintock, 1970; Loeb and Lennartz, 1989). This decline has been attributed to the decline of the mature mixed pine and hardwood forest, replaced by the pure pine forest (Loeb and Lennartz, 1989). S. niger is the primary seed disperser of many species of hardwood trees, particularly black walnuts (Juglans nigra), hickories, and many oaks (Stapanian and Smith, 1978, 1984, 1986). However, the effects of anthropogenic disturbance on tree species richness in the mid-canopy alone and frequencies of individuals mid-canopy species cannot be accurately quanti®ed with the data from this study. Several common mid-canopy species in the southeast, including mountain laurel (Kalmia latifolia) and rhododendron (Rhododendron spp.) are not considered trees in FHM (Tallent-Halsell, 1994) but are considered trees by other researchers (e.g. Little, 1971). However, anthropogenic disturbances are associated
249
with signi®cantly higher proportions of introduced plant species in the ground layer of vegetation (i. e. 0±1 m high) in Georgia and Alabama (Stapanian et al., in press). In particular, species originating in far eastern and sub-tropical Asia occur more frequently in disturbed forested areas than in undisturbed. Therefore, anthropogenic disturbance has a profound, region-wide effect on the structure and composition of at least two forest layers. Since most plots had more than one type of disturbance, determining which types are `causal' in decreasing a species' frequency is beyond the scope of this study. However, more species experienced signi®cant decreases in the loblolly-shortleaf pine forest than in any other cover type. The most widespread (>50% of the forested area of the region) disturbance in this forest type is planting monocultures of Pinus. From a purely economics and management standpoint, a mixture of pine and hardwoods in stands could mean a higher net rate of return because of reduced `start-up' and maintenance costs (Lentz et al., 1989) and other resource and economic bene®ts (Leopold et al., 1989), including wildlife diversity. This is especially important for private landowners in the region because mixed pine-hardwood stands also meet multiple-use objectives (Leopold et al., 1989). However, the volume of growth of pines per unit area would be lower in mixed pine-hardwood. Short-term rate of economic return may be less in mixed stands, because it takes most hardwoods longer to reach marketable volume. However, the lower volume may be offset by the higher values per board foot associated with selected hardwood species (Leopold et al., 1989). Such a management plan would require managers to assess value of a stand of timber based on both hardwood and pine. The results of this study strongly suggest that the long-term economics, including those from multiple use, of mixed stands warrant investigation. Acknowledgements We thank D. White and J. Tappiener for comments on an earlier draft of the manuscript. C. Liff and B. Cordova provided assistance with information management. The Forest Health Monitoring Program,
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an inter-agency program, funded this study. Participating agencies include the U.S.D.A. Forest Service, U.S.D.I. Bureau of Land management, and the U.S. Environmental Protection Agency. The research in this paper has been subjected to review by some of the participating agencies. It does not necessarily re¯ect the views of any of the agencies. No of®cial endorsement should be inferred. The USGS/BRD Ohio Cooperative Fish and Wildlife Research Unit provided support to work on this manuscript. References Alexander, S.A., Barnard, J.E., 1993. Forest health monitoring 1992 activities plan. EPA/620/R-93/002. US Environmental Protection Agency, Washington, DC. Alig, R.J., Hohenstein, W.G., Murphy, B.C., Haight, R.C., 1990. Changes in area of timberland in the United States 1952±2040 by ownership, forest type, and region and state. General Technical Report SE-64, USDA Forest Service, Southeastern Forest Experiment Station, Asheville, NC. Atkeson, T.D., Johnson, A.S., 1979. Succession of small mammals on pine plantations in the Georgia piedmont. A. Midland Naturalist 101, 385±392. Bazzaz, F.A., 1983. Characteristics of populations in relation to disturbance in natural and modified ecosystems. In: Mooney, H.A., Godron, M. (Eds.). Disturbance and Ecosystems. Springer, New York, pp. 259±277. Binkley, D., Richter, D., David, M.B., Caldwell, B., 1992. Soil chemistry in a loblolly/longleaf pine forest with interval burning. Ecol. Appl. 2, 157±164. Boyce, S.G., Martin, W.H., 1993. The future of terrestrial communities in the south-eastern coastal plain. In: Martin, W.H., Boyce, S.G., Echternacht, A.C. (Eds.), Biodiversity of the Southeastern United States: Upland Terrestrial Communities. Wiley, New York, pp. 339±366. Brown, M.J., 1996. Forest Statistics for Florida, 1995. USDA Forest Service Resource Bulletin. SE-109. Southeastern Forest Experiment Station, Asheville, NC. Cassell, D.L., 1992. Estimating optimal resource allocation for environmental monitoring networks. In: Proc. Section on Statistics and the Environment 1991. A. Statistical Assoc., Alexandria, VA, pp. 225±229. Cassell, D.L., 1993. Inclusion probabilities for environmental monitoring in FHM and EMAP. In: Proc. Section of Statistics and the Environment 1992. A. Statistical Assoc., Alexandria, VA, pp. 82±85. Childers, E.L., Sharik, T.L., Adkisson, C.S., 1986. Effects of loblolly pine plantations on songbird dynamics in the Virginia piedmont. J. Wildlife Manage. 50, 406±413. Cochran, W.G., 1977. Sampling Techniques, 3rd edn. Wiley, New York..
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