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Arboreal composition changes following white-tailed deer restoration to urban park forests without off-trail park visitor trampling
Urban Forestry & Urban Greening 10 (2011) 305–310
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Arboreal composition changes following white-tailed deer restoration to urban park forests without off-trail park visitor trampling Robert E. Loeb a,∗ , Jesse Germeraad b , Laura Griffin c , Steve Ward b a b c
Biology and Forestry, The Pennsylvania State University – DuBois, DuBois, PA 15801, United States Radnor Lake State Natural Area, Nashville, TN 37220, United States Environmental Engineering, Colorado State University, Fort Collins, CO 80521, United States
a r t i c l e
i n f o
Keywords: Forest resampling Long-term arboreal change Odocoileus virginianus browsing Restricted human access Urban natural area forests
a b s t r a c t Human trampling destroys seedlings and saplings without regard to species in urban park forests and the addition of deer browsing compounds the losses. The unexamined research question is: what is the effect of white deer browsing in the absence of human trampling? Radnor Lake State Natural Area, Nashville, TN, USA has been protected from off-pathway human transit since 1973 and white-tailed deer (Odocoileus virginianus Zimmerman) were restored to the Natural Area in 1980. From 1976 to 2007, a plot with tagged trees in the mesic slope forest showed the tree population for 16 species decreased, two remained stable, and one increased. The pattern of increase for sugar maple (Acer saccharum Marsh) was 57 new trees added but 46 trees were lost, which appears as a population increase from 1976 to 1996 and a decrease in 2007. In all five forest communities, the total tree stems per ha declined from 1974 to 2008. During the period 1994–2008, O. virginianus over browsing decimated the seedling population of all species and caused the total for stems per ha for saplings to become smaller than the total stems per ha for trees in each community except the ravine forest, which had the greatest loss of trees. The only consistent change in trees across the five communities when comparing 1974–2008 was the significant tree importance value increase for A. saccharum. The conflicting significant changes for major species, other than A. saccharum, across the Natural Area forest communities informs management for other urban park forests—browsing by O. virginianus results in increased numbers for trees and saplings in the communities the species are well adapted to grow and reproduce in and fewer trees and saplings in the communities with environmental conditions that are not well suited to the species. © 2011 Elsevier GmbH. All rights reserved.
Introduction Over time arboreal species composition and forest structure of urban park forests tend to diverge from arboreal species composition and forest structure of natural areas in more rural settings. The unique challenges of the urban environment contribute to these changes. For example, in urban park forests of Australia, New Zealand, United States, and Europe, trampling by people causes soil compaction as well as the loss of seedlings and saplings without regard to species (Brown et al., 1977; Bagnall, 1979; Florgård, 2000; Lehvävirta and Rita, 2002; Malmivaara et al., 2002; Quigley, 2002; Stenhouse, 2004; Sukopp, 2004; Asnani et al., 2006; Nováková, 2008). Browsing by deer species in urban park forests of New Zealand, United States and Europe compound the losses of seedlings and saplings caused by human trampling (Conover and Kania, 1998; Peck and Stahl, 1997; Crampton et al., 1998; Coomes
∗ Corresponding author. Tel.: +1 814 5743036; fax: +1 814 8143754784. E-mail address: [email protected] (R.E. Loeb). 1618-8667/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ufug.2011.07.002
et al., 2003; Husheer et al., 2005; Perrin et al., 2006; MalmivaaraLämsä et al., 2008; Krueger et al., 2009; Mihók et al., 2009). Urban park forest managers have three management options to address the potential future forest losses by deer browsing and trampling by people: (1) permit the arboreal changes to occur (Millward et al., 2011); (2) remove the deer but not the people (Latham et al., 2005) and (3) remove the people but not the deer (Loeb et al., 2010). In the eastern United States, permitting the changes to occur results in pervasive damage to seedlings and saplings (Robertson and Robertson, 1995) and a wide breadth of species composition changes (Peck and Stahl, 1997; Rossell et al., 2007; Morgan, 2009). Extensive trampling by people when Odocoileus virginianus (mammal nomenclature follows Kays and Wilson, 2009) are absent in urban park forests of the eastern United States was found to result in Prunus serotina Ehrh. and Quercus spp. L. (plant nomenclature follows Gleason and Cronquist, 1991) dominance among seedlings, saplings, and trees (Stalter and Kincaid, 2008). The third option of causing people to virtually cease trampling and permitting O. virginianus to browse has been proposed (Millward et al., 2011) but not examined. If deer brows-
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ing without human trampling does not effect changes in species composition or forest structure then the management of urban park forests can be designed to incorporate white-tailed deer while restricting human access in order to protect the forest over the long-term. The objective of this research is to identify changes in the trees, saplings, and seedlings of the five urban park forest communities of Radnor Lake State Natural Area (RLSNA), Nashville, TN (which strictly prohibits off-trail forest access by visitors) that are related to the long-term effects of browsing by O. virginianus. Methods Site description RLSNA is located in Nashville, TN, USA (36◦ 03 N, 86◦ 47 W) and encompasses approximately 473 ha including the 34 ha artificial lake. European land clearance began circa 1835 with agriculture occurring throughout the property, except on steep slopes. The demand for wood prior to the Battle of Nashville during the American Civil War (December 15–16, 1864) caused forest clearance in all of the RLSNA area. The core property forming RLSNA was acquired in 1912 to create a reservoir for the steam engine facility of the Louisville and Nashville Railroad. The lake was protected from poaching fish and the land was closed to the public as a private hunting area. Logging for Juglans nigra L., Populus spp. L., and Quercus alba L. in 1918 and for primarily Fraxinus spp. L. and Quercus spp. was done in 1950. Limited extent fires occurred in 1948 and 1954 and ice storms damaged many trees in 1957 and 1994 (Schibig, 1996). In 1973, RLSNA was the first Natural Area established under the Tennessee State Natural Areas Preservation Act. The Act prohibits human activities that are not conducive to wildlife observation or modify the natural state of the ecosystem. O. virginianus were overhunted in Tennessee and were not observed in RLSNA until 1980 following reintroduction of the species to Tennessee in the mid 1970s (Tennessee Wildlife Resources Agency, 1991). Population estimates for O. virginianus in RLSNA are not available. The forests of RLSNA are part of the Western Mesophytic Region described by Braun (1950). Windstorms and torrential rain induced landslides cause the loss of trees and form many canopy gaps in RLSNA (Loeb and King, in press). Carpenter et al. (1976) identified five forest communities in RLSNA: lakeshore, ravine, mesic slope, xeric slope, and ridge. The palatability of tree species for browsing mammals was classified as “high,” “medium” or “low” according to the Plants Database (Natural Resources Conservation Service, 2010). The soils underlying RLSNA are well drained silt loams. Lakeshore forests grow on a flood plain soil with a soil reaction pH range of 6.1–7.8. Ravine and mesic slope forests are found on soils that are underlain by phosphatic limestone and have a soil reaction pH range of 4.5–6.0. Ridge and xeric slope forests grow on cherty soils that overlay shale bedrock and have a soil reaction pH range of 3.6–5.5. The soils of both the mesic slope and xeric slope forests are thin, contain rock outcrops, and are on hillsides with greater than 20% slopes (North, 1981). Forest sampling Temporal coincidence studies of the effects of O. virginianus browsing on the arboreal composition of forest communities involve measurement of trees, saplings, and seedlings before and after the reintroduction of O. virginianus to forests (Russell et al., 2001). Unlike exclusion plots research which enables comparison of adjacent areas with O. virginianus browsing and no browsing, a tem-
poral coincidence study does not involve observation of browsing damage presence and absence. The advantage provided by temporal coincidence research is the examination of the effects of O. virginianus browsing in large areas and over long periods of time. When the goal of the research is to identify changes caused by the restoration of O. virginianus in urban forests without human trampling, then the ideal situation is to conduct the research in a site with several forest communities protected from human trampling before O. virginianus was reintroduced. Also, the site must have sample plots that were measured before O. virginianus restoration and measured again decades later for comparison. The changes in the seedlings, saplings, and trees can be associated with the browsing preferences of O. virginianus. However, the conclusions drawn from the comparison must consider both changing environmental conditions such as unusual drought and heat and biological conditions including diseases and canopy openings during the intervening period. Radnor Lake State Natural Area provides many components of the ideal conditions for a temporal coincidence study but what is not present limits the scope of the analysis. As noted above, an individual O. virginianus was first observed to have returned to RLSNA in 1980 following reintroduction of the species to Tennessee in the mid 1970s. However, no O. virginianus population estimates are available; therefore, relationships between densities for O. virginianus and trees, saplings, and seedlings cannot be determined. The research of Carpenter et al. (1976) on the five forest communities in RLSNA was performed in 1974 and the tagged trees plot of Eickmeier (1988) measurements began in 1976 and thusly permits comparison of forest composition before and after O. virginianus restoration. Although the tagged trees (dbh > 10.2 cm) were only in a 75 m by 75 m permanent plot (Eickmeier, 1988), the data enables comparison of tree species recruitment, death, and total population from archival data for 1976, 1986, and 1996 and a measurement in 2007 for one forest community, the mesic slope forest. Total population was the sum of the trees found in the plot. Recruitment was the sum of the new trees present in 1986, 1996, and 2007, while death was the total for the trees not found in 1986, 1996, and 2007 that were present in the respective previous year. To describe the five forest communities of RLSNA, Carpenter et al. (1976) measured trees with eight circular plots (7.93 m in diameter) in each of four forest samples per community but none of the original plot stakes could be found in 1994 (Schibig, 1996). The field data from the 1974 sample is not available (Schibig, personal communication), which limits species level and forest variability analysis because only tree importance values are available for 1974 at the species level. In 1994, Schibig placed forest sample sites in the area of the 1974 sites (Schibig, 1996). The sampled forests distribution map available in the Schibig (1996) study report also represents the location of the forests sampled in 2007 (mesic slope; Eickmeier’s tagged tree plot is adjacent to one of Schibig’s mesic slope plots) and 2008 (lakeshore, ravine, ridge, and xeric slope). Schibig placed stakes marking the center of the circular plots (8.025 m in radius), which were distributed five per forest and four forests were measured for each of the five communities. Tree and sapling resampling in 2007 and 2008 was done with five, 8 m radius circular plots. Schibig sampled seedlings in 0.8025 m radius circular plots at the center of the larger plots for sampling trees and saplings; which was repeated in 2007 and 2008 except the radius was 0.8 m. In virtually all of the forests only some of the stakes were found and the location of the missing stakes were interpolated; however, interpolation was not possible for one mesic slope forest where none of the stakes were found and no measurements were done. All of the forest sample sites were located > 50 m from the border of RLSNA except for two which are discussed in the results. Stems per ha were reported for trees, saplings,
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Stems Per Hectare Saplings
Trees
1500
1200
Stems Per Hectare
and seedlings by community. Importance values (total of relative density, relative dominance, and relative frequency = 300) were given for tree species. The research of Carpenter et al. (1976) and Schibig (1996) were not total inventories of the forest communities in RLSNA. The area resampled represents less than 1% of RLSNA. Also, the researchers did not report whether the placement of plots provided a representative sample of the forest communities or provided measurements of species distribution variability in the forests. No effort was made to determine if the samples were representative at the time of the recent sampling because no alternative sites were available for the comparison. If the forest sample sites could have been consistently identified in 2007–2008, then at least the comparison of results would represent changes in particular sections of RLSNA. Instead of clear measurements of changes in arboreal composition through time, the differences in results among years also represent variation in species composition in space and time. In light of this limitation, the significance of changes in importance values for trees and saplings of major (>30 importance value in at least one forest community) species was determined using the Chi-square test by comparing the importance value of a species to all of the other species. The Chi-square test is appropriate to account for confounding temporal and spatial differences when the location of the historical sample is unknown. The expected values are generated by comparing among spatially separated contemporaneous samples and the observed values are generated by comparing the modern versus historical sampling results (Loeb, 1990). The Chisquare tests were performed using PASW Statistics (formerly SPSS Statistics) version 17 and the significance level selected was 0.01.
307
900
600
300
0
'74 '94 '08
'74 '94 '08
'74 '94 '07
Lakeshore
Ravine
Mesic Slope
'74 '94 '08
Xeric Slope
'74 '94 '08
Ridge
Forests Fig. 1. Stems per ha for trees and saplings in the five forest communities of Radnor Lake State Natural Area.
Results Tagged trees plot In the mesic slope forest with tagged trees, all of the species had a decline in total population from 1976 to 2007 (Table 1) except for A. saccharum (medium palatable), P. serotina (low palatable), and Liriodendron tulipifera L. (low palatable). The one L. tulipifera was recruited between 1976 and 1986 and the two P. serotina trees remained throughout the 31-year period. The A. saccharum population rose from 87 in 1976 to 107 in 1996 but declined to 98 in 2007. Considering recruitment, the total loss of trees from 1976 to 2007 was 36 for Quercus spp. (all species are medium palatable except Quercus prinus L. which is low palatable) and 46 for A. saccharum. For all of the species, the trees lost over the 31 years were 63% recruits (10.2 cm > dbh > 20 cm) and 37% canopy trees (dbh ≥ 20 cm). Acer saccharum, Celtis occidentalis, and Quercus rubra losses were close to 70% recruits. Among the three Carya species, C. cordiformis losses were all recruits while for both C. glabra and C. ovata each stem lost was a tree. Aesculus flava, Ailanthus altissima, J. nigra, Q. prinus, Sassafras albidum, Tilia americana, and Ulmus americana had nearly equal numbers of tree and recruit losses. Five forest communities Tree stems per ha decreased from 1974 to 2007 or 2008 in all five communities and the largest decrease occurred for the ravine forests from 1974 to 1994. The decline in sapling stems per ha from 1994 to 2007 or 2008 was greater than the respective tree stems per ha decrease for each community. Sapling stems per ha was greater than tree stems per ha for all of the communities in 1994; however, the reverse was found in 2007 and 2008 except for the ravine forests (Fig. 1). The lakeshore forests having the greatest decrease in sapling and tree stems per ha from 1994 to 2008 should be discounted because the lake level was raised during the late 1990s and
was only recently lowered, which resulted in the loss of saplings and trees from part of each lakeshore forest plot. The only species with increased tree and sapling stems per ha in all five communities was A. saccharum. In 1994, the total stems per ha for seedlings in the lakeshore, ravine, mesic slope, xeric slope, and ridge forests were 5800, 12,350, 12,800, 8050, and 14,000, respectively. The total density for seedlings in the mesic slope forest in 2007–2008 was 517 stems per ha. There were 457 stems per ha for A. flava Aiton. (low palatable; 1050 stems per ha in 1994) and 60 for Acer rubrum L. (medium palatable) and A. saccharum (all Acer spp. L. were 1100 stems per ha in 1994) in mesic slope forest. Seedlings were not found in the seedling plots for the remaining four communities in 2008. Of the 16 major tree species (>30 importance value in at least one forest for either 1974 or 2007–2008) in Table 2 only four were medium palatable (A. saccharum, Fraxinus americana L., Populus deltoides Bartram ex Marsh., and Q. rubra L.). F. americana L. and Q. prinus had inconsistent changes with a significant importance value increase in at least one forest community and a significant importance value decrease in another community (Chi-square test of significance results are presented in Table 2). The three species with only significant decreases in importance value were P. deltoides, Salix nigra Marsh. (low palatable), and U. americana L. (low palatable). A. saccharum had significant increases in all five forests while Acer saccharinum L. (low palatable) had a significant increase in the Lake forest, which was the only community in which the species was found. A. flava trees attained an importance value of 33.6 (no saplings were present) for the xeric slope community in 2008 but the trees were present in only one forest sample located on the border of RLSNA, which was disturbed by residential construction and plot placement was based on interpolation from just two adjacent stakes. The A. flava seedlings noted above occurred only in one mesic slope forest sample and no evidence of disturbance was
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Table 1 Tree species recruitment and death, during the period 1976–2007 and total population in 1976, 1986, 1996 and 2007 for a mesic slope forest, Radnor Lake Natural Area, Nashville, TN. Recruitment and death are based on comparisons from results in 1976, 1986, 1996 and 2007. Palatability for browsing species is rated as low or medium following information from the Plants Database of the Natural Resources Conservation Service (2010). Species
Acer saccharum Marsh. Aesculus flava Aiton. Ailanthus altissima (Miller.) Swingle. Carya cordiformis (Wangenh.) K. Koch. Carya glabra (Miller) Sweet. Carya ovata (Miller) K. Koch. Celtis occidentalis L. Cornus florida L. Fraxinus americana L. Juglans nigra L. Liriodendron tulipifera L. Prunus serotina Ehrh. Quercus muehlenbergii Englen. Quercus prinus L. Quercus rubra L. Sassafras albidum (Nutt.) Nees. Tilia americana L. Ulmus americana L. Ulmus rubra Muhl.
Palatability
1976–2007
Medium Low Medium Low Low Low Medium Medium Medium Low Low Low Medium Low Medium Medium Medium Low Low
Total
Recruitment
Death
1976
1986
1996
2007
57 1 0 0 0 1 4 0 2 0 1 0 0 0 0 0 2 3 0
46 2 7 9 6 7 10 0 13 2 0 0 3 6 19 11 6 12 1
87 3 8 9 11 16 14 2 28 3 0 2 8 8 31 18 9 12 2
101 2 3 1 9 12 10 1 27 2 1 2 7 4 20 13 7 6 2
107 3 2 0 8 11 9 1 22 1 1 2 5 3 17 9 5 5 2
98 2 1 0 5 10 8 0 17 1 1 2 5 2 12 7 5 3 1
observed. The second forest sample located close to the border of RLSNA is a ridge forest and is remarkable in being the only forest sample that contains the invasive shrub Lonicera macckii (Rupr.) Maxim., which has been present in RLSNA for more than 30 years (Loeb et al., 2010).
rum was discovered to be a second choice to other browse species in mixed mesophytic forests (Strole and Anderson, 1992), which includes the five forest communities of RLSNA. Although the first O. virginianus returned to RLNSA in 1980, an O. virginianus population takes years to grow to a sufficient size to over browse a forest and cause the dramatic reductions in seedling and sapling stems per ha found in RLSNA. The population increase for A. saccharum trees in the tagged tree plot through 1996 and the decline found in 2007 indicates over browsing of A. saccharum occurred after 1996. Explanation of the significant changes in the forest canopy for the remaining species in Table 1 should focus on the whether the trees are classified as medium or low palatable because this would be related to the survival of saplings to grow into the canopy. However, the classification system fails to inform which species have had significant increases. For examples, F. americana is a medium palatable species but had significant increases in the mesic slope and xeric slope forests and Carya glabra is a low palatable species which had a significant decrease in the ridge forest. Perhaps a clas-
Discussion The long-term forest changes in RLSNA do not include the large increases for P. serotina and Quercus spp. that typify urban park forests with high levels of unrestricted pedestrian traffic but no O. virginianus population. The one consistent long-term change across the five forest communities in RLSNA was significant increased importance values for A. saccharum trees and saplings, which is not reported for urban park forests with extensive human trampling and no O. virginianus browsing (Fitzgerald and Loeb, 2008). A. saccharum is rated as a medium palatable species for browsing mammals, but the specific preference of O. virginianus for A. saccha-
Table 2 Importance values for tree species in the five forest communities of Radnor Lake State Natural Area, Nashville, TN for major species (species had an importance value >30 in at least one forest community) in 1974 and 2008. Palatability for browsing species is rated as low or medium following information from the Plants Database of the Natural Resources Conservation Service (2010). Taxa
Palatability
Forests Ridge 1974
Acer negundo L. Acer saccharum Marsh. Acer saccharinum L. Aesculus flava Aiton. Carya cordiformis (Wangenh.) K. Koch Carya glabra (Miller) Sweet. Carya ovata (Miller) K. Koch. Celtis laevigata Willd. Fraxinus americana L. Fraxinus quadrangulata Michx. Populus deltoides Bartram ex Marsh. Quercus prinus L. Quercus rubra L. Quercus shumardii Buckley. Salix nigra Marsh. Ulmus americana L. *
Low Medium Low Low Low Low Low Low Medium Low Medium Low Medium Low Low Low
0 0 0 0 0 46 17 4 32 49 0 74 0 0 0 0
Xeric slope 2008 0 24* 0 0 0 12* 37* 6 6* 47 0 110* 0 0 0 0
Mesic slope
Ravine
Lake
1974
2008
1974
2008
1974
2008
1974
2008
0 46 0 12 0 16 21 5 12 26 0 69 27 0 0 0
0 95* 0 34* 0 14 23 0 28* 8* 0 25* 0* 0 0 0
0 73 0 12 32 0 13 0 15 0 0 0 36 7 0 7
0 119* 0 7 0* 0 0* 3 50* 0 0 0 44 13 0 0*
3 49 0 3 30 0 0 3 11 0 0 0 12 22 0 64
5 112* 0 8 36 0 0 0 7 0 0 0 5 35 0 4*
24 0 32 0 0 0 0 35 8 0 37 0 0 0 42 74
33 7* 112* 0 0 0 0 28 0* 0 0* 0 0 11* 3* 39*
Chi-square test significant difference between importance values for 1974 and 2008 at the 0.01 level.
R.E. Loeb et al. / Urban Forestry & Urban Greening 10 (2011) 305–310
sification system based on only O. virginianus browsing preference would be better but O. virginianus has variable browsing preferences depending upon geographic region (Latham et al., 2005). To explain the conflicting significant changes among forest communities for Carya ovata, F. americana, and Q. prinus, O. virginianus browsing is a stress on the population, which can be overcome in the forests where the species is best adapted to grow and reproduce but causes losses in forests where the species is less well adapted to survive. Unaccounted for factors including the effects of other herbivores as well as diseases and unnoticed infestations could possibly provide explanations for some of the changes in particular species. For example, canopy closure is related to a shift in arboreal reproduction to predominantly shade tolerant species such as A. saccharum, especially when fire is rare as is the situation in RLSNA. Although canopy gaps are created by windstorms and landslides in the forests, the increases for A. saccharum in the tagged trees plot from 1976 to 1996 and the five forest communities from 1974 to 2008 suggest a transition to lower light levels reaching the forest floor started prior to 1974. However, the virtual loss of A. saccharum seedlings after 1994 and the decline for trees in the tagged trees plot since 1996 indicates increased shading has been supplanted by another factor—O. virginianus overbrowsing. For two species, P. deltoides and S. nigra, the trees were cut down by three beavers (Castor canadensis Kuhl.) which became residents at RLSNA in 2001. A. flava remained present among the seedlings, possibly because O. virginianus avoids browsing the species (Latham et al., 2005; Asnani et al., 2006) but a significant increase in the canopy only occurred because of a change in one xeric forest sample. The decrease in U. americana is related to Dutch elm disease (Ophiostoma ulmi (Buisman) Nannf.) which has devastated the species in other urban park forests (Fitzgerald and Loeb, 2008; Loeb, 2010). Tree stems per ha being higher than sapling stems per ha in 2007 and 2008 indicates reduced recruitment to the canopy is possible. The relative absence of seedlings in 2007 and 2008 as compared to 1994 and the decline in sapling stems per ha from 1994 to 2007 and 2008 also indicate the possibility of fewer new trees in the canopy. Climatic variation cannot explain declines in tree recruitment at RLSNA. Considerably lower mean annual rainfall and higher mean annual temperatures in 1994 through 2007 as compared to 1974 through 1993 is not an explanation for the seedling and sapling stems per ha decreases since 1994. Specifically, mean annual precipitation for 1974 through 1993 was 122.0 cm (28.7 cm standard deviation) versus 128.7 cm (19.7 cm standard deviation) for 1994 through 2007 (National Weather Service, 2009). The mean yearly temperature for 1974 through 1993 was 15.2 ◦ C (0.7 ◦ C standard deviation) while for 1994 through 2007 mean yearly temperature was 15.6 ◦ C (0.7 ◦ C standard deviation). Many hot summers with drought also do not appear to be a reason for the losses since the mean monthly summer temperature was only 0.2 ◦ C less during 1974 through 1993 than the 25.8 ◦ C in 1994 through 2007 (0.9 ◦ C standard deviation for both periods). Average monthly summer precipitation was 3.4 cm (1.8 cm standard deviation) in 1974 through 1993 as versus 3.8 cm (2.0 cm standard deviation) in 1994 through 2007. Furthermore, differences in cold and dryness in the winters are not an apparent cause of the losses since from 1974 through 1993 mean monthly temperature was 4.2 ◦ C (3.2 ◦ C standard deviation) and mean monthly precipitation was 10.3 cm (6.3 cm standard deviation) while for 1994 through 2007 the mean monthly temperature was 5.1 ◦ C (4.0 ◦ C standard deviation) and mean monthly precipitation was 10.4 cm (2.7 cm standard deviation). RLSNA will continue to experience tree deaths as occurs in other urban park forests with human trampling. The decimation of the seedling layer by O. virginianus suggests greater losses for all species
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are likely to occur in the future, except perhaps for A. saccharum and A. flava. The lower stems per ha for saplings in 2007–2008 as compared to 1994 points to the importance of how O. virginianus has affected saplings directly by browsing and indirectly through seedling losses. The combination of no human trampling and O. virginianus browsing at RLSNA results in selective advancement of arboreal species that O. virginianus does not prefer to browse and species that are well adapted to grow and reproduce in the environmental conditions of the particular forest. In contrast, urban park forests with human trampling but no O. virginianus have representation in the seedling and sapling size classes in areas where people do not walk (Lehvävirta and Rita, 2002). However, when the O. virginianus population is large enough browsing becomes over browsing and all seedlings if not also many saplings can be lost because deer browsing is not spatially limited, unlike human trampling. When O. virginianus over browsing is combined with human trampling, the result is small populations of tree species avoided by O. virginianus as observed by Robertson and Robertson (1995), Peck and Stahl (1997), and Rossell et al. (2007). The declines in tree stems per ha across the five forest communities of RLSNA reflect what has occurred in other urban park forests (Zipperer and Zipperer, 1992; Peck and Stahl, 1997; Quigley, 2002; Loeb, 2010) but unlike the other urban park forests, RLSNA has not and will not have landscape plantings done to replace the lost trees (barring a change in state law). The following management recommendations for increasing tree recruitment and diversity are analyzed in regard to RLSNA but are applicable to other similar settings. The direct management response of restricted O. virginianus hunting as has been done in other urban parks (Rossell et al., 2007) is not a viable choice for RLSNA because the Tennessee Natural Areas Act prohibits actions against native species. Hunting is not supported by many urban park forest patrons because viewing animals is an important part of the visit experience. An O. virginianus population drop should not be expected in the future because private homes with extensive landscaped gardens along the open boundaries of RLSNA supplement the O. virginianus food resources (Swihart et al., 1995). The prospect of canopy density decline can be addressed by increasing the population of rare native trees such as Fagus grandifolia Ehrh. which is not preferred by O. virginianus (Tubbs and Houston, 2010). Also introducing native shrubs which grow densely enough to protect seedlings and saplings from O. virginianus browsing is another remedy (Cipollini et al., 2009). Alternative methods which deter O. virginianus browsing and increase soil fertility (Odin et al., 2005) while meeting the provisions of natural areas laws also should be tested. Conclusions Tree losses occur in RLSNA as has been reported for other urban park forests. However, the seedling and sapling stems per ha decreases since 1994 indicate over browsing by O. virginianus has shaped long-term arboreal composition changes. A. saccharum is becoming the dominant tree because O. virginianus has a preference against browsing the species. A. flava may become more frequent among the saplings and trees of the forest communities because of O. virginianus browsing avoidance. Canopy species representation in forest communities will decrease except for the community which fits the ecological characteristics of the tree species. Finally, the prospect of less canopy recruitment in Radnor Lake State Natural Area can lead to research into expanding the population of native species that O. virginianus avoids browsing. Acknowledgements We gratefully acknowledge the provision of archival data, permission to use archival data, and advice given by William Eickmeier
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and Joe Schibig. The permission provided by the Tennessee Department of Environment and Conservation Division of Natural Areas to study Radnor Lake Natural Area is appreciated. The efforts of the Radnor Lake Natural Area staff in assisting with our research are deeply valued. Also, we are very appreciative of the support provided by Kent Gallaher and Jon Lowrance of Lipscomb University. The funding provided by the DuBois Educational Foundation and the Friends of Radnor Lake to support the research is appreciated. Critiques by two anonymous reviewers provided valuable changes for the manuscript. References Asnani, K.M., Klips, R.A., Curtis, P.S., 2006. Regeneration of woodland vegetation after deer browsing in Sharon Woods Metro Park, Franklin County, Ohio. Ohio Journal of Science 106, 86–92. Bagnall, R.G., 1979. A study of human impact on an urban forest remnant: Redwood Bush, Tawa, near Wellington, New Zealand. New Zealand Journal of Botany 17, 117–126. Braun, E., 1950. Deciduous Forests of Eastern North America. Hafner, New York. Brown, J.H., Kalisz, S.P., Wright, W.R., 1977. Effects of recreational use on forested sites. Environmental Geology 1, 425–431. Carpenter, L., Turner, J., Schibig, J., 1976. Forest communities of the Radnor Lake Natural Area, Davidson County, Tennessee. Journal of the Tennessee Academy of Science 51, 68–72. Cipollini, K., Ames, E., Cipollini, D., 2009. Amur honeysuckle (Lonicera maackii) management method impacts restoration of understory plants in the presence of white-tailed deer (Odocoileus virginianus). Invasive Plant Science and Management 2, 45–54. Conover, M.R., Kania, G.S., 1998. Browsing preference of white-tailed deer for different ornamental species. Wildlife Society Bulletin 16, 175–179. Coomes, D.A., Allen, R.B., Forsyth, D.M., Lee, W.G., 2003. Factors preventing the recovery of New Zealand forests following control of invasive deer. Conservation Biology 17, 450–459. Crampton, A.B., Stutter, O., Kirby, K.J., Welch, R.C., 1998. Changes in the composition of Monks Woods National Nature Preserve (Cambridgeshire, UK) 1964–1996. Arboricultural Journal 22, 229–245. Eickmeier, W., 1988. Ten years of forest dynamics at Radnor Lake, Tennessee. Bulletin of the Torrey Botanical Club 115, 100–107. Fitzgerald, J., Loeb, R.E., 2008. Historical ecology of Inwood Hill Park, Manhattan, New York. Journal of the Torrey Botanical Society 135, 281–293. Florgård, C., 2000. Long-term changes in indigenous vegetation preserved in urban areas. Landscape and Urban Planning 52, 101–116. Gleason, H., Cronquist, A., 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada, second edition. New York Botanical Gardens, Bronx, NY. Husheer, S.W., Hansen, Q.W., Urlich, S.C., 2005. Effects of red deer on tree regeneration and growth in Aorangi Forest, Wairarapa. New Zealand Journal of Ecology 29, 271–277. Kays, R., Wilson, D., 2009. Mammals of North America. Princeton University Press, Princeton, NJ. Krueger, L.M., Peterson, C.J., Royo, A., Carson, W.P., 2009. Evaluating relationships among tree growth rate, shade tolerance, and browse tolerance following disturbance in an eastern deciduous forest. Canadian Journal of Forest Research 39, 2460–2469. Latham, R.E., Beyea, J., Benner, M., Dunn, C.A., Fajvan, M.A., Freed, R.R., Grund, M., Horsley, S.B., Rhoads, A.F., Shissler, B P., 2005. Managing White-tailed Deer in Forest Habitat from an Ecosystem Perspective: Pennsylvania Case Study. Report by the Deer Management Forum for Audubon Pennsylvania and Pennsylvania Habitat Alliance, Harrisburg, Pennsylvania. Lehvävirta, S., Rita, H., 2002. Natural regeneration of trees in urban woodlands. Journal of Vegetation Science 13, 57–66. Loeb, R.E., 1990. Measurement of vegetation changes through time by resampling. Bulletin of the Torrey Botanical Club 116, 173–175. Loeb, R.E., 2010. Diversity gained, diversity lost: long-term changes in woody plants in Central Park, New York City and Fairmount Park, Philadelphia. Studies in the History of Gardens and Designed Landscapes 30, 124–151. Loeb, R., Germeraad, J., Treece, T., Wakefield, D., Ward, S., 2010. Effects of 1-year vs annual treatment of Amur honeysuckle in forests. Invasive Plant Science and Management 3, 334–339.
Loeb, R.E., King, S. Landslides in Urban Forests. Arboriculture and Urban Forestry, in press. Malmivaara, M., Löfström, I., Vanha-Majamaa, I., 2002. Anthropogenic effects on understory vegetation in Myrtillus type urban forests in southern Finland. Silva Fennica 36, 367–381. Malmivaara-Lämsä, M., Hamberg, L., Löfström, I., Vanha-Majamaa, I., Niemelä, J., 2008. Trampling tolerance of understorey vegetation in different hemiboreal urban forest site types in Finland. Urban Ecosystems 11, 1–16. Mihók, B., Kenderes, K., Kirby, K.J., Paviour-Smith, K., Elbourn, C.A., 2009. Forty-year changes in the canopy and the understorey in Wytham Woods. Forestry 82, 515–527. Millward, A.A., Paudel, K., Briggs, S.E., 2011. Naturalization as a strategy for improving soil physical characteristics in a forested urban park. Urban Ecosystems 14, 261–278. Morgan, E.C., 2009. The vegetation and vascular flora of the Bartlett Arboretum forest. Journal of the Torrey Botanical Society 136, 532–540. National Weather Service, 2009. Climate Data at Nashville, Tennessee 1871present. Retrieved February 20, 2009 from http://www.weather.gov/climate/ local data.php?wfo=ohx. Natural Resources Conservation Service, 2010. Plants Database. USDA Forest Service. Retrieved May 2, 2010 from http://plants.usda.gov/index.html. North, O.L., 1981. Soil survey of Davidson County, Tennessee. United Department of Agriculture, Soil Conservation Service and University of Tennessee Agricultural Experiment Station, Knoxville, TN. Nováková, J., 2008. Vegetation changes in Prague’s suburban forest during the last 40 years—human impact and legislative protection. Urban Forestry and Urban Greening 7, 301–314. Odin, L., Stephens, M.T., Mengak, G.G., Miller, K.V., 2005. Using Milorganite® to repel white-tailed deer from ornamental plantings. Wildlife Management Series No. 3, Warnell School of Forest Resources, University of Georgia, Athens, Georgia. Retrieved February 20, 2009 from http://www.walterreeves.com/uploads/ pdf/usingmilorganitetorepelwhitetaileddeerfromornamentalplantings.pdf. Peck, L.J., Stahl, J.F., 1997. Deer management techniques employed by the Columbus and Franklin County Park District, Ohio. Wildlife Society Bulletin 25, 440–442. Perrin, P.M., Kelly, D.L., Mitchell, F.J.G., 2006. Long-term deer exclusion in yew-wood and oakwood habitats in southwest Ireland: Natural regeneration and stand dynamics. Forest Ecology and Management 236, 356–367. Quigley, M., 2002. Franklin Park: 150 years of changing design, disturbance, and impact on tree growth. Urban Ecosystems 6, 223–235. Robertson, D.J., Robertson, M.C., 1995. Eastern mixed mesophytic forest restoration. Restoration and Management Notes 13, 64–70. Rossell, C., Patch, S., Salmons, S., 2007. Effects of deer browsing on native and nonnative vegetation in a mixed oak-beech forest on the Atlantic coastal plain. Northeastern Naturalist 14, 61–72. Russell, F.L., Zippin, D.B., Fowler, N.L., 2001. Effects of white-tailed deer (Odocoileus virginanus) on plants, plant populations and communities: a review. American Midland Naturalist 146, 1–26. Schibig, J., 1996. Twenty years of forest change at Radnor Lake Natural Area, Davidson County, middle Tennessee. Journal of the Tennessee Academy of Science 71, 57–72. Stalter, R., Kincaid, D., 2008. A 70-year history of arborescent vegetation of Inwood Hill Park, Manhattan, New York, US. Arboriculture and Urban Forestry 34, 245–251. Stenhouse, R.H., 2004. Fragmentation and internal disturbance of native vegetation reserves in the Perth metropolitan area, Western Australia. Landscape and Urban Planning 68, 389–401. Strole, T., Anderson, R., 1992. Odocoileus virginianus browsing: species preferences and implications for central Illinois forests. Natural Areas Journal 12, 139–144. Sukopp, H., 2004. Human-caused impact on preserved vegetation. Landscape and Urban Planning 68, 347–355. Swihart, R.K., Picone, P.M., DeNicola, A.J., Cornicelli, L., 1995. Ecology of urban and suburban white-tailed deer. In: Aninch, J.B. (Ed.), Urban Deer: A Manageable Resource? Symposium, 55th Midwest Fish and Wildlife Conference. St. Louis, MO, pp. 35–44. Tennessee Wildlife Resources Agency, 1991. Deer restoration in Tennessee, 1940–1985: a final report. Wildlife Research Report No. 91-1, Nashville, TN. Tubbs, C.H., Houston, D.R., 2010. Fagus grandifolia Ehrh American Beech. Retrieved May 2, 2010 from http://na.fs.fed.us/pubs/silvics manual/volume 2/ fagus/grandifolia.html. Zipperer, W.C., Zipperer, C.E., 1992. Vegetation responses to changes in design and management of an urban park. Landscape and Urban Planning 22, 1–10.
Contents lists available at ScienceDirect
Urban Forestry & Urban Greening journal homepage: www.elsevier.de/ufug
Arboreal composition changes following white-tailed deer restoration to urban park forests without off-trail park visitor trampling Robert E. Loeb a,∗ , Jesse Germeraad b , Laura Griffin c , Steve Ward b a b c
Biology and Forestry, The Pennsylvania State University – DuBois, DuBois, PA 15801, United States Radnor Lake State Natural Area, Nashville, TN 37220, United States Environmental Engineering, Colorado State University, Fort Collins, CO 80521, United States
a r t i c l e
i n f o
Keywords: Forest resampling Long-term arboreal change Odocoileus virginianus browsing Restricted human access Urban natural area forests
a b s t r a c t Human trampling destroys seedlings and saplings without regard to species in urban park forests and the addition of deer browsing compounds the losses. The unexamined research question is: what is the effect of white deer browsing in the absence of human trampling? Radnor Lake State Natural Area, Nashville, TN, USA has been protected from off-pathway human transit since 1973 and white-tailed deer (Odocoileus virginianus Zimmerman) were restored to the Natural Area in 1980. From 1976 to 2007, a plot with tagged trees in the mesic slope forest showed the tree population for 16 species decreased, two remained stable, and one increased. The pattern of increase for sugar maple (Acer saccharum Marsh) was 57 new trees added but 46 trees were lost, which appears as a population increase from 1976 to 1996 and a decrease in 2007. In all five forest communities, the total tree stems per ha declined from 1974 to 2008. During the period 1994–2008, O. virginianus over browsing decimated the seedling population of all species and caused the total for stems per ha for saplings to become smaller than the total stems per ha for trees in each community except the ravine forest, which had the greatest loss of trees. The only consistent change in trees across the five communities when comparing 1974–2008 was the significant tree importance value increase for A. saccharum. The conflicting significant changes for major species, other than A. saccharum, across the Natural Area forest communities informs management for other urban park forests—browsing by O. virginianus results in increased numbers for trees and saplings in the communities the species are well adapted to grow and reproduce in and fewer trees and saplings in the communities with environmental conditions that are not well suited to the species. © 2011 Elsevier GmbH. All rights reserved.
Introduction Over time arboreal species composition and forest structure of urban park forests tend to diverge from arboreal species composition and forest structure of natural areas in more rural settings. The unique challenges of the urban environment contribute to these changes. For example, in urban park forests of Australia, New Zealand, United States, and Europe, trampling by people causes soil compaction as well as the loss of seedlings and saplings without regard to species (Brown et al., 1977; Bagnall, 1979; Florgård, 2000; Lehvävirta and Rita, 2002; Malmivaara et al., 2002; Quigley, 2002; Stenhouse, 2004; Sukopp, 2004; Asnani et al., 2006; Nováková, 2008). Browsing by deer species in urban park forests of New Zealand, United States and Europe compound the losses of seedlings and saplings caused by human trampling (Conover and Kania, 1998; Peck and Stahl, 1997; Crampton et al., 1998; Coomes
∗ Corresponding author. Tel.: +1 814 5743036; fax: +1 814 8143754784. E-mail address: [email protected] (R.E. Loeb). 1618-8667/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ufug.2011.07.002
et al., 2003; Husheer et al., 2005; Perrin et al., 2006; MalmivaaraLämsä et al., 2008; Krueger et al., 2009; Mihók et al., 2009). Urban park forest managers have three management options to address the potential future forest losses by deer browsing and trampling by people: (1) permit the arboreal changes to occur (Millward et al., 2011); (2) remove the deer but not the people (Latham et al., 2005) and (3) remove the people but not the deer (Loeb et al., 2010). In the eastern United States, permitting the changes to occur results in pervasive damage to seedlings and saplings (Robertson and Robertson, 1995) and a wide breadth of species composition changes (Peck and Stahl, 1997; Rossell et al., 2007; Morgan, 2009). Extensive trampling by people when Odocoileus virginianus (mammal nomenclature follows Kays and Wilson, 2009) are absent in urban park forests of the eastern United States was found to result in Prunus serotina Ehrh. and Quercus spp. L. (plant nomenclature follows Gleason and Cronquist, 1991) dominance among seedlings, saplings, and trees (Stalter and Kincaid, 2008). The third option of causing people to virtually cease trampling and permitting O. virginianus to browse has been proposed (Millward et al., 2011) but not examined. If deer brows-
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ing without human trampling does not effect changes in species composition or forest structure then the management of urban park forests can be designed to incorporate white-tailed deer while restricting human access in order to protect the forest over the long-term. The objective of this research is to identify changes in the trees, saplings, and seedlings of the five urban park forest communities of Radnor Lake State Natural Area (RLSNA), Nashville, TN (which strictly prohibits off-trail forest access by visitors) that are related to the long-term effects of browsing by O. virginianus. Methods Site description RLSNA is located in Nashville, TN, USA (36◦ 03 N, 86◦ 47 W) and encompasses approximately 473 ha including the 34 ha artificial lake. European land clearance began circa 1835 with agriculture occurring throughout the property, except on steep slopes. The demand for wood prior to the Battle of Nashville during the American Civil War (December 15–16, 1864) caused forest clearance in all of the RLSNA area. The core property forming RLSNA was acquired in 1912 to create a reservoir for the steam engine facility of the Louisville and Nashville Railroad. The lake was protected from poaching fish and the land was closed to the public as a private hunting area. Logging for Juglans nigra L., Populus spp. L., and Quercus alba L. in 1918 and for primarily Fraxinus spp. L. and Quercus spp. was done in 1950. Limited extent fires occurred in 1948 and 1954 and ice storms damaged many trees in 1957 and 1994 (Schibig, 1996). In 1973, RLSNA was the first Natural Area established under the Tennessee State Natural Areas Preservation Act. The Act prohibits human activities that are not conducive to wildlife observation or modify the natural state of the ecosystem. O. virginianus were overhunted in Tennessee and were not observed in RLSNA until 1980 following reintroduction of the species to Tennessee in the mid 1970s (Tennessee Wildlife Resources Agency, 1991). Population estimates for O. virginianus in RLSNA are not available. The forests of RLSNA are part of the Western Mesophytic Region described by Braun (1950). Windstorms and torrential rain induced landslides cause the loss of trees and form many canopy gaps in RLSNA (Loeb and King, in press). Carpenter et al. (1976) identified five forest communities in RLSNA: lakeshore, ravine, mesic slope, xeric slope, and ridge. The palatability of tree species for browsing mammals was classified as “high,” “medium” or “low” according to the Plants Database (Natural Resources Conservation Service, 2010). The soils underlying RLSNA are well drained silt loams. Lakeshore forests grow on a flood plain soil with a soil reaction pH range of 6.1–7.8. Ravine and mesic slope forests are found on soils that are underlain by phosphatic limestone and have a soil reaction pH range of 4.5–6.0. Ridge and xeric slope forests grow on cherty soils that overlay shale bedrock and have a soil reaction pH range of 3.6–5.5. The soils of both the mesic slope and xeric slope forests are thin, contain rock outcrops, and are on hillsides with greater than 20% slopes (North, 1981). Forest sampling Temporal coincidence studies of the effects of O. virginianus browsing on the arboreal composition of forest communities involve measurement of trees, saplings, and seedlings before and after the reintroduction of O. virginianus to forests (Russell et al., 2001). Unlike exclusion plots research which enables comparison of adjacent areas with O. virginianus browsing and no browsing, a tem-
poral coincidence study does not involve observation of browsing damage presence and absence. The advantage provided by temporal coincidence research is the examination of the effects of O. virginianus browsing in large areas and over long periods of time. When the goal of the research is to identify changes caused by the restoration of O. virginianus in urban forests without human trampling, then the ideal situation is to conduct the research in a site with several forest communities protected from human trampling before O. virginianus was reintroduced. Also, the site must have sample plots that were measured before O. virginianus restoration and measured again decades later for comparison. The changes in the seedlings, saplings, and trees can be associated with the browsing preferences of O. virginianus. However, the conclusions drawn from the comparison must consider both changing environmental conditions such as unusual drought and heat and biological conditions including diseases and canopy openings during the intervening period. Radnor Lake State Natural Area provides many components of the ideal conditions for a temporal coincidence study but what is not present limits the scope of the analysis. As noted above, an individual O. virginianus was first observed to have returned to RLSNA in 1980 following reintroduction of the species to Tennessee in the mid 1970s. However, no O. virginianus population estimates are available; therefore, relationships between densities for O. virginianus and trees, saplings, and seedlings cannot be determined. The research of Carpenter et al. (1976) on the five forest communities in RLSNA was performed in 1974 and the tagged trees plot of Eickmeier (1988) measurements began in 1976 and thusly permits comparison of forest composition before and after O. virginianus restoration. Although the tagged trees (dbh > 10.2 cm) were only in a 75 m by 75 m permanent plot (Eickmeier, 1988), the data enables comparison of tree species recruitment, death, and total population from archival data for 1976, 1986, and 1996 and a measurement in 2007 for one forest community, the mesic slope forest. Total population was the sum of the trees found in the plot. Recruitment was the sum of the new trees present in 1986, 1996, and 2007, while death was the total for the trees not found in 1986, 1996, and 2007 that were present in the respective previous year. To describe the five forest communities of RLSNA, Carpenter et al. (1976) measured trees with eight circular plots (7.93 m in diameter) in each of four forest samples per community but none of the original plot stakes could be found in 1994 (Schibig, 1996). The field data from the 1974 sample is not available (Schibig, personal communication), which limits species level and forest variability analysis because only tree importance values are available for 1974 at the species level. In 1994, Schibig placed forest sample sites in the area of the 1974 sites (Schibig, 1996). The sampled forests distribution map available in the Schibig (1996) study report also represents the location of the forests sampled in 2007 (mesic slope; Eickmeier’s tagged tree plot is adjacent to one of Schibig’s mesic slope plots) and 2008 (lakeshore, ravine, ridge, and xeric slope). Schibig placed stakes marking the center of the circular plots (8.025 m in radius), which were distributed five per forest and four forests were measured for each of the five communities. Tree and sapling resampling in 2007 and 2008 was done with five, 8 m radius circular plots. Schibig sampled seedlings in 0.8025 m radius circular plots at the center of the larger plots for sampling trees and saplings; which was repeated in 2007 and 2008 except the radius was 0.8 m. In virtually all of the forests only some of the stakes were found and the location of the missing stakes were interpolated; however, interpolation was not possible for one mesic slope forest where none of the stakes were found and no measurements were done. All of the forest sample sites were located > 50 m from the border of RLSNA except for two which are discussed in the results. Stems per ha were reported for trees, saplings,
R.E. Loeb et al. / Urban Forestry & Urban Greening 10 (2011) 305–310
Stems Per Hectare Saplings
Trees
1500
1200
Stems Per Hectare
and seedlings by community. Importance values (total of relative density, relative dominance, and relative frequency = 300) were given for tree species. The research of Carpenter et al. (1976) and Schibig (1996) were not total inventories of the forest communities in RLSNA. The area resampled represents less than 1% of RLSNA. Also, the researchers did not report whether the placement of plots provided a representative sample of the forest communities or provided measurements of species distribution variability in the forests. No effort was made to determine if the samples were representative at the time of the recent sampling because no alternative sites were available for the comparison. If the forest sample sites could have been consistently identified in 2007–2008, then at least the comparison of results would represent changes in particular sections of RLSNA. Instead of clear measurements of changes in arboreal composition through time, the differences in results among years also represent variation in species composition in space and time. In light of this limitation, the significance of changes in importance values for trees and saplings of major (>30 importance value in at least one forest community) species was determined using the Chi-square test by comparing the importance value of a species to all of the other species. The Chi-square test is appropriate to account for confounding temporal and spatial differences when the location of the historical sample is unknown. The expected values are generated by comparing among spatially separated contemporaneous samples and the observed values are generated by comparing the modern versus historical sampling results (Loeb, 1990). The Chisquare tests were performed using PASW Statistics (formerly SPSS Statistics) version 17 and the significance level selected was 0.01.
307
900
600
300
0
'74 '94 '08
'74 '94 '08
'74 '94 '07
Lakeshore
Ravine
Mesic Slope
'74 '94 '08
Xeric Slope
'74 '94 '08
Ridge
Forests Fig. 1. Stems per ha for trees and saplings in the five forest communities of Radnor Lake State Natural Area.
Results Tagged trees plot In the mesic slope forest with tagged trees, all of the species had a decline in total population from 1976 to 2007 (Table 1) except for A. saccharum (medium palatable), P. serotina (low palatable), and Liriodendron tulipifera L. (low palatable). The one L. tulipifera was recruited between 1976 and 1986 and the two P. serotina trees remained throughout the 31-year period. The A. saccharum population rose from 87 in 1976 to 107 in 1996 but declined to 98 in 2007. Considering recruitment, the total loss of trees from 1976 to 2007 was 36 for Quercus spp. (all species are medium palatable except Quercus prinus L. which is low palatable) and 46 for A. saccharum. For all of the species, the trees lost over the 31 years were 63% recruits (10.2 cm > dbh > 20 cm) and 37% canopy trees (dbh ≥ 20 cm). Acer saccharum, Celtis occidentalis, and Quercus rubra losses were close to 70% recruits. Among the three Carya species, C. cordiformis losses were all recruits while for both C. glabra and C. ovata each stem lost was a tree. Aesculus flava, Ailanthus altissima, J. nigra, Q. prinus, Sassafras albidum, Tilia americana, and Ulmus americana had nearly equal numbers of tree and recruit losses. Five forest communities Tree stems per ha decreased from 1974 to 2007 or 2008 in all five communities and the largest decrease occurred for the ravine forests from 1974 to 1994. The decline in sapling stems per ha from 1994 to 2007 or 2008 was greater than the respective tree stems per ha decrease for each community. Sapling stems per ha was greater than tree stems per ha for all of the communities in 1994; however, the reverse was found in 2007 and 2008 except for the ravine forests (Fig. 1). The lakeshore forests having the greatest decrease in sapling and tree stems per ha from 1994 to 2008 should be discounted because the lake level was raised during the late 1990s and
was only recently lowered, which resulted in the loss of saplings and trees from part of each lakeshore forest plot. The only species with increased tree and sapling stems per ha in all five communities was A. saccharum. In 1994, the total stems per ha for seedlings in the lakeshore, ravine, mesic slope, xeric slope, and ridge forests were 5800, 12,350, 12,800, 8050, and 14,000, respectively. The total density for seedlings in the mesic slope forest in 2007–2008 was 517 stems per ha. There were 457 stems per ha for A. flava Aiton. (low palatable; 1050 stems per ha in 1994) and 60 for Acer rubrum L. (medium palatable) and A. saccharum (all Acer spp. L. were 1100 stems per ha in 1994) in mesic slope forest. Seedlings were not found in the seedling plots for the remaining four communities in 2008. Of the 16 major tree species (>30 importance value in at least one forest for either 1974 or 2007–2008) in Table 2 only four were medium palatable (A. saccharum, Fraxinus americana L., Populus deltoides Bartram ex Marsh., and Q. rubra L.). F. americana L. and Q. prinus had inconsistent changes with a significant importance value increase in at least one forest community and a significant importance value decrease in another community (Chi-square test of significance results are presented in Table 2). The three species with only significant decreases in importance value were P. deltoides, Salix nigra Marsh. (low palatable), and U. americana L. (low palatable). A. saccharum had significant increases in all five forests while Acer saccharinum L. (low palatable) had a significant increase in the Lake forest, which was the only community in which the species was found. A. flava trees attained an importance value of 33.6 (no saplings were present) for the xeric slope community in 2008 but the trees were present in only one forest sample located on the border of RLSNA, which was disturbed by residential construction and plot placement was based on interpolation from just two adjacent stakes. The A. flava seedlings noted above occurred only in one mesic slope forest sample and no evidence of disturbance was
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Table 1 Tree species recruitment and death, during the period 1976–2007 and total population in 1976, 1986, 1996 and 2007 for a mesic slope forest, Radnor Lake Natural Area, Nashville, TN. Recruitment and death are based on comparisons from results in 1976, 1986, 1996 and 2007. Palatability for browsing species is rated as low or medium following information from the Plants Database of the Natural Resources Conservation Service (2010). Species
Acer saccharum Marsh. Aesculus flava Aiton. Ailanthus altissima (Miller.) Swingle. Carya cordiformis (Wangenh.) K. Koch. Carya glabra (Miller) Sweet. Carya ovata (Miller) K. Koch. Celtis occidentalis L. Cornus florida L. Fraxinus americana L. Juglans nigra L. Liriodendron tulipifera L. Prunus serotina Ehrh. Quercus muehlenbergii Englen. Quercus prinus L. Quercus rubra L. Sassafras albidum (Nutt.) Nees. Tilia americana L. Ulmus americana L. Ulmus rubra Muhl.
Palatability
1976–2007
Medium Low Medium Low Low Low Medium Medium Medium Low Low Low Medium Low Medium Medium Medium Low Low
Total
Recruitment
Death
1976
1986
1996
2007
57 1 0 0 0 1 4 0 2 0 1 0 0 0 0 0 2 3 0
46 2 7 9 6 7 10 0 13 2 0 0 3 6 19 11 6 12 1
87 3 8 9 11 16 14 2 28 3 0 2 8 8 31 18 9 12 2
101 2 3 1 9 12 10 1 27 2 1 2 7 4 20 13 7 6 2
107 3 2 0 8 11 9 1 22 1 1 2 5 3 17 9 5 5 2
98 2 1 0 5 10 8 0 17 1 1 2 5 2 12 7 5 3 1
observed. The second forest sample located close to the border of RLSNA is a ridge forest and is remarkable in being the only forest sample that contains the invasive shrub Lonicera macckii (Rupr.) Maxim., which has been present in RLSNA for more than 30 years (Loeb et al., 2010).
rum was discovered to be a second choice to other browse species in mixed mesophytic forests (Strole and Anderson, 1992), which includes the five forest communities of RLSNA. Although the first O. virginianus returned to RLNSA in 1980, an O. virginianus population takes years to grow to a sufficient size to over browse a forest and cause the dramatic reductions in seedling and sapling stems per ha found in RLSNA. The population increase for A. saccharum trees in the tagged tree plot through 1996 and the decline found in 2007 indicates over browsing of A. saccharum occurred after 1996. Explanation of the significant changes in the forest canopy for the remaining species in Table 1 should focus on the whether the trees are classified as medium or low palatable because this would be related to the survival of saplings to grow into the canopy. However, the classification system fails to inform which species have had significant increases. For examples, F. americana is a medium palatable species but had significant increases in the mesic slope and xeric slope forests and Carya glabra is a low palatable species which had a significant decrease in the ridge forest. Perhaps a clas-
Discussion The long-term forest changes in RLSNA do not include the large increases for P. serotina and Quercus spp. that typify urban park forests with high levels of unrestricted pedestrian traffic but no O. virginianus population. The one consistent long-term change across the five forest communities in RLSNA was significant increased importance values for A. saccharum trees and saplings, which is not reported for urban park forests with extensive human trampling and no O. virginianus browsing (Fitzgerald and Loeb, 2008). A. saccharum is rated as a medium palatable species for browsing mammals, but the specific preference of O. virginianus for A. saccha-
Table 2 Importance values for tree species in the five forest communities of Radnor Lake State Natural Area, Nashville, TN for major species (species had an importance value >30 in at least one forest community) in 1974 and 2008. Palatability for browsing species is rated as low or medium following information from the Plants Database of the Natural Resources Conservation Service (2010). Taxa
Palatability
Forests Ridge 1974
Acer negundo L. Acer saccharum Marsh. Acer saccharinum L. Aesculus flava Aiton. Carya cordiformis (Wangenh.) K. Koch Carya glabra (Miller) Sweet. Carya ovata (Miller) K. Koch. Celtis laevigata Willd. Fraxinus americana L. Fraxinus quadrangulata Michx. Populus deltoides Bartram ex Marsh. Quercus prinus L. Quercus rubra L. Quercus shumardii Buckley. Salix nigra Marsh. Ulmus americana L. *
Low Medium Low Low Low Low Low Low Medium Low Medium Low Medium Low Low Low
0 0 0 0 0 46 17 4 32 49 0 74 0 0 0 0
Xeric slope 2008 0 24* 0 0 0 12* 37* 6 6* 47 0 110* 0 0 0 0
Mesic slope
Ravine
Lake
1974
2008
1974
2008
1974
2008
1974
2008
0 46 0 12 0 16 21 5 12 26 0 69 27 0 0 0
0 95* 0 34* 0 14 23 0 28* 8* 0 25* 0* 0 0 0
0 73 0 12 32 0 13 0 15 0 0 0 36 7 0 7
0 119* 0 7 0* 0 0* 3 50* 0 0 0 44 13 0 0*
3 49 0 3 30 0 0 3 11 0 0 0 12 22 0 64
5 112* 0 8 36 0 0 0 7 0 0 0 5 35 0 4*
24 0 32 0 0 0 0 35 8 0 37 0 0 0 42 74
33 7* 112* 0 0 0 0 28 0* 0 0* 0 0 11* 3* 39*
Chi-square test significant difference between importance values for 1974 and 2008 at the 0.01 level.
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sification system based on only O. virginianus browsing preference would be better but O. virginianus has variable browsing preferences depending upon geographic region (Latham et al., 2005). To explain the conflicting significant changes among forest communities for Carya ovata, F. americana, and Q. prinus, O. virginianus browsing is a stress on the population, which can be overcome in the forests where the species is best adapted to grow and reproduce but causes losses in forests where the species is less well adapted to survive. Unaccounted for factors including the effects of other herbivores as well as diseases and unnoticed infestations could possibly provide explanations for some of the changes in particular species. For example, canopy closure is related to a shift in arboreal reproduction to predominantly shade tolerant species such as A. saccharum, especially when fire is rare as is the situation in RLSNA. Although canopy gaps are created by windstorms and landslides in the forests, the increases for A. saccharum in the tagged trees plot from 1976 to 1996 and the five forest communities from 1974 to 2008 suggest a transition to lower light levels reaching the forest floor started prior to 1974. However, the virtual loss of A. saccharum seedlings after 1994 and the decline for trees in the tagged trees plot since 1996 indicates increased shading has been supplanted by another factor—O. virginianus overbrowsing. For two species, P. deltoides and S. nigra, the trees were cut down by three beavers (Castor canadensis Kuhl.) which became residents at RLSNA in 2001. A. flava remained present among the seedlings, possibly because O. virginianus avoids browsing the species (Latham et al., 2005; Asnani et al., 2006) but a significant increase in the canopy only occurred because of a change in one xeric forest sample. The decrease in U. americana is related to Dutch elm disease (Ophiostoma ulmi (Buisman) Nannf.) which has devastated the species in other urban park forests (Fitzgerald and Loeb, 2008; Loeb, 2010). Tree stems per ha being higher than sapling stems per ha in 2007 and 2008 indicates reduced recruitment to the canopy is possible. The relative absence of seedlings in 2007 and 2008 as compared to 1994 and the decline in sapling stems per ha from 1994 to 2007 and 2008 also indicate the possibility of fewer new trees in the canopy. Climatic variation cannot explain declines in tree recruitment at RLSNA. Considerably lower mean annual rainfall and higher mean annual temperatures in 1994 through 2007 as compared to 1974 through 1993 is not an explanation for the seedling and sapling stems per ha decreases since 1994. Specifically, mean annual precipitation for 1974 through 1993 was 122.0 cm (28.7 cm standard deviation) versus 128.7 cm (19.7 cm standard deviation) for 1994 through 2007 (National Weather Service, 2009). The mean yearly temperature for 1974 through 1993 was 15.2 ◦ C (0.7 ◦ C standard deviation) while for 1994 through 2007 mean yearly temperature was 15.6 ◦ C (0.7 ◦ C standard deviation). Many hot summers with drought also do not appear to be a reason for the losses since the mean monthly summer temperature was only 0.2 ◦ C less during 1974 through 1993 than the 25.8 ◦ C in 1994 through 2007 (0.9 ◦ C standard deviation for both periods). Average monthly summer precipitation was 3.4 cm (1.8 cm standard deviation) in 1974 through 1993 as versus 3.8 cm (2.0 cm standard deviation) in 1994 through 2007. Furthermore, differences in cold and dryness in the winters are not an apparent cause of the losses since from 1974 through 1993 mean monthly temperature was 4.2 ◦ C (3.2 ◦ C standard deviation) and mean monthly precipitation was 10.3 cm (6.3 cm standard deviation) while for 1994 through 2007 the mean monthly temperature was 5.1 ◦ C (4.0 ◦ C standard deviation) and mean monthly precipitation was 10.4 cm (2.7 cm standard deviation). RLSNA will continue to experience tree deaths as occurs in other urban park forests with human trampling. The decimation of the seedling layer by O. virginianus suggests greater losses for all species
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are likely to occur in the future, except perhaps for A. saccharum and A. flava. The lower stems per ha for saplings in 2007–2008 as compared to 1994 points to the importance of how O. virginianus has affected saplings directly by browsing and indirectly through seedling losses. The combination of no human trampling and O. virginianus browsing at RLSNA results in selective advancement of arboreal species that O. virginianus does not prefer to browse and species that are well adapted to grow and reproduce in the environmental conditions of the particular forest. In contrast, urban park forests with human trampling but no O. virginianus have representation in the seedling and sapling size classes in areas where people do not walk (Lehvävirta and Rita, 2002). However, when the O. virginianus population is large enough browsing becomes over browsing and all seedlings if not also many saplings can be lost because deer browsing is not spatially limited, unlike human trampling. When O. virginianus over browsing is combined with human trampling, the result is small populations of tree species avoided by O. virginianus as observed by Robertson and Robertson (1995), Peck and Stahl (1997), and Rossell et al. (2007). The declines in tree stems per ha across the five forest communities of RLSNA reflect what has occurred in other urban park forests (Zipperer and Zipperer, 1992; Peck and Stahl, 1997; Quigley, 2002; Loeb, 2010) but unlike the other urban park forests, RLSNA has not and will not have landscape plantings done to replace the lost trees (barring a change in state law). The following management recommendations for increasing tree recruitment and diversity are analyzed in regard to RLSNA but are applicable to other similar settings. The direct management response of restricted O. virginianus hunting as has been done in other urban parks (Rossell et al., 2007) is not a viable choice for RLSNA because the Tennessee Natural Areas Act prohibits actions against native species. Hunting is not supported by many urban park forest patrons because viewing animals is an important part of the visit experience. An O. virginianus population drop should not be expected in the future because private homes with extensive landscaped gardens along the open boundaries of RLSNA supplement the O. virginianus food resources (Swihart et al., 1995). The prospect of canopy density decline can be addressed by increasing the population of rare native trees such as Fagus grandifolia Ehrh. which is not preferred by O. virginianus (Tubbs and Houston, 2010). Also introducing native shrubs which grow densely enough to protect seedlings and saplings from O. virginianus browsing is another remedy (Cipollini et al., 2009). Alternative methods which deter O. virginianus browsing and increase soil fertility (Odin et al., 2005) while meeting the provisions of natural areas laws also should be tested. Conclusions Tree losses occur in RLSNA as has been reported for other urban park forests. However, the seedling and sapling stems per ha decreases since 1994 indicate over browsing by O. virginianus has shaped long-term arboreal composition changes. A. saccharum is becoming the dominant tree because O. virginianus has a preference against browsing the species. A. flava may become more frequent among the saplings and trees of the forest communities because of O. virginianus browsing avoidance. Canopy species representation in forest communities will decrease except for the community which fits the ecological characteristics of the tree species. Finally, the prospect of less canopy recruitment in Radnor Lake State Natural Area can lead to research into expanding the population of native species that O. virginianus avoids browsing. Acknowledgements We gratefully acknowledge the provision of archival data, permission to use archival data, and advice given by William Eickmeier
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