PUBLISHED: DECEMBER 2005

The Changing Flora of the New York Metropolitan Region

by Steven E. Clemants and Gerry Moore

Brooklyn Botanic Garden, 1000 Washington Avenue, Brooklyn, NY 11225

Abstract

We statistically analyzed 100 years of herbarium specimen data for woody plants in the New York metropolitan region in order to measure the floristic changes of this area. Change index values were computed for 224 of the region's 556 woody species to provide a specific measure of whether these species are expanding, contracting, or stable. The results show that, in general, nonnative invasive species are spreading rapidly in the region, while native species are in slight decline.

Key words: Chimaphila, ecological change; Ericaceae; herbarium; invasive plants; Lonicera; New York City, urban flora

Introduction

Plant species differ in their ability to adapt to environmental changes brought on by urban development and spread. Yet there are few studies that attempt to quantify the differences in adaptability among species (but see, for example, Dickson et al., 2000). In this study, we use current and historical data on woody plants in the New York metropolitan region to develop a change index measuring the relative degree to which species have expanded or contracted their ranges over the past century. The findings help us gain a better understanding of exactly how the flora of this urban region is changing and should prove useful to those attempting to improve and restore the ecosystems of the region.

It is difficult to quantify changes in the flora of the New York metropolitan region because the region, like other urban areas in the United States, has not been subjected to any long-term plant studies using standard sampling methods. In our study, we used herbarium specimen data from about a dozen herbaria in the northeastern United States. Botanists do not use a standard sampling method when collecting herbarium specimens: Some collect every plant they see, while others collect only the plants they are studying or those that are of particular interest at a site. But although there are a variety of sampling strategies, the strategies themselves have not changed significantly over the past century, and the data should be adequate for carrying out a comparison of the relative changes in the ranges of species.

Although our technique only analyzes the change in range of a species, it has been shown that there is a relationship between range and abundance of species (Hanski, Kouki & Halkka, 1993; He, Gaston & Wu, 2002). Therefore, an expanding range for a species is a good indication that the species may be increasing in abundance. Likewise, a range contraction is an indicator that a species may be declining in abundance.

Methods

This study is comparable to a study done for plants in Great Britain. We have predominantly used techniques developed by Telfer, Preston, and Rothery (2002), with a few modifications, spelled out in detail here.

The distributional data comes from the New York Metropolitan Flora (NYMF) project database (Moore, Steward, Clemants, Glenn & Ma, 2002; and see http://www.bbg.org/sci/nymf/). This database currently has nearly 250,000 records of plant occurrences from the New York metropolitan region. Each record is geo-coded to five-kilometer-square cells in a grid, with 964 cells total. We will call these cells "blocks." (The names used in this study are those adopted by the NYMF project; see Moore et al., 2002.)

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Number of unique specimens of woody species collected over the past century.

Figure 1. Number of unique specimens of woody species collected over the past century.

In this study, we used the woody-species data from the NYMF database. The woody-plant data set is the most complete one in the database and has over 145,000 records, representing 556 species. In our analysis, we only used records of woody species based upon herbarium specimens collected between 1901 and 2000. Once we narrowed the data to meet this criterion and eliminated duplicate records, there were 24,795 records remaining for this study. These records were made relatively evenly over the first half of the 20th century, but for the second half of the century, the bulk of the data is from the last decade (the 1990s), when the NYMF project began actively collecting (Figure 1).

The data were partitioned into two cohorts (time periods): the early cohort, containing data from 1901 to 1950, and the later cohort, containing data from 1951 to 2000. Following Telfer, Preston, and Rothery (2002), we only included blocks for which there were occurrences of a species in both cohorts. This reduced the number of blocks used in the analysis to 647. These 647 blocks are distributed throughout the New York metropolitan region (Figure 2). The Telfer, Preston, and Rothery study excluded species with fewer than five occurrences in the early cohort. In our study, we modified the procedure by excluding species with fewer than five occurrences in either the early or late cohort. This reduced the number of species in our study to 224.

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Distribution of blocks used in this study.

Figure 2. Distribution of blocks used in this study.

The statistical methods for developing the change index are outlined in Telfer, Preston, and Rothery (2002). All statistics were calculated using Systat 10.2 statistical software (SPSS, 2000).

Results and Discussion

Table 1 lists the 224 species studied in this analysis, the raw sampling block counts for each cohort, species provenance (native or introduced), and the change index. Please note that the raw counts for some species show an increase over time, while their change indices show a decrease. This is because there are many more records in the later period (from the 1990s). The statistic essentially corrects for this overabundance of data. This means that a species showing no change in distribution will have a larger raw count in the later period than the earlier, and that some species may show a decrease in distribution while showing an increase in the raw count.

The first, unweighted least-squares regression equation was y = –1.05 + 0.66x, with r2 = 0.444. Following two iterations of the weighing procedure, we arrived at a weighted regression equation of y = –1.00 + 0.68x, with r2 = 0.467. We believe that the relatively low r2 is the result of two things. First, unlike in Telfer, Preston, and Rothery (2002), our data were not collected following a uniform procedure. Therefore, we suspect that there is greater statistical error in the data. Second, we believe we are studying a much more rapidly changing flora (an urban flora) than the one in the studies used by Telfer, Preston, and Rothery (a country-wide flora). Therefore, we would expect larger change indices in general.

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A dual histogram of the change indices for introduced (nonnative) and native species. These graphs show the distribution of change index values for the 226 species studied.

Figure 3. A dual histogram of the change indices for introduced (nonnative) and native species. These graphs show the distribution of change index values for the 224 species studied.

Figure 3 shows the distribution of change indices in relation to the provenance of the plant species. Because the data for natives are right-skewed, we used a Mann-Whitney U test to determine if the native and nonnative (introduced) species data are significantly different. The Mann-Whitney test statistic was 5054, which is significant (p = 0.014). This indicates that the nonnative species are increasing relative to the native species. In general, native species are showing slight decline, and introduced species are showing much greater expansion of their ranges, with only a few species showing any decline.

The change index in this study is valuable because it provides species-specific information about what is changing in the flora. For instance, nearly all the members of the heath family (Ericaceae) in the region are showing contraction of their ranges. There are probably many reasons why these species appear sensitive to urbanization, but three stand out: 1) most heath family species are acidophilic (Kron & Chase, 1993), and urban soils are generally more basic (Craul, 1992; Scheyer & Hipple, 2005); 2) many Ericaceae species are hydrophytes, and much wetland habitat has been lost over the past century (e.g., New Jersey lost an estimated 39% of its wetlands between 1870 and 1970, with half that loss occurring between 1950 and 1970; see New Jersey Sustainable State Institute, 2004); 3) the overabundance of white-tailed deer (Odocoileus virginianus) in suburban regions may impact some species through overgrazing (Department of Environmental Protection, Division of Fish, Game and Wildlife, 1999), though we expect this impact would be broad across many taxa.

The results show that several congeneric species have very different change indices. For example, Celastrus scandens, the native American bittersweet, has a change index of –1.15, while Celastrus orbiculata, the nonnative Oriental bittersweet, has a change index of +3.24. This wide disparity—indicative of a dramatic decline for the American bittersweet and a dramatic spread by the Oriental bittersweet—reinforces the results of a previously published account of these two species (Steward, Clemants & Moore, 2003).

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Figure 4

Figure 4. Range map of Lonicera sempervirens for the New York metropolitan area. (Native, Change Index = –1.93)

Figure 5

Figure 5. Range map of Lonicera japonica for the New York metropolitan area. (Introduced, Change Index = +1.60)

Figure 6

Figure 6. Range map of Lonicera dioica for the New York metropolitan area.

Figure 7

Figure 7. Range map of Lonicera morrowii for the New York metropolitan area.

Figure

Figure 8. Range map of Lonicera maackiii for the New York metropolitan area.

Figure 9

Figure 9. Range map of Chimaphila umbellata for the New York metropolitan area.

Figure 10

Figure 10. Range map of Chimaphila maculata for the New York metropolitan area.

Nonnative honeysuckles are significantly increasing, while native species are undergoing significant decline. The native Lonicera dioica and L. sempervirens have change indices of –2.87 and –1.93, respectively, and the nonnative L. japonica and L. morrowii have change indices of +1.60 and +1.73, respectively (see Figures 4–7). (In the case of L. japonica and L. sempervirens, the nonnative's growth architecture may be giving it a competitive advantage over its native congener and allowing it to increase its range; see Schweitzer & Larson, 1990; Larson, 2000). Another nonnative species, L. maackii, not included in this study because of its more recent date of introduction (and thus lack of any pre-1950 records), is also rapidly spreading in the region (Figure 8).

Other native-nonnative congeneric species groups also reflect this pattern, such as the following (change index in parentheses): nonnative Clematis terniflora (+1.33), native C. virginiana (–0.32); nonnative Morus alba (+2.41), native M. rubra (–1.71); nonnative Ribes rubrum (+0.28), native R. americanum (–0.41), native R. hirtellum (–1.92), and native R. rotundifolium (–0.54).

A striking pattern is observed for the New York metropolitan region's two native Chimaphila species (which are not being impacted by non-native congeners), with C. umbellata having a change index of –2.51 and C. maculata having a change index of –0.29 (Figures 9 and 10). While there have not been any studies aimed at better understanding why C. umbellata is declining at a greater rate than C. maculata, field botanists have hypothesized that C. umbellata may be more significantly affected by deer browsing than C. maculata, perhaps as a result of differences in leaf chemistry between the two species (Lamont & Young, 2004). Cowan (1945) reported that C. umbellata was casually eaten by deer.

Conclusion

Without question, the flora of the New York metropolitan region is rapidly changing. Most notably, nonnative invasive species are rapidly spreading in the area, while native species are generally in decline. Monitoring programs such as the NYMF project provide a mechanism by which these changes can be quantitatively measured. They may, in the future, be used to identify potentially invasive species before these species spread throughout the range. Also, these programs provide baseline data that future generations can use in comparative analysis to track floristic change.

Literature Cited

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Department of Environmental Protection Division of Fish, Game, and Wildlife (1999). Governor's report on deer management in New Jersey. Retrieved June 21, 2005, from the New Jersey Department of Environmental Protection website: http://www.state.nj.us/dep/fgw/derrpt99.htm.

Craul, P.J. (1992). Urban soil in landscape design. New York: J. Wiley & Sons.

Dickson, J.H., Macpherson, P., Watson, K.J. & Tait, N. (2000). The changing flora of Glasgow: urban and rural plants through the centuries. Edinburgh: Edinburgh University Press.

Hanski, I., Kouki, J. & Halkka, A. (1993). Three explanations of the positive relationship between distribution and abundance of species. In Ricklefs, R.E & Schluter, D. (Eds.), Species diversity in ecological communities (pp. 108–116). Chicago: University of Chicago Press.

He, F., Gaston, K.J. & Wu, J. (2002). On species occupancy–abundance models. Ecoscience, 9(1), 119–126.

Kron, K.A. & Chase, M.W. (1993). Systematics of the Ericaceae, Empetraceae, Epacridaceae, and related taxa based upon rbcL sequence data. Annals of the Missouri Botanical Garden, 80(3), 735–741.

Lamont, E.E. & Young, S.M. (2004). Noteworthy plants reported from the Torrey range–2002 and 2003. Journal of the Torrey Botanical Society, 131(4), 394–402.

Larson, K.C. (2000). Circumnutation behavior of an exotic honeysuckle vine and its native congener: influence on clonal mobility. American Journal of Botany, 87, 533–538.

Moore, G., Steward, A., Clemants, S.E., Glenn, S. & Ma, J. (2002). An overview of the New York Metropolitan Flora project. Urban Habitats, 1(1), 17–24. Retrieved June 21, 2005, from the Urban Habitats website: http://www.urbanhabitats.org/v01n01/nymf_full.html.

New Jersey Sustainable State Institute (2004). Living with the future in mind. New Brunswick, New Jersey: New Jersey Sustainable State Institute (Third ed.). Retrieved June 21, 2005, from the New Jersey Sustainable State Institute website: http://www.njssi.net/gi/.

Scheyer, J.M. & Hipple, K.W. (2005). Urban soil primer. Lincoln, Nebraska: United States Department of Agriculture, Natural Resources Conservation Service, National Soil Survey Center. Retrieved June 21, 2005, from the Nature Resources Conservation Service website: http://soils.usda.gov/use.

Schweitzer, J.A. & Larson, K.C. (1999). Greater morphological plasticity of exotic honeysuckle species may make them better invaders than native species. Journal of the Torrey Botanical Society, 126(1), 15–23.

SPSS, Inc. (2000). Systat for Windows, Version 10.2 [Computer Software]. Chicago: SPSS Inc.

Steward, A.M., Clemants, S.E. & Moore, G. (2003). The concurrent decline of the native Celastrus scandens and spread of the non–native Celastrus orbiculatus in the New York City metropolitan area. Journal of the Torrey Botanical Society, 130(2), 143–146.

Telfer, M.G., Preston, C.D. & Rothery, P. (2002). A general method for measuring relative change in range size from biological atlas data. Biological Conservation, 107(1), 99–109.

Table 1. The change index for each species in the study along with the raw data and the provenance of each species. (Names follow Moore et al., 2002.)

Species name Provenance 1901-1950 raw count 1951-2000 raw count Change Index
Acer negundo Native 22 65 1.86
Acer pensylvanicum Native 18 23 0.20
Acer platanoides Introduced 22 58 1.64
Acer pseudoplatanus Introduced 13 23 0.57
Acer rubrum Native 93 117 1.26
Acer saccharinum Native 22 39 0.91
Acer saccharum Native 45 69 1.12
Acer spicatum Native 26 11 -1.48
Aesculus hippocastanum Introduced 6 8 -0.37
Ailanthus altissima Introduced 16 54 1.88
Akebia quinata Introduced 6 6 -0.84
Alnus incana Native 17 20 0.03
Alnus serrulata Native 85 67 0.26
Amelanchier arborea Native 29 43 0.76
Amelanchier canadensis Native 47 90 1.59
Amelanchier stolonifera Native 15 16 -0.22
Amorpha fruticosa Native 22 37 0.81
Ampelopsis brevipedunculata Introduced 8 40 2.12
Aralia spinosa Introduced 6 34 2.14
Arctostaphylos uva-ursi Native 43 13 -1.81
Aronia arbutifolia Native 71 59 0.25
Aronia melanocarpa Native 39 21 -0.86
Baccharis halimifolia Native 37 39 0.30
Berberis thunbergii Introduced 25 65 1.71
Berberis vulgaris Introduced 17 12 -0.84
Betula alleghaniensis Native 34 21 -0.69
Betula lenta Native 69 64 0.44
Betula nigra Native 30 21 -0.55
Betula papyrifera Native 12 7 -1.35
Betula populifolia Native 82 74 0.50
Broussonetia papyrifera Introduced 15 10 -1.01
Campsis radicans Introduced 10 15 0.13
Carpinus caroliniana Native 53 66 0.83
Carya cordiformis Native 20 40 1.06
Carya glabra Native 42 46 0.44
Carya ovalis Native 15 12 -0.70
Carya ovata Native 28 43 0.80
Carya tomentosa Native 50 51 0.42
Castanea dentata Native 63 50 0.10
Catalpa bignonioides Introduced 10 24 0.94
Ceanothus americanus Native 61 25 -1.10
Celastrus orbiculata Introduced 8 71 3.24
Celastrus scandens Native 81 30 -1.15
Celtis occidentalis Native 68 56 0.21
Cephalanthus occidentalis Native 53 63 0.74
Chamaecyparis thyoides Native 27 20 -0.51
Chamaedaphne calyculata Native 49 23 -0.98
Chimaphila maculata Native 107 59 -0.29
Chimaphila umbellata Native 39 8 -2.51
Clematis terniflora Introduced 8 26 1.33
Clematis virginiana Native 36 27 -0.32
Clethra alnifolia Native 101 63 -0.08
Comptonia peregrina Native 63 46 -0.05
Cornus alternifolia Native 41 34 -0.07
Cornus amomum Native 75 75 0.64
Cornus florida Native 87 83 0.65
Cornus foemina Native 77 64 0.30
Cornus rugosa Native 31 18 -0.85
Cornus sericea Native 12 21 0.50
Corylus americana Native 60 56 0.37
Corylus cornuta Native 21 16 -0.60
Crataegus crusgalli Native 17 14 -0.58
Crataegus pruinosa Native 21 13 -0.95
Diervilla lonicera Native 38 19 -1.00
Diospyros virginiana Native 20 16 -0.54
Dirca palustris Native 8 6 -1.15
Elaeagnus umbellata Introduced 12 53 2.18
Epigaea repens Native 67 26 -1.16
Euonymus europaea Introduced 19 12 -0.97
Fagus grandifolia Native 42 71 1.26
Fraxinus americana Native 50 63 0.82
Fraxinus nigra Native 21 27 0.31
Fraxinus pennsylvanica Native 45 46 0.36
Gaultheria procumbens Native 41 24 -0.69
Gaylussacia baccata Native 102 65 -0.04
Gaylussacia frondosa Native 59 28 -0.86
Hamamelis virginiana Native 66 73 0.75
Hibiscus syriacus Introduced 7 10 -0.16
Hudsonia ericoides Native 30 8 -2.19
Hudsonia tomentosa Native 60 26 -1.01
Hydrangea arborescens Native 16 8 -1.45
Ilex glabra Native 32 15 -1.20
Ilex laevigata Native 24 17 -0.65
Ilex opaca Native 16 26 0.55
Ilex verticillata Native 78 69 0.43
Iva frutescens Native 34 33 0.10
Juglans cinerea Native 21 23 0.03
Juglans nigra Native 21 47 1.30
Juniperus communis Native 19 10 -1.28
Juniperus virginiana Native 74 57 0.14
Kalmia angustifolia Native 64 35 -0.57
Kalmia latifolia Native 67 49 -0.02
Larix laricina Native 14 11 -0.77
Leucothoe racemosa Native 76 39 -0.59
Ligustrum vulgare Introduced 13 14 -0.28
Lindera benzoin Native 73 97 1.18
Liquidambar styraciflua Native 42 35 -0.05
Liriodendron tulipifera Native 32 61 1.29
Lonicera dioica Native 35 6 -2.87
Lonicera japonica Introduced 33 73 1.60
Lonicera morrowii Introduced 14 77 2.73
Lonicera sempervirens Native 20 7 -1.93
Lycium barbarum Introduced 13 10 -0.85
Lyonia ligustrina Native 104 54 -0.41
Lyonia mariana Native 68 33 -0.75
Magnolia virginiana Native 18 16 -0.42
Malus coronaria Native 8 8 -0.68
Malus pumila Introduced 13 22 0.49
Menispermum canadense Native 48 42 0.12
Morus alba Introduced 20 81 2.41
Morus rubra Native 20 8 -1.71
Myrica gale Native 34 13 -1.52
Myrica pensylvanica Native 112 63 -0.22
Nemopanthus mucronatus Native 25 10 -1.60
Nyssa sylvatica Native 62 75 0.88
Ostrya virginiana Native 46 39 0.03
Parthenocissus quinquefolia Native 58 84 1.19
Parthenocissus vitacea Native 7 7 -0.75
Paulownia tomentosa Introduced 8 18 0.69
Philadelphus coronarius Introduced 10 16 0.24
Physocarpus opulifolius Native 25 17 -0.70
Picea rubens Native 10 8 -0.92
Pinus echinata Native 8 6 -1.15
Pinus rigida Native 44 34 -0.16
Pinus strobus Native 33 33 0.13
Pinus virginiana Native 20 7 -1.93
Platanus occidentalis Native 13 32 1.16
Populus alba Introduced 11 13 -0.22
Populus deltoides Native 16 42 1.41
Populus grandidentata Native 73 54 0.05
Populus tremuloides Native 56 43 -0.03
Potentilla fruticosa Native 28 12 -1.42
Prunus avium Introduced 23 42 0.99
Prunus maritima Native 46 32 -0.32
Prunus pensylvanica Native 14 12 -0.62
Prunus pumila Native 18 9 -1.39
Prunus serotina Native 74 101 1.25
Prunus virginiana Native 39 31 -0.18
Ptelea trifoliata Native 8 10 -0.31
Pyrus communis Introduced 6 10 0.00
Quercus alba Native 57 77 1.04
Quercus bicolor Native 49 46 0.26
Quercus coccinea Native 40 49 0.62
Quercus ilicifolia Native 70 42 -0.35
Quercus marilandica Native 32 25 -0.32
Quercus montana Native 48 53 0.54
Quercus muhlenbergii Native 6 9 -0.17
Quercus palustris Native 31 53 1.07
Quercus phellos Native 11 15 0.02
Quercus prinoides Native 51 26 -0.81
Quercus rubra Native 50 78 1.23
Quercus stellata Native 36 26 -0.39
Quercus velutina Native 60 76 0.95
Rhamnus cathartica Introduced 16 22 0.26
Rhamnus frangula Introduced 10 32 1.45
Rhododendron maximum Native 28 28 0.04
Rhododendron periclymenoides Native 94 56 -0.21
Rhododendron viscosum Native 105 59 -0.26
Rhus copallinum Native 55 51 0.30
Rhus glabra Native 72 65 0.42
Rhus hirta Native 44 48 0.46
Ribes americanum Native 23 19 -0.41
Ribes hirtellum Native 24 8 -1.92
Ribes rotundifolium Native 20 16 -0.54
Ribes rubrum Introduced 18 24 0.28
Robinia hispida Introduced 10 12 -0.25
Robinia pseudo-acacia Introduced 21 60 1.76
Robinia viscosa Introduced 19 6 -2.13
Rosa carolina Native 105 58 -0.29
Rosa eglanteria Introduced 16 7 -1.67
Rosa multiflora Introduced 14 79 2.79
Rosa palustris Native 50 45 0.19
Rosa rugosa Introduced 11 14 -0.09
Rosa virginiana Native 38 16 -1.30
Rubus allegheniensis Native 67 43 -0.25
Rubus flagellaris Native 49 34 -0.29
Rubus hispidus Native 48 35 -0.21
Rubus laciniatus Introduced 13 15 -0.16
Rubus occidentalis Native 46 37 -0.06
Rubus odoratus Native 48 19 -1.29
Rubus pensilvanicus Native 34 29 -0.13
Rubus phoenicolasius Introduced 35 62 1.22
Salix alba Introduced 16 13 -0.64
Salix bebbiana Native 31 12 -1.54
Salix discolor Native 74 83 0.86
Salix eriocephala Native 43 41 0.21
Salix fragilis Introduced 12 11 -0.60
Salix humilis Native 84 19 -2.01
Salix nigra Native 42 63 1.03
Salix purpurea Introduced 15 10 -1.01
Salix sericea Native 54 24 -1.02
Sambucus canadensis Native 74 79 0.76
Sambucus racemosa Native 29 15 -1.08
Sassafras albidum Native 66 97 1.31
Smilax glauca Native 69 44 -0.25
Smilax rotundifolia Native 59 67 0.73
Solanum dulcamara Introduced 67 78 0.86
Spiraea alba Native 64 51 0.12
Spiraea tomentosa Native 63 33 -0.65
Staphylea trifolia Native 48 50 0.43
Symphoricarpos orbiculatus Introduced 9 9 -0.61
Tilia americana Native 35 57 1.06
Toxicodendron radicans Native 45 54 0.65
Toxicodendron vernix Native 30 31 0.13
Tsuga canadensis Native 34 40 0.44
Ulmus americana Native 36 59 1.09
Ulmus rubra Native 35 41 0.45
Vaccinium angustifolium Native 72 29 -1.05
Vaccinium corymbosum Native 159 87 -0.11
Vaccinium macrocarpon Native 66 21 -1.51
Vaccinium pallidum Native 104 70 0.08
Vaccinium stamineum Native 64 46 -0.07
Viburnum acerifolium Native 108 82 0.33
Viburnum dentatum Native 101 92 0.65
Viburnum lentago Native 38 39 0.26
Viburnum nudum Native 57 30 -0.70
Viburnum opulus Native 16 22 0.26
Viburnum prunifolium Native 74 85 0.90
Viburnum rafinesquianum Native 19 14 -0.71
Vitis aestivalis Native 82 66 0.28
Vitis labrusca Native 81 69 0.38
Vitis riparia Native 25 31 0.35
Vitis vulpina Introduced 19 17 -0.38
Zanthoxylum americanum Native 12 20 0.42