Journal of the Torrey Botanical Society 144(2): 125–138, 2017. Plant diversity and community composition in eastern North American serpentine barrens1 Kathryn M. Flinn2, Jennifer L. Mikes, and Hannah A. D. Kuhns Biology Department, Franklin and Marshall College P.O. Box 3003, Lancaster, PA 17604 Abstract. Serpentine barrens are globally rare, savanna habitats that support distinctive vegetation. In eastern North America, two major threats to serpentine barrens are the spread of invasive species and the encroachment of trees into open habitats. We assessed these threats by comparing serpentine savannas and woodlands in the State Line serpentine barrens, southeast Pennsylvania. We asked which invasive species were most frequent and abundant in each habitat and how they may affect plant diversity and community composition and how succession of savannas to woodlands may affect plant diversity and community composition. We compared species richness, evenness, diversity, and beta diversity between savannas and adjacent woodlands and quantified differences in community composition. Invasive species comprised 16% of total plant cover, and the most frequent invasive species were Microstegium vimineum (Trin.) A. Camus, Lonicera japonica Thunb., and Elaeagnus umbellata Thunb. Savannas had substantially greater species richness, diversity, and beta diversity than woodlands. Of all species we found, 31% were unique to savannas. The cover of invasive species and the native greenbrier Smilax rotundifolia L. were negatively associated with the cover of characteristic native grasses and forbs. Although invasive species threaten diversity in serpentine barrens, tree encroachment may pose the greater threat. Savannas that become woodlands lose diversity and change composition. Fighting tree encroachment and thickets of Smilax spp. is evidently necessary to maintain the distinctive vegetation of serpentine barrens. These habitats are worth protecting because they harbor many rare species, increase landscape-level beta diversity, and make a unique contribution to regional biological diversity. Key words: beta diversity, indicator species, rank-abundance, savanna, succession In forested landscapes, places that are too wet, naturally occurring, persistently open habitats too dry, or otherwise inimical to closed forests contribute disproportionately to regional diversity provide habitat for a flora that contrasts strongly (e.g., Flinn et al. 2008). Serpentine barrens in with surrounding vegetation, and they thus in- eastern North America are perhaps the best example of persistently open habitats in contrast- crease the landscape-level biological diversity ing matrices that support exceptional flora (Raja- more than their area might suggest (Anderson et karuna et al. 2009). al. 1999, Wright et al. 2002, Marks et al. 2008). Serpentine barrens are globally rare habitats that Because many parts of eastern North America occur on the unusual soils derived from ultramafic were largely forested for most of the Holocene, bedrock; these soils tend to be thin and nutrient poor, with low Ca:Mg ratios and high heavy metal 1 The authors thank Franklin and Marshall College for levels, especially nickel (Whittaker 1954, Walker supporting this project. We thank Tim Draude for help 1954, Harrison and Rajakaruna 2011). So called with plant identification; Jay Gregg for unpublished data; Roger Latham for advice and unpublished data; Eric because of their lack of utility for timber or Lonsdorf for help with beta diversity analyses; Kirk Miller agriculture, barrens consist of mosaics of plant for statistical discussions; and Janet Fischer, Sybil Gotsch, communities, ranging from savannas, in which Afshan Ismat, Roger Latham, Peter Marks, Mark Olson, scattered trees make up 10–25% of total cover, Tim Sipe, and two reviewers for comments on the manuscript. Thanks to Lancaster County Conservancy, through woodlands, with 25–60% tree cover, to The Nature Conservancy, Nottingham County Park forests, with . 60% tree cover, as well as several (Chester County, PA), Pennsylvania Department of wetland types (Latham and McGeehin 2012, Conservation and Natural Resources, and the Friends of the State Line Serpentine Barrens for their stewardship of Burgess et al. 2015a). Although all these commu- our sites. nities are distinct from surrounding nonserpentine 2 Author for correspondence: kfl[email protected]. Present vegetation, savannas have the greatest conserva- address: Biology Department, Baldwin Wallace tion value because they harbor nearly all the rare University, 275 Eastland Road, Berea, OH 44017. species characteristic of serpentine barrens (Lath- doi: 10.3159/TORREY-D-16-00030 am and McGeehin 2012). Eastern North American ÓCopyright 2017 by The Torrey Botanical Society Received for publication May 17, 2016, and in serpentine barrens have many rare plants, includ- revised form August 30, 2016; first published April 3, ing at least one endemic variety (Cerastium 2017. velutinum Raf. var. villosissimum (Pennell) J.K. 125 126 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 144 Morton; Gustafson et al. 2003), one near-endemic the most detailed vegetation history. Using aerial species (Symphyotrichum depauperatum (Fernald) photographs, historical accounts, and ground Nesom) that occurs only on serpentine barrens in truthing, MacLachlan (1993) and Arabas (2000) Pennsylvania and Maryland and three mafic documented a general shift from open to closed diabase glades in North Carolina (Gustafson and habitats in the Nottingham barrens from 1937 to Latham 2005), and possibly several other endemic 1993. Nottingham barrens lost half of its savanna species (Harris and Rajakaruna 2009, Boufford et habitat during that period, with 30% shifting to al. 2014). There is also evidence of ecotypic oak-pine (Quercus-Pinus) woodland, 21% to pine differentiation on eastern North American serpen- woodland, and 8% to nonoak-hardwood forest tine soils (Burgess et al. 2015b). Many other (Arabas 2000). Historical, aerial photographs show plants, including some species whose main range a similar shift from savanna habitats to closed is in western prairies, are regionally restricted to forests in the Pilot barrens (Tyndall 1992b, Latham serpentine barrens and are, thus, rare or endan- 1993), New Texas barrens (Barton and Wallenstein gered at the regional, state, or global level (Latham 1997), and Cherry Hill barrens (Burgess et al. 1993, Rajakaruna et al. 2009). Based on both the 2015a). At Cherry Hill, dendrochronology further uniqueness of their vegetation and the rarity of shows an increase in tree recruitment from 1850 to individual species, serpentine barrens represent a 2000 and a shift from xerophytic oaks through high priority for conservation in eastern North conifers to mesophytic, shade-tolerant tree species America. Here, we focus on the State Line (Burgess et al. 2015a). Overall, Maryland serpen- serpentine barrens on the Pennsylvania-Maryland tine barrens lost 75% of their savanna area to tree border, the largest occurrence of serpentine barrens encroachment between 1938 and 1988 (Tyndall in the eastern United States and a prime target for 1992b). conservation efforts (Latham 1993, Rajakaruna et This loss of open habitat is generally thought to al. 2009). result from fire suppression, although the relative One challenge to the conservation of eastern importance of natural and human-set fires, as well serpentine barrens—hardly unique to these habi- as grazing, remains controversial (Tyndall 1992b, tats—is the spread of invasive species. Robinia Latham 1993, Barton and Wallenstein 1997, pseudoacacia L., a North American tree not native Tyndall and Hull 1999, Arabas 2000, Burgess et to eastern Pennsylvania or Maryland (Little 1971), al. 2015a). The rapid afforestation of the 20th has been considered a problem at some State Line century coincided with a period of fire suppression; sites (Latham 1993). A number of other widely problematic invasive species have been recorded at Nottingham, for example, active fire suppression as present at Nottingham barrens, which are the began with a 1936 fire and became increasingly largest barrens in the State Line group and which effective in the 1950s and 1960s (Arabas 2000). At has the largest remaining area of serpentine the same time, increasing fragmentation by savanna (R.E. Latham, personal communication). agriculture, development, and roads created inad- Several of these are shrubs with the potential to vertent firebreaks (Latham 1993). Evidence from change the physiognomy of serpentine grasslands natural and prescribed burns also supports the idea and savannas. However, before this study, we that fire can maintain the openness of serpentine lacked data on which invasive species were present vegetation (Miller 1981, Tyndall 1994, R.E. across the State Line serpentine barrens, how Latham, personal communication). For example, frequent and abundant they were, and what a series of natural fires effectively removed Pinus impacts they may have on serpentine vegetation. virginiana Mill. and Juniperus virginiana L. from Another threat to serpentine vegetation in this the Rock Springs barrens (Miller 1981). In fact, the area is tree encroachment into open habitats. ubiquity of tree encroachment on eastern serpen- Historical evidence indicates that the grasslands tine barrens in the absence of fire leads some to and savannas of the State Line serpentine barrens conclude that ‘‘evidence for an edaphic climax declined drastically in size during the 20th century, community is lacking’’