Biodiversity at the HFR Long-term Ecological Research Site

Authors: Glenn Motzkin and David R. Foster

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Abstract

Glenn Motzkin and David R. Foster

Introduction

Ongoing research in the Harvard Forest LTER program focusses on documenting the influence of historical land-use activities on modern vegetation patterns and ecosystem processes, and on determining the mechanisms by which historical land- use exerts a persistent influence on species distribution and community composition, structure, and function. Our investigations of persistent historical influences on modern species distribution and community composition have important implications for interpreting local, landscape, and regional patterns of biodiversity.

Objectives

- To evaluate the effect of historical land-use activities on local to landscape scale patterns of biodiversity and to determine how this varies regionally across physical and cultural gradients.

- To assess the importance of human disturbances on regional patterns of biodiversity.

- To determine the degree to which the effects of disturbance history and physical gradients on species richness vary among life forms (i.e. bryophytes vs. vascular plants).

Studies of Land-use Impacts on Local Biodiversity

Previous studies in Britain, Europe, and North America have reported significant differences in species richness on ancient versus recent woodlands -differences that may persist for at least several centuries (Peterken and Game 1984, Hermy 1994, Matlack 1994). We are conducting detailed investigations of a number of sites to determine the effect of historical land-use on species richness and composition. Sites are selected across major physical and cultural gradients, ranging from poor sites with limited historical agriculture to rich sites with a long history of intensive agriculture.

Results to date indicate that:

1) Historical land-use strongly influences modern species composition at the local to landscape scales (Whitney and Foster 1988, Foster 1992, Motzkin et al. 1996).

2) The impact of historical land-use on species richness varies by site. For instance, on a xeric outwash plain, the number of species per 20 x 20 m plot does not differ among historical land-uses (plowed vs. unplowed), with the exception of the most recently abandoned and heavily-fertilized agricultural sites, which support > 2x the average number of species found on all other sites. However, because of the strong effect of land-use on species composition on this site, biodiversity at a landscape scale is strongly affected by land-use. Therefore, conservation of the representative flora must incorporate historical land-use variability. On moderate upland sites, historical woodlands that we have investigated are generally species poor relative to nearby secondary forest stands. However, this probably reflects the dominance of many primary woodlands by hemlock (Tsuga canadensis), a species that creates local environmental conditions (shade and thick, acidic litter) that are unsuitable for many species. As in the outwash site, compositional differences between primary and secondary woodlands suggest that at a landscape scale, land-use influences on patterns of biodiversity must be considered.

3) Land-use influences on biodiversity are also apparent in the persistent seed bank. Preliminary results suggest that several weedy or field species (e.g. Rubus, Panicum, Fragaria, and others) that are uncommon in the above ground flora of modern forests are well represented in the persistent seed bank and may emerge following disturbances such as windthrow, cutting, or fire. Ongoing studies are aimed at determining whether persistent seed banks vary among sites of differing land-use histories.

As described above, historical land-use appears to affect modern biodiversity in our study sites largely through long-term alteration of vegetation composition. Several studies are underway to determine the relative importance of biological versus edaphic factors in enabling these compositional effects to persist. On the outwash site investigated, modern edaphic conditions do not differ substantially between plowed and unplowed sites, although it is likely that this was not the case immediately after abandonment of agriculture. Persistent differences in vegetation on plowed versus unplowed sites may therefore result from biological mechanisms such as dispersal and establishment limitations, or from environmental conditions (e.g. light, depth and quality of litter) that developed as a result of early succession onto these abandoned fields. In our studies and in the related literature it appears that several species that have not re-colonized former agricultural areas in 50 to >100 years since abandonment spread sexually very slowly or infrequently, despite abundant production of viable seed. Ongoing investigations of the demography and reproductive ecology of these restricted species will provide detailed information on the mechanisms by which historical land-use may have long-term impacts on species richness and composition.

In addition to documenting the effect of land-use on plant species composition and richness and investigating the mechanisms by which these impacts persist, we are interested in determining the degree to which land-use versus physical environment influences biodiversity among different life forms. In an ongoing study, we are comparing land-use and environmental controls on species richness and composition in bryophytes versus vascular plants. Taxa may also be classified by functional traits in order to determine which components of the flora have been most affected by historical land-use activities.

Studies of Disturbance Impacts on Regional Biodiversity

Within central New England, broad patterns of biodiversity reflect, to a large degree, the concentration of uncommon or rare species in areas that are geologically and physiographically distinct. Much of the region is characterized by metamorphic or granitic bedrock with acidic, nutrient poor soils that support relatively few rare species. Concentrations of regionally rare species are found in areas of carbonate, sedimentary, or basaltic bedrock with circumneutral to alkaline soils, as well as in coastal areas. We are investigating vegetation dynamics and disturbance impacts on a range of uncommon plant communities throughout western Massachusetts, including pitch pine-scrub oak communities, floodplain forests, and calcareous fens.

Results to date suggest that the modern distribution, composition, and structure of many uncommon communities may be interpreted as having developed as a result of unusual initial site conditions in combination with a long history of human modification of the region. For example: xeric outwash deposits are widespread throughout the Connecticut Valley lowland in central Massachusetts, and may support a range of plant associations, including common mixed oak-pine associations and relatively uncommon pitch pine-scrub oak types. Throughout the region, the current distribution, composition, and dynamics of the uncommon associations is largely determined by land-use history (Motzkin et al. 1996). Regionally rare species characteristic of these uncommon associations are similarly restricted to sites with specific land-use histories. For instance, scrub oak shrublands are largely restricted to sites that have never been plowed but that were probably used as historical woodlots or pastures. Rare lepidopteran species that are characteristic of these shrublands are similarly restricted to these unplowed sites. Other rare species are found almost exclusively on formerly plowed sites or other heavily disturbed areas (e.g. Lupinus perennis).

In addition, our studies indicate that patterns of rare species distribution have been highly dynamic within the recent historical period in response to changing regional patterns of land-use and other disturbances. For instance, the distribution of uncommon wetland species characteristic of calcareous fens has changed dramatically within this century, apparently in response to regional changes in land-use and in beaver populations (Motzkin 1994). Some uncommon species that were formerly widespread in wet meadows or pastures have declined in abundance as a result of reforestation after agricultural abandonment. The re-establishment of beavers on the landscape in this century has also influenced the distribution of all of the uncommon wetland species investigated. However, patterns of response of calciphiles to regional changes in beaver and wetland dynamics over time are complex, and it is not possible to generalize to any great extent.

Conclusions

- Historical land-use strongly influences modern species composition and therefore must be evaluated in any interpretation of landscape to regional patterns of biodiversity.

- The influence of historical land-use on species richness varies by site and by vegetation type.

- Regional patterns of biodiversity reflect variation in physiographic and edaphic conditions as well as variation in the history of anthropogenic and natural disturbances.

- Rare species distribution and community composition have been highly dynamic within the historical period in response to changing patterns of land-use and other disturbances.

Foster, D. R. 1992. Land-use history (1730-1990) and vegetation dynamics in central New England, USA. Journal of Ecology 80:753-772.

Hermy, M. P. 1994. Effects of former land use on plant species diversity and pattern in European deciduous woodlands. Pages 123-144. in T. J. B. Boyle and C. E. B. Boyle, editors. Biodiversity, temperate ecosystems, and global change. Springer-Verlag. Berlin.

Matlack, G. R. 1994. Plant species migration in a mixed-history forest landscape in eastern North America. Ecology 75:1491-1502.

Motzkin, G., D. R. Foster, A. Allen, J. Harrod, and R. D. Boone. 1996. Controlling site to evaluate history: vegetation patterns of a New England sand plain. Ecological Monographs 66:345-365.

Motzkin, G. 1994. Calcareous fens of western New England and adjacent New York state. Rhodora 96:44-68.

Peterken, G. F. and M. Game. 1984. Historical factors affecting the number and distribution of vascular plant species in the woodlands of central Lincolnshire. Journal of Ecology 72:155-182.

Whitney, G. G. and D. R. Foster. 1988. Overstorey composition and age as determinants of the understorey flora of woods of central New England. Journal of Ecology 76:867-876.