Long-term Data

From a scientific standpoint, much of the value of the Duke Forest lies in data collected from hundreds of studies and surveys completed over the Forest’s 75+ year history. To appreciate the significance of these data, it is important to recognize that forests change and grow very slowly by human measures. Most of what has been learned regarding patterns of forest change has been inferred from static comparisons of forest stands at one point in time. However, these comparisons are insufficient for understanding the response of forests to external factors such as landscape fragmentation (e.g., land development), long-term chronic insults (e.g., acid rain or low-level ozone), or global climate change (e.g., elevated temperatures and C02 concentrations). Yet it is just these issues that are most important to ecosystem scientists and policy makers.

The Duke Forest long-term database provides a unique opportunity to address these issues, because it provides observations of the changes in various forest ecosystems over time scales exceeding the working lifetime of any single researcher. These sample plots have been largely protected from active management, enabling continued monitoring of natural forest processes. Few other sites in North America have been soextensively studied over such a long time period and across such a range of site conditions.

Original Permanent Sample Plots

From 1933 through 1947, Drs. Korstian and Coile established a set of 87 permanent sample plots (PSP’s) in stands of various ages and covertypes. The plots ranged in size from 0.1 to 1 acre. In plots 1 through 51, every tree was individually numbered, identified to species, and measured for diameter, height, crown length and other features. Remeasurement of the 34 existing PSP’s has been carried out at 4- to 9-year intervals, most recently in 2001 as part of a Long Term Research in Environmental Biology (LTREB) project funded by the National Science Foundation.

Originally these plots were designed to address the impacts of specific silvicultural practices on timber growth and quality. However, through the years these data have been used to answer a broader set of questions. Recent uses include examination of trends in tree growth and stand yields over the recent decades as a consequence of acid rain and atmospheric pollutants, and examination of the impact of hurricane disturbance and herbivory on forest structure and plant diversity.

Mapped Forest Stands

From 1950 through the mid 1980’s, eight intensively-mapped plots were established, varying in size from 1 to 10 acres. Within these plots each tree (> 1 cm DBH) was mapped with an X-Y coordinate system, and species and size data were collected. The larger size of these plots and the spatial information that was recorded has allowed researchers to examine the spatial patterns and processes associated with maturing forests. These tree plots are periodically resampled, most recently in 1997 and 1998 as part of a LTREB project. Use of these data has expanded to include research that utilizes the stands for ground-truthing remote sensing data.

Within five of these mapped forest stands, subplots were established to examine tree seedling and sapling demography. The seedlings and saplings in these intensively mapped plots were annually censused from 1978 to 1994 and from 1997 to 2001 as part of a LTREB project. This research examined the importance of the tree-establishment stage on forest dynamics, and also explored which aspects of tree seedling dynamics are most important for projecting future changes in forest composition.

Compositional Survey Plots

In 1977 an additional 230 permanent plots (0.1 hectare in size) were established in the Duke Forest to document the variation in composition and succession of forest stands under different site conditions. Compared to the original PSP’s, these plots cover a wider range of site histories and site conditions, and include unique habitats such as acidic soil monadnocks, clay-pan soils and wet forests. In addition to collection of data on tree size and composition, as with the original PSP’s, these survey plots were also sampled for herbaceous (non-woody) species composition and soil chemistry.

Data from these plots have been used for many purposes, including studies of forest composition, species diversity, and compositional convergence during succession. Because herbaceous plants have limited economic value, permanent plots such as these with detailed herbaceous and soil data are extremely uncommon in North America.

Herbaceous plants are known to be especially sensitive to changes in environmental conditions; therefore, these data provide invaluable information about the impact of chronic stresses on the forest ecosystem.

These survey plots are part of a long-term project aimed at understanding the nature and mechanisms of forest change following disturbance. The project was initiated by professor Robert Peet of the UNC Chapel Hill Department of Biology and professor Norman Christensen of Duke University’s Nicholas School of the Environment & Earth Sciences.

Forest Inventories

Upon appointment as director of the Duke Forest, one of the first actions taken by Dr. Korstian was to assess the Forest’s timber resources. In 1931 20% of the trees in each forest stand (approximately 600 stands) in the then 4,700 acres of forestland were censused using surveyed transects. The species and diameter of each tree was recorded, with the entire survey sample totaling 20% of the mature trees then alive in the forest. This mapped transect data provides the earliest inventory data for the Forest.

A second forest-wide survey was conducted in 1941 using 2,300 tenth-acre plots spaced 5 chains (330 feet) apart. A subset of these plots were resampled in 1952. In following years, as additional parcels were added to the Forest, these new properties were subsequently surveyed and recorded, providing additional data from the mid 1940’s to 1953. Additional inventories of timber resources were undertaken in 1979 using plotless methods, and again in 1990 using tenth-acre plots and stratified sampling.

While these inventories were undertaken to assess the volume and distribution of timber resources in the Forest, they have been very useful to latter-day ecologists. This is particularly true of data gathered on the plots that can still be relocated today. Most of the 1931 transects in the Durham Division have been resurveyed and censused. All of the extant plots sampled in 1941 and 1951 have been recensused. The resulting data have been used to develop and validate computer simulation models of changes in forest composition and structure over the past 70+ years.

Plantation Establishment

At the time that the Duke Forest was established, a significant proportion of the area was in recently abandoned agricultural fields. While much of this land was allowed to revegetate naturally, many areas were planted with a variety of silviculturally important species. Careful records of plantation establishment (site preparations, tree densities, etc.), as well as subsequent evaluations of success, were completed. The species planted included important timber species for this region (e.g. loblolly pine), but also included non-native species such as white pine, bald cypress, longleaf pine, Chinese chestnut and Douglas fir. The Office of the Duke Forest maintains a comprehensive collection of these plantation records for research and educational use.

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