Long-term disturbance history
Summary: From the early Archaeic peoples through European settlement, long-term disturbance history of the Coweeta basin, such as fires and Chestnut decline, has been studied in this area since 1933 when the Coweeta Experimental Forest opened for research.
The Coweeta basin has been occupied by humans for over 6000 years, from early Archaic peoples through the Cherokees and early European settlers; the latter two cleared the land for agriculture and stock grazing. Logging in the basin continued through the mid 1920s. Paleoecological analyses indicate 20th century "recovery" forests differ substantially from pre-settlement forests. Whether increased burning from 1450 to 1650 reflects Cherokee activities is not known, but it did not appear to have large impacts on forest composition. The research showed large changes in composition and increasing charcoal accumulation corresponds with the rise in European agriculture since the 19th century. Declines in the chestnut tree coincided with transient increases in maple trees and longer term increases in elm, hornbeam, hemlock, birch and pine trees. These taxa all appear to play a greater role in recovery forests than they did in pre-settlement forests. Jim Clark’s lab made a significant contribution to understanding the role of fire in land-use change by reconstructing the charcoal and pollen records from cores taken from small bogs and ponds in North Carolina and Virginia. They were analyzed to determine the importance of fire and human disturbance in shaping pre-settlement and 20th century forests in the southern Appalachians. They found that prior to European settlement, low charcoal accumulations occurred, indicating low amounts of burning during the past 2000 years. However, after the European settlement, charcoal peaks which suggests the presence of natural fires in forests. Furthermore, high charcoal concentrations occur at the transition between coniferous and deciduous forests of the Holocene and Pleistocene indicating a greater role of fire in these transitional forests. By investigating the effects and relative importance of fire use historically, we can gain a better understanding of how land use has shaped the environment we live in today. For more information on how this research is evolving currently, please visit Coweeta fire research.
Long-term hydrologic and biogeochemical cycling studies
Summary: Understanding biogeochemical cycles and ecosystem processes in both terrestrial and aquatic ecosystems has been a key element of Coweeta's LTER research activities.
Early research emphasized the role of severe disturbance (e.g. clearcutting, drought) and the resultant impacts on nutrient, carbon, and hydrologic cycles. We completed a 40-year summary of changes in water yield and timing of streamflow and a 20-year summary
Long-term hydrologic and solute responses to the 1977 commercial clearcut, cable logging experiment showed significant increases (>20 cm) in annual water yield with large increases in streamflow during the low flow months but with only small changes (10%) in storm hydrograph characteristics. Streamflow returned to pre-treatment levels by year 6. There was minor loss of soil nutrients as shown by small changes (<3 kg/ha) and rapid recovery to baseline stream chemistry. This study significantly altered management practices in the region: within 2 years, a quarter of timber sales on National Forest lands required cable logging. For current research on the topic please visit Coweeta LTER biogeochemical cycling research.
Long-term atmospheric chemistry effects
Atmospheric chemistry influences forest ecosystem processes and continues as an important research topic at Coweeta. High ozone levels reduced basal area increment, induced premature senescence and loss of foliage, stimulated pine seedling germination, and produced small but measurable increases in NO3 and K+ concentrations in a white pine plantation (Swank and Vose 1990/91). Oxidant (O3, SO2, HNO3) concentrations increase over the elevational gradient, which could have important consequences for forest biogeochemical cycles and for forest growth and health. For more information please visit Coweeta LTER biogeochemical cycling research.
Long-term changes in soils
Because soil chemical properties (% C and cation content) directly influence site productivity, we have monitored their changes in reference and managed watersheds over two decades (Knoepp and Swank 1994). On reference watersheds soil C levels remained stable. Changes in exchangeable soil cation content varied with aspect: concentrations decreased in a N-facing slope but were stable on a S-facing slope. The impact of forest management practices varied considerably. Soils in a white pine plantation showed stable C levels, but cations declined. Commercial sawlog harvest resulted in large increases in soil C and cation concentrations, which remained elevated for 17 years. Whole-tree harvest resulted in decreased soil C for the next 14 years. Clearly soil response to harvest varies with type of harvest and site. Long-term studies like these have proven useful in guiding ecosystem management projects in the Southern Appalachians (Meyer and Swank, 1996). Please follow the link to research on C-levels, for the new direction this research has taken.
Long-term studies of forest succession
We have examined the role of dominant early successional species in forest recovery and identified longer term patterns in composition, diversity, and richness of herbaceous and woody species (Elliott and Swank 1994). After two successive clearcuts, overstory diversity was highest in the early establishment stage of stand development, then declined at the intermediate stage with canopy closure (Elliott and Swank 1994). On a SW-facing watershed, woody species diversity was relatively stable through succession, although tree species richness increased. Ground flora diversity declined through succession. To view how this past research has developed through the millennium, please visit Coweeta research on site factors, plant history traits, and canopy gaps.
Vegetation responses to climate
Relationships between annual wood tissue ∂13 C, growing season, soil water potential and basal area growth were studied in a mature white pine (Pinus strobus) stand at Coweeta by extracting wood cores spanning the 1980s (McNulty and Swank 1995). Some of the hottest, driest and wettest years were recorded during this decade. The ∂13C of annual wood tissue was positively correlated with average growing season soil water potential and average annual basal area growth. Wood tissue ∂13C appears useful in estimating historic changes in soil water potential and interpreting patterns of basal area growth in mature forests.
Long-term research on streams
Summary: We have continued to assess long-term recovery of a stream after clear cutting its watershed.
Litterfall, leaf decay rates, benthic organic matter, stream geomorphology, nutrient and DOC concentrations, and invertebrate community structure and production have been measured periodically since 1975 (Webster et al. 1992). Benthic invertebrate abundance was 3 times higher and biomass and production were 2 times higher in the disturbed stream even 16 years after disturbance (Stone 1995). The proportion of scrapers decreased and shredders increased over the period. Invertebrate assemblages in the disturbed stream 16 years after clearcutting were similar to those in the reference stream.
Export of coarse particulate organic matter (CPOM) from 3 headwater streams was quantified for nearly a decade (Wallace et al. 1995). Annual CPOM export varied by 9- to 16-fold among years, much greater than the 3- to 5-fold variation in stream discharge. Annual export was poorly correlated with annual discharge but related to storm frequency and magnitude.
Stream geomorphology and biology changed in response to experimental additions of woody debris: depth increased, current velocity decreased, cobble substrate was covered by sand and silt, and benthic FPOM and CPOM standing stock increased (Wallace et al., 1996). Solute uptake lengths did not change dramatically, but invertebrate community structure did. We continue to follow changes in geomorphology and invertebrates in this long-term experiment. For more information on this research please visit Coweeta LTER research in land use and long term change in aquatic ecosystems.
To view all citations from past research web page, please see our past references page.