Community dynamics of streamside forests: Lessons from the Conestoga River drainage, Lancaster County, Pennsylvania, USA
Open Access
- Author:
- Kuhn, Robert Jeffrey
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 09, 2006
- Committee Members:
- Marc David Abrams, Committee Chair/Co-Chair
David Eissenstat, Committee Member
Kim C Steiner, Committee Member
Alan Taylor, Committee Member - Keywords:
- riparian
floods
forest ecology
dendrochronology
witness trees - Abstract:
- Abstract Riparian zones have come to be recognized as providers of critical ecosystem functions in watersheds, namely water pollution mitigation, erosion and flood control, wildlife habitat, carbon storage, recreation and aesthetics. Relatively few studies have focused on riparian systems in the northeastern United States where habitat degradation has been severe since settlement. The drainage of the Conestoga River in Lancaster County, Pennsylvania was selected for study, as this intensely farmed area is an extreme case in which very erodable soils and inadequate riparian buffers have resulted in the transport of remarkable loads of silt and agricultural enrichment into the watershed. Conservation of limited streamside forests and creation of new ones is vital for the region. This study focuses on four areas of streamside forest ecology: 1) The impacts of past land use from presettlement to the present on the forest community, 2) The composition of the present community and changes that have occurred over the last ten years, 3) The effects of climate on the growth of nine common riparian species, comparing growth responses of trees at streamside with that of trees at a distance from the stream, and 4) The effects of floods on the woody riparian community. Information on the presettlement forest was obtained from witness trees recorded in the early 18th century metes and bounds land surveys. Dramatic changes were seen when comparing the early forest inferred from the witness trees with the modern streamside forest. At settlement, the Lancaster County streamside forests were dominated by Quercus alba, Q. velutina and Carya, spp. Today Quercus alba and Q. velutina combined make up only 2% of the community in the same area. The forest is now dominated by early successional species, namely Acer negundo, A. saccharinum, Platanus occidentalis and Fraxinus pennsylvanica. To determine the composition and structure of the modern forest, 46 sampling locations were established throughout the Conestoga River drainage to include, where possible, all tributaries and all existing stream classes. Over 8000 trees were included in the study. Two distinct streamside communities were identified, associated with the two contrasting physiographic provinces in the study area. The more species rich northern community, an environment of first and second order streams, was dominated by Acer rubrum, F. pennsylvanica, Liriodendron tulipifera and P. occidentalis, with the late successional species Acer saccharum and Fagus grandifolia occurring prominently in the understory. P. occidentalis, A negundo, A. saccharinum and F. pennsylvanica dominate the southern community along the mainstem of the lower Conestoga River and lower reaches of the major tributaries. The effects of severe flooding limit this community to early successional and flood resistor species. Radial growth of nine common tree species was evaluated using a dendroclimatological approach, comparing growth responses in the streamside and contiguous upland environment for each species. Radial growth was correlated with Palmer Drought Severity Index (PDSI), and temperature and precipitation separately. Three guilds of trees emerged in the streamside environment: L. tulipifera and F. pennsylvanica correlated positively with climate variables (grew best in wet and warm conditions); P. occidentalis, A saccharum, A. saccharinum, A. negundo and Ailanthus altissima correlated negatively (grew best during the worst droughts); and A. rubrum and Celtis occidentalis showed weak and mixed correlations. These correlations were usually not seen with these species in the upland environments. The lower Conestoga River drainage is a region shaped by an extreme historical event, the Hurricane Agnes flood of 1972. Seven sites were selected in the lower drainage to represent a range of potential flood velocities, inferred from channel morphology. Tree core analysis revealed that most of the trees in the floodplain of the highest velocity sites dated to the Agnes event. In the lowest velocity site no evidence of this establishment pulse was seen. There is a gradient in between. A major release event is documented in the tree ring records of those trees that established prior to 1972 and survived the flood. Patterns seen in this study can be applied to other systems to infer flood histories and stand dynamics in the absence of hydrological records.