Long-term disturbance histories of managed and natural mixed-hardwood forests of central Indiana

Robert C Morrissey, Purdue University

Abstract

This research also explores composition and structure of forests over time under natural and management scenarios. I examined transitions of species composition and structure in a natural, relict forest as a contrast to stands that were subjected to partial, selective harvesting in the context of reduced disturbance regimes. All stands were grazed by domestic livestock until about the 1920s, at which time the forest stands were fenced to eliminate grazing. Dutch elm disease also impacted these stands around the middle of the 20th century by eliminating many large elms (Ulmus spp.) and reducing elms to a predominantly understory species. Partial, selective harvests resulted in increased species richness, diversity, and species mingling in different canopy strata relative to changes in the relict stand. All three stands experienced increases in stem density and basal area, particularly in the number of understory shade-tolerant trees, notably sugar maple (Acer saccharum Marsh.), elms, and basswood (Tilia americana L.). The disturbances of the partial harvest stands resulted in a retrogression of stand development by allowing portions of the stand to persist in an earlier stage of development and maintain more midseral, mast- producing species, notably oaks and hickories; this was considered a positive development by diversifying structure and maintaining a degree of functional resilience by ensuring the presence of mast-producing species. Dendroecological analysis of the relict and single-harvest stands indicated that disturbance was primarily characterized by small-scale (< 200 m2) overstory disturbances, but the relict stand had a significantly higher number of gaps and stand area in gaps annually. It was evident that after 1952 disturbance patterns within the two stands differed; approximately 27% of the sample area of the managed stand had newly-formed gaps because of the Dutch elm disease and harvest, stand structure likely accounted for the observed differences between the two stands. The managed stand had fewer gaps, less area in new gaps, and less variability in gap size because there were fewer large gaps formed after 1952. Natural and anthropogenic disturbance patterns not only influence patterns of stand growth and development, but they also contribute to the structure of the dead wood material that many organisms rely on. The individual piece size, density, and volume of down dead wood (DDW) and standing dead wood (SDW) were all higher in the relict stand compared to the double-harvest stand. In addition, higher densities and volumes of DDW and SDW were observed in the largest size and most highly decayed classes in the relict stand. Higher rates of disturbance and large pieces of DDW and SDW in the relict stand also meant higher connectivity across all size classes, generally fewer patches, and larger average patch size across most size classes. Even after almost 50 years, coarse woody debris in the partially harvested stands has not recovered to natural levels. The differences in abundance and connectivity were attributed to higher disturbance rates in the relict stand and what I termed the “storage effect,” which suggests that the larger xv piece sizes and more decay-resistant species will tend to persist on site longer, thus, allowing for greater accumulation and connectivity as trees die. This study highlighted both the importance of disturbances on species composition and stand structure, and the fate of “disturbance legacies” (e.g. CWD) over almost a century. That highlights the importance and value of long-term, detailed planning to ensure sustainability of structure and legacies. Further, efforts to replicate past disturbance regimes as a coarse filter for biodiversity conservation must be extended to match impacts on the understory composition as well as the overstory. Planning must also ensure the provision of suitable coarse woody debris over time to maintain, or even enhance, biodiversity in managed hardwood forests. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Jenkins, Purdue University.

Subject Area

Ecology|Environmental management|Forestry

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