Environmental conditions promoting plant diversity in some upland hardwood and hardwood-pine forests of the interior coastal plain ecoregion.

摘要

Effective ecological restoration of fire-dependent ecosystems requires an understanding of historical species composition and associated environmental conditions and the prospects for restoring environmental conditions that will prevent ongoing declines in fire-dependent species. To date, there have been very few well-conceived approaches to restoring oak- and oak-pine-dominated ecosystems in the interior coastal plain of the southeastern United States. The dissertation presents research that examines (1) historical tree species composition in north Mississippi and associated environmental conditions, (2) relationships between current groundcover plant diversity and composition and a variety of landscape-level and local environmental factors in mature upland forests, and (3) an initial attempt to restore historical plant composition and associated environmental conditions in upland hardwood forests in north Mississippi and west Tennessee.; Tree species composition of mature forests in uplands of northern Mississippi currently bears little resemblance to that which dominated the upland landscape in the early 1800s. Presettlement upland forests once consisted of fire-climax communities that were dominated by shade-intolerant, fire-tolerant shortleaf pine and oaks in both the overstory and midstory. Relative frequencies of these species were associated with variation in soil fertility, but not slope position or aspect. Shortleaf pine was restricted to infertile sandy and sandy clay-loam soils in uplands, whereas upland, fire-tolerant oaks occurred in both infertile and fertile soils in uplands.; Current upland forests now contain both fire-tolerant upland and fire-sensitive floodplain species in the overstory, and fire sensitive floodplain species in the mid- and understories. In addition, upland areas that support the highest plant diversity today (sandy lower slopes) were historically dominated by fire-tolerant pines and oaks, as well as white oak and American chestnut. Although many upland forests in northern Mississippi currently contain largely closed tree canopies (>80%), groundcover composition consists of a mixture of open-habitat and shade-tolerant species. However, many open-habitat groundcover species (e.g., Pityopsis graminifolia, Tephrosia virginiana, and Liatris aspera) are suppressed within these forests.; Differences in species richness and composition within second-growth upland forests in northern Mississippi were associated with a combination of landscape disturbance, canopy density, fire frequency, slope steepness and soil compaction. Species richness was greatest within relatively undisturbed landscapes located on well-drained, sandy soils, with relatively less dense canopies, which had experienced somewhat frequent fire. In addition, cover by the invasive species, Japanese honeysuckle (Lonicera japonica), was greatest within forests in which there was greater anthropogenic disturbance in the surrounding landscape, higher canopy density, more compacted soils and decreased fire frequency. However, there was no evidence of a competitive effect of Japanese honeysuckle on native groundcover vegetation.; To investigate the effects of reduced canopy closure on open-habitat groundcover species in fire-suppressed, second-growth hardwood forests, forest thinning and early-spring (March-April) prescribed burning was conducted at two restoration sites in north Mississippi and west Tennessee. Although deadening treatments did result in significant mortality of mesophytic off-site species, thinning and deadening treatments did not adequately reduce canopy closure to promote open-habitat groundcover species composition. No significant changes in species richness or evenness were observed after treatments; however, many native groundcover species showed mixed responses. Invasive species experienced mixed responses as well, with Japanese honeysuckle decreasing, and Japanese stiltgrass (Microstegium vimineum) and mimosa (Albi