C. Mark Cowell

Historical Change in Vegetation and Disturbance on the Georgia Piedmont

Abstract Due to the extensive removal of the forest cover of the southeastern piedmont during the 19th century and to the lack of systematic presettlement records for most of this region, there has been little basis for relating the piedmonts maturing postsettlement secondary forest to its pre-European condition. This study compares species composition patterns between pre-European and present periods for a portion of the Georgia piedmont that had a systematic presettlement land survey. Detrended correspondence analysis of presettlement, immature postsettlement, and mature postsettlement forests identifies a primary gradient that distinguishes between the vegetation of these periods on the basis of habitat moisture preferences and fire tolerance of species. A secondary gradient emphasizes life history characteristics of species that typically differentiate immature and mature postsettlement forests; presettlement forests were not dominated by the late successional species typical of mature postsettlement forests, but had abundant disturbance-favored taxa. Changes in the abundance of individual species from presettlement to mature postsettlement forests occurred across all habitat types. The xerophytic, fire-tolerant taxa that dominated presettlement forests (e.g., Pinus species, Quercus stellata, Q. velutina) are less important in mature postsettlement forests. Dominants of mature postsettlement forests (e.g., Q. alba, Liriodendron tulipifera, Carya species) are primarily mesophytic, fire-intolerant species that were far less prominent in presettlement forests. Marked contrasts in composition between presettlement and mature postsettlement forests of the piedmont have been produced by changes in the prevalent disturbance regime from fire-dominated dynamics to gap-phase processes.

Vegetation Development in a Modified Riparian Environment: Human Imprints on an Allegheny River Wilderness

Pristine floodplain forests are virtually nonexistent in the eastern United States, requiring that preservation efforts focus on relatively intact representatives of these unique ecosystems, many situated where hydrologic modifications are the norm. This article examines the vegetation dynamics for one such natural area, a wilderness island in northwestern Pennsylvania, to assess how the ecological processes of a riparian preserve are affected by changes to the surrounding environment. Ordination of a vegetation sample identifies several landscape patches on the island; the structure and historical development of these communities are analyzed using tree ring patterns, aerial photography, and the flood regime characteristics preceding and following construction of a large dam upstream. Research on natural riparian sites has emphasized the role of floods as a disturbance that generates early successional habitat. Here, however, moderation of the hydrologic regime has shifted the impact of floods from disturbance to stressor. Peak flows are no longer sufficient to open sites for colonization, while the duration of flooding has increased. Without flood disturbance, later stages of succession become more widely represented, and species regeneration occurs in the context of competitive—rather than open—sites. The altered disturbance regime thus favors species with life history characteristics atypical of the pre-dam environment, including nonnative species, resulting in altered composition and vegetation dynamics. Managerial expectations that natural successional processes will eventually restore degraded riparian habitats in these modified settings are therefore unlikely to be fulfilled.

Slope Aspect Mediates Fine-Scale Tree Establishment Patterns at Upper Treeline during Wet and Dry Periods of the 20th Century

Abstract Previous research collectively demonstrates the importance of taking local moisture availability and biotic threshold responses into account when seeking to reveal the ecological manifestations of climate change within upper treeline ecotones. Yet dendroecological studies that explicitly address the role of slope aspect in this context are non-existent. In this paper, we examine whether slope aspect and related temperature-precipitation interactions mediate abrupt increases in tree establishment and pulses of upper treeline advance (≥10 m) on contrasting north- and south-facing slopes during wet and dry periods of the 20th century. We used regime-shift analysis to quantify episodic changes in the rate of tree regeneration at each site (p < 0.05). We employed a climatic water deficit approach to define fine-scale moisture conditions to compare with dendroecological data from opposite aspects on eleven mountain peaks along a latitudinal gradient in the U.S. Rocky Mountains. Regime-shift analysis measured abrupt, yet asynchronous increases in tree establishment across contrasting slope aspects on 10 of 11 mountain peaks. Upper treeline advance was significantly greater (p < 0.05) during drought on north-facing slopes. On south-facing slopes, ecotonal dynamics varied more with respect to fluxes in the climatic water deficit; namely because of differences in local hydroclimate regimes. Collectively, these results underscore the importance of considering both slope aspect and temperature-moisture interactions when elucidating climate-vegetation interactions within upper treeline ecotones.

The Changing Geography of the U.S. Water Budget: Twentieth-Century Patterns and Twenty-First-Century Projections

Persistent changes in temperature and precipitation patterns have dramatic effects on the availability of surface water for natural vegetation, streamflow, agricultural production, and human consumption. We use a combination of historical observational climate data and water budget equations to develop time-series and maps of twentieth-century water variables within the contiguous United States and compare these with anticipated twenty-first-century patterns projected by global climate models. The results graphically demonstrate regional variation in hydroclimatic trends: areas that experienced convergent actual (AET) and potential evapotranspiration (PET) rates during the twentieth century (such as the Great Lakes and Gulf South) witnessed long-term increases in available moisture, whereas areas with divergent rates (such as the Mid-Atlantic and Great Plains) had greater water deficits. Increasing temperatures through the twenty-first century will produce higher PET across the United States; areas where AET similarly escalates will maintain average moisture levels within twentieth-century ranges, but where AET does not correspondingly increase, as in much of the South and West, average conditions will be comparable to those of extreme twentieth-century droughts. The findings highlight the importance of a regional approach to environmental change, as the impacts of climate on water in the United States will be spatially uneven.

Disturbance and riparian tree establishment in the Sespe Wilderness, California, USA

The riparian forests of southern California are subject to disturbance by both fire and flood. These agents are capable of causing pulses of mortality and recruitment, but it remains unclear how they interact to determine patterns of stand development. We use dendrochronology to identify establishment dates for stems of major riparian tree species in the Sespe Creek watershed, in order to examine their relationship to regional flooding and fire history. Our 11 study sites were burned by major fires in 1932 and 2002, with a smaller 1975 fire affecting only two sites; major floods were concentrated within the second half of the 1933–2009 streamflow record, with the largest floods occurring in 1969, 1978, and 1983. Three periods of stand development are evident: (1) the oldest alder (Alnus), cottonwood (Populus), and oak (Quercus) stems became established soon after the 1932 Matilija Fire, (2) minimal stem establishment between the 1940s and mid-1960s, and (3) continued, although irregular, recruitment of alder and cottonwood since the late 1960s. These patterns show episodes both of regeneration following a catastrophic site-clearing event (Matilija Fire) and of more localized stem replacement during the recent period of increased flood magnitude, with implications for changes in the composition of these forests.