Ecology and Decline of Red Spruce in the Eastern United States
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Schaberg , Gary J. Hawley , Christopher Michael Hansen.
Disentangling the effects of acidic air pollution, atmospheric CO2 , and climate change on recent growth of red spruce trees in the Central Appalachian Mountains. Justin M Mathias , Richard B. Isil , Thomas F. Lavery , Kristi A. Gebhart , Christopher M. Rogers , Carol Armbrust Wanta. Douglas Kaylor. Impacts of human disturbance on the temporal stability of climate—growth relationships in a red spruce forest, southern Appalachian Mountains, USA Philip B.
White , Peter T. Effects of soil calcium and aluminum on the physiology of balsam fir and red spruce saplings in northern New England Richard L. Boyce , Paul G. Hawley , Joshua M. Halman , Paula F. Therefore, hemlock would be expected to be under stress during the greater aridity that coincided with the early colonial period and consequently less able to recover from a regime of intense land use and selective harvesting for its tannins throughout the colonial period .
Indeed, an initial decline by hemlock in many pollen diagrams just before European settlement is followed by a more rapid decline with intensive land use .
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In the modern era, hemlock is being decimated by the invasive hemlock wooly adelgid HWA; Adelges tsugae. We saw the largest reductions of hemlock in the southern portion of our study area, where it frequently declined to zero, which is consistent with the northward spread of HWA from Richmond, Virginia since .
Spruce Fir Moss Spider
At present, cold winter temperatures appear to be limiting HWA's extension into northern New England . On the other end of the successional spectrum are species such as red maple Acer rubrum , black cherry Prunus serotina , and aspens Populus spp. Within our analysis maple had, by far, the largest absolute change in relative abundance — a nearly 20 percent rise in average regional abundance. Because the FIA data have species-level resolution, we know that the maple species that dominates the modern landscape is overwhelmingly red maple, a pioneer species with moderate to great shade tolerance and light, wind-dispersed seeds that readily invades open fields after farm abandonment as well as occupy forest gaps following disturbance.
It sprouts readily following damage and has great edaphic amplitude ranging from saturated wetland soils to drier uplands. The ecological versatility of red maple allows it to occupy a broad range of edaphic and climatic conditions throughout our study area  — . And, indeed, maple increased in 87 percent of the towns we examined, spanning all the environmental gradients within the region, as demonstrated by its central location within the cloud of modern data in the ordination Figure 6.
Similarly, cherry is positioned squarely in the modern data cloud within the ordination; indeed, it has the highest position along axis two of all taxa Figure 6. Cherry was absent in most of the pre-settlement surveys, but was present at low abundance throughout much of the modern forests. The prevalence of this relatively short-lived, pioneer taxon reflects the greater extent and intensity of forest disturbance in the modern era. It shows that disturbance, e. In the absence of anthropogenic influences we would expect succession to increase the similarity to the pre-colonial forests.
By examining our two snapshots of forest composition coupled with our understanding of past and ongoing disturbance processes, we can infer that this has not yet occurred because: 1 despite more the years of reforestation, these ecosystems are still in the early stages of recovery, and 2 through that year period there has been ongoing harvests and other land uses as well as exogenous disruptions, including pests e. Chestnut suffered a near complete extirpation as a tree-sized individual from the region due to the introduced fugal blight, Endothia parasitica in the early s.
Much has been written about the loss of chestnut and its replacement by oak and maple  ,  — . Conventional wisdom holds that chestnut was among the dominant overstory species throughout eastern forests  — . However, the average relative abundance of chestnut in the pre-colonial data was only 3. Its highest abundance was in the Appalachian ecoregion, and even there it was less than ten percent and just one-quarter that of oak.
Indeed, and in contrast to many other southern and northern taxa, no town in our analysis exceeded percent pre-colonial chestnut abundance. These data suggest that the historical dominance of chestnut is currently often overstated, at least in the Northeast.
Nevertheless, even with our comparatively modest estimate of pre-colonial abundance, the loss of chestnut initiated a significant change in forest composition. Like chestnuts, oaks are mid- to late-successional, hard-mast producing canopy tree. There has been growing concern among researchers and managers that oaks are losing their dominance on the landscape  , . And, indeed, oaks declined in 75 percent of the towns in which its abundance was greater than five percent.
Across the region, the relative abundance of oaks declined by almost seven percent. Declines were largest in the Eastern Broadleaf ecoregion where relative abundance fell from 40 to 18 percent. It is worth noting, however, that there were a few small pockets where the relative abundance of oak increased e.
McEwan et al. Important factors include a climatic shift toward increased moisture availability, a shift away from Native American burning and a shift first toward low human populations and then to European colonization, plus changes in populations of acorn consuming fauna, including white tailed deer Odocoileus virginianus. The regional stability in average pine abundance belies the fact that pine was among the most dynamic taxa we examined Figure S1. At the regional scale, the relative abundance of pine did not change significantly. However, in the Central Appalachian ecoregion, pines declined by 11 percent, a reduction that was offset by modest increases in pine in the other ecoregions.
The relative abundance of pines increased by more than ten percent in towns and decreased by more than ten percent in 99 towns. This high amplitude of change in pine abundance likely speaks to two countervailing characteristics of pine in the region: On one hand, pine is a pioneer taxon that establishes on disturbed sites including post-fire landscapes and abandoned agricultural fields, grows fast, and lives for a long time. On the other hand, pine is vulnerable to natural disturbance particularly wind and has always been among the most sought after timber species in the region  , .
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It is important to acknowledge the limitations of the datasets we employed and discuss how we dealt with them: 1 The WT data were collected over a period of more than years. While the surveys were conducted at a similar timeframe relative to colonization, they still potentially represent different levels of Native American and European influence and different survey methods.
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We minimized our exposure to this by excluding towns with explicit evidence of colonial influence e. Instead we assembled all witness tree records known to exist with the nine-state region. While we know of no particular bias in the towns with existing data, we also cannot say emphatically that our collection of towns is a statistically robust sample. We hedged against this by only including towns with at least two FIA plots and then putting the FIA plots through the rarefaction analysis.
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Having noted these issues, we still believe that historical survey records offer a singular resource for understanding widespread patterns of forest composition at the time of European colonization. This perspective has been borne out through several studies that have shown that these data provide an accurate account of landscape scale vegetation patterns e. In addition, consistency between WT data and contemporary pollen studies  ,  further confirm the utility of this resource.
By comparing the modern forest condition to regional database of WT, we have learned much about the changes regional forest composition. While most native taxa remain, the community composition has shifted dramatically. While significant compositional changes were ubiquitous throughout the region, the specific attributes of change varied at local scales. One important pattern throughout was the reduction of late successional species in favor of early successional species.
Additionally, the modern forest is more homogeneous and less coupled to local climatic controls. Among individual taxa, we found that late successional trees such as beech, hemlock, and spruce, were once predominant across the region but are now much less so, save for in some small refugia within isolated mountain regions with little history of land use. Oaks also declined throughout most, but not all, of its range. The year history of land use benefited maple the most and it is now a dominant taxon throughout most of the region.
Short-lived, early seral species are also much more common in the modern landscape, which indicates that land clearing disturbance is ongoing and widespread and the abandonment of agriculture years ago did not mark a return to the natural disturbance regime of small gap openings. The northeast is once again a predominantly forested landscape, but today's forest is not a facsimile of its predecessor. We find this to be at once disheartening and encouraging. On the one hand, the modern expense of forest is diminished in so many of the components and processes that once characterized the regional ecosystem; on the other, given the extent and magnitude of land use it is remarkable that native species predominate and the forests looks in many ways as it has for millennia.
We thank D. Orwig for a helpful review of an earlier version of this manuscript and Evelyn Strombom for editing and help with figures. Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract The northeastern United States is a predominately-forested region that, like most of the eastern U. Introduction The land use history of the northeastern United States is well documented but its ecological consequences remain poorly understood, especially at a regional scale  — .
Methods Study Region We examined changes in forest composition within a sample of colonial townships distributed throughout a nine state region of the northeastern U. Download: PPT. Figure 1. The nine-state study region in the northeastern United States. Pre-colonial data We used the relative abundance of witness tree taxa i. Table 1. Taxa groupings with stem counts and occurrences within colonial towns that met the minimum threshold to be included in the analysis. Figure 2. An illustrative example of one bootstrap sample of FIA plots or witness trees used to estimate the minimum sampling intensity necessary to capture the forest compositional diversity of a town.
Comparing pre-colonial to modern forest composition We first compared the relative abundance of each taxon across the study region and within each ecoregion. Results Of towns in the study area with witness tree data, contained two or more forested FIA plots Figure 1. Figure 3. Relative composition of pre-colonial era Witness Trees and modern inventory trees in colonial townships in the northeastern USA. Figure 4. Town-scale relative abundance of forest taxa within colonial townships. Table 3. Pre-colonial and modern relative abundance of taxa by ecoregion and throughout the entire study area.