No description available.

No description available.

Anthropogenic activities perturb the global carbon and nitrogen cycle with large implications for the earth’s climate. Land use activities deliver excess carbon and nitrogen to aquatic ecosystems. In the boreal biome, this is mainly due to forestry and atmospheric deposition. Yet, impacts of these anthropogenically mediated inputs of carbon and nitrogen on the processing and emissions of greenhouse gases from recipient streams and lakes are largely unknown. Understanding the ecosystem-scale response of aquatic greenhouse gas cycling to land use activities is critical to better predict anthropogenic effects on the global climate system and design more efficient climate change mitigation measures. This thesis assesses the effects of forest clearcutting and nitrate enrichment on greenhouse gas emissions from boreal inland waters. It also advances methods to quantify sources and sinks of these emissions. Short-term clearcut and nitrate enrichment effects were assessed using two whole-ecosystem experiments, carried out over four years in nine headwater catchments in boreal Sweden. In these experiments, I measured or modeled air-water fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), combining concentration, ebullition and gas-transfer velocity measurements in groundwater, streams and lakes. By using Swedish national monitoring data, I also assessed broad-scale effects of forest clearcutting by relating CO2 concentrations in 439 forest lakes to the areal proportion of catchment forest clearcuts. To improve quantifications of CO2 sources and sinks in lakes, I analyzed time series of oxygen concentrations and water temperature in five lakes on conditions under which whole-lake metabolism estimates can be inferred from oxygen dynamics given the perturbing influence of atmospheric exchange, mixing and internal waves. The experiments revealed that aquatic greenhouse gas emissions did not respond to nitrate addition or forest clearcutting. Importantly, riparian zones likely buffered clearcut-induced increases in groundwater CO2 and CH4 concentrations. Experimental results were confirmed by monitoring data showing no relationship between CO2 patterns across Swedish lakes and clearcut gradients. Yet, conclusions on internal vs. external CO2 controls largely depended on whether spatially or temporally resolved data was used. Partitioning CO2 sources and sinks in lakes using time series of oxygen was greatly challenged by physical transport and mixing processes. Conclusively, ongoing land use activities in the boreal zone are unlikely to have major effect on headwater greenhouse gas emissions. Yet, system- and scale specific effects cannot be excluded. To reveal these effects, there is a large need of improved methods and design of monitoring programs that account for the large spatial and temporal variability in greenhouse gas dynamics and its controls by abiotic and biotic factors.

A comparison of the soil fertility and relationships between soil fertility and early regenerating vegetation were examined following clearcut harvesting and wildfire in the black spruce-feathermoss zone of west-central Quebec. During the summer of 1997, sampling was conducted in wildfires burnt 2, 14, and 21 years ago, stands clearcut within +/-3 years of each fire, and undisturbed control stands. At each site an estimation of vegetative cover of each species present and a volumetric sample of the forest floor (FH), and mineral soil (0--10 cm) were collected from at least 8 fire and cut sites and at least 4 control sites in each study area. The comparison between the soil fertility of stands clearcut and burnt suggested that important differences exist following these two disturbance types in the boreal forest. The forest floor of clearcut sites had greater dry mass, mass of total nutrients and mineralized N than fire or control sites, whereas fire sites generally had higher pH and concentrations of total nutrients than clearcut or control sites. Partitioning of the variance of the vegetation data between soil fertility and general site characteristic variables was carried out to evaluate the direct influence of post-disturbance soil fertility on the composition of regenerating vegetation. All explanatory variables accounted for 53.6% of the variance, of which 23.2% was exclusively attributed to soil fertility variables. Disturbance severity is suggested as an important factor, directly affecting the initial composition of the regenerating vegetation, and indirectly affecting soil fertility and stand productivity in the later stages of regeneration.

Mémoire (M.Ress.Renouv.)-- Université du Québec à Chicoutimi, 1991. / Bibliogr.: f. 63-67. Document électronique également accessible en format PDF. CaQCU

Mémoire (M.Ress.Renouv.)--Université du Québec à Chicoutimi, 1998. / Document électronique également accessible en format PDF. CaQCU

Clearing of trees from the so-called caatinga woodland that characterizes the vegetation of the semi-arid region of northeast Brazil offers possibilities for increasing forage production. This research analyzed the first-year effects of clearing caatinga on dry season forage for goats and sheep. In addition, factors affecting litter decomposition on cleared and uncleared caatinga were assessed to evaluate the viability of deferring grazing of forages during the wet season for use later in the dry season. Removing the trees resulted in a sixfold increase in production of herbaceous vegetation, however, 88 percent of the increased yield on the cleared areas was in the form of stems from herbaceous vegetation. Seventy-two percent of the stems were unpalatable to goats and sheep because of the massive size of those stems. Leaf litter from trees was an important component of the diets of goats and sheep during the dry season and clearing reduced production of this forage threefold. Clearing resulted in increased decomposition of leaf litter. Changes in microclimate played only a minor role in this difference. The reduction in the amount of leaf litter from trees relative to litter from herbs had the greatest effect on decomposition rates of dry season forage because tree litter decomposed less rapidly than did herbaceous litter. The slow decomposition of leaf litter during the dry season suggests that deferment. of cleared or uncleared caatinga for use as forage in the latter part of the dry season is feasible. An analysis of the diets of esophageally fistulated goats and sheep indicated that clearing may be a viable alternative for improving the amount and the in vitro dry matter digestibility of the forage consumed during the dry season the first-year post-treatment. These increases were attributed to an absolute greater abundance of preferred herbaceous forages (i.e., foliage and leaf litter) and to the persistent green foliage on coppicing woody plants. Dietary nitrogen appeared to limit intake, and clearing did not improve availability of this nutrient to sheep and goats at the higher levels of grazing pressure applied in this study. Other ecosystem considerations such as watershed protection and long-term community stability must also be considered in decisions to remove the tree canopy of the caatinga.

No description available.

The objective of this study was to develop empirical regression equations for describing forest overstory-understory relationships in Arizona mixed conifer forests, and for predicting responses of potential mule deer (Odocoileus hemionus crooki) and elk (Cervus canadensis nelsoni) forage resources following thinning and clearcutting. Data were collected in the White Mountains from unlogged stands, a 2-year-old thinned stand, a 4-year-old thinned stand, an 8-year-old clearcut, and a 16-year-old clearcut. A combination of high variability in understory production (due presumably to inherent structurally-complex overstory conditions) and substantial intercorrelation between independent variables yielded multiple regression equations of questionable value. However, average understory production levels can be predicted using stand basal area alone. Relationships between basal area and understory production components were generally best described using log or log-log functions. Despite considerable diversity in slope and aspect conditions, potential insolation was generally not correlated with understory production; nor was potential insolation per unit of basal area often a better predictor than basal area alone. On unlogged sites production of potential deer forage was positively correlated with presence of spruce and fir, but negatively correlated with combined pine and Douglas-fir composition, and not significantly correlated with aspen composition. Potential elk forage was positively correlated with spruce and combined pine and Douglas-fir composition, but inversely correlated with aspen and fir composition. Understory production was generally inversely related to periodic annual forest growth and thickness of the forest floor. Browse production was generally less related to overstory conditions than herbage components, but was correlated with slope gradient and soil rockiness. Although total browse production could be estimated from basal area data, production of browse potentially useful to deer and elk could not be. Total understory production under virgin mixed conifer stands ranged from about 150 pounds per acre at 50 square feet of basal area (BA) to about 40 pounds at BA 400. The portion of this biomass potentially valuable as deer forage declined from 30% at BA 50 to 8% at BA 400, while elk forage declined from 49 to 11%. Forbs were the dominant understory component for all stand densities higher than about BA75, but were out-produced by graminoids below this level. Browse was the least productive understory component and increased only slightly with decreasing stand density. Total understory production on the 4-year-old thinned site was higher, but not statistically different than on unlogged sites; however, from 65 to 74% of the biomass consisted of forage potentially valuable to deer and elk. Depending on stand density, understory production under the 2-year-old thinning was comparable to or less than that of unlogged sites. Overstory-understory regression curves were often statistically different between thinned and unlogged stands. Diversity of principal deer and elk forage species increased as stand density declined under both unlogged and thinned stands. Unlogged stands produced slightly more potential elk than deer forage at lower stand densities, but production of deer and elk forage after thinning was nearly identical over the entire range of stand densities sampled on both thinned sites. The 8- and 16-year-old clearcuts produced about 750 and 1160 pounds per acre of biomass, respectively, with about 34% consisting of potential deer and elk forage on the younger site, versus 56% in the older clearcut. The 8-year-old clearcut produced 30% more elk than deer forage, and the 16-year-old site 7% more. The duration of beneficial deer and elk forage responses following clearcutting appears substantially longer than 16 years where tree regeneration is unsuccessful.

The forest floor represents the uppermost organic and organicenriched mineral soil horizons. They have been formed by the deposition of organic material and the subsequent biologically mediated decomposition. The forest floor influences rooting-zone temperature, aeration, moisture, and nutrient conditions, and hence, forest productivity. Considering the importance of the forest floor, and the fact that it is exposed to disturbance (being the surface layer), we need to assess the potential impacts our logging practices may have. Clearcutting, one of the contentious silvicultural practices used in British Columbia, is imputed to most adversely affect ecosystems and sustainability. We assessed the long-term impact of clearcutting on the forest floor by documenting changes in the thickness, chemical and biotic properties of the humus form across a chronosequence of forest stands. The study was located in the largest and most representative portion of the coastal rainforest the Very Wet Maritime Coastal Western Hemlock (CWHvm) subzone.