Land use history has altered natural disturbance dynamics, causing widespread modifications of the earth's forests. The aim of this study is to reconstruct a regional, spatially-explicit, fire and logging history for a large southern boreal forest landscape (6,050 km 2 ) of eastern Canada. We then examined the long-term influence of land use history, fires, and physiographical gradients on the area's disturbances regimes, present-day age structure and tree species composition. Spatially-explicit fire (1820-2005) and logging histories were reconstructed from forestry maps, terrestrial forest inventories and historical records (local newspapers, travel notes, regional historical reviews). Logistic regression was used to model the occurrence of major boreal tree species at the regional scale, in relation to their disturbance history and physiographical variables. The interplay of elevation and fire history was found to explain a large part of the present-day distribution of the four species studied. We conclude that human-induced fires following the colonization activities of the nineteenth and twentieth centuries have increased fire frequency and the dominance of fire-adapted species at lower elevations. At higher elevations, the low historical fire frequency has fostered the dominance of fire-sensitive species. Twentieth-century forestry practices and escaped settlement fires have generated a forest landscape dominated by younger forest habitats than in presettlement times. The expected increase of wildfire activity in North America's eastern boreal forest, in conjunction with continued forest management, could have significant consequences on the resilience of boreal forests.
Aims
Our study aimed to: (1) document the preindustrial (1925) forest composition prior to extensive logging; (2) document the magnitude of changes from 1925 to 2005; and (3) identify the relative influence of logging and natural disturbances as drivers of the present‐day forest composition.
Location
Boreal forest in central Quebec, eastern Canada.
Methods
We used a dense network of georeferenced historical (~1925) forest plots (n = 30 033) to document preindustrial forest composition. We evaluated the magnitude of changes with the present‐day using modern plots (1980s to 2000s). We reconstructed a long‐term, spatially explicit history of logging, spruce budworm outbreaks (Choristoneura fumiferana [Clem.], SBO), and fire using historical maps and field surveys.
Results
In the preindustrial period, late successional coniferous taxa (Abies balsamea and Picea spp.) dominated the landscape, whereas early successional deciduous taxa (Betula spp. and Populus spp.) were confined to recently burned areas. In the present‐day landscape, large areas dominated by late successional coniferous taxa have been replaced by early successional deciduous taxa. Forest communities dominated by early successional deciduous taxa increased sharply throughout the study area. Logging has been a minor driver of these changes compared to fire and SBOs.
Conclusions
This study demonstrates the importance of documenting the long‐term history of both anthropogenic and natural disturbances in order to assess their relative contributions to the development of the present‐day forest ecosystems. Natural disturbances have remained the main drivers of forest composition during the 20th century, whereas logging played a less important role. In the current context of global change, long‐term experimental research is required to help forecast impacts of natural disturbances and forest management on boreal forest composition.
To reduce the quantity of irrigation water used and the amount of mineral nutrients lost because of leaching, we used time domain reflectometry to monitor and maintain four irrigation regimes (15, 30, 45 and 60%, v/v) during the first growing season for 1+0 black spruce (Picea mariana (Mill.) BSP) seedlings. The seedlings were produced in air-slit containers (IPL 25350A), filled with a peat substrate and were grown under a polyethylene tunnel at a forest nursery. Similar fertility levels were maintained in all four irrigation regimes even though the water content of the substrate could be very low (15 and 30%). Irrigation regime did not affect growth, root architecture or tissue nutrient contents at the end of the growing season. Monitoring water use over the course of the growing season clearly showed that the amount of irrigation water could be reduced by 62 to 76% without compromising seedling quality relative to the 60% irrigation regime. Leachate losses varied exponentially as a function of irrigation regime. The mean amount of water leached, relative to the quantity of water applied during the sampling period, was 10, 7.1, 28.4, and 62.2% for the 15, 30, 45, and 60% irrigation regimes, respectively. The losses of mineral nitrogen at the beginning of August were 49.7, 35.9, 55.2, and 88.2%, respectively, for the 15, 30, 45, and 60% irrigation regimes. To optimize irrigation and decrease leaching, a dynamic model for irrigation management is proposed that accounts for the phenological development of black spruce seedlings grown under tunnel conditions in forest nurseries.
Abstract:In boreal forested wetlands, the observed increase in the water table level after clearcutting (watering-up) is often a threat to sustained ecosystem productivity. Hydrologic recovery refers to the processes by which a water table progressively drops back to its initial level after the cut. In eastern Canada, drainage is used operationally after clearcutting wet sites in order to lower the water table level and accelerate hydrologic recovery. The objective of this study was to evaluate the duration of the watering-up caused by timber harvesting and the extent to which drainage affected the water table recovery on five peatlands and three hydromorphic mineral sites located in the St. Lawrence Lowlands of Québec (Canada). The mixed wood stands studied are dominated by balsam fir (Abies balsamea (L.) Mill.), eastern white cedar (Thuja occidentalis L.), and red maple (Acer rubrum L). Results indicate that, 10 years after clearcutting, water table levels in undrained plots are still 5 to 7 cm higher than the pre-cut levels. The slight recovery in water table level plateaued after the third year. Rainfall interception by vegetation was also monitored, and after 10 years had reached nearly 50% of the pre-cut rate. The immediate water table drawdown following drainage mitigated watering-up within 40 m of a ditch. The persistent watering-up observed in this study should encourage using sylvicultural systems adapted to boreal forested wetlands in order to prevent productivity loss and stand conversion.
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