This study evaluated the potential compounding effect of salvage-logging and wildfire following wind disturbance on seedling regeneration, understory composition, and ecosystem processes in Routt National Forest, a subalpine forest that sustained a 10,000 ha blowdown in 1997, was partially salvage-logged in 1999, and burned in 2002.
From 2000 to 2002, I examined the effects of the blowdown and salvage-logging disturbances on
1) tree seedling regeneration dynamics,
2) understory vegetation composition and cover, and
3) nutrient cycling and ecosystem processes.
In terms of seedling regeneration dynamics, results show little evidence of new seedling establishment in either blowdown or salvage-logged areas, confirming the results of prior studies in Colorado subalpine forests that show that advanced regeneration of previously suppressed understory seedlings is the primary mechanism of regeneration in wind-disturbed subalpine ecosystems.
Density of residual seedlings, particularly Engelmann Spruce seedling density, was severely reduced in salvage-logged areas. Subsurface soil temperatures routinely exceeded 32 degrees C during the growing season in salvage-logged areas, and may have induced physiological shock in residual seedlings. Seedlings in blowdown areas showed enhanced growth rates in the period following the 1997 blowdown but otherwise did not differ significantly in their density or composition from seedlings in control stands. Seedling growth rates since the blowdown were most closely related to the biomass of woody debris, which may indicate the importance of woody debris in moderating temperature and light availability in a manner that favors seedling growth.
These results suggest salvage-logging following catastrophic wind disturbance disrupts the advanced regeneration processes that ensue following wind disturbance in subalpine forest ecosystems. If new seedling establishment remains low over time, salvage-logged areas with extremely low residual seedling densities may transition from a spruce-fir dominated ecosystem to a subalpine meadow.
Studies of the composition, cover, and diversity of understory plants revealed that understory species cover and diversity were greater in blown down areas than in salvage-logged or control areas. At a landscape level, the blowdown added 15 species to the assemblage found in intact spruce-fir forest, whereas salvage-logging only added 2 species not found in the other treatments, one of which (Spegularia rubra) is non-native. From a landscape perspective, the blowdown disturbance helps maintain understory diversity, while initially salvage logging does not contribute significantly to understory diversity.
Blowdown areas experienced an increase in cover of such early successional species as Rubus idaeus, Chamerion danielsii, and Distegia involucrata that thrive in disturbed areas and also favored late successional species, such as, Geranium richardsonii, Oreochrysum parryi, and Thalictrum fendleri. Species growing in salvage-logged areas were primarily early successional disturbance specialists, such as, Chlorocrepis gracilis, Spegularia rubra, Chamerion danielsii and shade-intolerant species, such as, Arnica cordifolia and Carex rossi.
Community composition of each treatment area was distinct and related to a gradient in organic soil depth, which reflected the severity of understory disturbance. Nonnative species cover was less than 1% in all treatment areas, though 73% of all plots contained at least one nonnative species. Nonnative cover was greater in blowdown areas than in control areas, where total vegetation cover was also greater. Proportional cover of nonnative species was greater in salvage-logged areas than in other areas. Composition and diversity in blowdown areas relative to control areas stabilized in the five years following the blowdown, but vegetation cover continued to increase. Salvage logged areas exhibited a shift towards graminoid dominance, which could prevent or delay future conifer seedling establishment.
The third part of this study evaluated the effects of the wind and salvage-logging disturbance on biotic control of nutrient cycling. Inorganic nitrogen availability in the top 10 cm of soil, and rates of nitrogen cycling in the soil (net nitrification, net and gross nitrogen mineralization), and decomposition of leaf material did not differ between blowdown and control areas of intact forest. This suggests that tight biotic control is maintained over these ecosystem processes despite massive structural disruption of the overstory in blowdown areas.
In contrast, salvage-logging resulted in erosion of soil organic matter, and soil compaction, which increased the carbon to nitrogen ratio of the top 10 cm of soil, which is an inverse measure of soil quality. Consequently, rates of nitrogen cycling (net and gross rates of nitrogen mineralization, and potential nitrification rates) were significantly lower in salvage-logged areas than in blowdown areas. Leaf decomposition rates in salvage-logged areas were elevated as a result of greater inorganic nitrogen availability and carbon limitation from the lack of fresh leaf material in sparsely-vegetated salvage-logged areas.
These results support the hypothesis that salvage-logging following wind disturbance converts a biologically intact system into a highly modified system, where reduced vegetation growth and abiotic factors such as soil erosion, compaction, and elevated surface temperatures are important factors driving ecosystem-level processes.
Overall results of this study showed that despite massive structural disruption of the overstory, tight biotic control was maintained over ecosystem processes in the blowdown. Recovery mechanisms included accelerated growth of understory seedlings and expansion of understory vegetation cover. Downed wood moderated light and temperature conditions in a manner that favored vegetation growth. In contrast, mechanized salvage-logging resulted in elevated soil temperatures, soil compaction and erosion, and reduced rates of nitrogen cycling in soil. Seedling density and understory vegetation cover were also reduced; new seedling establishment was minimal. Conditions in salvage-logged areas were similar to south-facing clearcuts, where failures in natural regeneration are common, suggesting that if new seedling establishment remains low, reestablishment of forest cover in salvage-logged areas will be delayed. Thus, salvage-logging following windthrow disrupts recovery mechanisms, converting a biologically intact ecosystem into a modified state, where a shift in ecosystem regime is possible.
Following the 2002 fires in the study area, I expanded the study to evaluate the effects of prior blowdown and salvage-logging disturbances on post-fire regeneration dynamics. In the first year following a wildfire in the study area, vegetation growth and seedling establishment were very low. Newly established tree seedlings were found in previously undisturbed burned areas, but not in burned blowdown or burned salvage-logged areas, possibly reflecting the lack of seed trees in these areas. Wildfire resulted in increased inorganic nitrogen availability in the top 10 cm of soil, and decreased soil carbon and nitrogen concentrations, but resulted in no difference in net nitrogen mineralization rates relative to unburned areas. There were few differences among burned areas as a result of pre-fire disturbance history. Only soil carbon concentrations were significantly lower in burned blowdown areas than in other burned areas, possibly as a result of greater fire intensity in burned blowdown areas. Differences in vegetation cover, seedling density, and soil properties between unburned blowdown, unburned salvage-logged, and control areas of intact forest were consistent with results of previous research. This study suggests that initially, the effects of a severe fire tend to “erase” the effects of previous disturbances on soil properties, and nitrogen cycling. However, with time, the effects of disturbances that occurred prior to the wildfire may become more pronounced.
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