Vegetative and edaphic responses in a Northern mixed conifer forest three decades after harvest and fire: Implications for managing regeneration and carbon and nitrogen pools

Research Highlights: This experiment compares a range of combinations of harvest, prescribed fire, and wildfire. Leveraging a 30-year-old forest management-driven experiment, we explored the recovery of woody species composition, regeneration of the charismatic forest tree species Larix occidentalis Nutt., and vegetation and soil carbon (C) and nitrogen (N) pools. Background and Objectives: Initiated in 1967, this experiment intended to explore combinations of habitat type phases and prescribed fire severity toward supporting regeneration of L. occidentalis. At onset of the experiment, a wildfire affected a portion of the 60 research plots, allowing for additional study. Our objective was to better understand silvicultural practices to support L. occidentalis regeneration and to better understand the subsequent impacts of silvicultural practices on C and N pools within the vegetation and soil. Materials and Methods: We categorized disturbance severity based on loss of forest floor depth; 11 categories were defined, including controls for the two habitat type phases involved. We collected abundance, biomass, and C and N concentrations for the herbaceous layer, shrubs, and trees using nested quadrats and 6 to 10 experimental units per disturbance category plot. Moreover, we systematically sampled woody residue from transects, and forest floor, soil wood, and mineral soil with a systematic grid of 16 soil cores per disturbance category plot. Results: We found that (1) disturbance severity affected shrub species richness, diversity, and evenness within habitat type phases; (2) L. occidentalis regenerates when fire is part of the disturbance; (3) N-fixing shrub species were more diverse in the hotter, drier plots; (4) recovery levels of C and N pools within the soil had surpassed or were closer to pre-disturbance levels than pools within the vegetation. Conclusions: We confirm that L. occidentalis regeneration and a diverse suite of understory shrub species can be supported by harvest and prescribed fire, particularly in southern and western aspects. We also conclude that these methods can regenerate L. occidentalis in cooler, moister sites, which may be important as this species’ climate niche shifts with climate change

Wood Decomposition After an Aerial Application of Hydromulch Following Wildfire in a Southern California Chaparral Shrubland

Severe wildfire can affect many soil processes, especially organic matter (OM) decomposition. Organic mulches are often applied on steep slopes to mitigate soil erosion, but little is known about how these surface organic additions affect subsequent soil OM decomposition. In 2003 the Cedar Fire burned 110,000 ha in southern California chaparral shrubland, after which hydromulch was aerially applied to reduce soil erosion. Subsequently, we established a 5-year study to assess the effect of hydromulch on OM decomposition at the burned soil surface and in the mineral soil using aspen (Populus tremuloides Michx.) and pine (Pinus taeda L.) wood stakes as standard substrates. Mass loss of both aspen and pine stakes in this dry Mediterranean ecosystem was lowest on the soil surface and increased with mineral soil depth. Decomposition was less in the hydromulched soil than in the untreated control, but the large loss of hydromulch from the soil surface within the first year after application make this result questionable. Subsequent analysis showed that subterranean termites had a major role in wood decomposition, but their variable activity in study replicates confounded the separation of hydromulch impacts on decomposition from other soil variables. Little is known on the role of termites in OM decomposition after wildfire, and they should be considered when designing studies in soils where termites are present. Our study results suggest that termite activity in mineral soil could also be a factor in root decomposition after a fire and affect soil stability on steep slopes

Coarse Woody Debris and Carbon Stocks in Pine Forests after 50 Years of Recovery from Harvesting in Northeastern California

The long-term effects of harvesting on stand carbon (C) pools were assessed in a dry, interior pine-dominated forest at the Blacks Mountain Experimental Forest in northeastern California. Six 8-hectacre plots, established in 1938–1943, were treated as either an uncut control or a heavy-cut harvest (three-quarters of the stand volume removed). Response variables included C pools in overstory tree and shrub, coarse woody debris (CWD), forest floor, mineral soil (to 30 cm depth), cubicle brown root fragments of wood, fine roots, and ectomycorrhizal root tips. CWD was further classified as intact wood or more highly decayed brown rot or white rot types. CWD nutrient stocks (N, P, K, Ca, and Mg) and soil N content were also measured. In 1992, 50 years after harvest, total ecosystem C was 188 and 204 Mg C ha−1 in the harvest and control treatments or 8% lower (p = 0.02) in the harvest stands. There were changes in the distributions of C pools between the treatments. After 50 years of recovery, most C pools showed statistically non-significant and essentially no change in C pool size from harvests. Notable reductions in C with harvests were declines of 43% in CWD including standing snags (p = 0.09) and a decline of 9% of live tree C (p = 0.35). Increases in C pools after harvest were in a 3-fold build-up of fragmented brown cubicle rot (p = 0.26) and an 11% increase in soil C (p = 0.19). We observed strong evidence of C transfers from CWD to soil C pools with two- to three-fold higher soil C and N concentrations beneath CWD compared to other cover types, and lower CWD pools associated with elevated cubicle brown rot are elevated soil C in the harvests. Our results showed that while harvest effects were subtle after 50 years of regrowth, CWD may play an important role in storing and transferring ecosystem C to soils during recovery from harvesting in these dry, eastside pine forests of California. This poses a tradeoff for managers to choose between keeping CWD for its contribution to C sequestration and its removal as the hazardous fuels

Coarse woody debris decomposition assessment tool: Model development and sensitivity analysis

Coarse woody debris (CWD) is an important component in forests, hosting a variety of organisms that have critical roles in nutrient cycling and carbon (C) storage. We developed a process-based model using literature, field observations, and expert knowledge to assess woody debris decomposition in forests and the movement of wood C into the soil and atmosphere. The sensitivity analysis was conducted against the primary ecological drivers (wood properties and ambient conditions) used as model inputs. The analysis used eighty-nine climate datasets from North America, from tropical (14.2° N) to boreal (65.0° N) zones, with large ranges in annual mean temperature (26.5°C in tropical to -11.8°C in boreal), annual precipitation (6,143 to 181 mm), annual snowfall (0 to 612 kg m-2), and altitude (3 to 2,824 m above mean see level). The sensitivity analysis showed that CWD decomposition was strongly affected by climate, geographical location and altitude, which together regulate the activity of both microbial and invertebrate wood-decomposers. CWD decomposition rate increased with increments in temperature and precipitation, but decreased with increases in latitude and altitude. CWD decomposition was also sensitive to wood size, density, position (standing vs downed), and tree species. The sensitivity analysis showed that fungi are the most important decomposers of woody debris, accounting for over 50% mass loss in nearly all climatic zones in North America. The model includes invertebrate decomposers, focusing mostly on termites, which can have an important role in CWD decomposition in tropical and some subtropical regions. The role of termites in woody debris decomposition varied widely, between 0 and 40%, from temperate areas to tropical regions. Woody debris decomposition rates simulated for eighty-nine locations in North America were within the published range of woody debris decomposition rates for regions in northern hemisphere from 1.6° N to 68.3° N and in Australia

Coarse Woody Debris Decomposition Assessment Tool: Model Validation and Application

Coarse woody debris (CWD) is a significant component of the forest biomass pool; hence a model is warranted to predict CWD decomposition and its role in forest carbon (C) and nutrient cycling under varying management and climatic conditions. A process-based model, CWDDAT (Coarse Woody Debris Decomposition Assessment Tool) was calibrated and validated using data from the FACE (Free Air Carbon Dioxide Enrichment) Wood Decomposition Experiment utilizing pine (Pinus taeda), aspen (Populous tremuloides) and birch (Betula papyrifera) on nine Experimental Forests (EF) covering a range of climate, hydrology, and soil conditions across the continental USA. The model predictions were evaluated against measured FACE log mass loss over 6 years. Four widely applied metrics of model performance demonstrated that the CWDDAT model can accurately predict CWD decomposition. The R2 (squared Pearson’s correlation coefficient) between the simulation and measurement was 0.80 for the model calibration and 0.82 for the model validation (P\u3c0.01). The predicted mean mass loss from all logs was 5.4% lower than the measured mass loss and 1.4% lower than the calculated loss. The model was also used to assess the decomposition of mixed pine-hardwood CWD produced by Hurricane Hugo in 1989 on the Santee Experimental Forest in South Carolina, USA. The simulation reflected rapid CWD decomposition of the forest in this subtropical setting. The predicted dissolved organic carbon (DOC) derived from the CWD decomposition and incorporated into the mineral soil averaged 1.01 g C m-2 y-1 over the 30 years. The main agents for CWD mass loss were fungi (72.0%) and termites (24.5%), the remainder was attributed to a mix of other wood decomposers. These findings demonstrate the applicability of CWDDAT for large-scale assessments of CWD dynamics, and fine-scale considerations regarding the fate of CWD carbon

Modelling the management of forest ecosystems: importance of wood decomposition

Scarce and uncertain data on woody debris decomposition rates are available for calibrating forest ecosystem models, owing to the difficulty of their empirical estimations. Using field data from three experimental sites which are part of the North American Long-Term Soil Productivity (LTSP) Study in south-eastern British Columbia (Canada), we developed probability distributions of standard wood stake mass loss of Populus tremuloides and Pinus contorta. Using a Monte Carlo approach, 50 synthetic decomposition rate values per debris type were used to calibrate the ecosystem-level forest model FORECAST. Significant effects of uncertainty of pine stake mass loss rates on estimated tree growth were found, especially in moderately managed forests, as estimations of available nitrogen were affected. Consequently, our work has shown that projections of tree growth under management conditions depend on accurate estimations of woody debris decomposition rates, and special effort should be done in create reliable databases of decomposition rates for their use in tree growth and yield modelling

Controls of Initial Wood Decomposition on and in Forest Soils Using Standard Material

Forest ecosystems sequester approximately half of the world’s organic carbon (C), most of it in the soil. The amount of soil C stored depends on the input and decomposition rate of soil organic matter (OM), which is controlled by the abundance and composition of the microbial and invertebrate communities, soil physico-chemical properties, and (micro)-climatic conditions. Although many studies have assessed how these site-specific climatic and soil properties affect the decomposition of fresh OM, differences in the type and quality of the OM substrate used, make it difficult to compare and extrapolate results across larger scales. Here, we used standard wood stakes made from aspen (Populus tremuloides Michx.) and loblolly pine (Pinus taeda L.) to explore how climate and abiotic soil properties affect wood decomposition across 44 unharvested forest stands located across the northern hemisphere. Stakes were placed in three locations: (i) on top of the surface organic horizons (surface), (ii) at the interface between the surface organic horizons and mineral soil (interface), and (iii) into the mineral soil (mineral). Decomposition rates of both wood species was greatest for mineral stakes and lowest for stakes placed on the surface organic horizons, but aspen stakes decomposed faster than pine stakes. Our models explained 44 and 36% of the total variation in decomposition for aspen surface and interface stakes, but only 0.1% (surface), 12% (interface), 7% (mineral) for pine, and 7% for mineral aspen stakes. Generally, air temperature was positively, precipitation negatively related to wood stake decomposition. Climatic variables were stronger predictors of decomposition than soil properties (surface C:nitrogen ratio, mineral C concentration, and pH), regardless of stake location or wood species. However, climate-only models failed in explaining wood decomposition, pointing toward the importance of including local-site properties when predicting wood decomposition. The difficulties we had in explaining the variability in wood decomposition, especially for pine and mineral soil stakes, highlight the need to continue assessing drivers of decomposition across large global scales to better understand and estimate surface and belowground C cycling, and understand the drivers and mechanisms that affect C pools, CO2 emissions, and nutrient cycles

Soil carbon and nitrogen pools in mid- to late-successional forest stands of the northwestern United States: Potential impact of fire

When sampling woody residue (WR) and organic matter (OM) present in forest floor, soil wood, and surface mineral soil (0-30 cm) in 14 mid- to late-successional stands across a wide variety of soil types and climatic regimes in the northwestern USA, we found that 44%-84% of carbon (C) was in WR and surface OM, whereas \u3e 80% of nitrogen (N) was in the mineral soil. In many northwestern forests fire suppression and natural changes in stand composition have increased the amounts of WR and soil OM susceptible to wildfire losses. Stands with high OM concentrations on the soil surface are at greater risk of losing large amounts of C and N after high-severity surface fires. Using the USDA Forest Service Regional Soil Quality Standards and Guidelines, we estimate that 6%-80% of the pooled C to a mineral-soil depth of 30 cm could be lost during a fire considered detrimental to soil productivity. These estimates will vary with local climatic regimes, fire severity across the burned area, the size and decay class of WR, and the distribution of OM in the surface organic and mineral soil. Estimated N losses due to fire were much lower ( \u3c 1%-19%). Further studies on the amounts and distribution of OM in these stands are needed to assess wildfire risk, determine the impacts of different fire severities on WR and soil OM pools, and develop a link between C and N losses and stand productivity. © 2006 NRC

Maintaining soil productivity during forest or biomass-to-energy thinning harvests in the Western United States

Forest biomass thinnings, to promote forest health or for energy production, can potentially impact the soil resource by altering soil physical, chemical, and/or biological properties. The extent and degree of impacts within a harvest unit or across a watershed will subsequently determine if site or soil productivity is affected. Although the impacts of stand removal on soil properties in the western United States have been documented, much less is known on periodic removals of biomass by thinnings or other partial cutting practices. However, basic recommendations and findings derived from stand-removal studies are also applicable to guide biomass thinnings for forest health, fuel reduction, or energy production. These are summarized as follows: (1) thinning operations are less likely to cause significant soil compaction than a stand-removal harvest, (2) risk-rating systems that evaluate soil susceptibility to compaction or nutrient losses from organic or mineral topsoil removal can help guide management practices, (3) using designated or existing harvesting traffic lanes and leaving some thinning residue in high traffic areas can reduce soil compaction on a stand basis, and (4) coarse-textured low fertility soils have greater risk of nutrient limitations resulting from whole-tree thinning removals than finer textured soils with higher fertility levels

Enhancing Soil Quality of Short Rotation Forest Operations Using Biochar and Manure

Biochar and manure may be used to enhance soil quality and productivity for sustainable agriculture and forestry operations. However, the response of surface and belowground wood decomposition (i.e., soil processes) and nutrient flux to soil amendments is unknown, and more site-specific information about soil property responses is also essential. In a split-plot design, the soil was amended with three rates of manure (whole plot; 0, 3, and 9 Mg ha−1) and three rates of biochar (split-plot; 0, 2.5, and 10 Mg ha−1). Soil physical properties, nutrients, and enzyme activities were evaluated in two years. In addition, wood stakes of three species (poplar, triploid Populus tomentosa Carr.; aspen, Populus tremuloides Michx.; and pine, Pinus taeda L.) were installed both horizontally on the soil surface and vertically in the mineral soil to serve as an index of soil abiotic and biotic changes. Wood stake mass loss, nitrogen (N), phosphorus (P), and potassium (K) flux were tested. The high rate of both manure and biochar increased soil water content by an average of 18%, but the increase in total soil P, K, organic carbon (C) content, and enzyme activities were restricted to single sample dates or soil depths. Wood stakes decomposed faster according to stake location (mineral > surface) and species (two Populus > pine). On average, soil amendments significantly increased the mass loss of surface and mineral stakes by 18% and 5%, respectively, and it also altered wood stake nutrient cycling. Overall, the decomposition of standard wood stakes can be a great indicator of soil quality changes, and 10 Mg ha−1 of biochar alone or combined with 9 Mg ha−1 of manure can be used for long-term carbon sequestration in plantations with similar soil conditions to the present study