Organic Layer Depth Research Articles

Long-term vegetation change in marshy pine forest Vaccinio uliginosi-Pinetum Kleist 1929

Długookresowe zmiany roślinności w zespole sosnowego boru bagiennegoVaccinio uliginosi-PinetumKleist 1929

Persistent effects of fire severity on early successional forests in interior Alaska

There has been a recent increase in the frequency and extent of wildfires in interior Alaska, and this trend is predicted to continue under a warming climate. Although less well documented, corresponding increases in fire severity are expected. Previous research from boreal forests in Alaska and western Canada indicate that severe fire promotes the recruitment of deciduous tree species and decreases the relative abundance of black spruce (Picea mariana) immediately after fire. Here we extend these observations by (1) examining changes in patterns of aspen and spruce density and biomass that occurred during the first two decades of post-fire succession, and (2) comparing patterns of tree composition in relation to variations in post-fire organic layer depth in four burned black spruce forests in interior Alaska after 10–20 years of succession. We found that initial effects of fire severity on recruitment and establishment of aspen and black spruce were maintained by subsequent effects of organic layer depth and initial plant biomass on plant growth during post-fire succession. The proportional contribution of aspen (Populus tremuloides) to total stand biomass remained above 90% during the first and second decades of succession in severely burned sites, while in lightly burned sites the proportional contribution of aspen was reduced due to a 40-fold increase in spruce biomass in these sites. Relationships between organic layer depth and stem density and biomass were consistently negative for aspen, and positive or neutral for black spruce in all four burns. Our results suggest that initial effects of post-fire organic layer depths on deciduous recruitment are likely to translate into a prolonged phase of deciduous dominance during post-fire succession in severely burned stands. This shift in vegetation distribution has important implications for climate-albedo feedbacks, future fire regime, wildlife habitat quality and natural resources for indigenous subsistence activities in interior Alaska.

Emergence of morel (Morchella) and pixie cup (Geopyxis carbonaria) ascocarps in response to the intensity of forest floor combustion during a wildfire

We studied the density of ascocarps (mushrooms) of morels (Morchella) and pixie cups (Geopyxis carbonaria) as a function of postfire duff (forest floor organic layer) depth in the first 4 y after a wildfire. The great majority of ascocarps of both species appeared in the first summer (2004) after an Aug 2003 fire in predominantly pine-spruce montane stands in Kootenay National Park, British Columbia. The spatial distribution of the ascocarps of both species was strongly biased toward (i) microsites with thin postfire duff and (ii) proximity to standing burned tree trunks. The bases of ascocarps of both species invariably were found just below the surface of the mineral soil. A field experiment in nearby intact forest showed that complete or partial duff removal in the absence of damage to the roots or crown did not lead to ascocarps of either species. We conclude that for both fungal species an unusually large abundance of ascocarps simultaneously requires damage to the associated trees and major duff reduction.

Changes in terpenes of three kinds of pine needles during litter decomposition

This study was designed to evaluate changes in the terpene composition of 3 types of pines (Pinus densiflora, Pinus thunbergii and Pinus rigida), while decomposing their leaf litter. Needle litters were placed at two different organic layer depths, one on the surface and the other beneath the litter layer. Changes in the terpene composition of this litter were detected using a gas chromatograph-mass spectrometer. Among the monoterpenes acquired from the fresh needles of P. densiflora and P. rigida, α-pinene (12.05% and 19.87%, respectively) was the major one, followed by β-pinene (2.90% and 14.07%). However, from the needles of P. thunbergii, β-pinene (20.77%) was the major one, followed by α-pinene (10.79%). Among the sesquiterpenes detected in P. densiflora , trans-caryophyllene (3.12%) was the highest composition compound, whereas germacrene-D (6.09%) for P. thunbergii and 1,6-cyclodecadiene (7.41%) and endo-1-bourbonanol (7.41%) for P. rigida were the highest content compounds. However, the total amounts of terpenes decreased sharply by 40-85.4% in all three types of pine needle after 90-120 days of the experiment. The concentration of each terpene differed during decomposition, and the majority of compounds disappeared from beneath the litter layer. It was determined that three types of reducing patterns of each compound appeared on the rate of loss of concentration during decomposition; one pattern decreasing sharply during the initial period, another pattern steadily or slowly decreasing, and a newly detected pattern at low concentration occurring during decomposition.

Does Soil Organic Layer Thickness Affect Climate–Growth Relationships in the Black Spruce Boreal Ecosystem?

The observed long-term decrease in the regional fire activity of Eastern Canada results in excessive accumulation of organic layer on the forest floor of coniferous forests, which may affect climate–growth relationships in canopy trees. To test this hypothesis, we related tree-ring chronologies of black spruce (Picea mariana (Mill.) B.S.P.) to soil organic layer (SOL) depth at the stand scale in the lowland forests of Quebec’s Clay Belt. Late-winter and early-spring temperatures and temperature at the end of the previous year’s growing season were the major monthly level environmental controls of spruce growth. The effect of SOL on climate–growth relationships was moderate and reversed the association between tree growth and summer aridity from a negative to a positive relationship: trees growing on thin organic layers were thus negatively affected by drought, whereas it was the opposite for sites with deep (>20–30 cm) organic layers. This indicates the development of wetter conditions on sites with thicker SOL. Deep SOL were also associated with an increased frequency of negative growth anomalies (pointer years) in tree-ring chronologies. Our results emphasize the presence of nonlinear growth responses to SOL accumulation, suggesting 20–30 cm as a provisional threshold with respect to the effects of SOL on the climate–growth relationship. Given the current climatic conditions characterized by generally low-fire activity and a trend toward accumulation of SOL, the importance of SOL effects in the black spruce ecosystem is expected to increase in the future.

Modeling fire severity in black spruce stands in the Alaskan boreal forest using spectral and non-spectral geospatial data

Biomass burning in the Alaskan interior is already a major disturbance and source of carbon emissions, and is likely to increase in response to the warming and drying predicted for the future climate. In addition to quantifying changes to the spatial and temporal patterns of burned areas, observing variations in severity is the key to studying the impact of changes to the fire regime on carbon cycling, energy budgets, and post-fire succession. Remote sensing indices of fire severity have not consistently been well-correlated with in situ observations of important severity characteristics in Alaskan black spruce stands, including depth of burning of the surface organic layer. The incorporation of ancillary data such as in situ observations and GIS layers with spectral data from Landsat TM/ETM+ greatly improved efforts to map the reduction of the organic layer in burned black spruce stands. Using a regression tree approach, the R 2 of the organic layer depth reduction models was 0.60 and 0.55 ( p < 0.01) for relative and absolute depth reduction, respectively. All of the independent variables used by the regression tree to estimate burn depth can be obtained independently of field observations. Implementation of a gradient boosting algorithm improved the R 2 to 0.80 and 0.79 ( p < 0.01) for absolute and relative organic layer depth reduction, respectively. Independent variables used in the regression tree model of burn depth included topographic position, remote sensing indices related to soil and vegetation characteristics, timing of the fire event, and meteorological data. Post-fire organic layer depth characteristics are determined for a large (> 200,000 ha) fire to identify areas that are potentially vulnerable to a shift in post-fire succession. This application showed that 12% of this fire event experienced fire severe enough to support a change in post-fire succession. We conclude that non-parametric models and ancillary data are useful in the modeling of the surface organic layer fire depth. Because quantitative differences in post-fire surface characteristics do not directly influence spectral properties, these modeling techniques provide better information than the use of remote sensing data alone.

Topographic controls on black carbon accumulation in Alaskan black spruce forest soils: implications for organic matter dynamics

There is still much uncertainty as to how wildfire affects the accumulation of burn residues (such as black carbon (BC)) in the soil, and the corresponding changes in soil organic carbon (SOC) composition in boreal forests. We investigated SOC and BC composition in black spruce forests on different landscape positions in Alaska, USA. Mean BC stocks in surface mineral soils (0.34 ± 0.09 kg C m−2) were higher than in organic soils (0.17 ± 0.07 kg C m−2), as determined at four sites by three different 13C Nuclear Magnetic Resonance Spectroscopy-based techniques. Aromatic carbon, protein, BC, and the alkyl:O-alkyl carbon ratio were higher in mineral soil than in organic soil horizons. There was no trend between mineral soil BC stocks and fire frequencies estimated from lake sediment records at four sites, and soil BC was relatively modern (<54–400 years, based on mean Δ14C ranging from 95.1 to −54.7‰). A more extensive analysis (90 soil profiles) of mineral soil BC revealed that interactions among landscape position, organic layer depth, and bulk density explained most of the variance in soil BC across sites, with less soil BC occurring in relatively cold forests with deeper organic layers. We suggest that shallower organic layer depths and higher bulk densities found in warmer boreal forests are more favorable for BC production in wildfire, and more BC is integrated with mineral soil than organic horizons. Soil BC content likely reflected more recent burning conditions influenced by topography, and implications of this for SOC composition (e.g., aromaticity and protein content) are discussed.

Does the post-fire organic layer compress beneath the snowpack?

It is well known that post‐fire duff layers that are thin or of lower porosity greatly enhance juvenile survivorship of sexually recruiting boreal plant species. Nonetheless, there has been no study on duff compaction by snow following charring. We examined post‐fire duff depth for the first 3 years (two winters) after a 2006 wildfire in the boreal forest of north‐western Quebec. We found that (1) significant compression was common, with (2) a positive relationship between the initial thickness of the burned organic layer depth and the subsequent amount of compression. The proportional compression rate was, however, roughly constant: ∼14% after 2 years regardless of initial post‐fire duff depth. We conclude that this amount of compression is too little to significantly impact post‐fire seedling recruitment.

Directional change in upland tundra plant communities 20‐30 years after seismic exploration in the Canadian low‐arctic

AbstractQuestion: What is the disturbance response of low‐arctic plant communities two to three decades after seismic exploration.Location: Mackenzie River Delta, low‐arctic, northwestern Canada.Methods: Plant communities in two upland tundra vegetation types were compared between winter seismic lines, created between 1970 and 1986, and adjacent “reference” tundra. Also, we used aerial surveys to quantify the total area impacted by visible linear features.Results: Vascular plant cover was significantly higher, and lichen cover significantly lower, on seismic lines than in reference tundra. The increase in vascular plant cover was attributable to deciduous shrubs and graminoids. There were significant differences in plant community composition between seismic lines and reference tundra but no differences in species diversity or richness. Betula glandulosa and Arctagrostis latifolia were significant indicator species for seismic lines, while Saussurea angustifolia was a significant indicator for reference tundra. Based on the aerial surveys, these effects apply to at least 90% of seismic lines from two‐dimensional programs in these habitat types during the 1970s.Conclusions: Vegetation composition and structure on 20‐30‐year‐old seismic lines differs from reference upland tundra despite no persistent differences in organic layer depth or depth to permafrost. We propose that this reflects: (1) successional redevelopment following changes in soil conditions and nutrient availability arising from the disturbance, and/or (2) disturbance‐initiated succession towards a community reflecting current climatic conditions.

Paludification dynamics in the boreal forest of the James Bay Lowlands: effect of time since fire and topography

In many northern forest ecosystems, soil organic matter accumulation can lead to paludification and forest productivity losses. Paludification rate is primarily influenced by topography and time elapsed since fire, two factors whose influence is often confounded and whose discrimination would help forest management. This study, which was conducted in the black spruce ( Picea mariana (Mill.) BSP) boreal forest of northwestern Quebec (Canada), aimed to (1) quantify the effect of slope and time since fire on paludification rates, (2) determine whether soil organic layer depth could be estimated by surface variables that can potentially be remotely sensed, and (3) relate the degree of paludification to tree productivity. In this study, soil organic layer depth was used as an estimator of the degree of paludification. Slope and postfire age strongly affected paludification dynamics. Young stands growing on steep slopes had thinner organic layers and lower organic matter accumulation rates compared with young stands growing on flat sites. Black spruce basal area and Sphagnum cover were strong predictors of organic layer depth, potentially allowing mapping of paludification degree across the landscape. Tree productivity was negatively related to organic layer depth (R2= 0.57). The equations developed here can be used to quantify forest productivity decline in stands that are undergoing paludification, as well as potential productivity recovery given appropriate site preparation techniques.

Sphagnum mosses limit total carbon consumption during fire in Alaskan black spruce forests

The high water retention of hummock-forming Sphagnum species minimizes soil moisture fluctuations and might protect forest floor organic matter from burning during wildfire. We hypothesized that Sphagnum cover reduces overall forest floor organic matter consumption during wildfire compared with other ground-layer vegetation. We characterized variability in soil organic layer depth and organic matter stocks in two pairs of burned and unburned black spruce ( Picea mariana (Mill.) BSP) stands in interior Alaska. In the unburned stands, microsites dominated by Sphagnum had more than twice as much soil organic matter·m–2 as microsites dominated by feather moss and (or) lichens. Whereas 20% of soil organic matter was consumed during fire in microsites dominated by Sphagnum, 45% was consumed in microsites dominated by the feather moss and (or) lichens. Across 79 recently burned black spruce stands, unburned moss abundance (primarily remnant Sphagnum hummocks), landscape position (backslope, flat upland, flat lowland classes), and the interaction among these variables explained 60% of postfire organic soil depths. We suggest that “Sphagnum sheep” could serve as a useful visual indicator of variability in postfire soil carbon stocks in boreal black spruce forests. Sphagnum mosses are important ecosystem engineers not only for their influence on decomposition rates, but also for their effect on fuel consumption and fire patterning.

Thermal Stability and Safe Venting of the Tri-N-Butyl Phosphate-Nitric Acid-Water (“Red Oil”) System - III: Predictions of Thermal Stability Boundaries and Required Vent Size

A Semenov-type analysis is made of the conditions for an exothermic runaway reaction in an “organic phase” (or “red oil”) made up of tri-n-butyl phosphate (TBP) saturated with nitric acid (HNO3). Also, a theoretical framework is developed to predict the critical organic layer depth above which a runaway will occur when the organic layer rests on a layer of aqueous nitric acid (“aqueous phase”). Available calorimetry data on peak pressurization rates during vented TBP/HNO3 reactions are rationalized using orifice flow theory, which provides a simple criterion for the required vent area for vessel pressure relief during a red oil runaway. Finally, it is shown that the Tomsk-7 accident can be explained by a combination of weak reaction tempering at the vessel relief valve set pressure and insufficient venting capacity. The formulations for determining the onset of a TBP/HNO3 runaway outlined in this paper rely heavily on the empirical and semiempirical equations developed in the companion paper “Thermal Stability and Safe Venting of the Tri-N-Butyl Phosphate-Nitric Acid-Water (“Red Oil”) System - II: Experimental Data on Reaction Self-Heat Rates and Gas Production and Their Correlation,” Nuclear Technology, Vol. 163, p. 294 (2008), which deals with the chemical self-heat rate in the organic phase, the gas production rate in the organic phase, and the superficial gas velocity across the aqueous-organic interface of a two-layer organic over aqueous configuration.

Evaluation of the composite burn index for assessing fire severity in Alaskan black spruce forests

We evaluated the utility of the composite burn index (CBI) for estimating fire severity in Alaskan black spruce forests by comparing data from 81 plots located in 2004 and 2005 fire events. We collected data to estimate the CBI and quantify crown damage, percent of trees standing after the fire, depth of the organic layer remaining after the fire, depth of burning in the surface organic layer (absolute and relative), and the substrate layer exposed by the fire. To estimate pre-fire organic layer depth, we collected data in 15 unburned stands to develop relationships between total organic layer depth and measures of the adventitious root depth above mineral soil and below the surface of the organic layer. We validated this algorithm using data collected in 17 burned stands where pre-fire organic layer depth had been measured. The average total CBI value in the black spruce stands was 2.46, with most of the variation a result of differences in the CBI observed for the substrate layer. While a quadratic equation using the substrate component of CBI was a relatively strong predictor of mineral soil exposure as a result of fire (R2 = 0.61, P &lt; 0.0001, F = 60.3), low correlations were found between the other measures of fire severity and the CBI (R2 = 0.00–0.37). These results indicate that the CBI approach has limited potential for quantifying fire severity in these ecosystems, in particular organic layer consumption, which is an important factor to understand how ecosystems will respond to changing climate and fire regimes in northern regions.

Impacts of postfire salvage logging on understory plant communities of the boreal mixedwood forest 2 and 34 years after disturbance

We compared understory vegetation composition and richness in aspen-dominated boreal mixedwood forest stands in Alberta, Canada, that had been burned by wildfire with those that burned and were subsequently salvage logged. Stands were examined at early and midsuccessional (2 and 34 years after disturbance(s), respectively) developmental stages. In comparison with wildfire stands, understory communities of early successional salvage-logged stands were characterized by greater species richness, weedy species presence, higher shrub abundance, and lower abundances of fire-specialist seed bank species. In constrained ordination, the understory community of early successional wildfire stands was related to greater canopy cover, sapling density, and moss depth, whereas that of salvage-logged stands was related to greater light, volume of downed deadwood, and litter and organic matter. Longer term effects of salvage logging on the understory community were minimal and, instead, reflected the influence of forest canopy redevelopment. In midsuccessional stands, understory composition was related to conifer density, litter cover, soil moisture, organic layer depth, tall shrub density, and bryophyte-covered microsite cover. Postfire salvage logging can have substantial short-term effects on the postfire understory plant community; in the longer term, effects will depend to a large extent on the influence of harvesting and subsequent management on canopy redevelopment.

Topographic influences on wildfire consumption of soil organic carbon in interior Alaska: Implications for black carbon accumulation

We measured characteristics of soil organic carbon (SOC) and black carbon (BC) along opposed north‐ and south‐facing toposequences in recent (2004) and old (∼1860–1950) burn sites throughout interior Alaska. Surface fuel consumption did not vary between different topographic positions, with 3.3 (±0.5) kg C m−2 being consumed on toe slope sites, 4.0 (±0.2) kg C m−2 on north‐slope sites, and 3.6 (±0.3) kg C m−2 on south‐slope sites. On a relative scale, 43% of the organic matter depth was removed through biomass burning on toe slopes, 62% on north‐slopes, and 77% on south‐slope sites. Mineral soil BC stocks (measured using a chemical/thermal oxidation method isolating more graphitic BC) ranged from 112 ± 24 g C m−2 to 173 ± 43 g C m−2 on north‐ and south‐facing aspects, respectively. These data supported the expectation that deeper and wetter ground fuels occurring in north‐facing and toe‐slope forests burned less completely, which decreased the amount of BC incorporated into mineral soil where it could be protected from subsequent fires. Thus, warmer and drier forests harbored less total SOC, but following wildfire aggraded larger BC stocks. BC was therefore an important component of stable soil C in warmer and drier forests which had higher C turnover rates in other soil pools (determined by density fractionation, δ13C, and Δ14C). These data provide a basis for understanding how variable patterns of organic layer depths and soil moisture mediate the consumption of SOC in wildfire, as well as the long‐term accumulation of BC.

The reduction of organic-layer depth by wildfire in the North American boreal forest and its effect on tree recruitment by seed

We compared prefire and postfire organic-layer depths in boreal forest types (14 fires) across Canada, and examined tree recruitment as a function of depth. There was extensive within-stand variation in depth, much of it due to clustering of thinner organic layers around boles. There were no significant differences in postfire organic-layer depth among sites with different prefire forest species composition, but sites in the eastern boreal region had thicker postfire organic layers than those in the western boreal region. Mean organic-layer depth was much greater in intact stands than after fires; overall, fire reduced organic-layer depth by 60%, largely because of increases in the area of thin (&lt;3 cm) organic layers (1% in intact stands vs. 40% in postfire stands). There was more variation in organic-layer depth within postfire than within prefire stands; notably, some areas in postfire stands were deeply combusted, while adjacent parts were only lightly combusted. We speculate that the diminished role of energy loss to latent heat around tree boles increased organic-layer consumption around tree boles. Seedlings were clustered around burned tree bases, where organic layers were thinner, and the dependence of a species on thin organic layers was an inverse function of seed size.

Carbon storage in the organic layers of boreal forest soils under various moisture conditions: A model study for Northern Sweden sites

A typical feature of the boreal forest landscape is a gradient from dry to wet sites, with associated increases in the depth of the soil organic layers. In this study, the coupled ecosystem–soil biogeochemistry model GUESS–ROMUL is used to explore how the specific features of soil organic matter decomposition and vegetation dynamics account for an observed difference between the soils formed under contrasting moisture conditions. Two sites, one mesic and one mesic-to-wet, representative of the natural forest in Northern Sweden, are simulated. In addition to the assumptions underlying the GUESS–ROMUL model, it is assumed that the fire frequency was higher at the mesic site. The model shows that with a natural fire regime, the soil organic layers at the mesic-to-wet site store 6.0 kg C m −2 compared to 3.1 kg C m −2 at the mesic site. Forty-seven percent of the difference between the sites in this respect is explained by suppressed decomposition under higher moisture conditions, 37% by the decreased litter input into the soil (more frequently disturbed ecosystems have lower productivity) and 16% by direct consumption of the forest floor in fires. It is predicted that due to anthropogenic fire suppression the organic soil layers of mesic sites will, in the future, sequester carbon at an average rate of 0.0103 kg C m −2 year −1 and have an equilibrium storage capacity of 5.4 kg C m −2. For the mesic-to-wet site, the model predicts an extremely slow sequestration rate of 0.0022 kg C m −2 year −1. The effect of increased precipitation on the carbon storage at the landscape level is also investigated.

Variation in postfire organic layer thickness in a black spruce forest complex in interior Alaska and its effects on soil temperature and moisture

This study investigated the relationship between climate and landscape characteristics and surface fuel consumption as well as the effects of variations in postfire organic layer depth on soil temperature and moisture in a black spruce (Picea mariana (Mill.) BSP) forest complex in interior Alaska. Mineral soil moisture and temperature at the end of the growing season and organic layer depth were measured in three burns occurring in different years (1987, 1994, 1999) and in adjacent unburned stands. In unburned stands, average organic layer and humic layer depth increased with stand age. Mineral soil temperature and moisture varied as a function of the surface organic layer depth in unburned stands, indicating that as a stand matures, the moisture content of the deep duff layer is likely to increase as well. Fires reduced the depth of the surface organic layers by 5 to 24 cm. Within each burn we found that significant variations in levels of surface fuel consumption were related to several factors, including mineral soil texture, presence or absence of permafrost, and timing of the fires with respect to seasonal permafrost thaw. While seasonal weather patterns contribute to variations in fuel moisture and consumption during fires, interactions among the soil thermal regime, surface organic layer depth, and previous fire history are also important in controlling patterns of surface fuel consumption.

Vegetation patterns resulting from spatial and temporal variability in hydrology, soils, and trampling in an isolated basin marsh, New Hampshire, USA

This study investigated the role of hydrology, soils, and trampling in determining the distribution of vegetation in an isolated wetland from 1996 to 1999. Grassy Pond, in Litchfield, New Hampshire, is a seasonally flooded basin marsh situated in sandy soils. It is a depressional wetland consisting of three connected basins; it has no surface-water inlets or outlets. This acidic, low-nutrient wetlands contains several rare species and represents an uncommon ecosystem type in New England. A network of wells and piezometers monitored from 1996 to 1999 established that the wetland receives an average of 95.4% of its growing season inputs from precipitation and the rest from shallow ground water flowing through the wetland; as a result, it experiences large fluctuations in water levels. Vegetation in the wetland fell into five major elevation zones. Variation in plant diversity within each zone, and differences between adjacent zones, result in part from differences in depth of organic layer, trampling by hikers, all-terrain vehicles, and native wildlife, and the extent of water-level fluctuations. Several of the low basin species are more likely to be found in trampled areas. Shrub invasion of the open basin areas is prevented by trampling in some areas and by high water levels in others. Variability in hydrology resulted in temporal, as well as spatial, variability in the plant community, as dry years yielded significantly greater diversity, including a large increase in tree seedlings. Both hydrologic variability and trampling are external factors that explain a significant portion of the variation in vegetation on a large scale and connect this geographically isolated wetland to the surrounding landscape. On a smaller scale, however, autogenic forces related to soil formation and plant species interactions may be more important in explaining the plant diversity in the wetland.

Wounding of aspen roots promotes suckering

In early May, 1-m sections of trembling aspen (Populus tremuloides Michx.) roots in a forest cutblock were carefully exposed and examined for damage. Undamaged roots were subjected to one of three wounding treatments (scrape, sever, or uninjured control) and were then reburied to either the full normal organic layer depth or to one third of the normal depth. Following one growing season, the roots were reexposed and assessed for aspen sucker numbers and growth rates. Results indicate that injured roots produced suckers nearly twice as often as uninjured roots. Further, injured roots produced more suckers per root, and these suckers were taller and had greater leaf area. Roots buried under shallow organic layers also generated more suckers, regardless of injury type. The side of injury (distal or proximal) did not affect any of the measured variables. The present study suggests that moderate wounding of aspen roots increases initial sucker numbers and growth rates.Key words: trembling aspen, root sucker, root injury, regeneration.