Stand-replacing Wildfire Research Articles

Fire regimes and forest structure in a sky island mixed conifer forest, Guadalupe Mountains National Park, Texas, USA

Fire is a key disturbance agent in the fire-prone mixed conifer and ponderosa pine forests of the southwestern United States. Human activities (i.e., livestock grazing, logging, and fire suppression) have resulted in the exclusion of fire from these forests for the past century and fire exclusion has caused changes in forest structure and composition. This study quantifies spatial and temporal variability in fire regimes and forest change in a 1000-ha area of mixed conifer forest in Guadalupe Mountains National Park (GMNP), an area with an uncommon history of grazing and fire suppression. Dendroecological methods were used to quantify fire frequency, season, severity, and extent, as well as forest structural and compositional change. The mean composite fire return interval (CFI) for the study area was 4 years. Widespread fires were less frequent. The mean CFI for fires recorded in at least 10% of the samples collected was 9.2 years, and mean CFI for fires scarring at least 25% of samples was 16.3 years. Many of these widespread fires occurred in the 19th century. The mean point fire return interval (PFI) was longer at 24 years. Fire scars were primarily formed in the earliest portion of earlywood in annual rings, indicating that fires burned mainly in the spring, at the beginning of the growing season. The onset of grazing in the 1920s dramatically reduced fire frequency. An increase in tree density and a compositional shift from southwestern white pine ( Pinus strobiformis Engelm.) to Douglas-fir ( Pseudotsuga menziesii [Mirb.] Franco) coincides with the grazing era. In addition, the pre-ranching era was characterized by low-severity fires, while structural changes have resulted in a contemporary forest that is prone to high severity fire, as evidenced by two stand-replacing wildfires in GMNP in the 1990s.

SPATIALLY SCALED RESPONSE OF A LAZULI BUNTING POPULATION TO FIRE

We examined the response of Lazuli Bunting (Passerina amoena) to fire in Gambel oak (Quercus gambelii) woodland at Camp Williams, Utah, during 1993–1998. Overall, Lazuli Bunting abundance on the study area increased significantly during the 2 years after a stand-replacing wildfire, which covered 800 ha of Gambel oak woodland. This increase suggested that Lazuli Buntings respond positively to fire. However, a comparison of pre- and post-fire abundance of Lazuli Bunting for 2 groups of monitoring plots with different fire histories showed that abundance was significantly greater during the post-fire period for both burned and unburned plots. When we examined our data at a spatial scale appropriate to Lazuli Bunting, we found that post-fire increases observed on unburned plots were limited to plots in close proximity to the burned area. A comparison of pre- and post-fire abundance of Lazuli Bunting for 3 groups of monitoring plots located at various distances from the burned area revealed that post-fire abundance was similar only for plots within the fire boundary and for those ≤1000 m from the fire boundary; plots located >1000 m from the fire boundary had fewer individuals per plot post-fire. However, prefire Lazuli Bunting abundance was similar among all 3 categories. This differential, spatially scaled response of Lazuli Bunting to fire at the landscape level may support a hierarchical view of habitat selection.

Past, Present, and Future Old Growth in Frequent-fire Conifer Forests of the Western United States

Old growth in the frequent-fire conifer forests of the western United States, such as those containing ponderosa pine (Pinus ponderosa), Jeffrey pine (P. jeffreyi), giant sequoia (Sequioa giganteum) and other species, has undergone major changes since Euro-American settlement. Understanding past changes and anticipating future changes under different potential management scenarios are fundamental to developing ecologically based fuel reduction or ecological restoration treatments. Some of the many changes that have occurred in these forests include shifts from historically frequent surface fire to no fire or to stand-replacing fire regimes, increases in tree density, increased abundance of fire-intolerant trees, decreases in understory productivity, hydrological alterations, and accelerated mortality of old trees. Although these changes are widespread, the magnitude and causes of changes may vary within and among landscapes. Agents of change, such as fire exclusion or livestock grazing, likely interacted and had multiple effects. For example, historical ranching operations may have altered both fire regimes and understory vegetation, and facilitated institutional fire exclusion through fragmentation and settlement. Evidence exists for large variation in presettlement characteristics and current condition of old growth across this broad forest region, although there are many examples of striking similarities on widely distant landscapes. Exotic species, climate change, unnatural stand-replacing wildfires, and other factors will likely continue to degrade or eradicate old growth in many areas. As a policy of fire exclusion is proving to be unsustainable, mechanical tree thinning, prescribed fire, or wildland fire use will likely be key options for forestalling continued eradication of old growth by severe crown fires. For many practical and societal reasons, the wildland—urban interface may afford some of the most immediate opportunities for re-establishing old growth typical of presettlement forests resistant to crown fires.

Influences of thinning, prescribed burning, and wildfire on soil processes and properties in southwestern ponderosa pine forests: A retrospective study

Following Euro-American settlement in the late 1800s, fire suppression and livestock grazing in ponderosa pine-bunchgrass ecosystems of the southwestern US resulted in the replacement of grass openings with dense stands of ponderosa pine. This, in turn, has led to apparent decreases in decomposition, net N mineralization, and soil respiration (i.e., net soil CO 2 efflux) rates, losses of floral and faunal diversity, and greater risk of the occurrence of stand-replacing wildfire. Alterations in ecosystem structure and function have prompted the implementation of restoration treatments throughout the region. Using a retrospective approach, we investigated the effects of thinning, thinning plus prescribed fire, and wildfire, 6–15 years following treatments, on ecosystem processes and properties across a 750-km 2 area in northern Arizona. We found that stands that had thinning or thinning plus burning treatments had lower in situ annual rates of net N mineralization in the mineral soil than unmanaged stands. However, stands burned by high-severity wildfire had net N mineralization rates that were about 60% higher than unmanaged stands. Because of similarities in net N mineralization rates among treatments under laboratory conditions, we speculate that variation in in situ net N transformation rates among stands were due to differences in C inputs (thinned and thinned plus burned) and soil microclimate (wildfire) among the stands. Size of the soil microbial C and N pools generally declined with decreases in litterfall (highest in unmanaged, intermediate in thinned and thinned plus burned, and lowest in wildfire stands); however, in situ rates of net soil CO 2 efflux did not follow this pattern. Our results contrast with some previous studies in southwestern ponderosa pine forests where restoration treatments (both thinning and thinning plus burning) increased net N transformation rates. We hypothesize that the dissimilarity in responses to treatments across studies is due to differences in the relative effect of these treatments on the quantity and quality of C inputs. Large increases in easily decomposable understory biomass following restoration treatments appear to result in increases in N cycling rates, whereas treatments that result in either no change or minimal increases in understory biomass, accompanied by a long-term decrease in pine needle input to the forest floor, lead to reductions in N cycling rates. We recommend that restoration and fire-hazard reduction treatments be applied preferentially to stands that currently have low understory production.

Wildfire disturbance and shallow landsliding in coastal British Columbia over millennial time scales: A numerical modelling study

An algorithm for wildfire occurrence is introduced for incorporation into a numerical model of drainage basin evolution. Within the model, fire return intervals are determined using a stochastic rule set and fire sizes are assigned according to a pareto distribution. A Weibull distribution was fit to millennial-scale lake charcoal data for calibration of fire return intervals in this study, while data for the Olympic Peninsula were used to determine the fire size distribution. Loss of tree root strength resulting from stand-replacing wildfires has been found to be comparable to loss after timber harvesting. Hence, data for increased shallow landslide activity associated with logging were used to calibrate transport equations for years following wildfire incidence. A weathering relation is incorporated in the model, which drives the supply of sediment. When sediment supply is depleted in the model, the shallow landsliding module shuts off until new material is available for transport. Time since last fire analysis indicates that the Weibull parameters defined for the Clayoquot charcoal data produce results that are weighted too heavily towards recent fires. Sensitivity analysis was undertaken, suggesting alternative values may be more reasonable for further model runs. The introduction of a weighted probability factor for aspect, whereby south-facing slopes have a higher likelihood of fire initiation, was not found to significantly affect patterns of fire age in the landscape. Patterns of net topographic change after 6000 years of model run time indicate that greatest net erosion is found on steep, upper slopes, with the highest deposition occurring in hillslope hollows and low-order channels. Depositional grid cells in interfluves show a variable response to the inclusion of wildfire in the model. Net erosion occurs over a relatively low proportion of hillslope area, with wildfire model runs showing a strong erosional signal in these locations. For stream grid cells, the additional deposition due to the inclusion of wildfire in model runs is about 9–10% for 1st to 3rd order streams. Overall, deposition rates increase with decreasing stream order, reflecting greater hillslope/channel coupling in low-order streams.

Changes in soil carbon and nitrogen cycling along a 72-year wildfire chronosequence in Michigan jack pine forests

We investigated the changes in soil processes following wildfire in Michigan jack pine (Pinus banksiana) forests using a chronosequence of 11 wildfire-regenerated stands spanning 72 years. The objective of this study was to characterize patterns of soil nutrients, soil respiration and N mineralization with stand development, as well as to determine the mechanisms driving those patterns. We measured in situ N mineralization and soil respiration monthly during the 2002 growing season and used multiple regression analysis to determine the important factors controlling these processes. Growing-season soil respiration rates ranged from a low of 156 g C/m2 in the 7-year-old stand to a high of 254 g C/m2 in the 22-year-old stand, but exhibited no clear pattern with stand age. In general, soil respiration rates peaked during the months of July and August when soil temperatures were highest. We used a modified gamma function to model a temporal trend in total N mineralization (total N mineralization=1.853-0.276xagexe-0.814xage; R2=0.381; P=0.002). Total N mineralization decreased from 2.8 g N/m2 in the 1-year-old stand to a minimum value of 0.5 g N/m2 in the 14-year-old stand, and then increased to about 1.5 g N/m2 in mature stands. Changes in total N mineralization were driven by a transient spike in N turnover in the mineral soil immediately after wildfire, followed by a gradual accrual of a slow-cycling pool of N in surface organic horizons as stands matured. Thus, in Michigan jack pine forests, the accumulation of surface organic matter appears to regulate N availability following stand-replacing wildfire.

Fire and the origin of Table Mountain pine – pitch pine communities in the southern Appalachian Mountains, USA

The prevalence of stand-replacing fire in the formation of Table Mountain pine – pitch pine (Pinus pungens Lamb. and Pinus rigida Mill., respectively) communities was investigated with dendrochronological techniques. Nine stands in Georgia, South Carolina, and Tennessee were analyzed for age structure, species recruitment trends, and radial growth patterns to determine whether they had originated as a result of stand-replacing fires. The oldest pines date from the late 1700s or early 1800s. Continuous or frequent episodic pine regeneration from those times to the early to mid 1900s was evident at all sites. During the first half of the 20th century, all sites experienced large surges in pine regeneration. However, no clear evidence of stand-replacing wildfires could be definitively linked to these surges. Rather, the regeneration appeared to have been caused by noncatastrophic surface fires and canopy disturbances occurring together or by the cessation of a frequent fire regime. For the past 25–50 years, there has been little pine regeneration at any of the sites. Restoring the dual disturbance regime of periodic fires and canopy disturbances should help sustain Table Mountain pine – pitch pine communities in southern Appalachian Mountains landscapes.

Carbon Storage on Landscapes with Stand-replacing Fires

Abstract Many conifer forests experience stand-replacing wildfires, and these fires and subsequent recovery can change the amount of carbon released to the atmosphere because conifer forests contain large carbon stores. Stand-replacing fires switch ecosystems to being a net source of carbon as decomposition exceeds photosynthesis—a short-term effect (years to decades) that may be important over the next century if fire frequency increases. Over the long term (many centuries), net carbon storage through a fire cycle is zero if stands replace themselves. Therefore, equilibrium response of landscape carbon storage to changes in fire frequency will depend on how stand age distribution changes, on the carbon storage of different stand ages, and on postfire regeneration. In a case study of Yellowstone National Park, equilibrium values of landscape carbon storage were resistant to large changes in fire frequency because these forests regenerate quickly, the current fire interval is very long, the most rapid chan...

Effects of post-fire conditions on germination and seedling success of diffuse knapweed in northern Arizona

Introduction of exotic plant species is confounding treatments designed to reduce unnaturally high tree densities in ponderosa pine ( Pinus ponderosa P. & C. Lawson) forests of the western United States. Also, the recent increase in large, stand-replacing wildfires may promote introduction and spread of species, such as diffuse knapweed ( Centaurea diffusa Lam.), which may lead to reduced forage and land values and major ecosystem changes. We hypothesized that diffuse knapweed germination and seedling growth would respond positively to burned conditions in northern Arizona ponderosa pine forests. Our first field study was based on a May 2002 wildfire near Flagstaff, Arizona (Site I). For the field study at Site I, we buried diffuse knapweed seeds in sealed mesh packets in four forest floor conditions: severely burned, moderately burned, unburned bare soil and unburned. We removed packets monthly and measured percent germination. Germination under severely burned conditions at Site I (overall average 76%) was higher than in unburned conditions (59%) ( P = 0.017). Our second field study was based on a fuel reduction treatment with slash piles burned in February 2003 (Site II). Germination in seed packets was higher ( P = 0.003) in pile burn scars (67%) than in adjacent unburned locations (38%) at Site II. In fall 2002, we planted knapweed seeds in pots containing intact soil cores from Site I from unburned and severely burned conditions. We allowed competition to grow in half the pots, and removed competition in the other half. Diffuse knapweed biomass/pot was greater ( P = 0.035) in soil from the severely burned (2.6 g) compared to the unburned condition (0.6 g) from Site I. Knapweed biomass/pot also appeared greater when grown in soil from pile burn scars (3.3 g) compared with unburned locations (2.5 g) from Site II, although the difference was not significant ( P = 0.317). We conclude that severe wildfire or pile burning can promote germination and seedling growth of diffuse knapweed in northern Arizona pine forests. Moreover, diffuse knapweed may also threaten unburned forests, as average germination was 48% in unburned locations. Failure to consider exotic invasive species in fuel reduction and wildfire rehabilitation may result in trading one undesirable condition for another.

Restoration treatments in a Montana ponderosa pine forest: Effects on soil physical, chemical and biological properties

Low-elevation ponderosa pine ecosystems of the inland northwestern United States experienced frequent, low-severity fire that promoted open stands dominated by large diameter ponderosa pine ( Pinus ponderosa). Fire exclusion has led to increased stand densities, often due to proliferation of less fire-tolerant species and an increased risk of stand-replacing wildfire. These fundamental changes have spurred interest in forest restoration treatments, including thinning, prescribed burning and thinning combined with prescribed burning. We examined the response of numerous soil physical, chemical and biological parameters to these treatments 1 and 3 years post-treatment, using a replicated field experiment. Individual restoration treatments were implemented in 9 ha units. We observed significantly lower C:N in the O horizon and higher O horizon and mineral soil NH 4 + concentrations in both BURN and THIN/BURN treatments during year 1. Soil NH 4 + remained elevated through year 3 in the THIN/BURN treatment. Net N mineralization, nitrification and NO 3 – concentration were significantly greater in the THIN/BURN than all other treatments during year 1 and net nitrification rates remained elevated through year 3. A high C:N substrate decomposed more rapidly in both BURN treatments relative to the unburned treatments. Treatments had no immediate effect on the soil microbial community; however, phospholipid fatty acid profiles differed 16–18 weeks following treatments due to higher actinomycetes in the THIN/BURN treatment. The large scale of our treatment units resulted in significant variation in fire severity among prescribed burns as a function of variation in fuel quantity and distribution, and weather conditions during burn days. Correlation analysis revealed that variation in fine fuel consumed was tightly correlated with net N mineralization and net nitrification. These differences in soil characteristics may influence stand productivity and understory species composition in the future.

Postfire Soil N Cycling in Northern Conifer Forests Affected by Severe, Stand-Replacing Wildfires

Severe, stand-replacing fires affect large areas of northern temperate and boreal forests, potentially modifying ecosystem function for decades after their occurrence. Because these fires occur over large extents, and in areas where plant production is limited by nitrogen (N) availability, the effect of fire on N cycling may be important for long-term ecosystem productivity. In this article, we review what is known about postfire N cycling in northern temperate and boreal forests experiencing stand-replacing fires. We then build upon existing literature to identify the most important mechanisms that control postfire N availability in systems experiencing severe, stand-replacing fires compared with fires of lower severity. These mechanisms include changes in abiotic conditions caused by the opening of the canopy (for example, decreased LAI, increased solar radiation), changes in ground layer quantity and quality (for example, nutrient release, permafrost levels), and postfire plant and microbial adaptations affecting N fixation and N uptake (for example, serotiny, germination cues). Based on the available literature, these mechanisms appear to affect N inputs, internal N cycling, and N outputs in various ways, indicating that severe fire systems are variable across time and space as a result of complex interactions between postfire abiotic and biotic factors. Future experimental work should be focused on understanding these mechanisms and their variability across the landscape.

Associations between Forest Fire and Mexican Spotted Owls

Abstract In 1993, the US Fish and Wildlife Service listed the Mexican spotted owl (Strix occidentalis lucida) as threatened, in part because of the rising threat to its habitat from stand-replacing wildfires. In 1997, we surveyed 33 owl sites that, in the previous four years, had burned at various levels ranging from light controlled burns to stand-replacing fires. We compared owl occupancy and reproduction in these burned sites to 31 unburned owl sites with similar habitat and topography. Although unburned sites showed higher proportions of both occupancy and reproduction, the negative relationship observed between recent fire occurrence and owl occupancy rank was statistically weak (Test for Marginal Homogeneity, P = 0.110). Owls tended not to be present where pure pine stands (Pinus spp.) comprised a large proportion (38–85%) of burned sites, but no other factors relating to habitat or fire severity had a significant, biologically interpretable influence on occupancy rank. We suspect that relatively low-intensity ground fires, including most prescribed fires, probably have little or no short-term impact on Mexican spotted owl presence or reproduction, but we have no data on long-term effects of fire. We recommend proactive fuels-management treatments in areas not currently occupied by owls as a means of reducing fire risk in areas occupied by owls. Within areas occupied by owls, judicious treatments may be appropriate after case-by-case evaluations of potential benefits and risks within those sites. FOR. SCI. 50(6):765–772.

Loss and recovery of ecosystem carbon pools following stand-replacing wildfire in Michigan jack pine forests

We used a 72-year chronosequence to study the loss and recovery of ecosystem C pools following stand-replacing wildfire in Michigan, USA, jack pine (Pinus banksiana Lamb.) forests. We quantified the amount of C stored in aboveground plant biomass, standing dead timber, downed dead wood, surface organic soil, and mineral soil in 11 jack pine stands that had burned between 1 and 72 years previously. Total ecosystem C ranged from a low of 59 Mg C·ha–1 in the 4-year-old stand to 110 Mg C·ha–1 in the 72-year-old stand. Changes in total ecosystem C across the chronosequence conformed to theoretical predictions, in which C stocks declined initially as decomposition of dead wood and forest-floor C exceeded production by regenerating vegetation, and then increased asymptotically with the development of a new stand of jack pine. This pattern was well described by the following "gamma" function: total ecosystem C (Mg·ha–1) = 112.2 – 39.6 × age0.351 × exp(–0.053 × age01.039); mean-corrected R2 = 0.976. Using the first derivative of this parameterized gamma function, we estimated that jack pine stands function as a weak source of C to the atmosphere for only ca. 6 years following wildfire, and reach a maximum net ecosystem productivity of 1.6 Mg C·ha–1·year–1 by year 16. We attribute the rapid transition from carbon source to carbon sink in these ecosystems to two factors: (i) stand-replacing wildfires in these xeric forests leave behind little respirable substrate in surface organic horizons, and (ii) jack pine is able to rapidly reestablish following wildfires via serotinous cones. Jack pine stands remained net sinks for C across the chronosequence; however, net ecosystem productivity had declined to 0.12 C ha–1·year–1 by year 72. Carbon sequestration by mature jack pine ecosystems was driven primarily by continued growth of overstory jack pine, not by accumulation of detrital C.

Using History to Plan the Future of Old-Growth Ponderosa Pine

Abstract Historical forest records, combined with the Forest Vegetation Simulator and a geographic information system for a land planning area on the Kaibab National Forest in northern Arizona, can suggest management approaches to restoring old-growth forest structures. Our analysis indicated that although more than 50 percent of ponderosa pine stands in the planning area are at or near an old-growth stage, they are at historically high tree densities. Stand-replacing wildfires have recently burned some old-growth stands. Silvicultural methods to restore desired old-growth structure and reduce fire danger can be evaluated with easy-to-use forest simulation models and validated with available historical and ground truth data.

Structural characteristics of wet montane forests in east-central British Columbia

Structural characteristics of forest stands were examined along a post-fire age chronosequence for wet montane sub-boreal and sub-alpine forests in the northern portion of the Rocky Mountains in British Columbia, Canada. The objective was to develop criteria that could be used to assess the extent to which managed stands approximate the structural characteristics of natural stands. Twelve and fifteen stands were sampled in wet montane sub-boreal and high-elevation subalpine forests, respectively. Tree density, variation in tree size, snag density by size class and coarse woody debris volume were examined for young (0–70 years), mature (71–140 years), and old (> 140 years) stands. Apart from a general increase in average tree size and a decrease in snag density, changes in other stand attributes over time since disturbance were limited, especially when compared to forests in drier climates at similar latitudes. The combination of low density and large variability in tree size of the young wet montane sub-boreal stands appear to be unusual for low elevation forests that originate from stand-replacing wildfire. In the study area, current management practices of salvage-logging fire-killed stands and planting relatively high densities of spruce on harvested sites should be examined in light of our data. This study illustrates the importance of developing area-specific ecosystem management guidelines relating to stand structure. Key words: stand structure, woody debris, snags, Picea engelmannii, Abies lasiocarpa, wildlife habitat

Survival of Suillus pungens and Amanita francheti ectomycorrhizal genets was rare or absent after a stand-replacing wildfire.

• We looked for evidence of mycelial survival by Suillus pungens and Amanita francheti following a stand-replacing wildfire. These species were selected because we had previously mapped and genotyped their fruiting bodies in the pre-fire forest. • Mycelial survival was investigated in two ways. First, we sampled seedlings in areas where these species had fruited abundantly before the fire, and second, we collected and genotyped mushrooms of S. pungens . • Neither species was detected on seedlings within the areas sampled, and A. francheti was not detected in any above- or below-ground samples after the fire. Genetic evidence from S. pungens revealed that post-fire genets were small and numerous, and none were found to be identical to the genets sampled prior to the fire. • From these results we conclude that A. francheti was not a common survivor or an early colonist of the post-fire forest, and that spores are the primary means by which S. pungens recolonized. If mycelial survival occurred in either species, it must have been relatively rare.

Impact of fire behavior on postfire forest development in a homogeneous boreal landscape

Although behavior of stand-replacing wildfire has significant impacts on initial tree regeneration in the fire-prone boreal landscape, the unknown behavior of most past wildfires has precluded any evaluation of these impacts on the progressive development of late-successional forest ecosystems. In this study, the effects of fire behavior on long-term ecosystem development were evaluated by linking the banding pattern of tree density in a jack pine (Pinus banksiana Lamb.) - black spruce (Picea mariana (Mill.) BSP) forest on a flat and homogeneous landform in northern Quebec to a similar, previously documented pattern of unburned strips of tree crowns. Complex wildfire-atmosphere interactions during the spread of a 1941 stand-replacing wildfire created this pattern in stem density, most likely by differentially damaging the canopy-stored seed bank between areas of contrasting fire severity. Sites with initial differences in seedling densities have followed different recovery pathways and developed markedly different forest structures, as well as differences in species abundance. Compared with areas of severe crown fire, the present-day vegetation in areas of low crown fire severity shows a higher density of living pines in the canopy layer, higher spruce and dead pine densities in the subcanopy layer, a lower pine density in the understory layer, and a higher abundance of Cladina rangiferina (L.) and Cladina stellaris Opiz (Brodo) in the lichen mat. This close spatial connection between crown fire severity and the ecological processes driving ecosystem recovery may explain large differences in vegetation among sites in the boreal landscape.

Understory response to management treatments in northern Arizona ponderosa pine forests

A range of forest management treatments was applied to ponderosa pine ( Pinus ponderosa) in northern Arizona. The treatments represented the full range of existing stand disturbance conditions and included forest stands that were unmanaged, thinned, thinned and prescribed burned, and burned by stand-replacing wildfire in order of increasing disturbance intensity. We assessed differences in understory diversity associated with these treatments. We identified 195 species of understory plants, focusing on the distribution of natives and exotics. The abundance of understory plants was more sensitive to changes in management treatments than the overall species richness. Exotics on the whole, responded statistically more strongly to disturbance treatments than did natives. Both the richness and abundance of exotic forbs increased significantly with treatment intensity. Species richness remained stable while abundance of native graminoids increased significantly with treatment intensity through thinned and burned stands. Both then decreased significantly in stands that experienced wildfire. The number of native shrub species decreased significantly with treatment intensity. Overall plant diversity was least in the unmanaged stands and progressively increased with intensity of disturbance/stand treatments. Both prescribed burning and wildfire increased plant diversity; however, stand-replacing wildfire also appeared to substantially increase the diversity of exotic plants.

Structural features of old-growth Australian montane ash forests

We examined structural features of montane ash stands that varied from 15 to 300+ years of age in the Central Highlands of Victoria, south-eastern Australia. Extensive field data were gathered for our investigation and were drawn from >3700 survey plots on >625 sites located throughout the study region. Much of our study focussed on understorey features because extensive past studies in these forests have highlighted their importance as key habitat components for wildlife. A wide range of stand features, including the abundance of shrubs, the prevalence of tree ferns, the presence of Acacia spp., the presence of Myrtle Beech ( Nothofagus cunninghamii), and the number of vegetation strata (a measure of vertical heterogeneity) varied considerably between age classes; most attributes were significantly ( p < 0.001) less likely to occur or were significantly less abundant in young logged forests. Highly significant differences in these structural features also were found in stands dominated by different tree species (Mountain Ash [ Eucalyptus regnans] and Alpine Ash [ E. delegatensis]). The traditional view of disturbance in montane ash forests is that of high-intensity stand-replacing wildfires which produce even-aged regrowth forests. However, our data suggest that low-intensity, non-stand-replacing fires or only partial stand-replacing fires, have occurred in many old-growth montane ash stands, both to create multi-aged stands and to re-juvenate components of the understorey — a process leading to asynchrony in the ages of understorey and some elements of the overstorey. Thus, the structural variability, and the patterns of natural disturbance leading to such conditions, appears to be more complex than often previously recognised. This has important implications for both nature conservation strategies and timber harvesting practices in montane ash forests. Existing high-intensity clearfell logging operations produce an even-aged dominant overstorey as well as understorey and overstorey plants of the same age. If logging effects are to more closely resemble natural disturbance regimes and promote structural complexity in stands of harvested forest to enhance wildlife habitat values, then a wider range of types of harvesting methods (in addition to clearfelling) need to be employed.

Vegetation Development in a Southern Maine Pitch Pine-Scrub Oak Barren

oak, mixed deciduous woodland, scrub oak, and open-canopy pitch pine. We investigated five factors that potentially influence vegetation distribution in the 856-ha preserve: soil texture, moisture, and fertility; topography; and disturbance history. Although analysis of variance revealed significant differences in soils and topography among plots in the five communities studied, multivariate analyses indicated a weak relationship between these environmental variables and species composition and structure. In contrast, disturbance history clearly influenced community and species distribution within the preserve. Effects of historic logging, charcoaling, and cultivation for blueberries persist despite a stand-replacing wildfire that swept through most of barrens in October, 1947. Pollen and charcoal analysis of sediments from a pond near the preserve suggests fire has influenced the barrens for at least the last several centuries, but that prior to European settlement of the area in the 18th century the character of the vegetation differed somewhat from that of today. Our results demonstrate that both fire and edaphic factors have influenced the vegetation of the area, but that historic land use has also played a strong role in determining the present character of the plant communities studied.