The Effects of Land-use History on Soil Properties and Nutrient Dynamics in Northern Hardwood Forests of the Adirondack Mountains

Soil nutrient pools and nitrogen dynamics in old-growth forests were compared with selectively logged stands and stands that were selectively logged and then burned approximately 100 years ago to test the hypothesis that land-use history exerts persistent controls on nutrient capital and nitrogen (N) transformation rates. We provide estimates of net N mineralization and nitrification rates for old-growth forests from the northeastern United States, a region in which few old-growth forests remain and for which few published accounts of mineralization rates exist. At the plot level, no effects of the dominant tree species were observed on any measured soil properties or N-cycling rates. Effects of alternate disturbance histories were detected in soil carbon (C) and N pools. Old-growth forest soils had higher total C (67 Mg·ha^–1) and N capital (3.3 Mg·ha^–1) than that of historically logged then burned soils (C = 50 Mg·ha^–1 and N = Mg·ha^–1), with intermediate values (C = 54 Mg·ha^–1 and N = 2.7 Mg·ha^–1) in the stands that were historically logged. Despite these differences in C and N content, corresponding differences in C–N ratio, net N mineralization rates, and net nitrification rates were not observed. The N concentration in the green foliage of American beech trees (Fagus grandifolia) was also highest from canopy trees growing in old-growth stands (3.0%), followed by logged stands (2.6%), and lowest in the logged/burned stands (2.2%). These data suggest that some legacies of light harvesting on ecosystem processes may be detected nearly 100 years following the disturbance event. These results are discussed in the context of how multiple forest disturbances act in concert to affect forest dynamics.     

Fire as a Recurrent Event in Tropical Forests of the Eastern Amazon: Effects on Forest Structure,Biomass, and Species Composition1

The effects of fire on forest structure and composition were studied in a severely fire-impacted landscape in the eastern Amazon. Extensive sampling of area forests was used to compare structure and compositional differences between burned and unburned forest stands. Burned forests were extremely heterogeneous, with substantial variation in forest structure and fire damage recorded over distances of <50 m. Unburned forest patches occurred within burned areas, but accounted for only six percent of the sample area. Canopy cover, living biomass, and living adult stem densities decreased with increasing fire inrensiry / frequency, and were as low as 10–30 percent of unburned forest values. Even light burns removed >70 percent of the sapling and vine populations. Pioneer abundance increased dramatically with burn intensity, with pioneers dominating the understory in severely damaged areas. Species richness was inversely related to burn severity, but no clear pattern of species selection was observed. Fire appears to be a cyclical event in the study region: <30 percent of the burned forest sample had been subjected to only one burn. Based on estimated solar radiation intensities, burning substantially increases fire susceptibility of forests. At least 50 percent of the total area of all burned forests is predicted to become flammable within 16 rainless days, as opposed to only 4 percent of the unburned forest. In heavily burned forest subjected to recurrent fires, 95 percent of the area is predicted to become flammable in <9 rain-free days. As a recurrent disturbance phenomenon, fire shows unparalleled potential to impoverish and alter the forests of the eastern Amazon.     

秦岭南坡松栎林群落演替过程中物种组成及多样性动态分析

为揭示秦岭南坡植物群落演替与物种多样性之间的复杂关系,采用空间代替时间的方法在秦岭南坡油松林、松栎混交林和锐齿栎林群落典型分布区域设置45个样地进行了群落学调查,统计分析了油松林→松栎混交林→锐齿栎林这一演替序列过程中草、灌、乔3层的物种组成和α物种多样性动态。结果表明:(1)秦岭南坡油松林、松栎混交林和锐齿栎林3种群落共有维管植物312种,隶属于73科183属,其中草本植物136种,灌木(含木质藤本)98种,乔木78种;油松林群落包含52科117属190种,松栎混交林群落含有60科95属129种,锐齿栎林群落则为50科109属178种。(2)沿油松林→松栎混交林→锐齿栎林群落演替序列,草本层物种均匀度和物种多样性显著降低,物种丰富度呈"V"形变化趋势;灌木层物种丰富度、均匀度以及物种多样性均无显著差异;乔木层物种多样性呈明显的单峰型变化,其中松栎混交林群落物种多样性指数最高。研究表明,秦岭南坡松栎林群落物种多样性随演替进展总体上呈现单峰型变化,与中期物种多样性假说一致;物种多样性可能仅仅是群落稳定的一个基础或前提条件。     

Succession of a Southern Appalachian Mountain Wetland Six Years following Hydrologic and Microtopographic Restoration

Microtopography, which is known to play a key role in the structure and function of wetland communities, is receiving increasing attention in wetland restoration projects. One goal of the Tulula Wetlands Mitigation Bank, which was the first large-scale wetland restoration project in the Blue Ridge Province, was to restore the microtopography in a degraded swamp forest–bog complex. This wetland type has become increasingly rare in the southern Appalachians and is characterized by a distinct microtopography of depressions and low ridges. We examined vegetation and soils in depression and ridge plots over a 6-year period, during which the hydrology and microtopography of the floodplain were restored. Our results showed that the flora of the depression and ridge plots differed, with greater coverage by obligate wetland species in the depression plots but greater overall taxonomic richness in the ridge plots. The edaphic characteristics that we measured varied very little during the study period. Creating microtopographic relief during the restoration of this wetland seems to have provided a variety of niches and may have increased the likelihood that the site will succeed back to a swamp forest–bog complex, rather than as a forested terrace of Tulula Creek.     

The Relative Impact of Harvest and Fire upon Landscape-Level Dynamics of Older Forests: Lessons from the Northwest Forest Plan

Interest in preserving older forests at the landscape level has increased in many regions, including the Pacific Northwest of the United States. The Northwest Forest Plan (NWFP) of 1994 initiated a significant reduction in the harvesting of older forests on federal land. We used historical satellite imagery to assess the effect of this reduction in relation to: past harvest rates, management of non-federal forests, and the growing role of fire. Harvest rates in non-federal large-diameter forests (LDF) either decreased or remained stable at relatively high rates following the NWFP, meaning that harvest reductions on federal forests, which cover half of the region, resulted in a significant regional drop in the loss of LDF to harvest. However, increased losses of LDF to fire outweighed reductions in LDF harvest across large areas of the region. Elevated fire levels in the western United States have been correlated to changing climatic conditions, and if recent fire patterns persist, preservation of older forests in dry ecosystems will depend upon practical and coordinated fire management across the landscape. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions: The study was conceived and designed by S.P.H., W.B.C., T.A.S, and M. M. The paper was written primarily by S.P.H, with editorial help by T.A.S. and W.B.C. The research itself, including mapping and analysis, was performed by S.P.H., D.P., M.G.W., and M.L. M.M. contributed additional methodologies related to the creation and validations of maps of historical older forests.     

Fire Severity and Long-term Ecosystem Biomass Dynamics in Coniferous Boreal Forests of Eastern Canada

The objective of this study was to characterize the effects of soil burn severity and initial tree composition on long-term forest floor dynamics and ecosystem biomass partitioning within the Picea mariana [Mill.] BSP-feathermoss bioclimatic domain of northwestern Quebec. Changes in forest floor organic matter and ecosystem biomass partitioning were evaluated along a 2,355-year chronosequence of extant stands. Dendroecological and paleoecological methods were used to determine the time since the last fire, the soil burn severity of the last fire (high vs. low severity), and the post-fire tree composition of each stand (P. mariana vs. Pinus banksiana Lamb). In this paper, soil burn severity refers to the thickness of the organic matter layer accumulated above the mineral soil that was not burned by the last fire. In stands originating from high severity fires, the post-fire dominance by Pinus banksiana or P. mariana had little effect on the change in forest floor thickness and tree biomass. In contrast, stands established after low severity fires accumulated during the first century after fire 73% thicker forest floors and produced 50% less tree biomass than stands established after high severity fires. Standing tree biomass increased until approximately 100 years after high severity fires, and then decreased at a logarithmic rate in the millennial absence of fire. Forest floor thickness also showed a rapid initial accumulation rate, and continued to increase in the millennial absence of fire at a much slower rate. However, because forest floor density increased through time, the overall rate of increase in forest floor biomass (58 g m⁻² y⁻¹) remained constant for numerous centuries after fire (700 years). Although young stands (< 200 years) have more than 60% of ecosystem biomass locked-up in living biomass, older stands (> 200 years) sequester the majority (> 80%) of it in their forest floor. The results from this study illustrate that, under similar edaphic conditions, a single gradient related to time since disturbance is insufficient to account for the full spectrum of ecosystem biomass dynamics occurring in eastern boreal forests and highlights the importance of considering soil burn severity. Although fire severity induces diverging ecosystem biomass dynamics in the short term, the extended absence of fire brings about a convergence in terms of ecosystem biomass accumulation and partitioning.     

Modeling Ecological Restoration Effects on Ponderosa Pine Forest Structure

FIRESUM, an ecological process model incorporating surface fire disturbance, was modified for use in southwestern ponderosa pine ecosystems. The model was used to determine changes in forest structure over time and then applied to simulate changes in aboveground biomass and nitrogen storage since exclusion of the natural frequent fire regime in an unharvested Arizona forest. Dendroecological reconstruction of forest structure in 1876, prior to Euro‐American settlement, was used to initialize the model; projections were validated with forest measurements in 1992. Biomass allocations shifted from herbaceous plants to trees, and nitrogen was increasingly retained in living and dead tree biomass over the 116‐year period (1876–1992). Forest conditions in 1992 were substantially degraded compared to reference presettlement conditions: old‐growth trees were dying at accelerated rates, herbaceous production was reduced nearly 90%, and the entire stand was highly susceptible to high‐intensity wildfire. Following an experiment initiated in 1993 to test ecological restoration treatments, future changes were modeled for the next century. Future forest structure remained within the natural presettlement range of variability under the full restoration treatment, in which forest biomass structure was thinned to emulate presettlement conditions and repeated low‐intensity fire was reintroduced. Simulation of the control treatment indicated continuation of exceptionally high tree density, probably culminating in stand‐replacing ecosystem change through high‐intensity wildfire or tree mortality from pathogens. Intermediate results were observed in the partial restoration treatment (tree thinning only); the open forest structure and high herbaceous productivity found immediately after treatment were gradually degraded as dense tree cover reestablished in the absence of fire. Modeling results support comprehensive restorative management as a long‐term approach to conservation of key indigenous ecosystem characteristics.     

A Historical Perspective and Future Outlook on Landscape Scale Restoration in the Northwest Wisconsin Pine Barrens

The concurrent discussions of landscape scale restoration among restoration ecologists, and of historic disturbance pattern as a guideline for forest management among forest scientists, offer a unique opportunity for collaboration between these traditionally separated fields. The objective of this study was to review the environmental history, early restoration projects, and current plans to restore landscape patterns at broader scales in the 450,000 ha northwest Wisconsin Pine Barrens. The Pine Barrens offer an example of a landscape shaped by fire in the past. In northwestern Wisconsin historically the barrens were a mosaic of open prairie, savanna, and pine forests on very poor, sandy soils. The surrounding region of better soils was otherwise heavily forested. Six restoration sites have been managed since the middle of this century using prescribed burns to maintain the open, barrens habitat. However, these sites are not extensive enough to mimic the shifting mosaic of large open patches previously created by fire. Extensive clear‐cuts may be used as a substitute for these large fire patches so that presettlement landscape patterns are more closely approximated in the current landscape. We suggest that such silvicultural treatments can be suitable to restore certain aspects of presettlement landscapes, such as landscape pattern and open habitat for species such as grassland birds. We are aware that the effects of fire and clear‐cuts differ in many aspects and additional management tools, such as prescribed burning after harvesting, may assist in further approximating the effect of natural disturbance. However, the restoration of landscape pattern using clear‐cuts may provide an important context for smaller isolated restoration sites even without the subsequent application of fire, in this formerly more open landscape.     

林火干扰对油松针叶林物种多样性与土壤理化性质的影响

探究不同程度林火干扰对油松(Pinus tabuliformis)针叶次生林植物物种多样性特征与土壤理化性质的影响以及二者之间的关系,对灾后油松针叶次生林的可持续经营具有重要意义。采用典型样地调查法对山西省药林寺森林公园不同程度林火干扰后的油松次生林群落进行物种多样性分析,并测定土壤理化性质。结果表明：(1)在调查研究样方中共记录到维管植物38种,在不同程度林火干扰下物种种类表现为对照(CK)>重度>中度。(2)乔木层的Simpson指数、Margalef指数与灌木层的α多样性指数在重度林火干扰后与CK相比显著升高;草本层的变化规律与灌木层相反。(3)样地群落稳定性表现为：中度>CK>重度。(4)在重度林火干扰后0—10cm土层中的含水率、有机质、全氮、碱解氮、速效磷含量与CK相比显著升高35.42%、102.89%、29.41%、21.67%、48.50%,10—20cm土层中的全氮、全磷、全钾、速效磷、速效钾含量与CK相比显著降低54.17%、28.94%、7.47%、1.74%、14.13%。(5)冗余分析结果表明：0—10cm土层中全氮、碱解氮、全磷及1...     

Short-Term Effects of Fire and Other Fuel Reduction Treatments on Breeding Birds in a Southern Appalachian Upland Hardwood Forest

Abstract: We compared the effects of 3 fuel reduction techniques and a control on breeding birds during 2001-2005 using 50-m point counts. Four experimental units, each >14 ha, were contained within each of 3 replicate blocks at the Green River Game Land, Polk County, North Carolina, USA. Treatments were 1) prescribed burn, 2) mechanical understory reduction (chainsaw-felling of shrubs and small trees), 3) mechanical + burn, and 4) controls. We conducted mechanical treatments in winter 2001-2002 and prescribed burns in spring 2003. Tall shrub cover was substantially reduced in all treatments compared to controls. Tree mortality and canopy openness was highest in the mechanical + burn treatment after burning, likely due to higher fuel loading and hotter burns; tree mortality increased with time. Many bird species did not detectably decrease or increase in response to treatments. Species richness, total bird density, and some species, including indigo buntings (Passerina cyanea) and eastern bluebirds (Sialia sialis), increased in the mechanical + burn treatment after a 1-year to 2-year delay; eastern woodpewees (Contopus virens) increased immediately after treatment. Hooded warblers (Wilsonia citrina), black-and-white warblers (Mniotilta varia), and worm-eating warblers (Helmitheros vermivorus) declined temporarily in some or all treatments, likely in response to understory and (or) leaf litter depth reductions. Densities of most species affected by treatments varied with shrub cover, tree or snag density, or leaf litter depth. High snag availability, open conditions, and a higher density of flying insects in the mechanical + burn treatment likely contributed to increased bird density and species richness. In our study, fuel reduction treatments that left the canopy intact, such as low-intensity prescribed fire or mechanical understory removal, had few detectable effects on breeding birds compared to the mechanical + burn treatment. High-intensity burning with heavy tree-kill, as occurred in our mechanical + burn treatment, can be used as a management tool to increase densities of birds associated with open habitat while retaining many forest and generalist species, but may have short-term adverse effects on some species that are associated with the ground- or shrub-strata for nesting and foraging.     

Effects of Chronic Human Activities on Invasion of Longleaf Pine Forests by Sand Pine

Chronic human activities may result in new and permanent successional trajectories in certain ecosystems. The invasion of longleaf pine ecosystems by sand pine in the Florida Panhandle is one such change in the landscape. This study examined the spatial pattern of sand pine expansion and explored the natural and anthropogenic disturbances that fostered this invasion. Aerial photographs (1949, 1994) and Geographic Information Systems analyses confirmed sand pine expansion at Eglin Air Force Base. In 1949, there were 8,982 ha of sand pine in the southern portion of the study area near riparian and coastal lowland forests. By 1994, sand pine had expanded further upland and inland, for a total of 17,147 ha in the study area. Sand pine age data showed that this expansion had started by 1920 but increased rapidly in the 1940s. Historical accounts and structural data from stands suggest that land-use activities associated with the extraction of turpentine promoted the invasion by sand pine. Fires were suppressed in longleaf pine forests to protect turpentine trees, resulting in increased vegetation cover and decreased regeneration of longleaf pine. In addition, stands were typically harvested after turpentining, and there was little or no advanced regeneration of longleaf pine. Sand pine age histograms showed that the onset of high establishment rates (1940s) coincided with changes in land ownership and widespread fire suppression. Sand pine is likely to persist in these ecosystems due to its abundant regeneration. Received 17 March 1999; accepted 28 January 2000.     

Long-Term Effects of Prescribed Fire and Thinning on Residual Tree Growth in Mixed-Oak Forests of Southern Ohio

Long-term (10 years) growth responses of residual trees to prescribed fire and thinning were evaluated using standard dendrochronological protocols to understand the broader effects of the treatments on mixed-oak forest ecosystems in southern Ohio. Analysis of 696 increment cores (348 trees ≥ 25 cm DBH; five species) from 80 0.1 ha permanent plots distributed evenly across four treatments (control, thin, thin + burn, burn) indicated substantial increase in tree basal area increment (BAI) following the treatment. Post-treatment mean BAI of trees from the three active treatments ranged from 20.52 to 23.55 cm² y^?1 compared with pre-treatment values of 16.86–17.07 cm² y^?1. BAI rates (averaging 15.13 and 16.33 cm² y^?1, respectively, for pre- and post-treatments) in the control plots did not change much over time. Mechanical treatments were more effective than prescribed fire at enhancing BAI of trees. However, basal area growth depended to some degree on the severity of prescribed fire. Analysis of percent BAI change revealed an interesting temporal trend with moderate to major growth releases during the first 5-year post-treatment period, and a slight attenuation thereafter, suggesting the need for periodic application of treatments to sustain growth over a longer timescale. Growth responses varied greatly among species, with yellow-poplar and hickories exhibiting the highest and lowest post-treatment BAI rates of 31.11 and 15.71 cm² y^?1, respectively. Given their variable growth responses, integrating residual trees into current monitoring programs may help in elucidating the consequences of prescribed fire and thinning on forest dynamics and development.     

Postfire Vegetation Recovery in Highland Pine Forests of the Dominican Republic

We surveyed postfire vegetation at five sites at high elevations (> 2000 m) in the Cordillera Central, Dominican Republic. Highlands of the Cordillera Central are dominated by a single pine species, Pinus occidentalis, but plant communities are rich with endemics and conservation and fire management efforts in these systems are ongoing. The burns were 2–7 yr in age and had consumed nearly all shrub crowns. Pines suffered high mortality (> 50%, all sites combined), but shrubs resprouted at high rates (88%, N = 957) after fire. All shrub taxa produced basal resprouts; eight of 11 shrub taxa measured had resprouting rates > 90 percent, while Baccharis myrsinites had the lowest (56%). Most taxa grew to prefire height quickly (within 5–7 yr), with regrowth of stem diameters lagging behind. Patterns and rates of shrub recovery resembled those documented in high elevation shrublands in Costa Rica and Brazil. Pinus occidentalis does not resprout, but larger individuals can survive fire. Survival increases dramatically when trees attain > 13-cm dbh, when bark becomes thick enough to protect cambial tissue. Overall, pines are regenerating much more slowly than shrubs, but seedling establishment varied considerably between sites. Frequent fires may cause a decline in pines and an increase in shrub- or grass-dominated communities. Succession in these high elevation fire-dependent pine forests favors taxa already present in the preburn vegetation, with woody composition changing little after fire, in contrast to lower-elevation cloud forest, where postfire vegetation has been shown to bear little resemblance to mature forest even after several years.     

Effects of Stream Restoration on Woody Riparian Vegetation of Southern Appalachian Mountain Streams,North Carolina,U.S.A

Riparian revegetation, such as planting woody seedlings or live stakes, is a nearly ubiquitous component of stream restoration projects in the United States. Though evaluations of restoration success usually focus on in‐stream ecosystems, in order to understand the full impacts of restoration the effects on riparian ecosystems themselves must be considered. We examined the effects of stream restoration revegetation measures on riparian ecosystems of headwater mountain streams in forested watersheds by comparing riparian vegetation structure and composition at reference, restored, and degraded sites on nine streams. According to mixed model analysis of variance (ANOVA), there was a significant effect of site treatment on riparian species richness, basal area, and canopy cover, but no effect on stem density. Vegetation characteristics at restored sites differed from those of reference sites according to all metrics (i.e. basal area, canopy cover, and species composition) except species richness and stem density. Restored and degraded sites were structurally similar, with some overlap in species composition. Restored sites were dominated by Salix sericea and Cornus amomum (species commonly planted for revegetation) and a suite of disturbance‐adapted species also dominant at degraded sites. Differences between reference and restored sites might be due to the young age of restored sites (average 4 years since restoration), to reassembly of degraded site species composition at restored sites, or to the creation of a novel anthropogenic ecosystem on these headwater streams. Additional research is needed to determine if this anthropogenic riparian community type persists as a resilient novel ecosystem and provides valued riparian functions.     

Recommendations for Integrating Restoration Ecology and Conservation Biology in Ponderosa Pine Forests of the Southwestern United States

Over the past century, ponderosa pine–dominated landscapes of the southwestern United States have been altered by human activities such as grazing, timber harvest, road building, and fire exclusion. Most forested areas within these landscapes now show increased susceptibility to stand‐replacing fires, insect outbreaks, and drought‐related mortality. Recent large wildfires in the region have spurred public interest in large‐scale fuel reduction and restoration programs, which create perceived and real conflicts with the conservation of biodiversity. Conservation concerns include the potential for larger road networks, soil and understory disturbance, exotic plant invasion, and the removal of large trees in treated areas. Pursuing prescribed burning, thinning, or other treatments on the broad scale that many scientists and managers envision requires the reconciliation of ecological restoration with biodiversity conservation. This study presents recommendations from a workshop for integrating the principles and practices of restoration ecology and conservation biology, toward the objective of restoring the composition, structure, and function of dry ponderosa pine forests. Planning on the scale of hundreds of thousands of hectares offers opportunities to achieve multiple objectives (e.g., rare species protection and restoration of ecological structures and processes) that cannot easily be addressed on a site‐by‐site basis. However, restoration must be coordinated with conservation planning to achieve mutual objectives and should include strict guidelines for protection of rare, declining, and sensitive habitats and species.     

Long-Term Persistence and Fire Resilience of Oak Shrubfields in Dry Conifer Forests of Northern New Mexico

Extensive high-severity fires are creating large shrubfields in many dry conifer forests of the interior western USA, raising concerns about forest-to-shrub conversion. This study evaluates the role of disturbance in shrubfield formation, maintenance and succession in the Jemez Mountains, New Mexico. We compared the environmental conditions of extant Gambel oak (Quercus gambelii) shrubfields with adjoining dry conifer forests and used dendroecological methods to determine the multi-century fire history and successional dynamics of five of the largest shrubfields (76–340 ha). Across the study area, 349 shrubfields (5–368 ha) occur in similar topographic and climate settings as dry conifer forests. This suggests disturbance, rather than other biophysical factors, may explain their origins and persistence. Gambel oak ages and tree-ring fire scars in our sampled shrubfields indicate they historically (1664–1899) burned concurrently with adjoining conifer forests and have persisted for over 115 years in the absence of fire. Aerial imagery from 1935 confirmed almost no change in sampled shrubfield patch sizes or boundaries over the twentieth century. The largest shrubfield we identified is less than 4% the size of the largest conifer-depleted and substantially shrub-dominated area recently formed in the Jemez following extensive high-severity wildfires, indicating considerable departure from historical patterns and processes. Projected hotter droughts and increasingly large high-severity fires could trigger more forest-to-shrub transitions and maintain existing shrubfields, inhibiting conifer forest recovery. Restoration of surface fire regimes and associated historical forest structures likely could reduce the rate and patch size of dry conifer forests being converted to shrubfields.     

Strategies for Savanna Restoration in the Southern Great Plains: Effects of Fire and Herbicides

Woody plant encroachment has degraded grassland and savanna ecosystems worldwide by decreasing herbaceous production and diversity, and altering these physiognomies toward woodlands. This study evaluated the long-term efficacy of fire and herbicide restoration strategies used in the southern Great Plains to reduce Honey mesquite ( Prosopis glandulosa ) dominance, restore a grassland/savanna physiognomy, and increase herbaceous production and diversity. Three treatments were evaluated: high-intensity winter fire, aerial spray of clopyralid + triclopyr (C + T), and aerial spray of clopyralid and were compared to untreated mesquite woodland (control). Post-treatment mesquite stand physiognomy was different between fire (low mortality, high basal sprouting), C + T (high mortality, high basal sprouting of surviving plants), and clopyralid (moderate mortality, low basal sprouting of surviving plants) treatments. From 6 to 8 years post-treatment, herbaceous production was increased in C + T and clopyralid treatments but not in the fire treatment. Mesquite regrowth in the fire treatment exerted a competitive influence that limited herbaceous production. Herbaceous functional group diversity was increased in fire and C + T treatments due to a decrease in C₃ perennial grass dominance and an increase in C₄ perennial grasses and/or C₃ forbs. Treatments that maintained mesquite overstory (control and clopyralid) had lower herbaceous diversity due to C₃ perennial grass dominance and lower C₄ perennial grass cover. The clopyralid treatment demonstrated greatest potential for long-term restoration of southern Great Plains savanna by reducing mesquite canopy cover to historic levels, limiting mesquite basal regrowth and increasing grass production.     

Fire Severity in Conifer Forests of the Sierra Nevada, California

Natural disturbances are an important source of environmental heterogeneity that have been linked to species diversity in ecosystems. However, spatial and temporal patterns of disturbances are often evaluated separately. Consequently, rates and scales of existing disturbance processes and their effects on biodiversity are often uncertain. We have studied both spatial and temporal patterns of contemporary fires in the Sierra Nevada Mountains, California, USA. Patterns of fire severity were analyzed for conifer forests in the three largest fires since 1999. These fires account for most cumulative area that has burned in recent years. They burned relatively remote areas where there was little timber management. To better characterize high-severity fire, we analyzed its effect on the survival of pines. We evaluated temporal patterns of fire since 1950 in the larger landscapes in which the three fires occurred. Finally, we evaluated the utility of a metric for the effects of fire suppression. Known as Condition Class it is now being used throughout the United States to predict where fire will be uncharacteristically severe. Contrary to the assumptions of fire management, we found that high-severity fire was uncommon. Moreover, pines were remarkably tolerant of it. The wildfires helped to restore landscape structure and heterogeneity, as well as producing fire effects associated with natural diversity. However, even with large recent fires, rates of burning are relatively low due to modern fire management. Condition Class was not able to predict patterns of high-severity fire. Our findings underscore the need to conduct more comprehensive assessments of existing disturbance regimes and to determine whether natural disturbances are occurring at rates and scales compatible with the maintenance of biodiversity.