Showing papers by "W. Wallace Covington published in 2006"

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

Abstract: 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 droughtrelated 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.

Assessing Targets for the Restoration of Herbaceous Vegetation in Ponderosa Pine Forests

Abstract: A restoration project is considered a success when the initial target is met, but many targets are plausible. We evaluated the success of a restoration project in its 11th year since treatment in a southwestern ponderosa pine–bunchgrass community and the appropriateness of several targets. We measured the responses of (1) total standing crop; (2) standing crop of five functional groups (C3 and C4 graminoids, leguminous forbs, and nonleguminous perennial and annual forbs); (3) graminoid community composition; and (4) standing crop of five common graminoid species (Festuca arizonica, Muhlenbergia montana, Elymus elymoides, Carex geophila, and Poa fendleriana). Targets were quantified in remnant grass patches, which provided the standards for these targets, and were assessed in three other forest patch types (pre-settlement tree patches, post-settlement tree patches, and patches where all post-settlement trees were removed). Patches where all post-settlement trees were removed reached target levels for total standing crop, C3 and C4 graminoid standing crop, graminoid community composition, and M. montana, E. elymoides, and C. geophila standing crops. Standing crop of legumes and of F. arizonica did not increase over time in any patch type. Targets were not met in pre-settlement patches or in patches where some post-settlement trees were left standing, suggesting that it is unrealistic to expect equal responses across all patch types. If increasing herbaceous standing crop is a major goal, practitioners should create gaps within the pine forest canopy.

Forest ecosystems of an Arizona Pinus ponderosa landscape: multifactor classification and implications for ecological restoration

Abstract: Aim We developed an ecosystem classification within a 110,000-ha Arizona Pinus ponderosa P. & C. Lawson (ponderosa pine) landscape to support ecological restoration of these forests. Specific objectives included identifying key environmental variables constraining ecosystem distribution and comparing plant species composition, richness and tree growth among ecosystems.

Diameter caps for thinning southwestern ponderosa pine forests: Viewpoints, effects, and tradeoffs

Abstract: Upper size limits of trees allowed to be cut, termed diameter caps, have resulted in polarization, litigation, and delays and alterations to thinning projects in many western forests. Using southwestern ponderosa pine forests as an example, we summarize viewpoints on caps, simulate effects of caps on thinning prescriptions, and provide examples of ecosystem-level tradeoffs of leaving extra trees during thinning projects. The importance placed on trees versus other ecosystem components primarily differentiates those who support caps and those who do not. We conclude that diameter caps may enhance some ecosystem components, such as densities of large trees, but they negatively impact many nontree components.

Vegetation-environment relationships and ecological species groups of an Arizona Pinus ponderosa landscape, USA

Abstract: Pinus ponderosa forests occupy numerous topographic and soil complexes across vast areas of the southwestern United States, yet few data exist on species distributions and vegetation–environment relationships for these environmentally diverse landscapes. We measured topography, soils, and vegetation on 66, 0.05-ha plots within a 110,000-ha P. ponderosa landscape in northern Arizona, USA, to discern vegetation–environment relationships on this landscape. We analyzed associations of environmental variables with plant communities and with single-species distributions, and we classified ecological species groups (co-occurring plant species exhibiting similar environmental affinities). Gradients in community composition paralleled gradients in soil texture, available water, organic C, total N, and geographic precipitation patterns. Soil parent material, affected by the presence or absence of volcanic activity, is a primary factor constraining vegetation patterns on this landscape. Using discriminant analysis, we built a model that correctly classified the most important of four grasses (Bouteloua gracilis, Muhlenbergia montana, Sporobolus interruptus, or Festuca arizonica) on 70–80% of plots based on five environmental variables related to soil moisture and resource levels. We also classified 52 of the 271 detected plant species into 18 ecological species groups. Species groups ranged from Phacelia and Bahia groups occupying xeric, volcanic cinder soils low in organic C and total N, to Festuca and Lathyrus groups characterizing moist, loam and silt loam soils. We applied the species groups by estimating P. ponderosa diameter increment in a regression tree using abundances of species groups. The most rapid P. ponderosa diameter growth of 5 mm/year occurred on plots with high importance of the Festuca and Lathyrus groups. Our results on this semi-arid landscape support several general ecological species group principles chiefly developed in temperate regions, and suggest that vegetation–environment research has great potential for enhancing our understanding of P. ponderosa forests occupying vast areas of the southwestern United States.

“Minimal‐Impact” Restoration Treatments Have Limited Effects on Forest Structure and Fuels at Grand Canyon, USA

Abstract: Restoration treatments were tested on the South Rim (ponderosa pine) and North Rim (mixed conifer) of Grand Canyon National Park, never-harvested forests where the historically frequent surface regime was interrupted in the late 19th century. Treatments were designed for ‘‘minimal impact’’ by limiting the size of trees to be thinned and minimizing mechanical equipment. Treatments included (1) thinning of small trees (diameter • 12.7 cm) and prescribed fire; (2) thinning of small trees located close to large old trees and prescribed fire; (3) prescribed fire only; and (4) control. On the South Rim, density declined by 23–45% but basal area only by 9– 14%. On the North Rim, density was reduced 32–68% and basal area declined 18–31%. Declines of 5–8% were observed in these same variables in the controls. Surface fuels were significantly reduced in all burned treatments (70–84% reduction in forest floor, 66–76% reduction in woody debris). However, canopy cover was nearly unchanged, and canopy fuel loads were not significantly reduced, although canopy base heights increased. The experiment accomplished the goal of minimally impacting the forest ecosystem but the effects of small-tree thinning plus burning were nearly indistinguishable from those of burning alone. All the treated units were left with modest gain in resistance to severe wildfires but in conditions still far removed from prefire-exclusion conditions.

Fire histories in ponderosa pine forests of Grand Canyon are well supported: reply to Baker

Abstract: Fire scars and other paleoecological methods are imperfect proxies for detecting past patterns of fire events. However, calculations of long fire rotations in Grand Canyon ponderosa pine forests by Baker are not convincing in methodology or assumptions compared with fire-scar evidence of frequent surface fires. Patches of severe disturbance are a possible hypothesis to explain the relatively short age structure at the park, where ∼12% fewer trees were older than 300 years compared with another unharvested northern Arizona site. However, mapped patterns of old trees as well as the evidence for frequent surface fire from fire scars, charcoal deposition studies, and evolutionary history are more consistent with the dominance of surface fire prior to c. 1880. The most relevant available evidence of fire recurrence at a given point, mean point fire intervals, had median values <16 years at all five study sites, close to filtered composite fire interval statistics (∼6-10 years), but much lower than Baker's calculated fire rotation values (55-110 years). The composite fire interval is not a uniquely important statistic or a numerical guideline for management, but one of many lines of evidence underscoring the ecological role of frequent surface fire in ponderosa pine forests.

Pinyon-juniper fire regime: Natural range of variability

Abstract: In this study, we used a variety of methods to quantify and describe historical patterns of fire and forest structure in two pinyon-juniper ecosystems of the Southwest. Sites were located on the Kaibab National Forest in Arizona, south of Grand Canyon National Park (Tusayan), and on the Carson National Forest in New Mexico, north of Espanola (Canjilon). Methodological approaches included analysis of fire scars, contemporary forest structure, fire evidence, modern fire records, and forest reconstruction. GIS surface maps, constructed using inverse distance weighted interpolation, were used to assess spatial patterns of fire and forest structure. Results indicated distinct fire histories and recent forest changes at the two sites. At Tusayan, surface fires burned historically at frequencies of 7.2-7.4 years (Weibull median probability) in canyons and draws dominated by ponderosa pine. On uplands dominated by pinyon-juniper communities, longer point fire intervals suggested fires occurred at a mean frequency of 41.6 years. Point intervals stratified by species indicated longer return periods for Utah juniper than pinyon or ponderosa pine. Fire evidence in the form of charred tree structures was ubiquitous at the site and there was no clear relationship between stand age and fire evidence. Live, old trees (300 yr) were prevalent and averaged 26 trees per hectare (TPH). Stands were all ages up to 400 yr and patch sizes were generally small (30 ha). Reconstructions showed a moderate overall increase (39(percent)) in stand density since the late 19th century. Ponderosa pine increases were responsible for the majority of recent structural changes although pinyon density also had apparently increased. We found no evidence of extensive stand replacing fire over the last 400 years at Tusayan and concluded that the historical pattern has been one of frequent surface fires in ponderosa pine communities and small severe fires on pinyon-juniper uplands. At Canjilon, fire scar analysis showed longer mean fire intervals (81.1 yr), suggesting that infrequent crown fires or severe surface fires were an important component of the historical regime. Like Tusayan, charred structures were found across the Canjilon site, although they appeared to be more abundant at lower elevations where stands ages were younger. Few live old trees (300 yr) were found at the site (4.2 TPH). The site dominated by stands in the 200-250-yr and 250-300-yr age classes. Mean patch sizes for stands of these age classes were 24 and 79 ha, respectively. Reconstructions showed relatively greater increases in tree density (61(percent)) with Rocky Mountain juniper and pinyon pine both showing positive changes since the late 19th century. Evidence of stand replacing fire was seen along the eastern edge of the study site and young trees appeared to be encroaching into previously open areas, particularly around big sagebrush meadows. Woodland treatments that may parallel historical patterns of fire and forest structure at these sites include targeted tree thinning and/or use of prescribed fire to create canopy openings of various sizes.