Northern Arizona Ponderosa Pine Forests Research Articles

Saving the Forest from the Trees: Expert Views on Funding Restoration of Northern Arizona Ponderosa Pine Forests through Registered Carbon Offsets

Ponderosa pine forests in the southwestern United States of America are overly dense, increasing the risk of high-intensity stand-replacing wildfires that result in the loss of terrestrial carbon and release of carbon dioxide, contributing to global climate change. Restoration is needed to restore forest structure and function so that a more natural regime of higher frequency, lower intensity wildfires returns. However, restoration has been hampered by the significant cost of restoration and other institutional barriers. To create additional revenue streams to pay for restoration, the National Forest Foundation supported the development of a methodology for the estimation and verification of carbon offsets generated by the restoration of ponderosa pine forests in northern Arizona. The methodology was submitted to the American Carbon Registry, a prominent carbon registry, but it was ultimately rejected. This paper presents a post-mortem examination of that methodology and the reasons it was rejected in order to improve the development of similar methodologies in the future. Using a mixed-methods approach, this paper analyzes the potential atmospheric carbon benefits of the proposed carbon offset methodology and the public and peer-reviewed comments from the associated review of the methodology. Results suggest a misalignment between the priorities of carbon registries and the context-specific ecosystem service benefits of this type of restoration; although findings confirm the potential for reductions in released carbon due to restoration, these results illuminate barriers that complicate registering these reductions as voluntary carbon offsets under current guidelines and best practices, especially on public land. These barriers include substantial uncertainty about the magnitude and timing of carbon benefits. Overcoming these barriers will require active reflexivity by the institutions that register voluntary carbon offsets and the institutions that manage public lands in the United States. Such reflexivity, or reconsideration of the concepts and purposes of carbon offsets and/or forest restoration, will allow future approaches to better align objectives for successfully registering restoration-based voluntary carbon offsets. Therefore, the results of this analysis can inform the development of future methodologies, policies, and projects with similar goals in the same or different landscapes.

Changes in potential wildland fire suppression costs due to restoration treatments in Northern Arizona Ponderosa pine forests

Wildfire suppression costs have been increasing since the early 1970's. With growing concern over wildfire suppression costs, our analysis addresses restoration treatment effectiveness in reducing wildfire suppression costs. We examine past fires across the Northern Arizona landscape to determine fire behavior characteristics that are significant in predicting wildfire suppression costs and capable of being modeled in fire simulations prior to wildfire events. We find burn severity metrics to be significant in predicting wildfire suppression costs. Three proposed treatment alternatives for the Four Forest Restoration Initiative (4-FRI) are analyzed to determine treatment effectiveness and policy implications in reducing burn severity metrics and wildfire suppression costs. The more aggressive treatments are more effective in reducing wildfire suppression costs except in the case of sever wind and weather events.

Managing for ecosystem services in northern Arizona ponderosa pine forests using a novel simulation-to-optimization methodology

Forest managers are faced with the difficulty of promoting multiple ecosystem services (ES) with an incomplete understanding of the complex ways these ES interact. We developed a quantitative model to help managers better understand the effect of different management options on eight ES over time. These ES were (1) Mexican spotted owl (Strix occidentalis lucida) habitat, (2) the harvest of merchantable timber, (3) the harvest of woody biomass for potential conversion to bioenergy, (4) northern goshawk (Accipiter gentilis) habitat, (5) scenic beauty, (6) fire hazard reduction, (7) carbon storage, and (8) restoration of pre-European settlement forest structure represented through larger quadratic mean diameters (QMD). We integrated production functions for these eight ES with a forest growth-and-yield model to simulate the effects of three management actions over a 45-year period in northern Arizona, USA. We scaled ES values based on their observed maximum and minimum values as a means to compare achievement levels between ES. These scaled ES values were input into a goal-programming model to identify the optimal management regimes given a corresponding set of five different management objectives. The combination of simulation modeling with goal-programming is a novel approach for evaluating ES responses to management and for optimizing ES attainment objectives. To illustrate the value of this novel approach, we designed management objectives to reflect the likely goals a forest manager may choose for each ES in this region. ES goals were distinguished at the forest- and stand-level spatial extents. Results demonstrate the flexibility of the ES optimization model to plan for a variety of situations and preferences. For instance, two ES management goals viewed as conflicting are the protection of Mexican Spotted Owl habitat and the reduction of fire hazard risk. One ES optimization scenario (Scenario 5) produces a plan that reduces area considered as having a high fire hazard from 56% to 7% of the study area in 2015 without any reduction in owl habitat quality throughout the time horizon. We recommend that our methodology be applied and developed further by those in both the research and applied fields of ecosystem sciences.

Carbon Stocks and Climate Change: Management Implications in Northern Arizona Ponderosa Pine Forests

Researchers have observed climate-driven shifts of forest types to higher elevations in the Southwestern US and predict further migration coupled with large-scale mortality events proportional to increases in radiative forcing. Range contractions of forests are likely to impact the total carbon stored within a stand. This study examines the dynamics of Pinus ponderosa stands under three climate change scenarios in Northern Arizona using the Climate Forest Vegetation Simulator (Climate-FVS) model to project changes in carbon pools. A sample of 90 stands were grouped according to three elevational ranges; low- (1951 to 2194 m), mid- (2194 to 2499 m), and high- (2499 to 2682 m.) elevation stands. Growth, mortality, and carbon stores were simulated in the Climate-FVS over a 100 year timespan. We further simulated three management scenarios for each elevational gradient and climate scenario. Management included (1) a no-management scenario, (2) an intensive-management scenario characterized by thinning from below to a residual basal area (BA) of 18 m2/ha in conjunction with a prescribed burn every 10 years, and (3) a moderate-management scenario characterized by a thin-from-below treatment to a residual BA of 28 m2/ha coupled with a prescribed burn every 20 years. Results indicate that any increase in aridity due to climate change will produce substantial mortality throughout the elevational range of ponderosa pine stands, with lower elevation stands projected to experience the most devastating effects. Management was only effective for the intensive-management scenario; stands receiving this treatment schedule maintained moderately consistent levels of basal area and demonstrated a higher level of resilience to climate change relative to the two other management scenarios. The results of this study indicate that management can improve resiliency to climate change, however, resource managers may need to employ more intensive thinning treatments than currently proposed to achieve the best results.

Nonnative species influence vegetative response to ecological restoration: Two forests with divergent restoration outcomes

Changes in the vegetative structure and diversity of ponderosa pine forests have generated interest in conducting ecological restoration projects to improve the overall forest health of these ecosystems. Ecological restoration prescriptions often consist of thinning trees to emulate pre-1870s forest structure followed by prescribed burning. Disturbances associated with ecological restoration can, however, promote invasion by nonnative species. We compared two northern Arizona ponderosa pine forests treated for ecological restoration, one at the Fort Valley Experimental Forest and one at Mt. Trumbull on the Grand Canyon-Parashant National Monument. We examined the response of native and nonnative plant species, as well as all species combined, to treatments at the two forests. Both study sites showed a significant increase in native and nonnative species cover and richness by the fifth year post-treatment that remained significant by the tenth year post-treatment. Despite these general trends in native and nonnative community development, the understory vegetation at the two sites followed diverging successional patterns after treatment. By the tenth year post-treatment Fort Valley was dominated by native species and Mt. Trumbull was dominated by a single nonnative species, cheatgrass. The differences in post-treatment understory recovery are likely due to pretreatment forest conditions. At Fort Valley, nonnatives were present, but accounted for only 0.11% of the pretreatment cover. At Mt. Trumbull, nonnatives accounted for 5.26% of the pretreatment understory cover, with cheatgrass accounting for approximately 4% of the understory cover. Additionally, the soil seedbank at Fort Valley had greater overall species richness and greater native perennial grass richness than Mt. Trumbull. We propose that the application of ecological restoration treatments should be targeted to sites with low abundance of nonnatives prior to treatment. Sites containing high abundance of nonnatives prior to treatment should be managed for nonnative species mitigation before initiating any ecological restoration projects.

Planting Trials in Northern Arizona Ponderosa Pine Forests

Abstract Mechanical tree thinning and prescribed fire have been widely proposed for restoring ponderosa pine (Pinus ponderosa) forests in the southwestern United States. However, these restoration activities often result in intensely disturbed sites, such as landings for equipment and temporary roads, which managers may need to revegetate. Managers may also wish to augment the forest understory through revegetation after thinning and burning treatments. We present five planting trials conducted as part of restoration projects in northern Arizona ponderosa pine forests in order to assess survival of 11 species and the influence of various microsites created by restoration activities. In one trial, five-year survival of four transplanted graminoid species ranged from 1% to 23% among species and from 0% (unthinned forest) to 22% (unburned slash) among microsites. Also illustrating microsite effects, over three times more Fendler’s meadow-rue (Thalictrum fendleri) survived when outplanted below Gambel oak (Quercus gambelii) than in openings created by restoration pine thinning. Only 4% of purple locoweed (Oxytropis lambertii) survived in an unsuccessful attempt at revegetating a decommissioned road on dry cinder soils. Conversely, two years after outplanting on a different road on moister soil, mountain muhly (Muhlenbergia montana) survival was 72% (89% of which were fruiting), and 40% of plants of the biennial ragleaf bahia (Bahia dissecta) were surrounded by new seedlings. Results illustrate that successes occurred without supplemental watering even in dry years; outcomes hinged on species or planting microsite; and heavy mortality occurred in some contexts. Owing to financial costs and logistical constraints, revegetation may be most suited for localized disturbances caused by restoration activities or for creating revegetated islands for understory augmentation.

Assessing the Representativeness of the Oldest Permanent Inventory Plots in Northern Arizona Ponderosa Pine Forests

Abstract A network of permanent plots established between 1909 and 1913 (the Woolsey plots) contains the oldest measured data in northern Arizona ponderosa pine forests. These forest inventory data offer a unique opportunity to reconstruct pre‐settlement reference conditions, as well as detect and quantify changes in southwestern forest structure and composition. However, the selection of plot locations in the early 1900s followed a subjective nonrandom approach. To assess the applicability, or inference space, of results obtained from these historical plots, we compared their environmental characteristics (terrestrial ecosystem unit [TEU, based on a U.S. Forest Service (USFS) ecological classification system], site index, elevation, insolation index, and soil parent material) as well as contemporary forest structure (trees per hectare, basal area, and quadratic mean diameter) with two large inventory samples: USFS Forest Inventory and Analysis (FSFIA) and Arizona State Land Department Continuous Forest Inventory (AZCFI). Analytical methods included multivariate permutation tests, ratios of variance, and Kolmogorov–Smirnov two‐sample tests. Results indicated that the Woolsey plots (1) were neither historically nor contemporarily representative of the entire study area because of environmental and current forest structural differences with respect to the FSFIA and AZCFI and (2) may be considered historically representative of their corresponding TEUs. Our study supports the use of TEUs for defining the applicability of information obtained from the Woolsey plots.

Persistent effects of fire-induced vegetation change on energy partitioning and evapotranspiration in ponderosa pine forests

We compared energy fluxes between a site converted from ponderosa pine ( Pinus ponderosa) forest to sparse grassland by a severe wildfire 10 years ago and a nearby, unburned forest. We used eddy covariance and associated instruments to measure total radiation, net radiation, albedo, and fluxes of energy into latent heat, sensible heat, and the soil. Total radiation, vapor pressure deficit, and air temperature were similar for each site. Compared to the unburned site, net radiation efficiency (net radiation/total radiation) was 30% lower and albedo 30% higher at the burned site. The magnitude of sensible and latent heats varied seasonally at both sites. Sensible heat was the major component of the energy balance in cold or dry seasons, whereas latent heat was the major component in the warm and wet season. Soil heat flux was the smallest in magnitude of the measured energy fluxes. Compared with the unburned forest, the burn-created grassland generally had lower sensible and latent heats, but greater soil heat flux for both soil cooling in winter and warming in summer. The grassland had similar maximum air temperature as the forest, and warmer surface soil temperature during the summer. Thus, the lower albedo and greater sensible heat of the forest did not produce a warmer site compared with the grassland, apparently because of the cooling effect of greater latent heat in the forest. Our results suggest only small changes in site air temperature, but larger changes in site surface soil temperature by shifts from forest to grassland caused by severe fire in northern Arizona ponderosa pine forests.

Using Ground Foraging Ant (Hymenoptera: Formicidae) Functional Groups as Bioindicators of Forest Health in Northern Arizona Ponderosa Pine Forests

Reintroduction of fire and thinning have been suggested as the main practices to regain forest health in ponderosa pine forests of northern Arizona. Recent silvicultural programs and the occurrence of catastrophic wildfires have created a range of disturbance severities and a mosaic of forest conditions. Sixteen stands were randomly selected to create a completely randomized experimental design with four treatments, (1) unmanaged, (2) thinned, (3) thinned and burned, and (4) wildfire, with four replicates of each treatment. We assessed changes occurring in ground foraging ant functional groups at the stand scale as related to these treatments. A pitfall trapping scheme was implemented during the summer months of 2002 and 2003. A total of 18,009 specimens were collected representing 20 species from 10 genera. We found that traditional biodiversity measures, such as species richness, diversity, and dominance were a less satisfactory measure of treatment impact on ants than functional group analysis, which allowed us to consider the ecosystem role of each species. We found that different functional groups were dominant under different levels of disturbance severity and suppressed or excluded other functional groups that were less suited to the disturbance intensity. Maintaining a diversity of habitat types is suggested for supporting ecologically diverse ant functional groups and improve forest health.

Snag and woody debris dynamics following severe wildfires in northern Arizona ponderosa pine forests

Following severe wildfires in southwestern ponderosa pine forests, dead trees remain on the landscape and eventually fall, but relatively little is known about the quantity and quality of post-wildfire coarse woody debris (CWD). To describe post-fire conditions, we measured snags, CWD, and fine woody debris and forest floor depth on seven fires in a chronosequence from 3 to 27 years old in northern Arizona. Snags declined in density with increasing time since fire and generally followed expected patterns of decay, except that few snags stood long enough to reach a clean-bark state. The mean biomass of the surface CWD ranged from as low of 3.3 Mg ha −1 to a high of 41.3 Mg ha −1. Total CWD biomass in the surface fuel load remained roughly comparable from 8–9-year-old fires to a 27-year-old fire but the state of the CWD changed from sound to rotten. The change to a rotten condition suggests an increase in ignitability of the post-fire fuel load, but fine fuels that could support high fireline intensity were relatively low. The number of “jackstraws,” points where intersecting downed logs could create a hot spot if reburned, was slightly higher in the oldest fire. Few fire-created snags remained by the 27th year post-fire. Management options to reduce fuels after severe wildfire, such as salvage logging, prescribed burning, or passive management, should be addressed in a broad ecological context.

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.

Arbuscular mycorrhizal propagule densities respond rapidly to ponderosa pine restoration treatments

1 Mycorrhizae form a critical link between above-ground plants and the soil system by influencing plant nutrition, nutrient cycling and soil structure. Understanding how mycorrhizae respond to disturbances may lead to important advances in interpreting above-ground plant recovery. 2 The inoculum potential for arbuscular mycorrhizae (AM) and ectomycorrhizal (EM) fungi was investigated in thinned-only, thinned and prescribed burned (both restoration treatments) and unthinned and unburned control stands in northern Arizona ponderosa pine forests. The relationships between mycorrhizal fungal propagule densities and plant community and soil properties were quantified. 3 The relative amount of infective propagules of AM fungi was significantly higher in samples collected from both restoration treatments than their paired controls (unthinned and unburned stands). In contrast, the same restoration treatments had no significant effect on the relative amount of infective propagules of EM fungi. 4 The relative amount of infective propagules of AM fungi was significantly positively correlated with graminoid cover and herbaceous understorey species richness and negatively correlated with overstorey tree canopy cover and litter cover. 5 Synthesis and applications. These results indicate that population densities of AM fungi can rapidly increase following restoration treatments in northern Arizona ponderosa pine forests. This has important implications for restoring the herbaceous understorey of these forests because most understorey plants depend on AM associations for normal growth. These results also can be applied to other ecosystems that are in a state of restoration or where the role of fire is just beginning to be understood.

Ponderosa pine mortality following fire in northern Arizona

Sampling of 1367 trees was conducted in the Side wildfire (4 May 1996), Bridger-Knoll wildfire (20 June 1996) and Dauber prescribed fire (9 September 1995) in northern Arizona ponderosa pine forests (Pinus ponderosa). Tree mortality was assessed for 3 years after each fire. Three-year post-fire mortality was 32.4% in the Side wildfire, 18.0% in the Dauber prescribed fire, and 13.9% in the Bridger-Knoll wildfire. In the Dauber and Side fires, 95% and 94% of 3-year post-fire mortality occurred by year 2, versus 76% in the Bridger-Knoll wildfire. Compared with trees that lived for 3 years after fire, dead trees in all fires had more crown scorch, crown consumption, bole scorch, ground char, and bark beetle attacks. Logistic regression models were used to provide insight on factors associated with tree mortality after fire. A model using total crown damage by fire (scorch + consumption) and bole char severity as independent variables was the best two-variable model for predicting individual tree mortality for all fires. The amount of total crown damage associated with the onset of tree mortality decreased as bole char severity increased. Models using diameter at breast height (dbh) and crown volume damage suggested that tree mortality decreased as dbh increased in the Dauber prescribed fire where trees were smallest, and tree mortality increased as dbh increased in the Side and Bridger-Knoll wildfires where trees were largest. Moreover, a U-shaped dbh–mortality distribution for all fires suggested higher mortality for the smallest and largest trees compared with intermediate-size trees. We concluded that tree mortality is strongly influenced by interaction between crown damage and bole char severity, and differences in resistance to fire among different-sized trees can vary among sites.

Corrigendum to: Ponderosa pine mortality following fire in northern Arizona

Sampling of 1367 trees was conducted in the Side wildfire (4 May 1996), Bridger-Knoll wildfire (20 June 1996) and Dauber prescribed fire (9 September 1995) in northern Arizona ponderosa pine forests (Pinus ponderosa). Tree mortality was assessed for 3 years after each fire. Three-year post-fire mortality was 32.4% in the Side wildfire, 18.0% in the Dauber prescribed fire, and 13.9% in the Bridger-Knoll wildfire. In the Dauber and Side fires, 95% and 94% of 3-year post-fire mortality occurred by year 2, versus 76% in the Bridger-Knoll wildfire. Compared with trees that lived for 3 years after fire, dead trees in all fires had more crown scorch, crown consumption, bole scorch, ground char, and bark beetle attacks. Logistic regression models were used to provide insight on factors associated with tree mortality after fire. A model using total crown damage by fire (scorch + consumption) and bole char severity as independent variables was the best two-variable model for predicting individual tree mortality for all fires. The amount of total crown damage associated with the onset of tree mortality decreased as bole char severity increased. Models using diameter at breast height (dbh) and crown volume damage suggested that tree mortality decreased as dbh increased in the Dauber prescribed fire where trees were smallest, and tree mortality increased as dbh increased in the Side and Bridger-Knoll wildfires where trees were largest. Moreover, a U-shaped dbh–mortality distribution for all fires suggested higher mortality for the smallest and largest trees compared with intermediate-size trees. We concluded that tree mortality is strongly influenced by interaction between crown damage and bole char severity, and differences in resistance to fire among different-sized trees can vary among sites.

Ground Beetle (Coleoptera: Carabidae) Species Assemblage as an Indicator of Forest Condition in Northern Arizona Ponderosa Pine Forests

Reintroduction of fire and thinning have been suggested as the main practices to regain forest health in northern Arizona ponderosa pine (Pinus ponderosa Dougl. ex Lawson) forests. Criteria for assessing the impact of such management practices in the forest are based on benchmark reconstructed conditions resembling pre-European forest stand structure and on the enhancement of tree vigor. A range of forest conditions currently exists including stands that have been unmanaged, thinned only, thinned plus prescribed burned and burned by wildfire. A surrogate taxon was used to assess forest condition under criteria of maintaining habitat for native species operating at the soil level. We assessed changes occurring in ground beetle assemblages at the stand scale as related to changes that had occurred in forest stands previously treated with the above treatments. A pitfall-trapping scheme was deployed during the summer months of 1998, 1999, and 2000. A total of 4,452 specimens was caught representing 15 genera and 20 species of ground beetles. We found that species diversity increased as the level of disturbance increased. The indicator species assemblage found on the wildfire treatment was represented by species in the genera Amara and Harpalus that are characteristic of dry-open habitats. Unmanaged stands generally had the lowest diversity and the assemblage was dominated by the species Synuchus dubius (Leconte). The thinned only stands did not significantly vary from unmanaged stands in species assemblage. Cyclotrachelus constrictus (Say) was indicative of the thinning plus broadcast burned stands. Stands that were thinned plus burned were richer than both unmanaged and thinned only stands without a shift toward an open-area dominant assemblage as occurred in the stands burned by wildfire.

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.