Arizona Ponderosa Pine Forests Research Articles

Extent of recent fire-induced losses of ponderosa pine forests of Arizona and New Mexico, USA

Land management legacies coupled with climate change have led to shifts in western ponderosa pine (Pinus ponderosa) forest structure and fire regimes. A growing body of evidence suggests that under increasing area burned at high-severity, warming and drought, ponderosa pine forests are vulnerable to conversion to alternate forest types or non-forest vegetation. However, the extent and direction of recent fire-induced changes in southwestern US ponderosa pine forests have not been subject to region-wide evaluation. Here, we assess recent fire effects in ponderosa pine forests using long-term repeated samples of stand composition and structure from the US Forest Service’s Forest Inventory and Analysis (FIA) program, combined with metrics of satellite-derived burn severity (predicted Composite Burn Index, CBI; and differenced normalized burn ratio, dNBR). We compiled and analyzed FIA plots dominated by ponderosa pine and associated species within the southwestern states of Arizona and New Mexico to quantify regional trends for ponderosa pine (e.g., forest losses or gains), link changes to wildfire severity, and characterize vegetation shifts.Among 685 plots sampled three times between 1996 and 2017, 26% of plots burned at least once, with more than half of these burning at moderate to high severity. Plots that burned within the study period exhibited a net loss of 46% of ponderosa pine trees, but unburned plots also exhibited a net decline of 11%. While all tree size classes exhibited greater losses than gains, tree size was a positive predictor of post-fire survival. Satellite-derived burn severity (predicted CBI) was strongly related to tree mortality and shifts toward alternate post-fire vegetation types. Severely burned stands exhibited trajectories toward non-forest vegetation with high dominance by resprouting species, particularly Gambel oak (Quercus gambelii).These findings contribute to an emerging understanding of the potential ecological outcomes of growing forest vulnerability to changing climate and disturbance regimes. Methods employed herein offer scalable opportunities to quantify changes across forest biomes using long-term monitoring data. As importantly, our findings inform regional and local land management efforts to sustain these valued forest types in an era of change. We highlight two key themes for management to promote the long-term persistence of ponderosa pine forests in the southwestern US: restoration of low-severity fire regimes and retention of large, fire-resistant trees.

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.

Dwarf Mistletoe and Breeding Bird Abundance in Ponderosa Pine Forests

Southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum) parasitizes ponderosa pine (Pinus ponderosa). It can kill severely infected trees and induce the growth of dense masses of branches that can affect foraging and nesting habitat for wildlife. We tested the hypothesis that higher densities of breeding birds would be correlated with higher levels of southwestern dwarf mistletoe in ponderosa pine forests of Arizona. We estimated densities of 15 species of breeding birds and measured 26 habitat elements in 19 stands. Average dwarf mistletoe ratings (DMR) ranged from 0.0 (uninfested) to 3.7 (severely infested). Although we observed higher densities with higher infestation of dwarf mistletoe for 2 bird species and lower densities for 3 bird species, the effect of dwarf mistletoe on these was minor. Instead, higher snag density, an indirect measure of past mistletoe infestation, was a more important predictor of bird density. With greater snag size or density, the species richness and densities of 3 bird species increased. Because dwarf mistletoe infection can create snags, retaining groups of dwarf mistletoe-infected trees in ponderosa pine stands will provide a continued source of snags, which are important habitat for many wildlife species.

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.

Bottom-up control of a northern Arizona ponderosa pine forest fire regime in a fragmented landscape

Fire regimes often vary at fine spatial scales in response to factors such as topography or fuels while climate usually synchronizes fires across broader scales. We investigated the relative influence of top-down and bottom-up controls on fire occurrence in ponderosa pine (Pinus ponderosa) forests in a highly fragmented landscape at Mount Dellenbaugh, in northwestern Arizona. Our study area of 4,000 ha was characterized by patches of ponderosa pine forest in drainages that were separated by a matrix of pinyon–juniper woodlands, sagebrush shrublands, and perennial grasslands. We reconstructed fire histories from 135 fire-scarred trees in sixteen 25-ha sample sites placed in patches of mature ponderosa forest. We found that, among patches of ponderosa forest, fires were similar in terms of frequency but highly asynchronous in terms of individual years. Climate synchronized fire but only across broader spatial scales. Fires occurring at broader scales were associated with dry years that were preceded by several wet years. The remarkable level of asynchrony at finer scales suggests that bottom-up factors, such as site productivity and fuel continuity, were important in regulating fire at Mount Dellenbaugh. Understanding where bottom-up controls were historically influential is important for prioritizing areas that may best respond to fuel treatment under a warming climate.

Woody debris and tree regeneration dynamics following severe wildfires in Arizona ponderosa pine forests

Severe forest fires worldwide leave behind large quantities of dead woody debris and regenerating trees that can affect future ecosystem trajectories. We studied a chronosequence of severe fires in Arizona, USA, spanning 1 to 18 years after burning to investigate postfire woody debris and regeneration dynamics. Snag densities varied over time, with predominantly recent snags in recent fires and broken or fallen snags in older fires. Coarse woody debris peaked at > 60 Mg/ha in the time period 6–12 years after fire, a value higher than previously reported in postfire fuel assessments in this region. However, debris loadings on fires older than 12 years were within the range of recommended management values (11.2–44.8 Mg/ha). Overstory and regeneration were most commonly dominated by sprouting deciduous species. Ponderosa pine ( Pinus ponderosa C. Lawson var. scopulorum Engelm.) overstory and regeneration were completely lacking in 50% and 57% of the sites, respectively, indicating that many sites were likely to experience extended periods as shrublands or grasslands rather than returning rapidly to pine forest. More time is needed to see whether these patterns will remain stable, but there are substantial obstacles to pine forest recovery: competition with sprouting species and (or) grasses, lack of seed sources, and the forecast of warmer, drier climatic conditions for coming decades.

Impacts of Silvicultural Thinning Treatments on Beetle Trap Captures and Tree Attacks During Low Bark Beetle Populations in Ponderosa Pine Forests of Northern Arizona

Our research used a combination of passive traps, funnel traps with lures, baited trees, and surveys of long-term thinning plots to assess the impacts of different levels of stand basal area (BA) on bark beetle tree attack and on trap captures of Ips spp., Dendroctonus spp., and their predators. The study occurred at two sites in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests, from 2004 to 2007 during low bark beetle populations. Residual stand BA ranged from 9.0 to 37.0 m2/ha. More predators and bark beetles were collected in passive traps in stands of lower BA than in stands of higher BA; however, significance varied by species and site, and total number of beetles collected was low. Height of the clear panel passive traps affected trap catches for some species at some sites and years. When pheromone lures were used with funnel traps [Ips pini (Say) lure: lanierone, +03/-97 ipsdienol], we found no significant difference in trap catches among basal area treatments for bark beetles and their predators. Similarly, when trees were baited (Dendroctonus brevicomis LeConte lure: myrcene, exo-brevicomin and frontalin), we found no significant difference for days to first bark beetle attack. Surveys of long-term thinning treatments found evidence of bark beetle attacks only in unthinned plots (approximately 37 m2/ha basal area). We discuss our results in terms of management implications for bark beetle trapping and control.

Effects of Prescribed Fire on Winter Assemblages of Birds in Ponderosa Pine Forests of Northern Arizona

Abstract Studies of birds in winter are rare in wildlife ecology despite winter being a critical time for birds. We examined winter assemblages of birds in ponderosa pine (Pinus ponderosa) forests of northern Arizona following prescribed fire. We conducted point counts on two study sites in northern Arizona from mid-October to mid-March 2004–2006. Each site had one unit treated by prescribed fire a full growing season before the point counts began, paired with control unit(s) of similar structure. We detected 39 species during the survey. Nine species comprised 81% of all detections; eight of these were year-round residents of the area. Dark-eyed juncos (Junco hyemalis) were the most numerous, comprising 23% of all detections. Assemblages were similar between treatments (Sorenson similarity index = 0.85) and years (Sorenson similarity index = 0.85), and rank abundance of species between burn and control units were correlated (Spearman's ρ = 0.83). Therefore, assemblages of birds in winter were simil...

Relative and Seasonal Abundance of Three Bark Beetle Predators (Coleoptera: Trogositidae, Cleridae) Across an Elevation Gradient in Ponderosa Pine Forests of North Central Arizona

We examined abundance and flight periodicity of 3 predators of bark beetles (Coleoptera: Curculionidae, Scolytinae), Temnochila chlorodia (Mannerheim) (Coleoptera: Trogositidae), Enoclerus sphegeus (Fabricius) (Coleoptera: Cleridae), and E. lecontei (Wolcott) (Coleoptera: Cleridae), across an elevational gradient of ponderosa pine (Pinus ponderosa Lawson) forests in north central Arizona. Predator populations were estimated at 10 sites in each of 3 elevation bands (low: 1600–1736 m; mid: 2058–2230 m; high: 2505–2651 m) for 3 years (2004–2006) using pheromone-baited funnel traps targeting 3 primary bark beetle species. We also investigated how predator abundance and flight seasonality related to those of 5 bark beetle species: lps pini (Say), I. lecontei Swaine, Dendroctonus frontalis Zimmermann, D. brevicomis LeConte, and D. adjunctus Blandford. Temnochila chlorodia was most abundant in the low- and mid-elevation bands, whereas E. sphegeus was most abundant in the high-elevation band. Enoclerus lecontei showed no consistent elevational trend in abundance. Within each elevation band, changes in annual abundance of pooled predator species tracked shifts in abundance of pooled bark beetle species. In general, predator flight initiation coincided with or closely followed bark beetle flight initiation in the spring, but predator flight terminated before flight activity ended for most bark beetle species in the fall. In addition, the ratio of prey to predators was lowest in the summer and highest in the fall. This suggests that all bark beetle species examined may be provided temporal escape from their predators in the fall. For all 3 predator species, the pheromone-baited trap targeting D. brevicomis was less attractive than the pheromone-baited traps targeting I. pini and I. lecontei.

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.

The Role of Fire in the Establishment and Spread of Nonnative Plants in Arizona Ponderosa Pine Forests: A Review

Arizona ponderosa pine (Pinus ponderosa) forests have undergone dramatic changes since the late nineteenth century. Management practices such as fire exclusion have resulted in increased surface and tree crown fuel loads. Consequently, large and severe wildfires have been occurring more frequently in the ponderosa pine forests of Arizona. Furthermore, prescribed fire, both with and without tree thinning, is being applied to ponderosa pine forests to mitigate the risk of wildfire. Increasingly, normative plant species are spreading into burned forests regardless of fire type. In this article, we review recent literature on normative establishment in the post-fire plant community. Current research suggests an increasing risk of nonnative establishment is associated with increasing fire severity. Furthermore, wildfires seem more likely to promote nonnative species establishment than prescribed fires. The results are inconsistent, however, with some high-severity fires having few normatives and some prescribed fires strongly promoting invasion. Further research is needed to understand the mechanisms of invasion. In the 23 studies we examined, a total of 43 nonnative plant species were reported in post-fire communities. Of these, only three species were listed as noxious in Arizona: field bindweed (Convolvulus arvensis), Dalmatian toadflax (Linaria dalmatica), and Scotch cottonthistle (Onopordum acantium). Cheatgrass (Bromus tectorum), Kentucky bluegrass (Poa pratensis), and common mullein (Verbascum thapsus) were the most commonly observed species in burned areas, regardless of natural or anthropogenic ignition. Generally, nonnative presence increased after fire, but abundance of these species remained low (< 10% cover). Changes in nonnative populations reported in Arizona are consistent with those seen in ponderosa pine forests of other regions, with normatives tending to increase with increasing fire severity.

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.

Observations of Bird Numbers and Species Following a Historic Wildfire in Arizona Ponderosa Pine Forests

ABSTRACT The Rodeo-Chediski Wildfire, the largest in Arizona's history, damaged or destroyed ecosystem resources or disrupted ecosystem functioning in a mostly mosaic pattern throughout the ponderosa pine (Pinus ponderosa) forests exposed to the bum. Impacts of the wildfire on the occurrence of birds and their diversities were studied on two watersheds in the area, one burned by a high severity (stand-replacing) fire, while the other was burned by a low severity (stand-modifying) fire. This paper reports on the results of a five-year (2002–2007) evaluation of this wildfire on the bird species and numbers observed on these two watersheds to provide a record of the impacts of the historic fire on avifauna communities.

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.

Historical Stem-Mapped Permanent Plots Increase Precision of Reconstructed Reference Data in Ponderosa Pine Forests of Northern Arizona

Forest structural reference conditions are widely used to understand how ecosystems have been altered and guide restoration and management objectives. We used six stem-mapped permanent plots established in the early twentieth century to provide precise structural reference conditions for ponderosa pine forests of northern Arizona prior to Euro-American settlement. Reference conditions for these plots in 1873–1874 included the following historical attributes: tree densities of 45–127 trees/ha, mean tree diameter at breast height (dbh) of 43.8 cm with a corresponding quadratic mean diameter range of 41.5–51.3 cm, and a stand basal area of 9.2–18.0 m2/ha. The reconstructed diameter distributions (for live ponderosa pine trees with dbh ≥9.14 cm) prior to fire exclusion varied in shape but generally displayed an irregular unimodal distribution. We suggest that management objectives for the structural restoration of ponderosa pine forests of northern Arizona emphasize: (1) conservation and retention of all pre-settlement (>130 years) trees; (2) reduction of tree densities with a restoration objective ranging between 50 and 150 trees/ha having a large-tree component between 25 and 50% of the total trees per hectare, respectively; (3) manipulation of the diameter distribution to achieve a unimodal or irregular, uneven-aged shape (possibly targeting a balanced, uneven-aged shape on cinder soil types) through the use of harvest and thinning practices that mimic gap disturbances (i.e., individual tree selection system); and (4) retention of 3–11 snags and logs per hectare resulting from natural mortality.

Integrating Modeling and Field Experiments to Evaluate Impacts of Vegetative Practices on Ponderosa Pine Watersheds1

Abstract: Determining watershed response to vegetation treatment has been the subject of numerous hydrologic studies over the years. However, generalizing the information obtained from traditional paired‐watershed studies to other watersheds in a region is problematic because of the empirical nature of such studies and the context dependence of hydrologic responses. This paper addresses the issue of generalizing hydrologic information through integration of process‐based modeling and field observations from small‐scale watershed experiments. To this end, the results from application of a process‐based model were compared with the results from small‐scale watershed experiments in ponderosa pine forests of Arizona. The model simulated treatment impacts reasonably well when compared to the traditional paired‐watershed approach. However, the model tended to overestimate water yields during periods of low flow, and there was a significant difference between the two approaches in the estimation of treatment impacts during the first four years following treatment. The results indicate that the lumped‐parameter modeling approach used here may be limited in its ability to detect small changes, and tends to overestimate changes that occur immediately following treatment. It is concluded that watershed experiments can be highly informative due to their direct examination of cause‐effect relationships, while process‐based models are useful for their processing power and focus on functional relationships. The integrated use of both watershed experiments and process‐based models provides a way to generalize hydrologic information, illuminate the processes behind landscape treatment effects, and to generate and test hypotheses.

Evaluation of Funnel Traps for Characterizing the Bark Beetle (Coleoptera: Scolytidae) Communities in Ponderosa Pine Forests of North-Central Arizona

Lindgren funnel traps baited with aggregation pheromones are widely used to monitor and manage populations of economically important bark beetles (Coleoptera: Scolytidae). This study was designed to advance our understanding of how funnel trap catches assess bark beetle communities and relative abundance of individual species. In the second year (2005) of a 3-yr study of the bark beetle community structure in north-central Arizona pine (Pinus spp.) forests, we collected data on stand structure, site conditions, and local bark beetle-induced tree mortality at each trap site. We also collected samples of bark from infested (brood) trees near trap sites to identify and determine the population density of bark beetles that were attacking ponderosa pine, Pinus ponderosa Douglas ex Lawson, in the area surrounding the traps. Multiple regression models indicated that the number of Dendroctonus and Ips beetles captured in 2005 was inversely related to elevation of the trap site, and positively associated with the amount of ponderosa pine in the stand surrounding the site. Traps located closer to brood trees also captured more beetles. The relationship between trap catches and host tree mortality was weak and inconsistent in forest stands surrounding the funnel traps, suggesting that trap catches do not provide a good estimate of local beetle-induced tree mortality. However, pheromone-baited funnel trap data and data from gallery identification in bark samples produced statistically similar relative abundance profiles for the five species of bark beetles that we examined, indicating that funnel trap data provided a good assessment of species presence and relative abundance.