Southwestern Ponderosa Pine Forests Research Articles

Postfire futures in southwestern forests: Climate and landscape influences on trajectories of recovery and conversion.

Southwestern ponderosa pine forests are vulnerable to fire-driven conversion in a warming and drying climate, yet little is known about what kinds of ecological communities may replace them. To characterize postfire vegetation trajectories and their environmental determinants, plant assemblages (361 sample plots including 229 vascular plant species, surveyed in 2017) were sampled within eight burns that occurred between 2000 and 2003. I used nonmetric multidimensional scaling, k-means clustering, principal component analysis, and random forest models to assess relationships between vegetation pattern, topographic and landscape factors, and gridded climate data. I describe seven postfire community types, including regenerating forests of ponderosa pine, aspen, and mixed conifers, shrub-dominated communities of Gambel oak and mixed species, and herb-dominated communities of native bunchgrasses and mixtures of ruderal, native, and nonnative species. Forest recovery was generally associated with cooler, mesic sites in proximity to forested refugia; shifts toward scrub and grassland types were most common in warmer, dryer locations distant from forested refugia. Under future climate scenarios, models project decreases in postfire forest recovery and increases in nonforest vegetation. However, forest to nonforest conversion was partially offset under a scenario of reduced burn severity and increased retention of forested refugia, highlighting important management opportunities. Burning trends in the southwestern United States suggest that postfire vegetation will occupy a growing landscape fraction, compelling renewed management focus on these areas and paradigm shifts that accommodate ecological change. I illustrate how management decisions around resisting, accepting, or directing change could be informed by an understanding of processes and patterns of postfire community variation and likely future trajectories.

Southwestern ponderosa pine forest patterns following wildland fires managed for resource benefit differ from reference landscapes

ContextManagers aiming to utilize wildland fire to restore southwestern ponderosa pine landscapes require better understanding of forest cover patterns produced at multiple scales. Restoration effectiveness of wildland fires managed for resource benefit can be evaluated against natural ranges of variation.ObjectivesWe describe landscape patterns within reference landscapes, including restored and functioning ponderosa pine forests of northern Arizona, and compare them to wildland fires managed for resource benefit. We make comparisons along a gradient of extents and assess the effects of scale on landscape differences.MethodsUsing Sentinel-2 imagery, we classified ponderosa pine forest cover and calculated landscape metrics across a gradient of landscape extent within reference and managed landscapes. We used non-parametric tests to assess differences. We used random forest models to assess and explore which landscape metrics were most importance in differentiating landscape patterns.ResultsRestored forests exhibited landscapes patterns consistent with those of ecologically intact forest landscapes. Managed wildfire landscape patterns differed significantly when compared to reference landscape patterns among nearly all landscape metrics considered and became increasingly different with increasing landscape extent (15–840 ha), tending towards both denser and larger patch areas.ConclusionsLandscape patterns from wildland fires managed for resource benefit we examined differ from those of reference landscapes. Differences become more pronounced with increasing landscape size. Landscape patterns among large managed forest landscapes suggest that the predominately single-entry, low-severity disturbance regime from these managed fires is failing to reduce tree densities and break up large contiguous areas of canopy cover.

Airborne lidar provides reliable estimates of canopy base height and canopy bulk density in southwestern ponderosa pine forests

Accurate estimates of canopy base height (CBH) and canopy bulk density (CBD) are critical inputs for many fire modeling and simulation programs. Commonly used LANDFIRE estimates of CBH and CBD are only available at 30 m resolution and have relatively low accuracy, and traditional field-based estimates can lead to inaccurate spatial estimates by assuming averages across spatial extents. Discrete-return airborne light detection and ranging (lidar) has become increasingly common in forestry applications, with demonstrated success in measuring canopy metrics and forest structure across broad landscapes and varied forest types. However, few studies have investigated the potential of airborne lidar for estimating these metrics in southwestern forests. This study developed an approach for estimating CBH and CBD using airborne lidar data and evaluated the accuracy of estimates at 20 m resolution. We also evaluated the predicted accuracy between managed and unmanaged forest stands. We employed a quantile-based method to derive CBH and utilized the Fire and Fuels Extension of the Forest Vegetation Simulator to estimate CBD from a lidar-derived tree list. Our results indicate that airborne lidar produced more accurate estimates of both CBH (R2 = 0.4; RMSE = 1.25 m) and CBD (R2 = 0.76; RMSE = 0.021 kg−3) as compared to LANDFIRE and other regional remote-sensing based estimates. We also report that airborne lidar is more accurate at estimating CBH in unmanaged stands versus managed stands, but CBD estimates maintain similar accuracy regardless of management history. Compared to methods developed in other regions, our approach resulted in relatively low R2 values, but our RMSE values were found to be similar or slightly improved. Our approach for deriving both metrics are conceptually and computationally simple making them especially valuable for use by land managers and ecologists alike. This study provides clear evidence that airborne lidar can be used to derive broad-scale, fine-resolution fuel data layers, which are commonly required in fire modeling and forest management activities and decision-making.

FIRE HISTORY OF AN OLD-GROWTH PONDEROSA PINE STAND IN THE SHEEP RANGE, DESERT NATIONAL WILDLIFE REFUGE, NEVADA, USA

ABSTRACT Southwestern ponderosa pine forests have experienced reduced fire frequency since Euro-American settlement generally because of successful fire suppression policies. We report here on the ...

Evaluating Prescribed Fire Effectiveness Using Permanent Monitoring Plot Data: A Case Study

Since Euro-American settlement, ponderosa pine forests throughout the western United States have shifted from high fire frequency and open canopy savanna forests to infrequent fire and dense, closed canopy forests. Managers at Zion National Park, USA, reintroduced fire to counteract these changes and decrease the potential for high-severity fires. We analyzed existing permanent monitoring plot data collected between 1995 and 2010 to assess achievement of management objectives related to prescribed fire in ponderosa pine forests. Following first entry fire, ponderosa pine (Pinus ponderosa C. Lawson var. scopulorum Engelm.) and Gambel oak (Quercus gambelii Nutt.) overstory and midstory densities declined between 10% and 45% and effectively shifted the Gambel oak diameter distribution toward larger trees. Second entry fires had a greater effect, reducing ponderosa pine and Gambel oak overstory and midstory densities between 24% and 92%. Diameter distributions of both species shifted toward fewer, larger trees following second entry fires. Total fuel load was reduced by <20% in first entry fires and by half in second entry fires. Several objectives identified by the National Park Service (e.g., overstory ponderosa pine reduction) were not achieved with either fire entry; however, power analysis indicated that sample sizes were not adequate to fully detect long term changes following first entry fires. First entry, low intensity prescribed fire alone may not meet management objectives in southwestern ponderosa pine forests. We recommend using multiple fire entries or increased fire intensity if mechanical treatments are not also being utilized concurrently. Long term fire effects data is critical to adaptive management in national parks; however, the utility of this data could be improved through increased sample sizes, consistent data archiving, and regional scale analyses.

Issue Cover

Cover Photo: Fire is an ecologically important disturbance process. Since forests help regulate the climate system by sequestering carbon from the atmosphere, quantifying how forest treatments that restore regular surface fire impact carbon dynamics is central to understanding the impacts of management on the provision of this ecosystem service. This photo, used with permission by Brandon R. Oberhardt, illustrates the article “Restoring forest structure and process stabilizes forest carbon in wildfire-prone southwestern ponderosa pine forests” by Matthew Hurteau, Shuang Liang, Katherine Martin, Malcolm North, George Koch, and Bruce Hungate, published in Ecological Applications 26:382–391 (doi: 10.1890/15-0337). See the Photo Gallery in this issue of the Bulletin for more images related to this article.

Restoring Forest Structure and Process Stabilizes Forest Carbon in the Wildfire‐Prone Southwestern USA

Fire is an ecologically important disturbance process. In the dry forest types of the southwestern USA, regular surface fires help maintain forest structural heterogeneity. Fire exclusion has homogenized these ecosystems and increased the risk of stand-replacing wildfire. Since forests help regulate the climate system by sequestering carbon from the atmosphere, quantifying how forest treatments that restore regular surface fire impact carbon dynamics is central to understanding the impacts of management on the provision of this ecosystem service. We quantified the effects of thinning and burning treatments in a ponderosa pine forest and found that they initially reduced the amount of carbon stored in the forest relative to the control. When we accounted for the chance of wildfire, we found that the restored forest structure and regular surface fire stored more carbon over time than the control. This was the direct result of treatments modifying fire behavior and reducing the risk of stand-replacing wildfire. These photographs illustrate the article “Restoring forest structure and process stabilizes forest carbon in wildfire-prone southwestern ponderosa pine forests” by Matthew Hurteau, Shuang Liang, Katherine Martin, Malcolm North, George Koch, and Bruce Hungate, published in Ecological Applications 26(2):382–391. http://dx.doi.org/10.1890/15-0337.1

Restoring forest structure and process stabilizes forest carbon in wildfire‐prone southwestern ponderosa pine forests

Changing climate and a legacy of fire-exclusion have increased the probability of high-severity wildfire, leading to an increased risk of forest carbon loss in ponderosa pine forests in the southwestern USA. Efforts to reduce high-severity fire risk through forest thinning and prescribed burning require both the removal and emission of carbon from these forests, and any potential carbon benefits from treatment may depend on the occurrence of wildfire. We sought to determine how forest treatments alter the effects of stochastic wildfire events on the forest carbon balance. We modeled three treatments (control, thin-only, and thin and burn) with and without the occurrence of wildfire. We evaluated how two different probabilities of wildfire occurrence, 1% and 2% per year, might alter the carbon balance of treatments. In the absence of wildfire, we found that thinning and burning treatments initially reduced total ecosystem carbon (TEC) and increased net ecosystem carbon balance (NECB). In the presence of wildfire, the thin and burn treatment TEC surpassed that of the control in year 40 at 2%/yr wildfire probability, and in year 51 at 1%/yr wildfire probability. NECB in the presence of wildfire showed a similar response to the no-wildfire scenarios: both thin-only and thin and burn treatments increased the C sink. Treatments increased TEC by reducing both mean wildfire severity and its variability. While the carbon balance of treatments may differ in more productive forest types, the carbon balance benefits from restoring forest structure and fire in southwestern ponderosa pine forests are clear.

Short-Term Belowground Responses to Thinning and Burning Treatments in Southwestern Ponderosa Pine Forests of the USA

Microbial-mediated decomposition and nutrient mineralization are major drivers of forest productivity. As landscape-scale fuel reduction treatments are being implemented throughout the fire-prone western United States of America, it is important to evaluate operationally how these wildfire mitigation treatments alter belowground processes. We quantified these important belowground components before and after management-applied fuel treatments of thinning alone, thinning combined with prescribed fire, and prescribed fire in ponderosa pine (Pinus ponderosa) stands at the Southwest Plateau, Fire and Fire Surrogate site, Arizona. Fuel treatments did not alter pH, total carbon and nitrogen (N) concentrations, or base cations of the forest floor (O horizon) or mineral soil (0–5 cm) during this 2-year study. In situ rates of net N mineralization and nitrification in the surface mineral soil (0–15 cm) increased 6 months after thinning with prescribed fire treatments; thinning only resulted in net N immobilization. The rates returned to pre-treatment levels after one year. Based on phospholipid fatty acid composition, microbial communities in treated areas were similar to untreated areas (control) in the surface organic horizon and mineral soil (0–5 cm) after treatments. Soil potential enzyme activities were not significantly altered by any of the three fuel treatments. Our results suggest that a variety of one-time alternative fuel treatments can reduce fire hazard without degrading soil fertility.

Restoring forest structure and process stabilizes forest carbon in wildfire-prone southwestern ponderosa pine forests

Restoring forest structure and process stabilizes forest carbon in wildfire-prone southwestern ponderosa pine forests

Increasing weight of evidence that thinning and burning treatments help restore understory plant communities in ponderosa pine forests

For more than a century ecosystems around the world have experienced an increase in the dominance of woody species. While the drivers of woody plant proliferation are complex, interactions between climate and land-use change are commonly invoked as primary contributing factors. In ponderosa pine forests of western North America, substantial increases in tree densities are impacting overall forest health and increasing the risk for severe wildfires and insect and disease outbreaks. Addressing this problem through the use of ecological restoration projects is widely advocated. Our objective was to quantify understory vegetation response five years after thinning and burning treatments in a southwestern ponderosa pine forest. We remeasured plant species richness and cover on thinned+burned, burned only, and untreated controls replicated four time in a BACI design. An untreated control (Untreated) was used for comparison of two alternative restoration treatments (1) thinning followed by prescribed fire (Thin+Burn) and (2) prescribed fire only (Burn-only). Understory species richness and total plant cover increased significantly in Thin+Burn compared to the Untreated and Burn-only treatments. Analysis of functional group composition revealed increases in species richness and cover was driven primarily by changes in annual–biennial forbs and graminoids. We then compared our results with those of three Long-term Ecological Restoration and Assessment Network (LEARN) sites. We found total and native plant cover showed evidence of a consistent response to Thin+Burn treatments across all sites. However, results suggest that understory response to restoration treatments is under strong environmental control. As a consequence the range of variability associated with understory responses should be expected to expand or contract depending on where treatments are implemented. Overall our findings add to a growing body of literature that restoration treatments combining mechanical thinning and prescribed fire are useful for increasing native understory abundance and diversity.

Ecological restoration and fine-scale forest structure regulation in southwestern ponderosa pine forests

Fine-scale forest patterns are an important component of forest ecosystem complexity and spatial pattern objectives are an increasingly common component of contemporary silviculture prescriptions in dry fire-adapted forests of North America. Despite their importance, questions remain regarding the assessment of silvicultural treatments designed to meet spatial objectives. We initiated a replicated silvicultural assessment of two forest management approaches commonly applied in dense ponderosa pine forests of the Southwest United States: historical evidence-based ecological restoration guidelines (ERG) and northern goshawk (Accipiter gentilis) foraging area management recommendations (GMR). We compared stand-level characteristics, global tree location point patterns and tree group-level attributes resulting from the marking of these approaches to current forest conditions and patterns of historical forest remnants in six, 2.02ha stem mapped plots. We also assessed group-level Vegetative Structural Stage (VSS; a classification of fine-scale forest structural development used to regulate fine-scale spatial patterns in these forests). ERG and GMR-based treatments significantly reduced densities and basal area from the current condition, but did not significantly differ in density from historical forest remnant estimates. GMR-based treatments retained greater stand level basal area than ERG-based treatments, primarily in large, 28–48cm tree diameter classes. GMR-based treatments approximated global tree location point patterns of forest remnants better than ERG-based treatments, primarily due to a 5–6m minimum spacing of residual trees, but also likely due to specific aspects of ERG-based marking techniques. Despite this difference, both treatments resulted in group-level characteristics similar to those exhibited by historical forest remnants. Both treatments significantly altered group-level VSS area and reduced variation of tree diameters within classified VSS groups.Our study provides insight how tree marking techniques using historical forest remnants may have important effects on resulting fine-scale forest structure patterns. We also demonstrate how the use of global point or group-level pattern assessment methods can affect the evaluation of fine-scale spatial pattern objectives. Our analysis of VSS group characteristics highlights implementation and assessment issues associated with group-level spatial pattern identification, classification and regulation. We conclude that group-level classification and regulation is not necessary for maintaining fine-scale spatial patterns in complex ponderosa pine ecosystems subject to frequent fire disturbances and changing future climate conditions and societal demands. Both ERG and GMR-based approaches applied in this study may have utility in maintaining fine-scale spatial heterogeneity and promoting resiliency in Southwest ponderosa pine forests.

Forest structure and fuels dynamics following ponderosa pine restoration treatments, White Mountains, Arizona, USA

Southwestern ponderosa pine forest ecosystems have become uncharacteristically dense as a result of intensive livestock grazing, logging, and fire exclusion, which have contributed to a buildup of fuels and increased vulnerability to high-severity, landscape-scale crown fires. In 2002, we implemented a replicated ecological restoration experiment to (1) quantify site-specific reference conditions, (2) analyze effects of elevation on forest structure, surface fuels, and canopy fuels, (3) test responses to three treatments: control, full treatment, and burn-only, and (4) evaluate effectiveness of treatments for restoring attributes to near historical reference conditions. Reconstructed reference conditions showed that basal area (BA) averaged 9.2m2ha−1 and tree density averaged 86.2treesha−1 across the site in 1880 prior to widespread fire exclusion, with the highest tree density values occurring at the two upper elevation sites. In 2002, prior to treatment, BA averaged 28.9m2ha−1 and density averaged 927.9treesha−1, representing three- and ten-fold increases, respectively, compared to 1880 values with a higher proportion of the increase coming from mesic species at higher elevations. By 2013 (5-years post-treatment), the full treatment showed BA was reduced by 52% compared to pre-treatment values from 31m2ha−1 to 15m2ha−1 and density was reduced by 85% from 998treesha−1 to 153treesha−1. In the burn-only treatment, BA increased by 6% from 27.5m2ha−1 to 29.1m2ha−1 although density was reduced by 25% from 837treesha−1 to 625treesha−1 between 2002 and 2013. Canopy fuels dynamics were similar to forest structure responses: low values prior to fire exclusion, a marked increase by the pre-treatment measurement, substantial reductions following thinning plus burning, and minor reductions following burn-only treatments. Post-treatment diameter distributions in the full treatment closely resembled reference conditions; minor reductions occurred in the lower diameter classes in the burn-only. Mortality of trees established before European settlement circa 1880 varied by treatment: 25% of presettlement trees alive in 2002 died by 2013 in the full treatment, 31% died in the burn-only treatment, and 15% died in the control. Of the treatments tested, the full treatment was the only one that rapidly shifted forest structure, diameter distributions, and canopy fuels values to levels near or within the historical reference conditions. Results from this study showed that managers will have to consider tradeoffs between alternative treatments in the context of climate-induced, landscape-scale forest fires.

Snag Dynamics and Cavity Excavation after Bark Beetle Outbreaks in Southwestern Ponderosa Pine Forests

In the United States, ponderosa pine (Pinus ponderosa) forests provide habitat for more cavity-nesting wildlife than any other forest type. In 2002-2005, stresses from drought, changes in the fire regime, and increases in forest density contributed to the largest bark beetle (Ips or Dendroctonus spp.) epidemic ever recorded in Arizona. We identified characteristics that predict snag longevity and use by cavity nesters of bark beetle-killed ponderosa pine. We mapped snags and monitored use by wildlife in sites with bark beetle outbreaks in northern Arizona. We measured snag characteristics 3-9 years after outbreaks and used multimodel inference to predict whether a snag was standing or fallen or contained excavated cavities or not. We used spatial statistical tests to determine snag patterns. Although >99% of beetle-killed snags were standing 3 years after outbreaks, <20% remained standing after 7 years. Snags were clumped when initially measured and in subsequent remeasurements. Snags that remained standing had lower surrounding basal area, larger dbh, and more limbs, were more decayed, and were on west-facing lower slopes. Snags with cavities were larger in diameter, had broken tops before falling, and were in open (mean live tree basal area = 17 m(2)/ha) forest. Our data are indicative of snag dynamics likely to occur in ponderosa pine forests of the southwestern United States, given the occurrence of recent droughts and bark beetle outbreaks.

Allometry of Phloem Thickness and Resin Flow and Their Relation to Tree Chemotype in a Southwestern Ponderosa Pine Forest

We measured traits of 470 Pinus ponderosa in northern Arizona and constructed equations scaling phloem thickness and resin flow to tree size. We tested the hypothesis that these traits were related to tree monoterpene chemotype. The monoterpene composition of resin was analyzed for a subset of trees (n 146). Correlation analyses revealed that all tree phenotypic traits were positively correlated. Equations of phloem thickness and resin flow were nonlinear: phloem thickness was a logarithmic function of tree diameter, and diameter explained moderate variation in phloem thickness (R 2 0.234). Similarly, resin flow was a reciprocal function of diameter (R 2 0.214). Resin flow rapidly increased with tree diameter but plateaued as tree diameter exceeded 40 cm. Phloem thickness was only weakly linearly related to resin flow (R 2 0.078). A multivariate regression incorporating diameter, height, and phloem thickness did not explain substantially more variance in resin flow than diameter alone. We found no evidence to suggest that tree phenotype was related to tree chemotype. We conclude that there are positive but weak relationships between tree size, phloem thickness, and resin flow in southwestern ponderosa pine and that trees with thick phloem are not necessarily better defended from herbivory by resin flow.

Stand-replacing wildfires increase nitrification for decades in southwestern ponderosa pine forests

Stand-replacing wildfires are a novel disturbance within ponderosa pine (Pinus ponderosa) forests of the southwestern United States, and they can convert forests to grasslands or shrublands for decades. While most research shows that soil inorganic N pools and fluxes return to pre-fire levels within a few years, we wondered if vegetation conversion (ponderosa pine to bunchgrass) following stand-replacing fires might be accompanied by a long-term shift in N cycling processes. Using a 34-year stand-replacing wildfire chronosequence with paired, adjacent unburned patches, we examined the long-term dynamics of net and gross nitrogen (N) transformations. We hypothesized that N availability in burned patches would become more similar to those in unburned patches over time after fire as these areas become re-vegetated. Burned patches had higher net and gross nitrification rates than unburned patches (P < 0.01 for both), and nitrification accounted for a greater proportion of N mineralization in burned patches for both net (P < 0.01) and gross (P < 0.04) N transformation measurements. However, trends with time-after-fire were not observed for any other variables. Our findings contrast with previous work, which suggested that high nitrification rates are a short-term response to disturbance. Furthermore, high nitrification rates at our site were not simply correlated with the presence of herbaceous vegetation. Instead, we suggest that stand-replacing wildfire triggers a shift in N cycling that is maintained for at least three decades by various factors, including a shift from a woody to an herbaceous ecosystem and the presence of fire-deposited charcoal.

Stand structure and breeding birds: Implications for restoring ponderosa pine forests

Fire-adapted forests in the western United States have dramatically departed from the natural or evolutionary environment over the past century because of fire suppression, logging, grazing, and other management practices. In particular, most southwestern ponderosa pine (Pinus ponderosa) forests are characterized by dense stands of small-diameter trees that are susceptible to stand-replacing crown fires and ensuing damage to watersheds, wildlife habitat, and communities at the wildland-urban interface. Restoration treatments using mechanical thinning and prescribed fire have become the dominant management paradigm in this forest type, with landscape-scale projects being implemented over the next 20 years. Although many studies have examined wildlife responses to restoration treatments, single-species information is difficult for managers to synthesize and incorporate into management decisions made at large scales. Our goal was to investigate responses of breeding birds in ponderosa pine forests to forest stand characteristics modified by restoration treatments. We sampled birds in 23 stands for 3 years in northern Arizona, and used multi-season and multi-species occupancy models to determine relationships between avian occupancy and stand attributes affected by treatment. Increasing occupancy rates and species richness were associated with decreasing canopy cover, increasing density of mature trees, and increasing density of snags. Occupancy rates and species richness were negatively associated with quadratic mean diameter, medium-sized trees, and downed woody material. Associations of the avian community and individual species with herbaceous cover and tree clumping were less consistent. Our results support the implementation of forest restoration treatments for restoring avian community occupancy in ponderosa pine forests, and emulating the evolutionary environment is likely the best approach. We recommend reducing canopy cover via thinning, and retaining trees >45.7 cm diameter at breast height and snags. A commonly-proposed diameter cap for retaining large trees after thinning (40.6 cm dbh) seems unlikely to benefit nesting passerines, as resulting tree densities would remain too large. We recommend occupancy modeling as an efficient method for assessing wildlife-habitat relationships at large scales. © 2013 The Wildlife Society.

Stand-replacing wildfires alter the community structure of wood-inhabiting fungi in southwestern ponderosa pine forests of the USA

Increases in stand-replacing wildfires in the western USA have widespread implications for ecosystem carbon (C) cycling, in part because the decomposition of trees killed by fire can be a long-term source of CO2 to the atmosphere. Knowledge of the composition and function of decay fungi communities may be important to understanding how wildfire alters C cycles. We assessed the effects of stand-replacing wildfires on the community structure of wood-inhabiting fungi along a 32-yr wildfire chronosequence. Fire was associated with low species richness for up to 4 yr and altered species composition relative to unburned forest for the length of the chronosequence. A laboratory incubation demonstrated that species varied in their capacity to decompose wood; Hypocrea lixii, an indicator of the most recent burn, caused the lowest decomposition rate. Our results show that stand-replacing wildfires have long-term effects on fungal communities, which may have consequences for wood decomposition and C cycling.

Extreme late-summer drought causes neutral annual carbon balance in southwestern ponderosa pine forests and grasslands

We assessed the impacts of extreme late-summer drought on carbon balance in a semi-arid forest region in Arizona. To understand drought impacts over extremes of forest cover, we measured net ecosystem production (NEP), gross primary production (GPP), and total ecosystem respiration (TER) with eddy covariance over five years (2006–10) at an undisturbed ponderosa pine (Pinus ponderosa) forest and at a former forest converted to grassland by intense burning. Drought shifted annual NEP from a weak source of carbon to the atmosphere to a neutral carbon balance at the burned site and from a carbon sink to neutral at the undisturbed site. Carbon fluxes were particularly sensitive to drought in August. Drought shifted August NEP at the undisturbed site from sink to source because the reduction of GPP (70%) exceeded the reduction of TER (35%). At the burned site drought shifted August NEP from weak source to neutral because the reduction of TER (40%) exceeded the reduction of GPP (20%). These results show that the lack of forest recovery after burning and the exposure of undisturbed forests to late-summer drought reduce carbon sink strength and illustrate the high vulnerability of forest carbon sink strength in the southwest US to predicted increases in intense burning and precipitation variability.

A comparison of targeted and systematic fire-scar sampling for estimating historical fire frequency in south-western ponderosa pine forests

Fire history researchers employ various forms of search-based sampling to target specimens that contain visible evidence of well preserved fire scars. Targeted sampling is considered to be the most efficient way to increase the completeness and length of the fire-scar record, but the accuracy of this method for estimating landscape-scale fire frequency parameters compared with probabilistic (i.e. systematic and random) sampling is poorly understood. In this study we compared metrics of temporal and spatial fire occurrence reconstructed independently from targeted and probabilistic fire-scar sampling to identify potential differences in parameter estimation in south-western ponderosa pine forests. Data were analysed for three case studies spanning a broad geographic range of ponderosa pine ecosystems across the US Southwest at multiple spatial scales: Centennial Forest in northern Arizona (100ha); Monument Canyon Research Natural Area (RNA) in central New Mexico (256ha); and Mica Mountain in southern Arizona (2780ha). We found that the percentage of available samples that recorded individual fire years (i.e. fire-scar synchrony) was correlated strongly between targeted and probabilistic datasets at all three study areas (r=0.85, 0.96 and 0.91 respectively). These strong positive correlations resulted predictably in similar estimates of commonly used statistical measures of fire frequency and cumulative area burned, including Mean Fire Return Interval (MFI) and Natural Fire Rotation (NFR). Consistent with theoretical expectations, targeted fire-scar sampling resulted in greater overall sampling efficiency and lower rates of sample attrition. Our findings demonstrate that targeted sampling in these systems can produce accurate estimates of landscape-scale fire frequency parameters relative to intensive probabilistic sampling.