Ponderosa Pine Landscape Research Articles

Multidecadal vegetation transformations of a New Mexico ponderosa pine landscape after severe fires and aerial seeding.

Wildfires and climate change increasingly are transforming vegetation composition and structure, and postfire management may have long-lasting effects on ecosystem reorganization. Postfire aerial seeding treatments are commonly used to reduce runoff and soil erosion, but little is known about how seeding treatments affect native vegetation recovery over long periods of time, particularly in type-converted forests that have been dramatically transformed by the effects of repeated, high-severity fire. In this study, we analyze and report on a rare long-term (23-year) dataset that documents vegetation dynamics following a 1996 post-fire aerial seeding treatment and a subsequent 2011 high-severity reburn in a dry conifer landscape of northern New Mexico, USA. Repeated surveys between 1997 and 2019 of 49 permanent transects were analyzed for differences in vegetation cover, richness, and diversity between seeded and unseeded areas, and to characterize the development of seeded and unseeded vegetation communities through time and across gradients of burn severity, elevation, and soil-available water capacity. Seeded plots showed no significant difference in bare ground cover during the initial years postfire relative to unseeded plots. Postfire seeding led to a clear and sustained divergence in herbaceous community composition. Seeded plots had a much higher cover of non-native graminoids, primarily Bromus inermis, a likely contaminant in the seed mix. High-severity reburning of all plots in 2011 reduced native graminoid cover by half at seeded plots compared with both prefire levels and with plots that were unseeded following the initial 1996 fire. In addition, higher fire severity was associated with increased non-native graminoid cover and reduced native graminoid cover. This study documents fire-driven ecosystem transformation from conifer forest into a shrub-and-grass-dominated system, reinforced by aerial seeding of grasses and high-severity reburning. This unique long-term dataset illustrates that post-fire seeding carries significant risks of unwanted non-native species invasions that persist through subsequent fires-thus alternative postfire management actions merit consideration to better support native ecosystem resilience given emergent climate change and increasing disturbance. This study also highlights the importance of long-term monitoring of postfire vegetation dynamics, as short-term assessments miss key elements of complex ecosystem responses to fire and postfire management actions.

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.

Restoration benefits of re-entry with resource objective wildfire on a ponderosa pine landscape in northern Arizona, USA

Abstract Prescriptions for ecological restoration of ponderosa pine (Pinus ponderosa) forests of the American Southwest are generally aimed at approximating pre-fire-exclusion conditions by reducing tree densities and hazardous fuels, and reintroducing surface fire. Increasingly, land managers are utilizing natural fire ignitions to achieve restoration and hazardous fuels reduction objectives, but to minimize risk of escape and reduce potential for undesirable outcomes fires are commonly managed under weather and fuel moisture conditions that produce lower burn severities. Few studies have examined effectiveness of incremental approaches that attempt to restore these ecosystems by allowing repeated, low-severity wildfires to reburn sites. We analyzed a 12,035-ha landscape on the Kaibab National Forest in northern Arizona and compared forest structure, hazardous fuel loading, and potential fire behavior in areas that had burned once (single fire) to others that had burned twice (re-entry) over the preceding 13 years. Forest structure and hazardous fuels characteristics in re-entry areas tended to be closer to historical ranges of variability (HRV) than in single fire areas; however, we found few statistically significant differences and tree densities in both single and re-entry fires remained higher than HRV by up to 3.5-fold. Similarly, potential fire behavior variables were statistically similar between single and re-entry fire areas. Analysis of burn severity effects suggested that trends toward reduced tree density and canopy fuels in re-entry areas were consistently driven by “cleanup effects”, whereby re-entry fires produced low-severity effects in areas showing unburned/low severity after first-entry fires. Evidence of “additive effects”, whereby restoration and hazardous fuels reduction objectives were being met incrementally by repeated low-severity fire, was difficult to detect on the re-entry fire landscape. Thus, with similar management, several additional fire entries will be required to restore tree densities to within the historical range. Further, our findings suggest that managing resource objective wildfires to allow for more moderate-severity burning, even with a single entry, may be more effective for restoring these forests than repeated low-severity entries.

Sequentially contingent fires, droughts and pluvials structured a historical dry forest landscape and suggest future contingencies

AbstractQuestionsDry forests throughout the world were historically influenced by fires and climatic events, evidenced by tree recruitment pulses in forest age structures, but did these influences act randomly or were recruitment pulses contingent on the type, magnitude, order and timing of events? If recruitment was random or contingent, what are the implications for future forests?LocationUnlogged, old‐growth ponderosa pine landscape in Grand Canyon National Park, Arizona,US.MethodsWe spatially reconstructed and compared tree recruitment pulses evident in forest age structures within plots with tree ring reconstructions of pluvials, droughts, fires and fire quiescence (longer fire‐free periods). We used chi‐square analysis to test for sequential contingency of combinations and permutations of pulse influences.ResultsOf 20 recruitment pulses, 17 were influenced to some extent by fire quiescence, 13 by fires and droughts each, and 11 by pluvials. Prevalence of pulses across the landscape did not correspond to the pronounced spatial variability in fire rotation. Analysis of combinations and permutations of these influences showed that potentially mortality‐inducing influences of fire and drought likely initiated pulses, whereas pluvials and quiescence, which enhance recruitment conditions, sustained 75% of pulses.ConclusionsSuccessful tree recruitment pulses in dry forests historically were sequentially contingent on mortality‐inducing influences, followed by recruitment‐enhancing conditions. The impacts of climate change projections, including prolonged droughts and intense fires, on dry forests may depend on the order, timing and magnitude of influences.

Variability of historical forest structure and fire across ponderosa pine landscapes of the Coconino Plateau and south rim of Grand Canyon National Park, Arizona, USA

We undertook reconstructions of historical ponderosa pine forest structure and fire regimes across an entire landscape to expand understanding of spatial variability in forest structure and dynamics. The study area includes the ponderosa pine forests of the Coconino Plateau, Arizona, USA. Using General Land Office survey data and newly developed methods, we examined surveyor descriptions of overstory and understory composition and structure across 60,998 ha. For 41,214 ha of forests, we reconstructed density, basal area, diameter-class distributions, and fire severity and used GIS to analyse their relationships with topography. Ponderosa pine forests were continuous in only 34 % of the 60,998 ha landscape. In 40 %, they were mixed with pinon–juniper, 24 % was pure pinon–juniper and 2 % were grass or shrubs. In ponderosa pine forests, the focus of this study, mean tree density was 144.2 trees ha−1; 18.8 % of the landscape had low tree density ( 200 trees ha−1). Small trees ( 50 % of all trees on 47 % of the landscape and large trees (>40 cm) composed >50 % of all trees on 21 % of the landscape. Low-severity fire likely structured the forest on 59 % of the landscape while 39 % was structured by mixed-severity fire, likely including small, patchy crown fires. Forest parameters displayed wide variability across environmental gradients. Broader-scale reconstructions revealed a much more spatially complex forest that was structured by wide range of fire severities. Variability in ponderosa pine forests was often under-reported in past studies, but is important in perpetuating biological diversity.

Proximity to grasslands influences fire frequency and sensitivity to climate variability in ponderosa pine forests of the Colorado Front Range

This study examines the influence of grasslands on fire frequency and occurrence in the ponderosa pine (Pinus ponderosa)-dominated forests of the central and northern Colorado Front Range. Fire frequency based on tree-ring fire-scar data was compared between 34 fire history sites adjacent to grasslands and 34 fire history sites not adjacent to grasslands for the time period 1675–1920. Relationships were examined between fire occurrence and values of the Palmer Drought Severity Index and sea-surface temperatures from the NINO3 region of the tropical Pacific Ocean (positive values indicating El Niño-like conditions and negative values La Niña-like conditions). Ponderosa pine stands adjacent to grasslands experienced more frequent fire than stands not adjacent to grasslands (P < 0.05) owing to proximity to prevalent fine fuels able to support relatively frequent surface fires. Fire activity adjacent to grasslands showed a lagged positive relationship with moist years (positive Palmer Drought Severity Index and positive NINO3) antecedent to fire events whereas fire occurrence at sites not adjacent to grasslands showed no relationship to antecedent moist years. This study illustrates how the presence of grasslands in a ponderosa pine landscape results in increased fire frequency (a bottom–up influence) and also increases the sensitivity of fire activity to interannual climate variability (a top–down influence).

Bias and error in using survey records for ponderosa pine landscape restoration

AbstractAim Public land survey records are commonly used to reconstruct historical forest structure over large landscapes. Reconstruction studies have been criticized for using absolute measures of forest attributes, such as density and basal area, because of potential selection bias by surveyors and unknown measurement error. Current methods to identify bias are based upon statistical techniques whose assumptions may be violated for survey data. Our goals were to identify and directly estimate common sources of bias and error, and to test the accuracy of statistical methods to identify them.Location Forests in the western USA: Mogollon Plateau, Arizona; Blue Mountains, Oregon; Front Range, Colorado.Methods We quantified both selection bias and measurement error for survey data in three ponderosa pine landscapes by directly comparing measurements of bearing trees in survey notes with remeasurements of bearing trees at survey corners (384 corners and 812 trees evaluated).Results Selection bias was low in all areas and there was little variability among surveyors. Surveyors selected the closest tree to the corner 95% to 98% of the time, and hence bias may have limited impacts on reconstruction studies. Bourdo’s methods were able to successfully detect presence or absence of bias most of the time, but do not measure the rate of bias. Recording and omission errors were common but highly variable among surveyors. Measurements for bearing trees made by surveyors were generally accurate. Most bearings were less than 5° in error and most distances were within 5% of our remeasurements. Many, but not all, surveyors in the western USA probably estimated diameter of bearing trees at stump height (0.3 m). These estimates deviated from reconstructed diameters by a mean absolute error of 7.0 to 10.6 cm.Main conclusions Direct comparison of survey data at relocated corners is the only method that can determine if bias and error are meaningful. Data from relocated trees show that biased selection of trees is not likely to be an important source of error. Many surveyor errors would have no impact on reconstruction studies, but omission errors have the potential to have a large impact on results. We suggest how to reduce potential errors through data screening.

Future climate affects management strategies for maintaining forest restoration treatments

Forests adapted to frequent-fire regimes are being treated to reduce fuel hazards and restore ecosystem processes. The maintenance of treatment effects under future climates is a critical issue. We modelled forest change under different climate scenarios for 100 years on ponderosa pine landscapes in the south-western USA, comparing management regimes that included prescribed burning, tree cutting, and no-management. We applied the Forest Vegetation Simulator (1) in its standard form, and (2) with modifications of reduced tree growth and increased mortality to simulate the effects of two levels of climate change. Without climate change effects, several management regimes, including the use of frequent burning similar to the historical fire frequency (~5 year), maintained future forest structure within a target range of variability. In contrast, simulations that accounted for climate change effects indicated that burning intervals should be lengthened (~20 year) and future tree thinning should be avoided to minimise forest decline. Although it has been widely predicted that future climate conditions will support more burning (warmer, drier fuels, longer fire season), our modelling suggests that the production of fuels will decline, so there will eventually be a trade-off between increased fire, driven by climate, v. reduced fuel, also driven by climate.

Bark beetle-caused mortality in a drought-affected ponderosa pine landscape in Arizona, USA

Extensive ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) mortality associated with a widespread severe drought and increased bark beetle (Coleoptera: Curculionidae, Scolytinae) populations occurred in Arizona from 2001 to 2004. A complex of Ips beetles including: the Arizona fivespined ips, Ips lecontei Swaine, the pine engraver beetle, Ips pini (Say), Ips calligraphus (Germar), Ips latidens (LeConte), Ips knausi Swaine and Ips integer (Eichhoff) were the primary bark beetle species associated with ponderosa pine mortality. In this study we examine stand conditions and physiographic factors associated with bark beetle-caused tree mortality in ponderosa pine forests across five National Forests in Arizona. A total of 633 fixed-radius plots were established across five National Forests in Arizona: Apache-Sitgreaves, Coconino, Kaibab, Prescott, and Tonto. Prior to the bark beetle outbreak, plots with mortality had higher tree and stocking compared with plots without pine mortality. Logistic regression modeling found that probability of ponderosa pine mortality caused by bark beetles was positively correlated with tree density and inversely related with elevation and tree diameter. Given the large geographical extent of this study resulting logistic models to estimate the likelihood of bark beetle attack should have wide applicability across similar ponderosa pine forests across the Southwest. This is particularly true of a model driven by tree density and elevation constructed by combining all forests. Tree mortality resulted in significant reductions in basal area, tree density, stand density index, and mean tree diameter for ponderosa pine and for all species combined in these forests. Most of the observed pine mortality was in the 10–35 cm diameter class, which comprise much of the increase in tree density over the past century as a result of fire suppression and grazing practices. Ecological implications of tree mortality are discussed.

Fire history in ponderosa pine landscapes of Grand Canyon National Park: is it reliable enough for management and restoration?

Reconstructing fire regimes of the past can provide a valuable frame of reference for understanding the impact of human land uses on contemporary fire and forest structure, but methods for reconstructing past fire regimes are under re-evaluation. In the present article, a common method of characterizing surface fire regimes, using composite fire intervals from fire scars, is shown to significantly underestimate the length of the fire rotation and population mean fire interval in Grand Canyon landscapes where these parameters are known. Also, the evidence and interpretation that past high-severity fire was uncommon in ponderosa pine landscapes in Grand Canyon National Park are challenged. Together, these two concerns mean that an alternative characterization of the fire regime, which has very different implications, cannot be excluded. Management aimed at lowering fire risk, as a means of restoration, does not presently have a sound scientific basis, if it uses the composite fire interval as a measure of the fire regime or is based on fire history research that lacks adequate analysis of past high-severity fire.

Uncertainty in surface-fire history: the case of ponderosa pine forests in the western United States

Present understanding of fire ecology in forests subject to surface fires is based on fire-scar evidence. We present theory and empirical results that suggest that fire-history data have uncertainties and biases when used to estimate the population mean fire interval (FI) or other parameters of the fire regime. First, the population mean FI is difficult to estimate precisely because of unrecorded fires and can only be shown to lie in a broad range. Second, the interval between tree origin and first fire scar estimates a real fire-free interval that warrants inclusion in mean-FI calculations. Finally, inadequate sampling and targeting of multiple-scarred trees and high scar densities bias mean FIs toward shorter intervals. In ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) forests of the western United States, these uncertainties and biases suggest that reported mean FIs of 2-25 years significantly underestimate population mean FIs, which instead may be between 22 and 308 years. We suggest that uncertainty be explicitly stated in fire-history results by bracketing the range of possible population mean FIs. Research and improved methods may narrow the range, but there is no statistical or other method that can eliminate all uncertainty. Longer mean FIs in ponderosa pine forests suggest that (i) surface fire is still important, but less so in maintaining forest structure, and (ii) some dense patches of trees may have occurred in the pre-Euro-American landscape. Creation of low-density forest structure across all parts of ponderosa pine landscapes, particularly in valuable parks and reserves, is not supported by these results.

Nonequilibrium Dynamics between Catastrophic Disturbances and Old‐Growth Forests in Ponderosa Pine Landscapes of the Black Hills

An emerging goal of ecosystem management is to maintain ecosystems within their range of natural variability, which requires attention to pre‐EuroAmerican landscape‐scale processes and corresponding landscape structures (e.g., old‐growth forest distribution). The prevailing “equilibrium” view of ponderosa pine forest landscapes, for example, holds that frequent, low‐intensity surface fires maintained open, park‐like forests of large, old trees. Yet a contrasting “nonequilibrium” view suggests that some forest ecosystems are subject to unpredictable catastrophic disturbances that dramatically alter these ecosystems. To assess these views’ relevance, we examined early historical accounts and records of natural disturbances in the ponderosa pine forests of the Black Hills in South Dakota and Wyoming ( U.S.A.). There is evidence of frequent, low‐intensity surface fires and large, catastrophic disturbances before EuroAmerican influence. Several large, stand‐replacing fires occurred between 1730 and 1852, and, shortly after EuroAmerican settlement, a major outbreak of mountain pine beetles ( Dendroctonus ponderosae Hopk.) occurred. The location of these severe disturbances coincides geographically with early explorers’ reports of extensive tracts of relatively dense closed‐canopy forests, including some very large patches (5000+ ha) of dense old growth. This contrasts with sparse, open‐canopy forests thought to be maintained by periodic, low‐intensity surface fires. We suggest that the cooler, moister, central and northern Black Hills and topographically protected areas may have been dominated by infrequent, catastrophic disturbances that maintained large patches of dense forests, including large, contiguous patches of old growth, in a relative state of nonequilibrium. The warmer and drier southern Black Hills, south‐facing slopes, and exposed areas may have been dominated by frequent, low‐intensity surface fires and other small disturbances that maintained open‐canopy forests in a relative state of equilibrium. Proposed Black Hills National Forest management plans that exclusively endorse the equilibrium view are misdirected and will move the forest ecosystem farther outside its range of natural variability.Desequilibrio Dinámico entre Perturbaciones Catastróficas y Bosques Maduros en Paisajes de Pino Ponderosa de Black HillsUna meta del manejo de ecosistemas es mantenerlos dentro de su rango de variabilidad natural, lo cual requiere de considerar procesos pre‐euroamericanos a escala de paisaje y las correspondientes estructuras del paisaje (e.g., distribución de bosques maduros). Por ejemplo, la visión prevaleciente del “equilibrio” de paisajes de bosque de pino ponderosa sostiene que frecuentes fuegos superficiales de baja intensidad mantenían bosques “similares a parques” abiertos con árboles grandes y viejos. Pero, una contrastante visión de “desequilibrio” sugiere que algunos ecosistemas de bosque están sujetos a perturbaciones catastróficas impredecibles que los altera dramáticamente. Para evaluar la pertinencia de estas visiones examinamos informes históricos tempranos y registros de perturbaciones naturales en los bosques de pino ponderosa de Black Hills en Dakota del Sur y Wyoming (E.U.A.). Hay evidencia de frecuentes fuegos superficiales de baja intensidad y grandes perturbaciones catastróficas anteriores a la influencia euroamericana. Entre 1730 y 1852 ocurrieron varios incendios mayores y, poco después de la colonización euroamericana, el escarabajo de pino de montaña ( Dendroctonus ponderosae Hopk.) causó grandes daños. La localización de estas perturbaciones severas coincide geográficamente con los reportes de exploradores respecto a extensas zonas de bosques de dosel cerrado relativamente densos, incluyendo unas extensiones muy grandes (5000+ha) de bosque maduro denso. Esto contrasta con los bosques de dosel abierto dispersos que se pensaba eran mantenidos por los incendios de baja intensidad periódicos. Sugerimos que, por su mayor frescura y humedad, las Black Hills centrales y del norte y las áreas topográficamente protegidas pudieron haber sido dominadas por perturbaciones catastróficas infrecuentes que mantenían grandes extensiones de bosques densos, incluyendo grandes parches contiguos de bosque maduro, en un relativo estado de desequilibrio. Las Black Hills del sur y las laderas del sur, más cálidas y secas, pudieron haber sido dominadas por incendios de baja intensidad frecuentes y otras perturbaciones pequeñas que mantenían bosques de dosel abierto en un relativo estado de equilibrio. Los planes de manejo propuestos para el Bosque Nacional Black Hills que exclusivamente avalan la visión de equilibrio están mal orientados y llevarán al ecosistema del bosque más allá de su rango de variabilidad natural.