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Landsat time series (LTS) and associated change detection algorithms are useful for monitoring the effects of global change on Earth's ecosystems. Because LTS algorithms can be easily applied across broad areas, they are commonly used to map changes in forest structure due to wildfire, insect attack, and other important drivers of tree mortality. But factors such as initial forest density, tree mortality agent, and disturbance severity (i.e., percent tree mortality) influence patterns of surface reflectance and may influence the accuracy of LTS algorithms. And while LTS algorithms are widely used in areas with a history of multiple disturbance events during the Landsat record, the effectiveness of LTS algorithms in these conditions is not well understood. We compared products from the LTS algorithm LandTrendr (Landsat-based Detection of Trends in Disturbance and Recovery) with a unique field dataset from a landscape heavily influenced by both wildfire and spruce beetles (Dendroctonus rufipennis) since c. 2000. We also compared LandTrendr to other common methods of mapping fire- and spruce beetle-affected areas. We found that LandTrendr more accurately detected wildfire than spruce beetle-induced tree mortality, and both mortality agents were more easily detected when they occurred at high severity. Surprisingly, prior spruce beetle outbreaks did not influence the detectability of subsequent wildfire. Compared to alternative disturbance mapping approaches, LandTrendr predicted a c. 40% lower area affected by wildfire or spruce beetle outbreaks. Our findings indicate that disturbance type- and severity-specific differences in omission error may have broad implications for disturbance mapping efforts that utilize Landsat data. Gradual, low-severity disturbances (e.g., background tree mortality and non-stand replacing disturbance) are pervasive in forest ecosystems, yet they can be difficult to detect using automated LTS algorithms. Whenever possible, methods to account for these biases should be incorporated in LTS-based mapping efforts, including the use of multispectral ensembles and ancillary spatial data to refine predictions. However, our findings also indicate that LTS algorithms appear to be robust in areas with multiple disturbance events, which is important because these areas will increase as new acquisitions extend the length of the Landsat record.

Outbreaks of bark beetles cause widespread tree mortality, which have important consequences for wildlife, forest composition and structure, timber production, and water resources. Recent severe, extensive, and synchronous bark beetle outbreaks have motivated research into methods for surveying the location, extent, and attack stage of outbreak using remotely sensed data. In this paper, we adapt methods for classing mountain pine beetle-induced tree mortality to detect spruce beetle-induced tree mortality from single-date high-resolution imagery and multi-date moderate-resolution data. We use freely available aerial imagery from the National Agriculture Imagery Program (NAIP) to produce a tree-scale map of gray-stage spruce beetle kill across a broad spatial extent. This map was then used to train a stand-scale classification of spruce beetle outbreak from Landsat Climate Data Record (CDR) data. Gray-stage spruce beetle outbreak was characterized by higher values of the red–green index (RGI) at the tree-level. At the stand-level, gray stands were associated with high values of a Disturbance Index (DI) and low values of the Normalized Difference Vegetation Index (NDVI). Both tree- and stand-scale classifiers of spruce beetle-attributed tree mortality were highly accurate (overall accuracy and user's accuracy for gray trees/stands greater than 88%). Stand-level classification improved with post-classification correction, which removed high-frequency year-to-year variability in forest condition. Comparison of high-resolution and moderate-resolution classified imagery revealed variable amounts of tree mortality occur in areas of landscape-scale spruce beetle outbreak. Our results highlight the utility of both ecologically informed post-classification correction and coupling fine-scale and moderate-scale resolution imagery for mapping and studying spruce beetle outbreaks.

Global warming is expected to enhance radial tree growth at alpine treeline sites worldwide. We developed a well-replicated tree-ring chronology from Nothofagus pumilio near treeline in a high precipitation climate on Choshuenco Volcano (40S) in Chile to examine: (a) variation in tree radial growth in relation to interannual climatic variability; and (b) relationships of radial growth to variability in El Nino Southern Oscillation (ENSO) and the Antarctic Oscillation (AAO) at interannual and decadal time scales. A tree-ring chronology based on 99 tree-ring series from 80 N. pumilio trees near treeline showed a high series intercorrelation (0.48) indicating a strong common environmental signal. Radial growth is negatively correlated with precipitation in late spring (November–December). Temperature and tree growth are positively correlated during late spring and early summer (November–January). Interannual variability in both seasonal climate and in tree growth is strongly teleconnected to ENSO and AAO variability. Radial growth of N. pumilio in this humid high-elevation forest does not show a positive trend over the past half century as predicted from global treeline theory and broadscale warming in the Patagonian-Andean region. Instead, tree growth increased sharply from the 1960s to a peak in the early 1980s but subsequently declined for c. 30 years to its lowest level in >100 years. The shift to higher radial growth after c. 1976 coincides with a shift towards warmer sea surface temperatures in the tropical Pacific which in turn are associated with warmer growing season temperatures. The decline in tree growth since the mid-1990s is coincident with the increasingly positive phase of the AAO and high spring precipitation periods associated with El Nino conditions. The recent shift towards reduced growth of N. pumilio at this humid high-elevation site coincident with rising AAO mirrors the reduced tree growth beginning in the 1960s for trees growing in relatively xeric, lower elevation sites throughout the Patagonian-Andean region. The current study indicates that N. pumilio growth response in humid high-elevation environments to recent broad-scale warming has been non-linear, and that AAO and ENSO are key climatic forcings of tree growth variability.

While bark beetle disturbance is an inherent component of coniferous forest ecosystems throughout the northern hemisphere, associated tree mortality and ensuing changes in forest composition and structure may conflict with timber, wildlife, water and other resource management objectives. Therefore, host tree density reduction has been suggested as a management option to increase forest stand resistance to beetle infestation, protect remaining trees and maintain forest resources. However, little is known about the effectiveness of such treatments to mitigate spruce beetle (Dendroctonus rufipennis) infestation or their influence on the stand structural controls of beetle disturbance in subalpine spruce-fir forests in the Rocky Mountains. We addressed this research gap in a retrospective assessment of the impact of density reduction treatments on stand composition and structure and subsequent (ca. 5–20 years later) spruce beetle infestation in southwestern Colorado. The study area was located at the fringe of an ongoing spruce beetle outbreak and at the time of sampling was affected by endemic to incipient beetle pressure. Stand structural attributes and beetle infestation were measured in treated and untreated control stands at four sites. Classification tree analyses revealed spruce diameter and its interaction with spruce basal area percentage as the most important drivers of tree-level beetle infestation. The number, basal area and proportions of beetle-infested spruce were lower in treated stands at sites where treatments significantly reduced the abundance of large spruce trees and where the abundance of large spruce was relatively high prior to tree removal. However, spruce density reduction did not result in a reduction of infestation rates in the remaining large (>25 cm DBH) spruce during the ongoing beetle outbreak. While confirming previous assessments on the limited effectiveness of density reduction treatments for mitigating stand-level beetle infestation, this study provides further insights on the stand structural controls that mediate forest management effects on beetle disturbance dynamics. We conclude by suggesting that priority should be given to management practices that enhance resilience by increasing spruce advance regeneration in the understory as opposed to treatments aimed at achieving resistance to beetle disturbance.

Stand-level hazard ratings are widely used to determine the susceptibility of forests to insect outbreak. Many of these risk systems were developed from observations and empirical studies of one to a few insect outbreaks and consequently may not reflect variability expected under different climatic conditions. Given the strong and nonlinear linkages between bark beetle outbreaks and climate, there is increased need for understanding the applicability of existing hazard and risk classification systems under current and future climate conditions. Here we examine the susceptibility of 100+ year-old Engelmann spruce to spruce beetle (Dendroctonus rufipennis) infestation during the 2002–2012 period of stand-mesoscale outbreak (i.e. patchy distribution of affected stands rather than a continuous outbreak across a large outbreak). In addition, we examine the effects of pre-1900 disturbance history on stand structural traits related to spruce beetle outbreak. We document the effect of a 1840s spruce beetle outbreak on stand structure 170 years after the outbreak. In contrast, stand structure was not drastically different amongst stands with different last stand-replacing fire dates, which ranged from CE 1740 to 1879. Spruce beetle infestation in the early 2000s was influenced by tree-level characteristics (e.g. DBH), but not by topographic variables or stand structure; thus stand-level effects of pre-1800 disturbance did not discernibly affect the pattern of recent spruce beetle occurrence. Stand-level hazard ratings systems could be improved to consider warm and dry climate conditions, which may remove or relax stand constrains such as in the current outbreak.

During European colonization and settlement of southern hemisphere temperate ecosystems, historical fire regimes were often dramatically altered by either burning vast areas to create farmland, or reducing fire frequencies by suppressing fires or by eliminating aboriginal populations that formerly set fires. To determine the historical range of variability of wildfire and the potential human influences on wildfire activity in temperate rainforests of western Patagonia, we used tree rings to reconstruct fire history over the past ca. 400 years. Over a 6° latitudinal range, we examined spatiotemporal changes in fire history and compared it to ethnohistorical evidence of human activities. Time series of fire years were developed from fire-scars at 27 sites for comparison with a priori defined periods of land use in each of six areas of homogeneous land-use history. We also examined the influence of climate variability to discriminate the relative roles of human ignitions and decadal-scale climate variability on fire activity. Fires were relatively common in the forest-bog habitats sampled in our study, at least two centuries prior to any likely impact from Euro-Chilean settlers, implying that fires set by the indigenous peoples in this rainforest climate were much more common (and sometimes even widespread) than previously known. Our results also show that coincident with Euro-Chilean settlement, fire regimes shifted into long-lasting regimes of substantially higher fire frequency. However, decadal-scale climate variability, also clearly was a driver of shifts in fire regimes, and the second half of the 20th century has been a time of increased temperatures and drought throughout the region. Overall, our findings indicate that although in both pre-historic and modern times climate variability is the dominant control on years of widespread fires, aboriginals and Euro-Chilean settlers have amplified fire activity (particularly during the 20th–21st centuries) and shifted the region's fire regimes to new behaviors.

a b s t r a c t This study evaluated if present-day wildfire potential (i.e. potential fireline intensity and percentage crown fire) differs for residential parcels developed at different time periods in the north-central Col- orado Front Range. To answer this question, a model of wildfire potential was built based on 2001 fuels and vegetation and compared the output to actual fire severity of the 2002 Hayman and 2004 Picnic Rock fires (measured by satellite imagery). Except for low-load fuel types such as grass, the modeled wild- fire potential corresponded well to observed fire severity. Wildfire potential was then evaluated within 7 classes: developed (1880-1944, 1945-1959, 1960-1974, 1975-1989, 1990-2005) and undeveloped (either zoned or not zoned for development). The results suggest that there is one class characterized by relatively low wildfire potential (developed 1880-1944) and three classes characterized by relatively high wildfire potential (developed 1960-1974 and the two undeveloped parcel classes). These results hold both for 99th percentile (extreme) and 50th percentile (average) fuel conditions. The results sug- gest that under current zoning regulations, future structures are likely to be built on parcels that, on average, have somewhat higher potential fireline intensity and higher percentage of crown fire com- pared to currently developed parcels. However, the location of future development may be influenced by forest changes, such as the visual degradation and perceived fire hazard of trees killed by the continuing mountain pine beetle outbreak. Overall, this study introduces an improved method for quantifying wild- fire potential in the rapidly developing wildland-urban interface that could be applied to other areas.

A key challenge in modern wildfire mitigation and forest management is accurate mapping of forest fuels in order to determine spatial fire hazard, plan mitigation efforts, and manage active fires. This study quantified forest fuels of the montane zone of Boulder County, CO, USA in an effort to aid wildfire mitigation planning and provide a metric by which LANDFIRE national fuel maps may be compared. Using data from 196 randomly stratified field plots, pre-existing vegetation maps, and derived variables, predictive classification and regression tree models were created for four fuel parameters necessary for spatial fire simulation with FARSITE (surface fuel model, canopy bulk density, canopy base height, and stand height). These predictive models accounted for 56–62% of the variability in forest fuels and produced fuel maps that predicted 91.4% and 88.2% of the burned area of two historic fires simulated in the FARSITE model. Simulations of areas burned based on LANDFIRE national fuel maps were less accurate, burning 77.7% and 40.3% of the historic fire areas. Our results indicate that fuel mapping efforts that utilize local area information and biotic as well as abiotic predictors will more accurately simulate fire spread rates and reflect the inherent variability of forested environments than do current LANDFIRE data products.

In forest ecosystems where infrequent, severe fires have been an important process in shaping ecosystem structure, understanding the effects of introduced livestock on post-fire recovery of the vegetation is essential for effective forest resource management and preservation. In Nahuel Huapi National Park in northwestern Patagonia, we studied the effects of livestock on the post-fire recovery of a Nothofagus dombeyi-Austrocedrus chilensis forest that was burned in 1999. We experimentally excluded cattle by fencing plots and compared the vegetation characteristics of fenced and unfenced control plots over a 5-year period. Although cattle did not significantly reduce total plant cover or total species richness, they did reduce maximum heights of woody species including the dominant tree species. Chusquea culeou, a tall understory bamboo, can impede establishment and height growth of the dominant tree species. Although C. culeou accounts for the largest percentage of cattle diet, its mean cover and mean maximum height were not strongly affected by cattle. The reduction in the height growth of seedlings of N. dombeyi and Austrocedrus in the unfenced areas implies that presence of cattle in the recently burned areas may contribute to a post-fire transition from tall forest to bamboo-dominated shrubland that is already widespread in this landscape. Thus, these results provide support for the fencing of recently burned Nothofagus and Austrocedrus forests in the national parks for periods long enough to allow the dominant tree species to grow to heights at which they are no longer severely inhibited by cattle browsing.

This study investigates the roles of treefall-gaps in tree regeneration processes and the development of stand structure in a series of stands in which silver fir (Abies alba Mill.) co-occurs with other conifers and numerous angiosperm trees in central Poland. The study was conducted in the Jata Reserve, near the north-eastern limit of this silviculturally important tree species. Sixty-eight treefall-gaps (minimum canopy opening of 20 m 2 ) were objectively located along transects, sampled for gap characteristics and tree regeneration patterns, and compared to comparable sites from beneath closed canopies. Gap size did not influence the quantity of seedlings and saplings of different tree species. Numbers of seedlings and saplings in gaps were related to species composition of the surrounding tree canopy, especially the abundance of fir. The main potential successors in gaps were hornbeam (29% of the total), sycamore (20%) and fir (19%). Based on the current stand structure and gap-filling processes, silver fir is likely to remain present in the canopy but the relative proportion of the canopy composed of hornbeam will increase substantially. These results help to clarify the role of treefall disturbances in the dynamics of mixed stands near the north-eastern limit of silver fir, and, consequently can inform the evaluation of silvicultural practices.

The regeneration of Quaking aspen (Populus tremuloides) in the western U.S. in some habitats occurs after severe fire which removes competing conifers and also triggers root suckering of aspen. Consequently, fire exclusion during the 20th century, sometimes in combination with elk browsing, has often been argued to have resulted in a decline in the extent of aspen relative to its pre-20th century extent. However, aspen regeneration is not always dependent on fire and many stands have structures indicative of aspen persistence even in the absence of fire. Thus, to investigate regeneration dynamics of both persistent and seral aspen stands, we reconstructed patterns of stand development in 26 aspen stands in northwestern Colorado. Seral and persistent aspen stands were initially selected based on geographic information system-based analysis of historic vegetation and fire occurrence. Stand-level age structures were determined from 1919 increment cores and size structures from counts and diameters of all juvenile, subcanopy and canopy trees in 40 m × 40 m plots. Stand structures were interpreted to determine modes of tree regeneration and patterns of stand development. As expected, in the eight seral stands aspen regeneration generally depended on coarse-scale, severe disturbance by fire. However, most aspen stands in this study showed signs of aspen self-replacement despite presence of conifers. In the 16 persistent aspen stands showing no conifer invasion, aspen are able to regenerate through a variety of regeneration modes that did not appear to require severe disturbance. Over 70% of the persistent aspen stands sampled did not require coarse-scale disturbance to regenerate. Two (11%) of the persistent stands regenerated continuously through time, and 11 (61%) had aspen cohorts that appeared to develop episodically without being triggered by severe fire or other severe disturbance. In the persistent stands sampled in our study, episodic regeneration not dependent on severe fire is the main mode of stand re-initiation. At many sites in northwestern Colorado aspen regeneration occurs episodically and fits a general stand development model of cohort senescence. According to this model, many of the currently “decadent” or deteriorating aspen stands in western Colorado may be replaced by regenerating stands as the senescent stages experience high levels of canopy tree mortality, thus assuring aspen persistence in the future.

The primary objective of this study was the detection of possible climatic influences on the recent (i.e., past c. 40 years) establishment of ponderosa pine (Pinus ponderosa) at or near forest-grassland ecotones in the northern Front Range of Colorado. Germination dates were precisely determined for >500 juvenile ponderosa pine collected in six widely dispersed sample areas. All sites sampled were open areas lacking an overstory tree cover but located near seed sources. To evaluate the effects of recent climatic variation on recruitment and survival patterns, three types of climate data were used: (1) instrumental climate records from nearby local weather stations; (2) a multivariate index of El Nino/Southern Oscillation (ENSO); and (3) a regional, ponderosa pine tree-ring index sensitive to moisture variation. There is a strong association between episodic recruitment of ponderosa pine and years in which spring and fall moisture availability is high in the instrumental climate record. During the past 40 years, tree establishment was highly episodic and concentrated mainly in four years—1973, 1979, 1983, and 1990. These years are also associated with large-scale warming of sea-surface temperatures in the eastern tropical Pacific (i.e., El Nino events). These years of abundant seedling establishment also coincide with years of above average radial growth in mature ponderosa pine. Thus, at open sites suitable for the survival of shade-intolerant ponderosa pine, successful establishment of seedlings is highly episodic depending on local moisture availability related to broad-scale climatic variation. This study demonstrates the climatic sensitivity of ponderosa pine recruitment at low elevation sites along forest-grassland ecotones in the northern Colorado Front Range. # 2006 Elsevier B.V. All rights reserved.

Austrocedrus chilensis (D. Don) et Boutleje (Cupressaceae) is an endemic conifer of northern Patagonia, Argentina and south-central Chile, which has been affected by browsing by livestock and introduced deer (especially red deer). In northern Patagonia, we studied the effects of livestock and deer on the densities and heights of Austrocedrus saplings and on the structure and floristic composition of the vegetation in which this species occurs under a variety of different browsing regimes. Twenty-seven stands were sampled within the `Compact' forest type (i.e., dense and pure) of Austrocedrus in both national reserves and on unprotected properties. Principal components analysis (PCA) indicates that greater abundance of Austrocedrus saplings is associated with lower stand basal areas as expected for this relatively shade-intolerant species. Although browsing does not directly reduce the abundance of saplings, it has an important influence in terms of reducing height growth and producing deformed stems. Multiple correspondence analysis (MCA) indicates that saplings between 0.5 and 1 m in height were more frequently browsed than other height classes; saplings shorter than 0.3 m in height were not greatly influenced by browsing. Saplings of intermediate heights (0.5–1 m) were scarce at sites lacking any current browsing, but this may reflect release of saplings from earlier browsing and subsequent inhibition of continued recruitment into intermediate height classes. High browsing indexes occur beneath canopy gaps, which are also sites of higher sapling densities. In the present study, sites characterized by heavy browsing also have a high cover of spiny shrubs (Berberis spp.) and other unpalatable woody species (e.g., Lomatia hirsuta). Where the preferred browse species, Schinus patagonicus, is abundant, Austrocedrus is less influenced by browsing. However, as browsing reduces the abundance of S. patagonicus, Austrocedrus becomes a preferred species. Austrocedrus stands that are in a phase of fine-scale gap-phase regeneration are the most susceptible to inhibition of tree regeneration by livestock and deer browsing. In such stands, long-term persistence of a forest cover requires protection from large browsing animals.

In previous studies, evidence of tree invasion into grasslands has mainly been through comparison of historical terrestrial photographs and/or tree age data. The goal of this paper is to provide a quantitative description of the tree invasion process at a landscape scale using historical aerial photography, image processing and geographic information systems (GIS) approaches. Various map interpretive techniques provided evidence of shifts in the ponderosa pine-grassland ecotone along the Colorado Front Range since the late 1930s. Historical aerial photos were digitally scanned and the outlines of tree invasions into the grassland were determined based on gray tone density slicing. Image processing of digitized aerial photography identified areas of change in tree cover and quantified locations and total hectares of tree invasions into grassland areas. Overall, the results clearly show an increase in woodland areas where there formerly existed grasslands. GIS modeling was used to relate tree invasion patterns to topographic orientation and changes in settlement patterns. The importance of terrain aspect on rate of tree invasion is clearly shown by the greater rate of tree invasion on north-facing slopes (generally moister with less heat stress) versus south-facing slopes. The most dramatic change in the controls of vegetation patterns over the past one or two centuries has been the decline in fire frequency due to fire suppressing policy since ca. 1920. However, changes in grazing regimes may also have played an important role. When comparing these results to the instrumental climate record of the area, periods of favorable climatic conditions for seedling establishment generally correspond to periods of increased rate of tree invasion into grassland areas.

The process of old growth conservation would be greatly facilitated if remote sensing could be used to locate America's few remaining old growth forests. The use of multispectral remote sensing in the identification of old growth has been largely confined to efforts in the Pacific Northwest. The purpose of this study was to determine which image processing techniques best discriminate between old growth and younger spruce-fir stands in the southern Rocky Mountains. Various image processing techniques were applied to TM imagery of the Marvine Lakes valley in the Colorado White River National Forest. The study focused on two major structural differences between the forest types: 1) multistoried (old growth) vs. single-storied (young growth) canopies and 2) higher frequency of standing dead in old growth. Field measurements of photosynthetically active radiation confirmed the hypothesis that old growth canopies contain more gaps than the young post-fire stands. Texture analysis, color-ratio techniques, and vegetation indices successfully identified old growth. Evaluation of the success of old growth identification within a geographic information system suggested that digital per-pixel classification was inferior to visual interpretation. Spatial and spectral methods must be used in combination if structurally variable old growth forests are to be detected.

More species of conifers, especially pines, are present in highland Guatemala than in any other area of equally low latitude. Thus, the arborescent flora of highland Guatemala constitutss a unique genetic resource for use in afforestation projets of other tropical montane areas. This valuable genetic resource is seriously threatened by an alarming rate of deforestation. Although the past 30 years have been the period of most rapid population growth in Guatemalan history, the demise of the highland forests cannot be regarded simply as a consequence of the post-World War II demographic explosion. Significant pressure on resources can be traced back to the pre-Hispanic period; as early as the 18 th century, erosion resulting from forest destruction and over-grazing by sheep was a critical problem in highland Guatemala. The safest strategy for preserving the unique genetic resources of the absorescent flora of highland Guatemala probably lies in a massive programme of seed and pollen collection for storage in seed-banks and in the establishment of ex situ conservation stands.

The large conifer, Fitzroya cupressoides (Mol.) Johnston, which is native to southern Chile and Argentina, has been intensively exploited for its valuable wood since European colonization in the 16th century. Today F. cupressoides persists only in relatively small, remote stands and reports of inadequate regeneration have given rise to considerable concern for its survival. A reconnaissance study of virgin stands in the Cordillera Pelada, Chile, indicates that regeneration by both seedling establishment and root suckering is generally adequate to assure local persistence. Regeneration following very light burning is also adequate; however, following intense fires where soil peat burns and bare areas on the sterile, podzolic soils are created, forest re-establishment is exceedingly slow. The presence of regenerating virgin stands of F. cupressoides in the Cordillera Pelada is a strong reason for this area to be set aside for in situ preservation.