Recent research in isolated sites in southwestern North America suggests that spatial variation in land use history can result in differences in the dates of fire exclusion onset and the extent to which forests have changed in response to fire cessation. We investigated the relationships among fire, land use, and forest structure in three mountain ranges that straddle the Texas–Mexico border including The Davis Mountains Preserve of The Nature Conservancy and Big Bend National Park in west Texas, and The Maderas del Carmen Protected Area in northern Coahuila, Mexico. We quantified fire regime characteristics (frequency, size, severity, and seasonality) and forest stand structure using dendroecology. Frequent low severity fire was a dominant force shaping forest structure and species composition historically. Mean fire return intervals prior to fire exclusion (1700–1900) ranged from 1 to 29years for fires scarring >25% of the fire-scar samples. The per-sample fire return intervals ranged from 15 to 25years signifying historically frequent fires at any one location on the landscape. The contemporary period was characterized by a doubling of fire return intervals across all three sites, probably as a result of fire exclusion through livestock (sheep) grazing. The presence of older trees in the tree age data suggested that fires were predominantly low in severity and that tree regeneration occurred during fire free intervals. Our results show that the disruption of frequent, low severity fire regimes in our sites resulted in widespread tree regeneration. This study documents the importance of sheep grazing as a causal factor of fire exclusion in west Texas and northern Mexico, which occurred in the early to mid-1900s, well after the 1880s cattle boom.

Introduction Trees do not just die; there is always a primary cause, and often contributing factors. Trees need adequate quantities of water, heat, light, nutrients, carbon dioxide, oxygen, and other abiotic resources to sustain life, growth, and reproduction. When these factors are deficient or excessive, they cause mortality. According to the concept of baseline mortality (Chapters 1, 2, and 3), a certain number of trees must die as a forest ages to maintain a healthy condition. Abiotic factors kill trees in different ways, e.g., starvation, desiccation, uprooting, or stem breakage. The patterns of mortality and how the forest responds determine how changing stand structures impact sustainability and productivity. Here, we discuss abiotic factors, and how they influence diameter and age class distributions. We conclude this chapter by suggesting general principles about the impacts of abiotic disturbances on stand structures within forest ecosystems. Weather events Weather is the set of all phenomena occurring in a given atmosphere at a given time. Weather phenomena include wind, clouds, rain, snow, fog, dust storms, ice storms, hurricanes, tornadoes, and others. Some weather events can reset forest succession directly by killing trees. Weather events also can influence the rate and direction of forest succession indirectly by increasing fuels to enhance fire risk or by predisposing trees to other stresses.

Trees are a major threat to power line security across forested regions of the world. We developed a decision support system for identifying locations in Connecticut, USA where trees have grown tall enough to make contact with transmission lines during storms. We used the Random Forest algorithm, danger tree presence/absence data, and 25 raster environmental datasets to develop (1) an understanding of the abiotic environmental settings that host danger trees and (2) a spatially explicit map of danger tree distributions across Connecticut power line corridors. Danger trees were prevalent in locations (1) with an infrequent history of storms; (2) forested and residential land uses; and (3) low to middle elevations. Products from this research can be transferred to adaptive right-of-way management because they present managers with key information on where danger trees are likely to occur, and the methods presented herein have great potential for future application to other regions managers seek to identify high priority areas for danger tree removal.

Vegetation management is a critical component of rights-of-way (ROW) maintenance for preventing electrical outages and safety hazards resulting from tree contact with conductors during storms. Northeast Utility's (NU) transmission lines are a critical element of the nation's power grid; NU is therefore under scrutiny from federal agencies charged with protecting the electrical transmission infrastructure of the United States. We developed a decision support system to focus right-of-way maintenance and minimize the potential for a tree fall episode that disables transmission capacity across the state of Connecticut. We used field data on tree characteristics to develop a system for identifying hazard trees (HTs) in the field using limited equipment to manage Connecticut power line ROW. Results from this study indicated that the tree height-to-diameter ratio, total tree height, and live crown ratio were the key characteristics that differentiated potential risk trees (danger trees) from trees with a high probability of tree fall (HTs). Products from this research can be transferred to adaptive right-of-way management, and the methods we used have great potential for future application to other regions of the United States and elsewhere where tree failure can disrupt electrical power.

While piñon woodlands cover much of arid North America, surprisingly little is known about the role of fire in maintaining piñon forest structure and species composition. The lack of region-specific fire regime data for piñon–juniper woodlands presents a roadblock to managers striving to implement process-based management. This study characterized piñon–juniper fire regimes and forest stand dynamics in Big Bend National Park (BIBE) and the Davis Mountains Preserve of the Nature Conservancy (DMTNC) in west Texas. Mean fire return intervals were 36.5 and 11.2 years for BIBE and DMTNC, respectively. Point fire return intervals were 150 years at BIBE and 75 years at DMTNC. Tree regeneration in west Texas piñon–juniper woodlands occurred historically during favorable climatic conditions following fire years. The presence of multiple fire scars on our fire-scar samples and the multicohort stands of piñon suggested that low intensity fires were common. This study represents one of the few fire-scar-based fire regime studies for piñon–juniper woodlands. Our results differ from other studies in less topographically dissected landscapes that have identified stand-replacing fire as the dominant fire regime for piñon–juniper woodlands. This suggests that mixed-severity fire regimes are typical across southwestern piñon forests, and that topography is an important influence on fire frequency and intensity.

The largest roseate tern (Sterna dougalii) colony in the Northern Hemisphere, which is located on Great Gull Island, is no pristine haven. Indeed, Great Gull Island is an ecological anomaly—a weed-ridden 17 acres (6.8 ha) of scrubby grass, tall blinds for bird watching, and crumbling military infrastructure. Invasive and other weedy plants have threatened tern-nesting habitat throughout the past decades, despite admirable attempts at rehabilitating the island’s ecosystem. A new restoration project that integrates historical lessons with expanded resources, promises the best chance yet for successful restoration of the island and its unique roseate and common tern (S. hirunda) populations.

Age and diameter distributions are powerful tools for assessing changes in forest structure and composition over time. This study analyzed the age distribution of 2,345 trees from 487 plots in the eastern Washington Cascades to document increases in per-hectare tree densities and shifts in species composition since Euro-American settlement. Diameter distributions of these trees plus an additional 814 snags, stumps, and logs were analyzed to determine the extent and pattern of structural and compositional change resulting from post-settlement fire exclusion and preferential harvest of large ponderosa pine (Pinus pon-derosaDougl. Ex Laws.) and western larch (Larix occidentalisNutt.). Per-hectare densities of most species increased following settlement, with shade-tolerant/fire-intolerant species showing the biggest gain. A comparison of diameter distributions for live and dead trees indicates existing stands may not provide snags and logs of adequate dimensions for future habitat needs. Changes in forest structure and composition over the past century increase risk for insect outbreaks, diseases, and catastrophic wildfires.