Extent,distribution and origin of non-native forest tree species in Europe

ABSTRACT

The management of non-native tree species in European forests has a long history, but the information on the current number and geographic distribution of these species in European forests is incomplete and scattered across various datasets. This study aims to perform an inventory of the most frequent non-native tree species growing in European forests and analyse their current extent, geographic distribution and geographic origin. Our results show that at least 150 non-native tree species are currently growing in European forests and provenance trials. The genera represented by largest number of species are Eucalyptus, Pinus, Acacia and Abies. Species growing at the largest areas are Robinia pseudoacacia (2.44 million ha), Eucalyptus globulus (1.46 million ha), Picea sitchensis (1.16 million ha) and Pseudotsuga menziesii (0.83 million ha). In total, non-native tree species in Europe are found in an area of approximately 8.54 million ha, or 4.0% of the European forest area, and the five most abundant species account for up to 77% of this area. The largest number of these 150 species were introduced from North America (71), followed by Asia (45) and Australia (20). North American species occupy by far the largest area.     

Pollution-induced injury in North American forests: facts and suspicions

Toxic gases are the only airborne chemicals that have been demonstrated to cause detrimental effects on forest trees in North America. Ozone is the only regionally dispersed pollutant known to have injured foliage, decreased radial growth, decreased resistance to insects and diseases, and increased mortality of sensitive tree species over a wide geographic range. All of these effects have been observed in eastern white pine (Pinus strobus L.) throughout its natural range, in some conifers and hardwood trees in southern California, and in a few tree species in the central Appalachian Mountains. Very little research has been done to determine effects of airborne chemicals on forests as a whole. At present, there is no conclusive evidence that acid deposition or other regionally dispersed pollutants are affecting the productivity of any forest in North America.     

Fire resistance of tree species explains historical gallery forest community composition

Fire, climatic variability, and grazing by large herbivores have historically limited woody vegetation in the tallgrass prairie region of North America to gallery forests in protected areas along rivers and streams. Fire, in particular, has been a strong selective pressure against woody vegetation. Consequently, we expect that dominant tree species in these forests have developed mechanisms for tolerating periodic surface fires. Susceptibility of trees to fire damage depends in part on key properties of bark which influence heat transfer to the vascular cambium, including thickness, density, and moisture content. An historical (1983) survey of Konza Prairie Biological Station in northeast Kansas, USA indicated that gallery forests were co-dominated by Quercus macrocarpa and Quercus muehlenbergii, while Celtis occidentalis occurred as an important sub-dominant species. Populus deltoides, Gleditsia triacanthos, and Juniperus virginiana were relatively uncommon. To test the hypothesis that historically dominant gallery forest tree species are more resistant to fire damage than uncommon species, fire was applied to the bark of 10 individuals of each of these six species under conditions mimicking surface fires (400 °C for 120 s). Maximum temperature at the vascular cambium, bark thickness, bark moisture content, and bark density were measured. Trees were considered fire-resistant if the vascular cambium temperature remained below the thermal cell death threshold, 60 °C, throughout the treatment. Using logistic regression, bark thickness was found to be a significant predictor of lethal cambium temperatures (P = 0.002), while neither bark density nor moisture content were significantly related to lethal cambium temperature (P = 0.279 and P = 0.131, respectively). Across species, a minimum bark thickness of approximately 8.6 mm was necessary to maintain the vascular cambium temperature below 60 °C. Trees that produce thick bark quickly in juvenile size classes (P. deltoides, Q. macrocarpa, and Q. muehlenbergii) experienced lower temperatures at the vascular cambium than those which do not develop thick bark with increasing diameter (C. occidentalis, G. triacanthos, and J. virginiana). Ranking these tree species by either the DBH or age needed to develop the minimum protective bark thickness largely agreed with ranking based on historical relative importance. As fire frequency and intensity decrease in remnant tallgrass prairie of North America as a result of habitat fragmentation, fire suppression, and changing land management, fire-sensitive species may increase in relative importance in gallery forests because of increased juvenile survival.     

Forecasting vole population outbreaks in forest plantations: The rise and fall of a major mammalian pest

Voles of the genera Microtus and Myodes feed on tree seedlings planted on cutover forest land in temperate and boreal forests of North America and Eurasia. This damage may have serious economic implications as well as limit regeneration of appropriate tree species in certain forest ecosystems. Prediction of vole population outbreaks and feeding damage to forest plantations, across even a limited geographic range, has yet to be achieved in North America. Thus, a major objective was a detailed analysis of changes in population dynamics of long-tailed voles (Microtus longicaudus), and to test three hypotheses (H) that vole populations would: (H₁) rise and fall in accordance with the abundance of herbaceous plants (grasses and forbs) during early vegetative succession after forest harvesting, (H₂) be positively associated with grass-seeded sites; and (H₃) incidence of feeding damage to seedlings would be positively associated with vole abundance. Voles were live-trapped for 6 years (2004–2009) from the time of harvesting on intensive sites, as well as surveyed over a range of extensive sites. Population numbers were related to habitat characteristics and tree damage in young forest plantations near Golden, British Columbia, Canada.     

Drought and fire suppression lead to rapid forest composition change in a forest-prairie ecotone

Altered fire regimes and increased drought can lead to major vegetation changes, especially in ecotones. A decrease in fire can lead to woody species encroachment in prairies and increasing forest stand density. The threat of global climate change raises questions about potential increases in the length, severity, and incidence of droughts substantially altering species composition. Re-measured upland forests in south-central North America's midcontinent forest-prairie ecotone exhibited major changes in woody species composition and structure over fifty years and successional trajectories appeared to favor invasive Juniperus virginiana L. over the previous dominant Quercus species. The objective of this study was to determine whether climate and fire exclusion affected the recruitment history of dominant woody species in these upland forests located near the xeric western edge of the eastern deciduous forest biome of North America. We removed cores and cross-sections from 992 J. virginiana, Quercus marilandica Münchh. and Q. stellata Wangenh. trees from eleven forest stands located across central and northwest Oklahoma, and determined their ages using standard dendrochronological methods. Recruitment of all species increased following a severe mid-20th century drought, but a rapid increase in J. virginiana recruitment and decrease in Quercus recruitment appeared to be linked to a decrease in fire. Future fire regime changes and increased drought due to global climate change could lead to widespread shifts from Quercus- to Juniperus- dominated forests and cause substantial changes to ecosystem services.     

Bird community responses to a gradient of site preparation intensities in pine plantations in the Coastal Plain of North Carolina

Although intensively managed pine forests are common in the southeastern US, few studies describe how combinations of mechanical (MSP) and chemical site preparation (CSP) and herbaceous weed control (HWC) techniques affect bird communities that use early successional habitats within young pine forests. Therefore, we examined effects of six treatments of increasing management intensity via combinations of MSP (strip-shear and wide spacing or roller chop and narrow spacing) and CSP (application or no application) treatments with banded or broadcast HWC on bird communities in six loblolly pine (Pinus taeda) plantations in the Coastal Plain of North Carolina, USA, for 8 years following site preparation. Wide pine spacing and strip-shear MSP increased bird abundance and species richness over narrow spacing and chopped MSP for 6 years after planting. Chemical SP reduced bird abundance in year 2, increased bird abundance in year 6, had no effect on abundance after year 7, and did not affect species richness in any year. Total bird abundance and species richness were similar between banded and broadcast HWC. Site preparation and HWC had no effect on bird diversity and bird communities were most similar in treatments of similar intensity. Site preparation and HWC had few or no effects on birds based upon migratory status, habitat association, or conservation value. The addition of chemical site preparation or HWC had little effect on birds beyond pine spacing, and bird abundance was not proportional to management intensity. Although we observed treatment effects, all treatments provided habitat used by a variety of bird species, and pine plantations may play an increasingly important role in bird conservation as forests become fragmented and converted to other land uses and as natural processes that create early successional habitat, such as fire, are suppressed.     

Fuel loads,snag abundance,and snag recruitment in an unmanaged Jeffrey pine–mixed conifer forest in Northwestern Mexico

Management of downed woody fuels and snags (standing dead trees) is receiving increasing attention because of their ecosystem values and effects on potential fire behavior. Research has correlated the abundance of many wildlife species with snags and downed woody material but very little information exists of the abundance and arrangement of these forest structures, particularly in unmanaged forests. Conifer forests in northwestern Mexico have not experienced systematic fire suppression or harvesting making them unique in western North America. In 1998, average snag density in Jeffrey pine–mixed conifer forests in the Sierra San Pedro Martir (SSPM) National Park was 3.95 snags/ha but 35% of inventoried plots had no snags. In 2002, average snag density significantly increased to 5.10 snags/ha after a multiple-year drought. Average surface and ground fuel loads were 15.8 and 8.7 t/ha, respectively. High variability characterized all snag and fuel attributes measured in this forest. This high amount of variation is probably the result of the relatively intact frequent surface fire regime and because no harvesting has occurred in the sampled area. The patchy distribution of snags observed argues against the application of uniform targets for snag retention across similar forested landscapes. An improvement in management guidelines would be to manage for snag density and large fuels over moderate spatial scales (hundreds of hectares) instead of on a per hectare basis. Forest fragmentation and diverse ownerships in many western United States forests complicates this recommendation. Conservation of the forests in the SSPM is critical because it is the last landscape-scale, old-growth mixed conifer forest in western North America with a relatively intact frequent fire regime.     

Herbivory, pathogens, and epiphylls in Araucaria Forest and ecologically-managed tree monocultures

Over the last centuries, natural forests have been replaced by extensive and homogeneous tree monocultures that cause strong impacts on ecological interactions and ecosystem processes. We expect, however, that ecologically-sustainable management practices can help to mitigate these effects. This study investigates how the replacement of Araucaria Forest by ecologically-managed tree monocultures affects leaf damage patterns produced by external chewers, internal chewers, leaf miners, galling insects, and pathogens, as well as the incidence of epiphylls. The study was performed in the São Francisco de Paula National Forest (southern Brazil) on 12 1-ha plots of Araucaria Forest and ecologically managed plantations of Araucaria, Pinus and Eucalyptus. Overall, 9955 leaves of the understory community (up to 1 m tall) were sampled and analyzed. The mean and the frequency distribution pattern of leaf damage were very similar between Araucaria Forest and all three tree monocultures. Also, there was no difference between habitats in the percentage of leaves attacked by different feeding guilds of insect herbivores and by pathogens. Araucaria Forest had a higher percentage of leaves with epiphylls than Eucalyptus plantation, probably due to its shadier and moister microclimate. Ecologically managed tree monocultures sustain a wide variety of herbivorous insects, pathogens and epiphylls which helps the maintenance of key ecological interactions and the functioning of the ecosystem.     

Fuel and stand conditions in an isolated, unmanaged forest landscape in central Oregon

? Mature, unmanaged forests in western North America provide important reference conditions for managers, yet little is known about fuel composition and the factors that influence fuel accumulation in such stands. Our objectives were to characterize fuels in a passively managed landscape of dry forests in central Oregon and identify environmental factors influencing fuel accumulation.

? Ordination techniques and analysis of variance revealed no statistical differences in total fuel loads across a wide range of environmental conditions.

? Individual fuel size classes, however, did vary by stand location and composition. Interior stands had more 1- to 100-h fuels and snags than stands at or near the edge, stands dominated by ponderosa pine had fewer small-diameter fuels and snags, lodgepole pine stands had more 1-h fuels and snags, and white fir stands had more 10-h fuels, duff, and snag basal area.

? Tree species, density and age, and years since last fire were the most important environmental variables, explaining 23% of the variation in fuels.

? Our results present further evidence that fuel composition is highly variable at many spatial and temporal scales. They also provide useful baseline information for managers of fire-prone western forests.

     

Ecological consequences of Douglas fir (Pseudotsuga menziesii) cultivation in Europe

Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) was first introduced to Europe from North America more than 150 years ago, was then planted on a large scale and is now the economically most important exotic tree species in European forests. This literature review summarizes the current knowledge on the effects of Douglas fir on soil chemistry, plants, arthropods and fungi. Douglas fir shapes its abiotic environment similarly to native tree species such as Norway spruce, silver fir or European beech. In general, many organisms have been shown to be able to live together with Douglas fir and in some cases even benefit from its presence. Although the number of species of the ground vegetation and that of arthropod communities is similar to those of native conifer species, fungal diversity is reduced by Douglas fir. Special microclimatic conditions in the crown of Douglas fir can lead to reduced arthropod densities during winter with possible negative consequences for birds. The ecological impacts of Douglas fir are in general not as severe as those of other exotic tree species, e.g., Pinus spp. in South Africa and Ailanthus altissima, Prunus serotina and Robinia pseudoacacia in Europe. Nonetheless, Douglas fir can negatively impact single groups of organisms or species and is now regenerating itself naturally in Europe. Although Douglas fir has not been the subject of large-scale outbreaks of pests in Europe so far, the further introduction of exotic organisms associated with Douglas fir in its native range could be more problematic than the introduction of Douglas fir itself.     

Multi-scale nest-site selection by black-backed woodpeckers in outbreaks of mountain pine beetles

Areas of mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks in the Black Hills can provide habitat for black-backed woodpeckers (Picoides arcticus), a U.S. Forest Service, Region 2 Sensitive Species. These outbreaks are managed through removal of trees infested with mountain pine beetles to control mountain pine beetle populations and salvage timber resources. To minimize impacts to black-backed woodpeckers while meeting management objectives, there is a need to identify characteristics of these areas that support black-backed woodpeckers. We examined the habitat associations of this species nesting in areas of beetle outbreaks in the Black Hills, South Dakota in 2004 and 2005. We used an information theoretic approach and discrete choice models to evaluate nest-site selection of 42 woodpecker nests at 3 spatial scales—territory, nest area, and nest tree. At the territory scale (250 m around nest), availability and distribution of food best explained black-backed woodpecker selection of beetle outbreaks versus the surrounding forest. Selection at the territory scale was positively associated with densities of trees currently infested by mountain pine beetles and indices of wood borer (Cerambycidae and Buprestidae) abundance, and was greatest at distances of 50–100 m from the nearest patch of infestation. At the nest-area scale (12.5 m radius around the nest), densities of snags positively influenced nest-area selection. Finally, at the nest-tree scale, aspen (Populus tremuloides) and 3–5-year-old ponderosa pine (Pinus ponderosa) snags were important resources. The association between abundant wood-boring insects and black-backed woodpeckers creates a difficult challenge for forest managers. In the absence of fire, areas of beetle outbreak might serve as the only substantial source of habitat in the Black Hills. Regulating insect populations via salvage logging will reduce key food resources to black-backed woodpeckers during nesting. Therefore, given the relatively infrequent occurrence of large-scale fire in the Black Hills, management should recognize the importance of beetle-killed forests to the long-term viability of the black-backed woodpecker population in the Black Hills.     

Phylogeography and host range of Armillaria gallica in riparian forests of the northern Great Plains,USA

Root disease pathogens, including Armillaria, are a leading cause of growth loss and tree mortality in forest ecosystems of North America. Armillaria spp. have a wide host range and can cause significant reductions in tree growth that may lead to mortality. DNA sequence comparisons and phylogenetic studies have allowed a better understanding of Armillaria spp. taxonomic diversity. Genetic sequencing has facilitated the mapping of species distributions and host associations, providing insights into Armillaria ecology. These studies can help to inform forest management and are essential in the development of disease risk maps, leading to more effective management strategies for Armillaria root disease. Armillaria surveys were conducted on publicly owned lands in North Dakota, South Dakota, and Nebraska, U.S.A. Surveyed stands consisted of riparian forests ≥0.4 hectares in area. Armillaria was found at 78 of 101 sites. A total of 57 Armillaria isolates—associated with 12 host tree species—were used for DNA sequencing of the translation elongation factor‐1 alpha (tef1) gene. Armillaria gallica was the only species identified within the study sites. Results suggest that A. gallica is a common root pathogen of hardwood trees in riparian forests of the northern Great Plains with a wider host range and geographic distribution than previously recognized.     

The influence of prescribed crown fire on lodgepole pine dwarf mistletoe (Arceuthobium americanum) populations 33 years post‐fire

Dwarf mistletoes (Arceuthobium spp.) are a group of obligate, hemiparasitic plants that infect numerous species in the Pinaceae in North America. Wildland fire is considered to be the primary natural agent influencing the population and distribution of dwarf mistletoes across landscapes. Based on this understanding, prescribed fire has been suggested as a potential method for dwarf mistletoe sanitation and control; however, experimental work has primarily focused on prescribed surface fire. In this study, we report long‐term impacts of three experimental crown fires on dwarf mistletoe severity in infested lodgepole pine stands in Colorado 33 years post‐fire. The three fires achieved tree mortality rates ranging from 20% to 100%. Our results suggested a significant negative relationship between the amount of fire‐caused tree mortality and future dwarf mistletoe severity. These findings supported the presumed natural role of fire in altering dwarf mistletoe populations, which perhaps exhibits a linear relationship between fire‐caused host tree mortality and future dwarf mistletoe severity.     

Bird communities following high-severity fire: Response to single and repeat fires in a mixed-evergreen forest,Oregon, USA

Fire is a widespread natural disturbance agent in most conifer-dominated forests. In light of climate change and the effects of fire exclusion, single and repeated high-severity (stand-replacement) fires have become prominent land management issues. We studied bird communities using point counting in the Klamath-Siskiyou ecoregion of Oregon, USA at various points in time after one or two high-severity fires. Time points included 2 and 3 years after a single fire, 17 and 18 years after a single fire, 2 and 3 years after a repeat fire (15 year interval between fires), and >100 years since stand-replacement fire (mature/old-growth forest). Avian species richness did not differ significantly among habitats. Bird density was highest 17 and 18 years after fire, lowest 2 years after fire, and intermediate in repeat burns and unburned forest. Bird community composition varied significantly with habitat type (A = 0.24, P < 0.0001) with two distinct gradients in species composition relating to tree structure (live to dead) and shrub stature. Using indicator species analysis, repeat burns were characterized by shrub-nesting and ground-foraging bird species while unburned mature forests were characterized by conifer-nesting and foliage-gleaning species. Bird density was not related to snag basal area but was positively related to shrub height. Contrary to expectations, repeated high-severity fire did not reduce species richness, and bird densities were greater in repeat burns than in once-burned habitats. Broad-leaved hardwoods and shrubs appear to play a major role in structuring avian communities in the Klamath-Siskiyou region. In light of these results, extended periods of early seral broadleaf dominance and short-interval high-severity fires may be important to the conservation of avian biodiversity.     

Nitrogen cycling following mountain pine beetle disturbance in lodgepole pine forests of Greater Yellowstone

Widespread bark beetle outbreaks are currently affecting multiple conifer forest types throughout western North America, yet many ecosystem-level consequences of this disturbance are poorly understood. We quantified the effect of mountain pine beetle (Dendroctonus ponderosae) outbreak on nitrogen (N) cycling through litter, soil, and vegetation in lodgepole pine (Pinus contorta var. latifolia) forests of the Greater Yellowstone Ecosystem (WY, USA) across a 0-30 year chronosequence of time-since-beetle disturbance. Recent (1-4 years) bark beetle disturbance increased total litter depth and N concentration in needle litter relative to undisturbed stands, and soils in recently disturbed stands were cooler with greater rates of net N mineralization and nitrification than undisturbed sites. Thirty years after beetle outbreak, needle litter N concentration remained elevated; however total litter N concentration, total litter mass, and soil N pools and fluxes were not different from undisturbed stands. Canopy N pool size declined 58% in recent outbreaks, and remained 48% lower than undisturbed in 30-year old outbreaks. Foliar N concentrations in unattacked lodgepole pine trees and an understory sedge were positively correlated with net N mineralization in soils across the chronosequence. Bark beetle disturbance altered N cycling through the litter, soil, and vegetation of lodgepole pine forests, but changes in soil N cycling were less severe than those observed following stand replacing fire. Several lines of evidence suggest the potential for N leaching is low following bark beetle disturbance in lodgepole pine.     

Toward reference conditions: wildfire effects on flora in an old-growth ponderosa pine forest

Remote ponderosa pine (Pinus ponderosa) forests on the North Rim of Grand Canyon National Park, Arizona, USA provide valuable examples of reference conditions due to their relatively uninterrupted fire regimes, limited grazing history, and protection from logging. Wildfire is an important disturbance agent in upland forests of the Interior West, yet repeated measurements taken before and after lightning-ignited fires are rare. In 1999, a low-severity Wildland Fire Use fire burned 156 ha on Fire Point, a peninsula dominated by old-growth ponderosa pines, which had not burned for at least 76 years. We measured understory plant community and forest floor characteristics in 1998 (1 year before the fire) and 2001 (2 years after the fire) at this site and at nearby reference sites that did not burn in 1999 but have had continuing fire regimes throughout the past century. After the wildfire, the plant community at Fire Point shifted toward higher compositional similarity with the reference sites. Analysis of functional group composition indicated that this change was due primarily to an increase in annual and biennial forbs. Gayophytum diffusum, Polygonum douglasii, Chenopodium spp., Solidago spp., Elymus elymoides, Calochortus nuttallii, Hesperostipa comata, and Lotus spp. were indicative of forests influenced by recent fires. Species richness, plant cover, plant layer density and plant diversity were significantly lower at Fire Point than at the reference sites, possibly due to long-term fire exclusion, but the fire did not increase the rate of change in these variables after 2 years. Few exotic species were present at any site. Forest floor depths at Fire Point were reduced to depths similar to the reference sites, primarily due to consumption of the duff layer. There was a significant inverse relationship between the ratio of duff:litter and species richness. Compared to fire-excluded forests, old-growth ponderosa pine forests influenced by low-intensity surface fires generally have greater plant species richness (especially annual forbs) and lighter fuel loads. This study supports the continued application of the Wildland Fire Use strategy in old-growth montane forests to maintain and improve forest health by altering understory species composition and reducing fuel loads.     

The pine wood nematode Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle (=B. lignicolus Mamiya and Kiyohara): an assessment of the current position

Pine wood nematode, Bursaphelenchus xylophilus (Steiner andBuhrer) Nickle, is the casual organism of pine wilt disease,a major tree killer in the Far East. This paper describes thebiology of B. xylophilus, particularly its inter-relationshipswith vector insects in the genus Monochamus (Coleoptera: Cerambycidae),tree species and environmental conditions. The principal methodof transmission, and hence of international plant health significance,is introduction of nematodes to susceptible dying or dead treesduring female oviposition. This occurs in all countries wherethe nematode occurs but is the only significant method of transmissionand survival of B. xylophilus in North America. Extensive treemortality in Japan and China is associated with presence ofhighly susceptible tree species, suitable vector species andhigh summer temperatures. Pest risk assessments have been carriedout to determine the risks to Europe; it is concluded that thenematode would undoubtedly survive in Europe but that tree mortalityis likely only in the warmer southern countries. Methods toprevent transfer of nematodes to Europe are discussed in relationto European Union legislation.     

Decreases by disturbance-dependent tree species in the eastern and central USA

Open pine and oak forest ecosystems and floodplain forests have declined because of fire and flooding suppression. I determined tree species that have contracted in area occupied (where area occupied was ≥0.5 % of total species composition) or decreased in percent composition (i.e., percentage of all stems) during recent surveys (2000–2013) to ≤85 % of area occupied or composition during older surveys (1968–1999). I then identified whether species were associated with frequent fire, flooding, wetlands, postfire conditions, or other disturbances. Of 35 species that decreased in area occupied or composition, 16 species were associated with fire or postfire disturbance and fire-dependent species decreased in composition from 20 to 14 % during tree surveys. Even though species of floodplain forests and wetlands comprise a small percent of overall tree composition, 12 decreasing species were associated with floodplain forests or wetlands. Fire maintains open oak and pine ecosystems and flooding maintains floodplain forests, generally suspending succession. Suppression of fire in upland forests and flooding in floodplain forests has allowed disturbance-sensitive species to remain established and advance successional forests throughout the USA.     

匈牙利刺槐（（Robinia pseudoacacia L.）林的经营研究

匈牙利刺槐（（Robinia pseudoacacia L.)是在17世纪初从北美引入到欧洲的第个森林树种。在匈牙利，刺槐是非常重要的速生树种，为经济和生态目的，可以成功营造刺槐人工林。结合该树种的生长特点选择营林措施，可以最大限度的提高刺槐人工林的成活率和生产力。本文根据林分结构和森林产量长期实验林的研究成果，提出了刺槐林的抚育作业次序。制定好的营林计划和模式将会培育出效益刺槐林，并能使土地经营者更好地接受该树种。同时，刺槐也是一个非常有用的能源生产树种，本文展示了对这方面的研究成果。图2表1参8。     

Twentieth-century decline of large-diameter trees in Yosemite National Park,California, USA

Studies of forest change in western North America often focus on increased densities of small-diameter trees rather than on changes in the large tree component. Large trees generally have lower rates of mortality than small trees and are more resilient to climate change, but these assumptions have rarely been examined in long-term studies. We combined data from 655 historical (1932–1936) and 210 modern (1988–1999) vegetation plots to examine changes in density of large-diameter trees in Yosemite National Park (3027 km²). We tested the assumption of stability for large-diameter trees, as both individual species and communities of large-diameter trees. Between the 1930s and 1990s, large-diameter tree density in Yosemite declined 24%. Although the decrease was apparent in all forest types, declines were greatest in subalpine and upper montane forests (57.0% of park area), and least in lower montane forests (15.3% of park area). Large-diameter tree densities of 11 species declined while only 3 species increased. Four general patterns emerged: (1) Pinus albicaulis, Quercus chrysolepis, and Quercus kelloggii had increases in density of large-diameter trees occur throughout their ranges; (2) Pinus jeffreyi, Pinus lambertiana, and Pinus ponderosa, had disproportionately larger decreases in large-diameter tree densities in lower-elevation portions of their ranges; (3) Abies concolor and Pinus contorta, had approximately uniform decreases in large-diameter trees throughout their elevational ranges; and (4) Abies magnifica, Calocedrus decurrens, Juniperus occidentalis, Pinus monticola, Pseudotsuga menziesii, and Tsuga mertensiana displayed little or no change in large-diameter tree densities. In Pinus ponderosa–Calocedrus decurrens forests, modern large-diameter tree densities were equivalent whether or not plots had burned since 1936. However, in unburned plots, the large-diameter trees were predominantly A. concolor, C. decurrens, and Q. chrysolepis, whereas P. ponderosa dominated the large-diameter component of burned plots. Densities of large-diameter P. ponderosa were 8.1 trees ha⁻¹ in plots that had experienced fire, but only 0.5 trees ha⁻¹ in plots that remained unburned.