Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still ...limited. We compiled a new pan‐continental tree‐ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1–100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long‐lasting declines were found for gymnosperms, shade‐ and drought‐tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark‐beetles) typically showed relatively small and short‐term growth reductions. Our analysis did not highlight any universal trade‐off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark‐beetle attack, while long‐term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth‐based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark‐beetle outbreaks.
Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological ...mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.
As a tropical nation with ~40% forested land area and 290 protected areas in the Indo-Burma Biodiversity Hotspot, Vietnam holds an important part of global forests. Despite a complex history of ...multiple colonial rules, war, rapid economic development and societal growth, Vietnam was one of a few Southeast Asian countries to reverse deforestation trends and sustain net forest cover gain since the 1990s. However, a considerable amount of Vietnam’s forest gain has been from plantation forestry, as Vietnam’s policies have promoted economic development. In the Central Highlands region of Vietnam, widespread forest degradation and deforestation has occurred recently in some areas due to plantation forestry and other factors, including fire-linked deforestation, but protected areas here have been largely effective in their conservation goals. We studied deforestation, wildfires, and the contribution of fire-linked deforestation from 2001 to 2020 in an area near the Da Lat Plateau of the Central Highlands of Vietnam. We stratified our study area to distinguish legally protected areas and those in the surrounding landscape matrix without formal protection. Using satellite-derived data, we investigated four questions: (1) Have regional deforestation trends continued in parts of the Central Highlands from 2001 to 2020? (2) Based on remotely sensed fire detections, how has fire affected the Central Highlands and what proportion of deforestation is spatiotemporally linked to fire? (3) Were annual deforestation and burned area lower in protected areas relative to the surrounding land matrix? (4) Was the proportion of fire-linked deforestation lower in protected areas than in the matrix? To answer these questions, we integrated the Global Forest Change and FIRED VIETNAM datasets. We found that 3794 fires burned 8.7% of the total study area and 13.6% of the area became deforested between 2001 and 2020. While nearly half of fires were linked to deforestation, fire-linked deforestation accounted for only a small part of forest loss. Across the entire study area, 54% of fire-linked deforestation occurred in natural forests and 46% was in plantation forests. Fire ignitions in the study area were strongly linked to the regional dry season, November to March, and instrumental climate data from 1971 to 2020 showed statistically significant increasing trends in minimum, mean, and maximum temperatures. However, the total area burned did not have a significant increasing trend. Regional trends in deforestation continued in Vietnam’s Central Highlands from 2001 to 2020, and nearly half of all detected fires can be spatially and temporally linked to forest loss. However, protected areas in the region effectively conserved forests relative to the surrounding landscape.
Fire is a concern for the sustainability of dry forests such as those of the Mediterranean region, especially under warming climate and high human use. We used data derived from Landsat and MODIS ...sensors to assess forest changes in the Talassemtane National Park (TNP) in North Africa from 2003–2018. The Talassemtane National Park is a protected area in northern Morocco, a biodiverse, mountainous region with endemic species of concern such as the Moroccan fir (Abies marocana) and Barbary macaque (Macaca sylvanus). To help the managers of the TNP better understand how the forest has been impacted by fire vs. other disturbances, we combined information from remotely derived datasets. The Hansen Global Forest Change (GFC) data are a global resource providing annual forest change, but without specifying the causes of change. We compared the GFC data to MODIS wildfire data from Andela’s Global Fire Atlas (GFA), a new global tool to identify fire locations and progression. We also analyzed surface reflectance-corrected Landsat imagery to calculate fire severity and vegetation death using Relative Differenced Normalized Burn Ratio analysis (RdNBR). In the park, GFC data showed a net loss of 1695 ha over 16 years, corresponding to an approximately 0.3% annual loss of forest. The GFA identified nine large fires that covered 4440 ha in the study period, coinciding with 833 ha of forest loss in the same period. Within these fires, detailed image analysis showed that GFA fire boundaries were approximately correct, providing the first quantitative test of GFA accuracy outside North America. High-severity fire, as determined by RdNBR analysis, made up about 32% of burned area. Overall, the GFA was validated as a useful management tool with only one non-detected wildfire in the study period; wildfires were linked to approximately 49% of the forest loss. This information helps managers develop conservation strategies based on reliable data about forest threats.
•The Hualapai forest differs from similar ponderosa pine forests in the US Southwest.•Indigenous perspectives inform a long history of forest management and fire use.•Fewer ladder and surface fuels ...may better protect against high-severity fire.•As climate changes, ongoing management helps sustain socio-cultural and ecological values.
Tribal nations in the US have worked to uphold their long history of managing forests in ways that reduce fuels, support ecosystem functioning, and enhance Indigenous livelihoods. Forests on the Hualapai Nation at the western end of the Grand Canyon have been actively managed for decades using fire and other treatments. We collected data on tree size and age structure, forest understory characteristics, and surface fuels to explore how the legacy of forest management, historical surface fire, and recent prescribed fire, have influenced the contemporary structure of the ponderosa pine-Gambel oak forest on the Hualapai tribal lands (hereafter Hualapai forest). Current overstory tree density (range: 361.1 to 1664.0 trees ha−1) and basal area (16.0 to 29.8 m2 ha−1) place the Hualapai forest in an intermediate state among more open vs. denser southwestern forests, but additional characteristics set the Hualapai forest apart from others in the region. In particular, diameter distributions of live ponderosa pine in the Hualapai forest are dominated by mid-diameter trees (trees ∼18-33 cm dbh), while diameter distributions of live ponderosa pine on forest lands across Arizona indicate a higher number of small-diameter trees (trees <23 cm dbh) and a lower number of large-diameter trees. This finding of a relatively lower number of small-diameter trees, which serve as ladder fuels, in the Hualapai forest may indicate better protection against severe wildfire, although some individual sites might be at greater risk due to higher numbers of small live trees and snags, seedlings and saplings, along with higher levels of coarse woody debris. Beyond fire risk, the Hualapai forest is lacking in large live trees and snags that support cultural needs and sustain its ecological functioning. Current management approaches are helping support the recovery of large trees and snags. While the Hualapai forest is at risk due to its location at the lower elevational limit of ponderosa pine in the Southwest, the legacy of past management and prescribed fire has differentiated it from similar forests in the region. Continued management informed by Indigenous perspectives can best position the Hualapai forest to sustain its structure and function as climate warms as well as achieve socio-cultural and ecological outcomes most important to the people of the Hualapai Tribe.
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Fire regimes often vary at fine spatial scales in response to factors such as topography or fuels while climate usually synchronizes fires across broader scales. We investigated the relative ...influence of top-down and bottom-up controls on fire occurrence in ponderosa pine (
Pinus ponderosa
) forests in a highly fragmented landscape at Mount Dellenbaugh, in northwestern Arizona. Our study area of 4,000 ha was characterized by patches of ponderosa pine forest in drainages that were separated by a matrix of pinyon–juniper woodlands, sagebrush shrublands, and perennial grasslands. We reconstructed fire histories from 135 fire-scarred trees in sixteen 25-ha sample sites placed in patches of mature ponderosa forest. We found that, among patches of ponderosa forest, fires were similar in terms of frequency but highly asynchronous in terms of individual years. Climate synchronized fire but only across broader spatial scales. Fires occurring at broader scales were associated with dry years that were preceded by several wet years. The remarkable level of asynchrony at finer scales suggests that bottom-up factors, such as site productivity and fuel continuity, were important in regulating fire at Mount Dellenbaugh. Understanding where bottom-up controls were historically influential is important for prioritizing areas that may best respond to fuel treatment under a warming climate.
Forests on tribal lands in the western United States have seen the return of low-intensity surface fires for several decades longer than forests on non-tribal lands. We examined the surface fire ...regime in a ponderosa pine-dominated (Pinus ponderosa) forest on the Hualapai tribal lands in the south-western United States. Using fire-scarred trees, we inferred temporal (frequency and seasonality) and spatial (synchrony) attributes and regulators of the fire regime over three land-use periods (historical, suppression, modern) between 1702 and 2007. Patterns of fire frequency and synchrony were similar, but fire seasonality was dissimilar, between the historical and modern periods. Logistic regression and generalised linear mixed models identified a suite of variables representing fuels, climate and human land uses that were associated with the probability of a site burning. Combined, these results allow for valuable insights regarding past fire spread and variability in fire frequency throughout our study area. In some respects, the current distinct fire regime in our study area, which predominately consists of prescribed fires implemented since the 1960s, resembles the past frequent surface fire regime that occurred here and in similar forest types on non-tribal lands in the south-western United States. Our results will be useful for informing adaptive management throughout the region as climate warms.
•We assess growth releases of trees growing along north–south axes of canopy gaps.•We examine the duration and magnitude of growth releases.•Duration and magnitude of releases decreased as distance ...from gap center increased.•The effect of distance on duration of releases was greater for trees north of gap center.•Results can help inform ecologically sustainable forest management approaches.
Canopy gaps created by the death of one to a few trees can exert a dominant influence on forest structure and composition by affecting the growth of nearby trees. Previous research in old-growth forests of coastal British Columbia, Canada indicated that most western redcedar (Thuja plicata Donn ex D. Don), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and Pacific silver fir (Abies amabilis Dougl. ex J. Forbes) growing around, or within, 20 natural canopy gaps of known timing of origin experienced a release (i.e., abrupt increase in radial growth) following gap formation. In that study, tree diameter, growth rate prior to a release, species identity, and interactions between some of these variables influenced the duration and magnitude of releases. In this study, we use trees growing along north–south transects that extend through the 20 gaps and into the adjacent forest to clarify the influence of two additional variables on growth releases: tree distance from the gap center and tree north–south position relative to the gap center. For all trees, predicted duration and magnitude of releases showed a decreasing trend with increasing distance from the gap center. Interestingly, the effect of distance on predicted duration of releases was greater for trees north versus south of the gap center, suggesting a response to additional light availability north of the gap center. These findings, combined with those from our previous study, indicate a strong influence of canopy gaps on growth releases of nearby trees of varying sizes and species in our study area, even trees that extend into the surrounding forest matrix. Our results can help inform ecologically sustainable forest management approaches, including variable retention approaches that aim to emulate the fine-scale, low-intensity disturbances that dominate the wetter forests of coastal British Columbia.
Climate is an important driver of forest dynamics. In this paper, we present three case studies from the forests of British Columbia to illustrate the direct and indirect effects of climatic ...variation and global warming on forest composition and function. (1) Tree mortality rates in old forests of western North America have doubled in recent decades. Regional warming and water deficits directly affected tree death rates or indirectly increased insect and pathogen activity and wind storms causing tree deaths. Concurrently, tree density and basal area declined significantly, indicating lagged growth responses of surviving trees or long-term decline of these forests. (2) Yellow-cedar decline along coastal British Columbia and Alaska shows that small changes in average climatic conditions, coupled with extreme weather events, can have large ecological effects. A small persistent increase in mean temperatures has reduced snow-cover depth and duration. Coupled with extreme cold events which damage unprotected tree roots, these climatic changes are considered the primary cause of widespread yellow-cedar mortality. (3) Interactions between climate and disturbance are complex in the mountain forests of the East Kootenay region. Understanding historic climate–fire interactions is key to anticipating future frequent and severe fires. Here, climate change effects may be exacerbated by the cumulative effects of human land use, fire exclusion and mountain pine beetle outbreaks. We conclude that understanding past climate variation and its effects on forests help us to anticipate the potential effects of global warming.
Yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach) forests of coastal British Columbia are apparently experiencing decline in a manner similar to that observed in southeastern Alaska. In this ...pilot study, we collect tree-ring data from live and standing dead yellow-cedar trees from four declining sites on the North Coast of British Columbia. We use this data to compare growth patterns at our sites to those of yellow-cedar trees at non-declining and declining sites in southwestern British Columbia and southeastern Alaska and, in addition, to assess the possibility of reconstructing yellow-cedar population dynamics in declining stands using dendrochronology. We found coherent growth patterns (i.e. marker years and periods of suppression) among yellow-cedar chronologies from non-declining and declining sites across a broad geographic range as well as unique growth patterns between our chronologies from declining sites and those from declining sites in nearby Alaska. Using outer-ring dates of increment cores, we were able to estimate time since death of decade- to century-old standing dead yellow-cedar trees, although the precision of the estimates was influenced by partial cambial mortality and erosion of outer rings. Our results provide baseline dendrochronological information that will be useful for planning future studies that assess growth-climate relations and reconstruct the long-term population dynamics of yellow-cedar in declining stands.
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