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Amplified by warming temperatures and drought, recent outbreaks of native bark beetles (Curculionidae: Scolytinae) have caused extensive tree mortality throughout Europe and North America. Despite their ubiquitous nature and important effects on ecosystems, forest recovery following such disturbances is poorly understood, particularly across regions with varying abiotic conditions and outbreak effects.To better understand post‐outbreak recovery across a topographically complex region, we synthesized data from 16 field studies spanning subalpine forests in the Southern Rocky Mountains, USA. From 1997 to 2019, these forests were heavily affected by outbreaks of three native bark beetle species (Dendroctonus ponderosae, Dendroctonus rufipennis and Dryocoetes confusus). We compared pre‐ and post‐outbreak forest conditions and developed region‐wide predictive maps of post‐outbreak (1) live basal areas, (2) juvenile densities and (3) height growth rates for the most abundant tree species – aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta) and subalpine fir (Abies lasiocarpa).Beetle‐caused tree mortality reduced the average diameter of live trees by 28.4% (5.6 cm), and species dominance was altered on 27.8% of field plots with shifts away from pine and spruce. However, most plots (82.1%) were likely to recover towards pre‐outbreak tree densities without additional regeneration. Region‐wide maps indicated that fir and aspen, non‐host species for bark beetle species with the most severe effects (i.e. Dendroctonus spp.), will benefit from outbreaks through increased compositional dominance. After accounting for individual size, height growth for all conifer species was more rapid in sites with low winter precipitation, high winter temperatures and severe outbreaks.Synthesis. In subalpine forests of the US Rocky Mountains, recent bark beetle outbreaks have reduced tree size and altered species composition. While eventual recovery of the pre‐outbreak forest structure is likely in most places, changes in species composition may persist for decades. Still, forest communities following bark beetle outbreaks are widely variable due to differences in pre‐outbreak conditions, outbreak severity and abiotic gradients. This regional variability has critical implications for ecosystem services and susceptibility to future disturbances. [ABSTRACT FROM AUTHOR]

Lack of tree fecundity data across climatic gradients precludes the analysis of how seed supply contributes to global variation in forest regeneration and biotic interactions responsible for biodiversity. A global synthesis of raw seedproduction data shows a 250‐fold increase in seed abundance from cold‐dry to warm‐wet climates, driven primarily by a 100‐fold increase in seed production for a given tree size. The modest (threefold) increase in forest productivity across the same climate gradient cannot explain the magnitudes of these trends. The increase in seeds per tree can arise from adaptive evolution driven by intense species interactions or from the direct effects of a warm, moist climate on tree fecundity. Either way, the massive differences in seed supply ramify through food webs potentially explaining a disproportionate role for species interactions in the wet tropics. [ABSTRACT FROM AUTHOR]

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution. [ABSTRACT FROM AUTHOR]

Warming temperatures and rising moisture deficits are expected to increase the rates of background tree mortality–low amounts of tree mortality (~0.5%–2% year−1), characterizing the forest demographic processes in the absence of abrupt, coarse‐scale disturbance events (e.g. fire). When compounded over multiple decades and large areas, even minor increases in background tree mortality (e.g. <0.5% year−1) can cause changes to forest communities and carbon storage potential that are comparable to or greater than those caused by disturbances.We examine how temporal variability in rates of background tree mortality for four subalpine conifers reflects variability in climate and climate teleconnections using observations of tree mortality from 1982 to 2019 at Niwot Ridge, Colorado, USA. Individually marked trees (initial population 5,043) in 13 permanent plots—located across a range of site conditions, stand ages and species compositions—were censused for new mortality nine times over 37 years.Background tree mortality was primarily attributed to stress from unfavourable climate and competition (71.2%) and bark beetle activity (23.3%), whereas few trees died from wind (5.3%) and wildlife impacts (0.2%). Mean annualized tree mortality attributed to tree stress and bark beetles more than tripled across all stands between initial censuses (0.26% year−1, 1982–1993/1994) and recent censuses (0.82% year−1, 2008–2019). Higher rates of tree mortality were related to warmer maximum summer temperatures, greater summer moisture deficits, and negative anomalies in ENSO (La Niña), with greater effects of drought in some subpopulations (tree size, age and species). For example, in older stands (>250 years), larger and older trees were more likely to die than smaller and younger trees. Differences in tree mortality rates and sensitivity to climate among subpopulations that varied by stand type may lead to unexpected shifts in stand composition and structure.Synthesis. A strong relationship between higher rates of tree mortality and warmer, drier summer climate conditions implies that climate warming will continue to increase background mortality rates in subalpine forests. Combined with increases in disturbances and declining frequency of moist‐cool years suitable for seedling establishment, increasing rates of tree mortality have the potential to drive declines in subalpine tree populations. [ABSTRACT FROM AUTHOR]

Understanding how severe disturbances and their interactions affect forests is key to projecting ecological change under a warming climate. Substantial increases in some biotic disturbances, such as bark beetle outbreaks, in temperate forest ecosystemsmay compromise recovery to a forest vegetation type (i.e., physiognomic recovery or resilience), especially if subsequent biotic disturbances (e.g., herbivory) alter recovery mechanisms. From 2005 to 2017, severe outbreaks (>90% mortality) of spruce bark beetles (SB, Dendroctonus rufipennis) affected Engelmann spruce (Picea engelmannii) across 325,000 ha of spruce and subalpine fir (Abies lasiocarpa) forest in the southern Rocky Mountains, USA. Concurrently, an outbreak of western balsam bark beetle (WBBB, Dryocoetes confuses) infested subalpine fir across at least 47,000 of these hectares. We explored the capacity of 105 stands affected by one or two bark beetle outbreaks and browsing of juvenile trees by ungulates to return to a forest vegetation type in the context of pre‐outbreak forest conditions and topography. Nine initial forest trajectories (i.e., at least several decades) were identified from four pre‐outbreak forest types affected by three biotic disturbances that occurred at different spatial scales and severities. Most stands (86%) contained surviving nonhost adult trees in the main canopy (fir and aspen [Populus tremuloides]) and many surviving juveniles of all species, implying that they are currently on a trajectory for physiognomic recovery. Stands composed exclusively of large‐diameter spruce were affected by a severe SB outbreak and were most vulnerable to a transition to a low‐density forest, below regional stocking levels (<370 trees/ha). Greater pre‐outbreak stand structural complexity and species diversity were key traits of stands with a higher potential for physiognomic recovery. However, all multispecies stands shifted in relative composition of the main canopy to nonhost species, suggesting low potential for compositional recovery over the next several decades. Most post‐outbreak stands (86%) exceeded regional stocking levels with trees taller than the browse zone (<2 m). As such, ungulate browsing on over half of all juveniles will primarily affect the rate of infilling of the forest canopy and preferential browsing of more palatable species will influence the composition of the future forest canopy. [ABSTRACT FROM AUTHOR]

Climate-induced increases in tree mortality are reported for many forests worldwide. Understanding the potential effects on carbon pools requires long-term monitoring of changes in forest biomass. We measured aboveground biomass (AGB) of living trees over a 34-year period (1982–2016) in permanent plots with varying stand ages, species compositions, and topographic settings in a subalpine forest in the Colorado Front Range. Stand-level and species-level AGB varied spatially and temporally in relation to stand age, successional processes, and site moisture classification. Young (ca. 122 years) postfire stands composed of lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson) had lower mean AGB than older (>250 years) mixed-species stands. Mesic stands had higher AGB than xeric or hydric stands of similar age. At the level of individual species, significant shifts in AGB among species were primarily explained by successional replacement of shade-intolerant pines by shade-tolerant Engelmann spruce (Picea engelmannii Parry ex Engelm.) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.). The permanent plot network recorded significant shifts in species dominance and tree densities between 1982 and 2016, reflecting successional patterns developing over several centuries and the effects of recent localized windthrow, insects, and pathogens. Despite increases in tree mortality, there was a general pattern of increasing AGB across the forest. [ABSTRACT FROM AUTHOR]

Studies of forest dynamics commonly assume that species composition of the seedling bank reflects the composition of the future forest canopy. However, many forest types exhibit persistent differences in relative dominance of species in the seedling bank versus the forest canopy. Species-specific differences in tree vital rates (e.g., in-growth, mortality, height growth, canopy residence time) across canopy positions may explain this discord in dominance between seedling banks and forest canopies. We tested for differences in tree vital rates for two widely distributed, coexisting species in subalpine forests of the Rocky Mountains, North America. We quantified seedling bank dynamics (>950 aged seedlings) and vital rates in permanent plots (>2500 trees) from 1982 to 2017 to determine if differences in vital rates explained the shift from seedling bank dominance by subalpine fir (Abies lasiocarpa (Hook.) Nutt.) to codominance of the main canopy by subalpine fir and Engelmann spruce (Picea engelmannii Parry ex Engelm.). Higher rates of fir recruitment into the main canopy were balanced by equally high rates of mortality, whereas spruce exhibited higher rates of net population increase and longer residence time in the main canopy. Projections of future forest trajectories from seedling bank composition can be improved by considering species-specific differences in vital rates. [ABSTRACT FROM AUTHOR]

Abstract: In the absence of broad‐scale disturbance, many temperate coniferous forests experience successful seedling establishment only when abundant seed production coincides with favorable climate. Identifying the frequency of past establishment events and the climate conditions favorable for seedling establishment is essential to understanding how climate warming could affect the frequency of future tree establishment events and therefore future forest composition or even persistence of a forest cover. In the southern Rocky Mountains, USA, research on the sensitivity of establishment of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa)—two widely distributed, co‐occurring conifers in North America—to climate variability has focused on the alpine treeline ecotone, leaving uncertainty about the sensitivity of these species across much of their elevation distribution. We compared annual germination dates for >450 Engelmann spruce and >500 subalpine fir seedlings collected across a complex topographic‐moisture gradient to climate variability in the Colorado Front Range. We found that Engelmann spruce and subalpine fir established episodically with strong synchrony in establishment events across the study area. Broad‐scale establishment events occurred in years of high soil moisture availability, which were characterized by above‐average snowpack and/or cool and wet summer climatic conditions. In the recent half of the study period (1975–2010), a decrease in the number of fir and spruce establishment events across their distribution coincided with declining snowpack and a multi‐decadal trend of rising summer temperature and increasing moisture deficits. Counter to expected and observed increases in tree establishment with climate warming in maritime subalpine forests, our results show that recruitment declines will likely occur across the core of moisture‐limited subalpine tree ranges as warming drives increased moisture deficits. [ABSTRACT FROM AUTHOR]

Remote sensing is widely used to detect forest disturbances (e.g., wildfires, harvest, or outbreaks of pathogens or insects) over spatiotemporal scales that are infeasible to capture with field surveys. To understand forest ecosystem dynamics and the ecological role of human and natural disturbances, researchers and managers would like to characterize spatiotemporal patterns of several types of disturbance. Recent advances include the development and testing of algorithms to automatically detect and attribute an array of disturbance types across large forest landscapes from remotely sensed imagery. We reviewed the scientific literature for automated disturbance type attribution in forest ecosystems to synthesize current knowledge and inform future work. We created metrics that characterize study contexts, methods, and outcomes to evaluate 34 studies that automated the attribution of several (two or more) forest disturbance types. Reported accuracies of ∼80% for up to eight disturbance types at local (500 km2) to continental (6.5 × 106 km2) spatial extents reaffirm the potential for attributing forest disturbances from remote sensing data at scales relevant to ecological research and forest management. Generally, greater accuracies were reported for attributing disturbance types that affect most of a pixel and exhibit spectral signatures distinct from other disturbances or the prior (undisturbed) condition. Accuracy of attributing disturbance type tends to be notably lower for disturbances with smaller spatial extents or resulting in subtle spectral changes, such as small patches of tree mortality covering <40% of a pixel or areas of tree damage without evident mortality (e.g., defoliator outbreaks). Nearly all (33 of 34) studies used Landsat image time series to attribute disturbance types because the Landsat archive is freely accessible, spans several decades (allowing longer-term change analyses), is calibrated to facilitate change analyses, and has multiple spectral bands that aid algorithms. In our synthesis, most studies focused on specific disturbance types of interest within a particular study area, rather than a comprehensive list of types that would make the algorithm more generally applicable at broader spatial extents. We recommend several future research areas to improve the thematic resolution, accuracy, and potentially the transferability of attribution algorithms. Further development and testing of algorithms will continue to reveal the most effective approaches for attributing disturbance types that have been more difficult to detect with available spectral data to date, such as insect damage or low severity wildfire, and to evaluate model sensitivity as well as accuracy. The lessons that collectively emerge from automated forest disturbance attribution studies have the potential to guide future research and management applications. [Display omitted] • We reviewed 34 studies that automated identification of forest disturbance types. • All reviewed studies except two used Landsat image time series as source data. • Studies attributed two to eight disturbance types with ∼22–100% accuracy per type. • Most studies focused on areas <75,000 km2 and separated 3–5 disturbance types. • More work is needed to refine algorithm sensitivity, accuracy, and transferability. [ABSTRACT FROM AUTHOR]

Aim: Our understanding of the mechanisms that maintain forest diversity under changing climate can benefit from knowledge about traits that are closely linked to fitness. We tested whether the link between traits and seed number and seed size is consistent with two hypotheses, termed the leaf economics spectrum and the plant size syndrome, or whether reproduction represents an independent dimension related to a seed size–seed number trade‐off. Location: Most of the data come from Europe, North and Central America and East Asia. A minority of the data come from South America, Africa and Australia. Time period: 1960–2022. Major taxa studied: Trees. Methods: We gathered 12 million observations of the number of seeds produced in 784 tree species. We estimated the number of seeds produced by individual trees and scaled it up to the species level. Next, we used principal components analysis and generalized joint attribute modelling (GJAM) to map seed number and size on the tree traits spectrum. Results: Incorporating seed size and number into trait analysis while controlling for environment and phylogeny with GJAM exposes relationships in trees that might otherwise remain hidden. Production of the large total biomass of seeds [product of seed number and seed size; hereafter, species seed productivity (SSP)] is associated with high leaf area, low foliar nitrogen, low specific leaf area (SLA) and dense wood. Production of high seed numbers is associated with small seeds produced by nutrient‐demanding species with softwood, small leaves and high SLA. Trait covariation is consistent with opposing strategies: one fast‐growing, early successional, with high dispersal, and the other slow‐growing, stress‐tolerant, that recruit in shaded conditions. Main conclusions: Earth system models currently assume that reproductive allocation is indifferent among plant functional types. Easily measurable seed size is a strong predictor of the seed number and species seed productivity. The connection of SSP with the functional traits can form the first basis of improved fecundity prediction across global forests. [ABSTRACT FROM AUTHOR]

Since the late 1990s, extensive outbreaks of native bark beetles (Curculionidae: Scolytinae) have affected coniferous forests throughout Europe and North America, driving changes in carbon storage, wildlife habitat, nutrient cycling, and water resource provisioning. Remote sensing is a crucial tool for quantifying the effects of these disturbances across broad landscapes. In particular, Landsat time series (LTS) are increasingly used to characterize outbreak dynamics, including the presence and severity of bark beetle-caused tree mortality, though broad-scale LTS-based maps are rarely informed by detailed field validation. Here we used spatial and temporal information from LTS products, in combination with extensive field data and Random Forest (RF) models, to develop 30-m maps of the presence (i.e., any occurrence) and severity (i.e., cumulative percent basal area mortality) of beetle-caused tree mortality 1997–2019 in subalpine forests throughout the Southern Rocky Mountains, USA. Using resultant maps, we also quantified spatial patterns of cumulative tree mortality throughout the region, an important yet poorly understood concept in beetle-affected forests. RF models using LTS products to predict presence and severity performed well, with 80.3% correctly classified (Kappa = 0.61) and R2 = 0.68 (RMSE = 17.3), respectively. We found that ≥10,256 km2 of subalpine forest area (39.5% of the study area) was affected by bark beetles and 19.3% of the study area experienced ≥70% tree mortality over the twenty-three year period. Variograms indicated that severity was autocorrelated at scales < 250 km. Interestingly, cumulative patch-size distributions showed that areas with a near-total loss of the overstory canopy (i.e., ≥90% mortality) were relatively small (<0.24 km2) and isolated throughout the study area. Our findings help to inform an understanding of the variable effects of bark beetle outbreaks across complex forested regions and provide insight into patterns of disturbance legacies, landscape connectivity, and susceptibility to future disturbance. [ABSTRACT FROM AUTHOR]

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. • Wildfire was easier to detect than spruce beetle outbreak using LTS. • Omission error decreased with greater tree mortality for each disturbance type. • Prior spruce beetle disturbance did not affect the detection of wildfire events. • LTS algorithms detected a c. 40% lower disturbance-affected area than other methods. [ABSTRACT FROM AUTHOR]

Despite its importance for forest regeneration, food webs, and human economies, changes in tree fecundity with tree size and age remain largely unknown. The allometric increase with tree diameter assumed in ecological models would substantially overestimate seed contributions from large trees if fecundity eventually declines with size. Current estimates are dominated by overrepresentation of small trees in regression models. We combined global fecundity data, including a substantial representation of large trees. We compared size-fecundity relationships against traditional allometric scaling with diameter and two models based on crown architecture. All allometric models fail to describe the declining rate of increase in fecundity with diameter found for 80% of 597 species in our analysis. The strong evidence of declining fecundity, beyond what can be explained by crown architectural change, is consistent with physiological decline. A downward revision of projected fecundity of large trees can improve the next generation of forest dynamic models. [ABSTRACT FROM AUTHOR]

Ongoing changes in fire regimes have the potential to drive widespread shifts in Earth's vegetation. Plant traits and vital rates provide insight into vulnerability to fire‐driven vegetation shifts because they can be indicators of the ability of individuals to survive fire (resistance) and populations to persist (resilience) following fire.In 15 study sites spanning climatic gradients in the southern Rocky Mountains, USA, we quantified variation in key traits and vital rates of two co‐occurring, widely distributed conifers (Pinus ponderosa Douglas ex. P. Lawson & C. Lawson and Pseudotsuga menziesii (Mirb.) Franco). We used mixed‐effects models to explain inter‐ and intraspecific variation in tree growth, survival, bark thickness and seed cone production, as a function of species, tree life stage (i.e. diameter, height and age), average climate, local competition and site conditions.Pinus ponderosa was predicted to survive low‐severity fire at a 23% earlier age than P. menziesii. Pinus ponderosa had thicker bark and more rapid juvenile height growth, traits conferring greater fire resistance. In contrast, P. menziesii was predicted to produce seed cones at a 28% earlier age than P. ponderosa. For both species, larger individuals were more likely to survive fire and to produce cones. For P. ponderosa, cone production increased where average actual evapotranspiration (AET) was higher and local competition was lower. More frequent cone production on productive sites with higher AET is an important and underappreciated mechanism that may help to explain greater resilience to fire in these areas.Synthesis. Our analyses indicated that many plant traits and vital rates related to fire differed between Pinus ponderosa and Pseudotsuga menziesii, with trade‐offs between investment in traits that promote individual defence to fire and those that promote recolonization of disturbed sites. Future changes in fire regimes will act as a filter throughout North American forests, with our findings helping to infer which individuals and populations of two iconic species are most vulnerable to future change and offering a framework for future inquiry in other forests facing an uncertain future. [ABSTRACT FROM AUTHOR]

Recent extreme events of drought and heat have been associated with insect-driven tree mortality. However, there is substantial uncertainty about the impact of climate variability and extreme climatic episodes on insect–host dynamics, especially over species biogeographical ranges. Here, we use climatic suitability indices derived from species distribution models to analyze the spruce beetle (Dendroctonus rufipennis) outbreak dynamics in spruce-fir forests across the Southern Rocky Mountains (USA) during a warm and dry episode (2000–2013). We estimated the historical climatic suitability of the host tree (1969–1998), its inter-annual variability, and the climatic suitability during the 2000–2013 episode for both beetle and host tree. Overall, outbreak was more likely to occur in host tree populations inhabiting areas with historically suitable climatic conditions that were also characterized by loss of suitability during the episode. Specifically, the outbreak initiation was located in areas with suitable climatic conditions for the beetle and high historical suitability for the host. However, the year-to-year analysis revealed that low–moderate amounts of outbreak initiation and spread were also determined by high host historical climatic suitability, with high historical inter-annual variability, and a modest reduction of suitability during the episode. Years with high amounts of outbreak initiation and spread mostly occurred in dense forests with large trees and were promoted by suitable climate conditions for the beetle. This study highlights the importance of considering the climatic suitability of the insect–host system to understand and anticipate outbreak dynamics at different temporal scales. [ABSTRACT FROM AUTHOR]

In contrast to abrupt changes caused by land cover conversion, subtle changes driven by a shift in the condition, structure, or other biological attributes of land often lead to minimal and slower alterations of the terrestrial surface. Accurate mapping and monitoring of subtle change are crucial for an early warning of long-term gradual change that may eventually result in land cover conversion. Freely accessible moderate-resolution datasets such as the Landsat archive have great potential to characterize subtle change by capturing low-magnitude spectral changes in long-term observations. However, past studies have reported limited success in accurately extracting subtle changes from satellite-based time series analysis. In this study, we introduce a supervised framework named 'PIDS' to detect subtle forest disturbance from a comprehensive Landsat data archive by leveraging disturbance-based calibration sites. PIDS consists of four components: (1) Parameter optimization; (2) Index selection; (3) Dynamic stratified monitoring; and (4) Spatial consideration. PIDS was applied to map the early stage of bark beetle infestations (i.e., a lower per-pixel fraction of trees cover that show visual signs of infestation), which are a typical example of subtle change in conifer forests. Landsat Analysis Ready Data were used as the time series inputs for mapping mountain pine beetle and spruce beetle disturbance between 2001 and 2019 in Colorado, USA. PIDS-detection map assessment showed that the overall performance of PIDS (namely 'F1 score') was 0.86 for mountain pine beetle and 0.73 for spruce beetle, making a substantial improvement (> 0.3) compared to other approaches/products including COntinuous monitoring of Land Disturbance, LandTrendr, and the National Land Cover Database forest disturbance product. A sub-pixel analysis of tree canopy mortality percentage was performed by linking classified high-resolution (0.3- and 1-m) aerial imagery and 30-m PIDS-detection maps. Results show that PIDS typically detects mountain pine beetle infestation when ≥56% of a Landsat pixel is occupied by red-stage canopy mortality (one year after initial infestation), and spruce beetle infestation when ≥55% is occupied by gray-stage mortality (two years after initial infestation). This study addresses an important methodological goal pertinent to the utility of event-based reference samples for detecting subtle forest change, which could be potentially applied to other types of subtle land change. • Develop a framework for detecting subtle changes from satellite time series. • Automatically calibrate model parameters and optimize spectral indices. • Assess results using multiple sources including field data and aerial survey. • Reach >0.3 F1 score improvement compared to other approaches. • Subpixel analysis for detected beetle stage using high-resolution imagery. [ABSTRACT FROM AUTHOR]