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![Figure A1. Examples of branchlets illustrating variability of different crown trait measurements. (A) Branchlet was clipped from a tree in a mesic (M) microsite. Four needle ages (WHL) are labeled by year, with CHL1 and CHL2 with 0% chlorosis for the first two years of needles (CHL1, CHL2), and CHL3 with 15% chlorosis. A needle defoliator, scale (Chionaspis, spp.; Chion), is present on the 2015 needles. Branch length (BRLNx) by year (x) is indicated by a dark mustard bar, with the foliated portion (%FOLLNx) of each year indicated by a dark green bar. An open red triangle indicates the position of the preformed 2018 terminal bud. The blue bar perpendicular to the branchlet axis indicates measurement point of prior year branchlet diameter (BRDIA2). (B) Branchlet was clipped from a tree in a xeric (X) microsite. Four needle ages were retained, with 40% and 60% chlorosis for CHL3 and CHL4, respectively. Needle length measured after elongation growth has ceased is presented relative to the average length of needles, of the whorl with the longest needles retained on the branchlet (%MxNLx): 100% for 2017 (total length not apparent in the photo); 5% for 2016; 60% for 2015, and 45% for 2014. The effects of a needle defoliator, Scythropus, spp. (Scyth) is present on the 4th year needles. (C) Branchlet from another X tree. The oldest needles are seven years old, but the 2014 needles were excised. Because the 2014 branchlet elongated, and the bracts for each needle fascicle (cluster of three needles bundled by a paper sheath) are widely spaced, it is likely that the needles elongated but were lost to reduce leaf area with increasing drought through 2016. The oldest WHL retained only one fascicle. % MxNL2 is 20% (compare to 5% in Panel (B)), 40% for MxNL3, and 60% for MxNL4. Trees differ in their respond to hydrologic deficits, especially in xeric microsites likely due to differences in the amount of trapped water in bedrock interstices. Whole tree examples of early senescence (ES) can be found in [53] and DMR in [57].](profile/Nancy-Grulke/publication/343110813/figure/fig3/AS:915792468119554@1595353347454/Figure-A1-Examples-of-branchlets-illustrating-variability-of-different-crown-trait.png)
Figure A1. Examples of branchlets illustrating variability of different crown trait measurements. (A) Branchlet was clipped from a tree in a mesic (M) microsite. Four needle ages (WHL) are labeled by year, with CHL1 and CHL2 with 0% chlorosis for the first two years of needles (CHL1, CHL2), and CHL3 with 15% chlorosis. A needle defoliator, scale (Chionaspis, spp.; Chion), is present on the 2015 needles. Branch length (BRLNx) by year (x) is indicated by a dark mustard bar, with the foliated portion (%FOLLNx) of each year indicated by a dark green bar. An open red triangle indicates the position of the preformed 2018 terminal bud. The blue bar perpendicular to the branchlet axis indicates measurement point of prior year branchlet diameter (BRDIA2). (B) Branchlet was clipped from a tree in a xeric (X) microsite. Four needle ages were retained, with 40% and 60% chlorosis for CHL3 and CHL4, respectively. Needle length measured after elongation growth has ceased is presented relative to the average length of needles, of the whorl with the longest needles retained on the branchlet (%MxNLx): 100% for 2017 (total length not apparent in the photo); 5% for 2016; 60% for 2015, and 45% for 2014. The effects of a needle defoliator, Scythropus, spp. (Scyth) is present on the 4th year needles. (C) Branchlet from another X tree. The oldest needles are seven years old, but the 2014 needles were excised. Because the 2014 branchlet elongated, and the bracts for each needle fascicle (cluster of three needles bundled by a paper sheath) are widely spaced, it is likely that the needles elongated but were lost to reduce leaf area with increasing drought through 2016. The oldest WHL retained only one fascicle. % MxNL2 is 20% (compare to 5% in Panel (B)), 40% for MxNL3, and 60% for MxNL4. Trees differ in their respond to hydrologic deficits, especially in xeric microsites likely due to differences in the amount of trapped water in bedrock interstices. Whole tree examples of early senescence (ES) can be found in [53] and DMR in [57].
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Drought, ozone (O 3), and nitrogen deposition (N) alter foliar pigments and tree crown structure that may be remotely detectable. Remote sensing tools are needed that pre-emptively identify trees susceptible to environmental stresses could inform forest managers in advance of tree mortality risk. Jeffrey pine, a component of the economically import...
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The 2012–2016 California Drought severely impacted natural vegetation across a wide range of environmental gradient. Although several studies have reported an increase in plant water stress and mortality, the spatiotemporal variations of ecosystem productivity responses and the associated environmental and biological drivers remain unclear. Here, u...
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... However, the largest group (n = 27) of studies with an exclusive focus on droughts refer to the mortality of a tree, stand, or forest. The focus of the studies on Germany [87,105,[127][128][129][130][131] and the USA [132][133][134][135][136][137][138] is noticeable. With seven study areas in each country, this proportion is distinctly higher than the overall distribution. ...
... Studies in the USA have focused on the Sierra Nevada region [133][134][135]137]. Again, several variables were shown to influence vulnerability to drought. ...
Temperate forests are particularly exposed to climate change and the associated increase in weather extremes. Droughts, storms, late frosts, floods, heavy snowfalls, or changing climatic conditions such as rising temperatures or more erratic precipitation are having an increasing impact on forests. There is an urgent need to better assess the impacts of climate change and extreme weather events (EWEs) on temperate forests. Remote sensing can be used to map forests at multiple spatial, temporal, and spectral resolutions at low cost. Different approaches to forest change assessment offer promising methods for a broad analysis of the impacts of climate change and EWEs. In this review, we examine the potential of Earth observation for assessing the impacts of climate change and EWEs in temperate forests by reviewing 126 scientific papers published between 1 January 2014 and 31 January 2024. This study provides a comprehensive overview of the sensors utilized, the spatial and temporal resolution of the studies, their spatial distribution, and their thematic focus on the various abiotic drivers and the resulting forest responses. The analysis indicates that multispectral, non-high-resolution timeseries were employed most frequently. A predominant proportion of the studies examine the impact of droughts. In all instances of EWEs, dieback is the most prevailing response, whereas in studies on changing trends, phenology shifts account for the largest share of forest response categories. The detailed analysis of in-depth forest differentiation implies that area-wide studies have so far barely distinguished the effects of different abiotic drivers at the species level.
... Recent research has demonstrated greater effectiveness of a simple green-red index in tracking conifer phenology and carbon up-Remote Sens. 2024, 16, 1005 4 of 24 take [31][32][33]. Stress in conifer crowns has been detected with four-band VNIR imagery [16], VNIR and thermal infrared (TIR) [18], hyperspectral data coupled with lidar [34], and red-edge (691-730 nm) data [35]. Thermographic instruments became widely available in the 1960s [36]. ...
... Tree crown attributes related to vigor and drought stress in PP [14] and a related species, Jeffrey pine [18], have been previously described and are presented in Appendix A. On each tree, 3 secondary branches and 3-4 branchlets back from the apex of primary branches in the upper third of the sunlit crown were used for morphological measures. Tree climbers accessed the crown of these mature trees to remove and drop sample branches for measurement. ...
... We also selected the RE, R, B, and NIR bands for analysis. This suite of bands and indices was chosen based on their utility in detecting stress and phenological status in conifers [18,19,56]. The final MS outputs were 16-bit unsigned 6 cm resolution img files, one file for each band/index. ...
The health of coniferous forests in the western U.S. is under threat from mega-drought events, increasing vulnerability to insects, disease, and mortality. Forest densification resulting from fire exclusion increases these susceptibilities. Silvicultural treatments to reduce stand density and promote resilience to both fire and drought have been used to reduce these threats but there are few quantitative evaluations of treatment effectiveness. This proof-of-concept study focused on such an evaluation, using field and remote sensing metrics of mature ponderosa pine (Pinus ponderosa Doug. Laws) in central Oregon. Ground metrics included direct measures of transpiration (sapflow), branch and needle measures and chlorosis; drone imagery included thermal (TIR) and five-band spectra (R, G, B, Re, NIR). Thermal satellite imagery was derived from ECOSTRESS, a space-borne thermal sensor that is on-board the International Space Station (ISS). All metrics were compared over 2 days at a time of maximum seasonal drought stress (August). Tree water status in unthinned, light, and heavy thinning from below density reduction treatments was evaluated. Tree crowns in the heavy thin site had greater transpiration and were cooler than those in the unthinned site, while the light thin site was not significantly cooler than either unthinned or the heavy thin site. There was a poor correlation (Adj. R2 0.10–0.13) between remotely sensed stand temperature and stand-averaged transpiration, and tree level temperature and transpiration (Adj. R2 0.04–0.19). Morphological attributes such as greater needle chlorosis and reduced elongation growth supported transpirational indicators of tree drought stress. The multispectral indices CCI and NDRE, along with the NIR and B bands, show promise as proxies for crown temperature and transpiration, and may serve as a proof of concept for an approach to evaluate forest treatment effectiveness in reducing tree drought stress.
... As a side effect of the research, a correlation between the NDVI and site trophism expressed by plant communities (Table 6) has been demonstrated. The Normalized Difference Vegetation Index (NDVI), a widely used method for estimating vegetation greenness, and other spectral indices are commonly used to assess the forest environment [27][28][29][30], but studies on this index show the complex relationship between NDVI and ecological factors such as meteorological data, soil moisture, and vegetation cover type. Creating a trophic grid of forest habitats based on NDVI could be a subject of separate research. ...
This study investigates the prospects for preserving the main Pinus sylvestris L. (Scots pine) ecotypes in Poland, considering the habitat conditions of their occurrence. Scots pine is known for its wide distribution and natural adaptability to various habitats. However, there is an increasing vulnerability of pine forests to damage from biotic factors and a decrease in natural regeneration, particularly in areas under legal protection. Additionally, projected climate change has raised concerns about the future of Pinus sylvestris, placing it in the “losing” group of tree species. The aim of the study was to analyze the habitat conditions of the seven main selected Pinus sylvestris L. ecotypes to assess the sustainability of pine stands in their natural habitat conditions. Out of the seven populations of studied pine ecotypes, only one grows under conditions representing a typical form of pine forest (Leucobryo–Pinetum plant association). Two populations grow under conditions corresponding to potential deciduous forests (Galio sylvatici–Carpinetum and Calamagrostio arundinaceae–Quercetum petraeae). The remaining populations represent potentially mixed oak–pine forests. Such a distribution of plant communities, except for Leucobryo–Pinetum, does not guarantee the continuity of the studied pine stands as a result of their natural regeneration. Therefore, it is necessary to preserve the offspring of the studied populations outside their occurrence sites, but the studied pine stands should be preserved until their natural death in their natural habitats. In the conducted research, the NDVI turned out to be very useful, showing a high correlation with the trophicity of habitat expressed in the diversity of plant communities, as well as with the height and diameter of the studied stands.
... Using remote sensing satellite data, it is possible to monitor the vegetative condition of large forest areas using derived vegetation indices such as NDVI, NDWI (Normalized Difference Water Index), NDRE (Normalized Difference Red Edge Index), GLI (Green Leaf Index), EVI, GNDVI (Green Normalized Differential Vegetation Index), and GCI (Green Chlorophyll Index). These indices can be used singly or in combination to map droughtinduced tree mortality in forests [29][30][31], where NDVI was identified as an index that differentiates various levels of tree mortality severity arising from droughts [32][33][34]. Details of such recent studies that have utilized existing or developed new vegetation indices for analyses of drought-induced tree mortality using various remotely sensed data can be found in Supplementary Material Table S1. ...
Protecting and enhancing forest carbon sinks is considered a natural solution for mitigating climate change. However, the increasing frequency, intensity, and duration of droughts due to climate change can threaten the stability and growth of existing forest carbon sinks. Extreme droughts weaken plant hydraulic systems, can lead to tree mortality events, and may reduce forest diversity, making forests more vulnerable to subsequent forest disturbances, such as forest fires or pest infestations. Although early warning metrics (EWMs) derived using satellite remote sensing data are now being tested for predicting post-drought plant physiological stress and mortality, applications of unmanned aerial vehicles (UAVs) are yet to be explored extensively. Herein, we provide twenty-four prospective approaches classified into five categories: (i) physiological complexities, (ii) site-specific and confounding (abiotic) factors, (iii) interactions with biotic agents, (iv) forest carbon monitoring and optimization, and (v) technological and infrastructural developments, for adoption, future operationalization, and upscaling of UAV-based frameworks for EWM applications. These UAV considerations are paramount as they hold the potential to bridge the gap between field inventory and satellite remote sensing for assessing forest characteristics and their responses to drought conditions, identifying and prioritizing conservation needs of vulnerable and/or high-carbon-efficient tree species for efficient allocation of resources, and optimizing forest carbon management with climate change adaptation and mitigation practices in a timely and cost-effective manner.
... The selection of tree-vigor attributes is based on previous work that established the relevance of the attributes in describing tree vigor [39][40][41]. The vigor attribute measurements are summarized in the Appendix A [32,42]. Tree foliage was sampled by clipping 3-5 secondary branchlets proximal to the terminus in mid-crown with a 13 m pole pruner. ...
... Across all treatments, upland stands had significantly greater proportions of LOW trees (p = 0.023) than in lowland stands. PP in uplands expressed reduced vigor due to increased physiological drought stress with hydrological drought, as was found for a related species, Jeffrey pine (Pinus jeffreyi Balf.) in the Sierra Nevada [42]. However, trees in 'marginal' microsites may be pre-adapted to environmental stress including drought and may not decline as much with stress as trees unacclimated to drought [58]. ...
A common forest restoration goal is to achieve a spatial distribution of trees consistent with historical forest structure, which can be characterized by the distribution of individuals, clumps, and openings (ICO). With the stated goal of restoring historical spatial patterns comes a need for effectiveness monitoring at appropriate spatial scales. Airborne light detection and ranging (LiDAR) can be used to identify individual tree locations and collect data at landscape scales, offering a method of analyzing tree spatial distributions over the scales at which forest restoration is conducted. In this study, we investigated whether tree locations identified by airborne LiDAR data can be used with existing spatial analysis methods to quantify ICO distributions for use in restoration effectiveness monitoring. Results showed fewer large clumps and large openings, and more small clumps and small openings relative to historical spatial patterns, suggesting that the methods investigated in this study can be used to monitor whether restoration efforts are successful at achieving desired tree spatial patterns.
Study Implications: Achieving a desired spatial pattern is often a goal of forest restoration. Monitoring for spatial pattern, however, can be complex and time-consuming in the field. LiDAR technology offers the ability to analyze spatial pattern at landscape scales. Preexisting methods for evaluation of the distribution of individuals, clumps, and openings were used in this study along with LiDAR individual tree detection methodology to assess whether a forest restoration project implemented in a Southern Oregon landscape achieved desired spatial patterns.
... The level of ph stress for each tree at MFm and MFx was well-characterized in 1999 unpubl. data), and in 2013 from thermal imagery [45]. All trees used i alive in September 2017 at MF and HM, and in September 2019 at SC. ...
... The significance of the DI coefficients varied among the fitted models, and the 1 yr lagged DI was significant for SPEI and AI but not for PDSI 152 or PDSI 100 . Across the DIs and AWC levels tested, and the source of the meteorological data, the model could not differentiate MFm-or MFx-dominated subsites, the latter with quantified differences in tree physiological drought stress between them [44,45]. Only SPEI and AI with on-site parameterizations differentiated SC from MFm. HM was not statistically distinguished from MFm in any of the models. ...
... Morphological crown response to drought (~30% loss of needles mid-crown in the 2007 drought; Grulke, unpubl. data at MF), physiological drought stress [44], elevated foliar oxidative stress [66], and as detected with thermal aerial imagery in 2013 ( [45]) clearly differentiated trees in the two subsites at MF. NDVI has been successfully used to validate the use of DIs in vastly different ecosystems (agricultural [77]; chaparral, [78]; forest [79]; and the African continent [80], but in the Californian mixed conifer zone with discontinuous woodlands and forest cover, neither mean August NDVI nor site-specific NDVI summed over the growing season could be used as a validation metric for DI parameterization (however, see [77]). ...
Tree mortality in Sierra Nevada’s 2012–2015 drought was unexpectedly excessive: ~152 million trees died. The relative performance of five drought indices (DIs: SPEI, AI, PDSI, scPDSI, and PHDI) was evaluated in the complex, upland terrain which supports the forest and supplies 60% of Californian water use. We tested the relative performance of DIs parameterized with on-site and modeled (PRISM) meteorology using streamflow (linear correlation), and modeled forest stand NDVI and tree basal area increment (BAI) with current and lagged year DI. For BAI, additional co-variates that could modify tree response to the environment were included (crown vigor, point-in-time rate of bole growth, and tree to tree competition). On-site and modeled parameterizations of DIs were strongly correlated (0.9), but modeled parameterizations overestimated water availability. Current year DIs were well correlated (0.7–0.9) with streamflow, with physics-based DIs performing better than pedologically-based DIs. DIs were poorly correlated (0.2–0.3) to forest stand NDVI in these variable-density, pine-dominated forests. Current and prior year DIs significantly predicted BAI but accounted for little of the variation in the model. In this ecosystem where trees shift seasonally between near-surface to regolithic water, DIs were poorly suited for anticipating the observed tree decline.
... Although it is not yet clear how to best implement and standardize remote sensing measures within the small and heterogeneous canopies of conifer seedlings, we postulate that there may be potential for remote sensing to enhance detection of seedling stress and mortality. Various remote sensing measures and indices including photochemical reflectance index (PRI), normalized difference vegetation index (NDVI), and hyperspectral band analysis have been used to identify adult conifer stress, often at relatively broad spatial scales and prior to onset of visual markers of stress [59][60][61][62]. Plant volumetric water content (VWC) is strongly related to mortality, and VWC is correlated with various remote sensing indices in angiosperms, but the connection between remote sensing indices and mortality has not been fully investigated in conifer seedlings or juveniles [63]. ...
Juvenile tree survival will increasingly shape the persistence of ponderosa pine forests in the western United States. In contrast to severe pulse disturbances that induce widespread adult and juvenile tree mortality, moderate periods of low rainfall and warm temperatures may reduce forest persistence by killing juvenile trees at the seedling stage. Intensification of these periods in a changing climate could therefore increasingly restrict both natural regeneration and artificial regeneration of planted seedlings. We conducted a controlled field experiment at a single site in the Front Range of Colorado, USA, to determine the responses and survival of 3 Colorado subpopulations of
... The selection of tree-vigor attributes is based on previous work that established the relevance of the attributes in describing tree vigor [39][40][41]. The vigor attribute measurements are summarized in the Appendix A [32,42]. Tree foliage was sampled by clipping 3-5 secondary branchlets proximal to the terminus in mid-crown with a 13 m pole pruner. ...
... Across all treatments, upland stands had significantly greater proportions of LOW trees (p = 0.023) than in lowland stands. PP in uplands expressed reduced vigor due to increased physiological drought stress with hydrological drought, as was found for a related species, Jeffrey pine (Pinus jeffreyi Balf.) in the Sierra Nevada [42]. However, trees in 'marginal' microsites may be pre-adapted to environmental stress including drought and may not decline as much with stress as trees unacclimated to drought [58]. ...
Ponderosa pine is an integral part of the forested landscape in the western US; it is the
dominant tree species on landscapes that provide critical ecosystem services. Moderate drought tolerance allows it to occupy the transition zone between forests, open woodlands, and grasslands. Increases in stand density resulting from wildfire suppression, combined with lengthening, intensifying, and more frequent droughts have resulted in reduced tree vigor and stand health in dry ponderosa pine throughout its range. To address a management need for efficient landscape-level surveys of forest health, we used Random Forests to develop an object-oriented classification of individual tree crowns (ITCs) into vigor classes using existing, agency-acquired four-band aerial imagery. Classes of tree vigor were based on quantitative physiological and morphological attributes established in a previous study. We applied our model across a landscape dominated by ponderosa pine with a variety of forest treatments to assess their impacts on tree vigor and stand health. We found that stands that were both thinned and burned had the lowest proportion of low-vigor ITCs, and that stands treated before the 2014–2016 drought had lower proportions of low-vigor ITCs than stands treated more recently (2016). Upland stands had significantly higher proportions of low-vigor trees than lowland stands. Maps identifying the low-vigor ITCs would assist managers in identifying priority stands for treatment and marking trees for harvest or retention. These maps can be created using already available imagery and GIS software.
... The selection of tree vigor attributes is based on previous work that established the relevance of the attributes in describing tree vigor [39][40][41]. The vigor attribute measurements are summarized in the appendix [32,42]. Tree foliage was sampled by clipping 3-5 secondary branchlets proximal to the terminus in mid-crown with a 13 m pole pruner. ...
... Across all treatments, upland stands had significantly greater proportion of LOW trees (p = 0.023) than in lowland stands. PP in uplands express reduced vigor due to increased physiological drought stress with hydrological drought, as was found for a related species, Jeffrey pine (Pinus jeffreyi) in the Sierra Nevada [42]. Trees in 'marginal' microsites may be pre-adapted to environmental stress including drought and may not decline as much with stress as trees unacclimated to drought [58]. ...
Ponderosa pine is an integral part of the forested landscape in the western US; it is the dominant tree species on landscapes that provide critical ecosystem services. Moderate drought tolerance allows it to occupy the transition zone between forests and open woodlands and grasslands. Increases in stand density resulting from wildfire suppression, combined with lengthening, intensifying and more frequent droughts have resulted in reduced tree vigor and stand health in dry ponderosa pine throughout its range. To address a management need for efficient landscape-level surveys of forest health, we used Random Forests to develop an object-oriented classification of individual tree crowns (ITCs) into vigor classes using existing, agency acquired 4-band aerial imagery. Classes of tree vigor were based on quantitative physiological and morphological attributes established in a previous study. We applied our model across a landscape dominated by ponderosa pine with a variety of forest treatments to assess their impacts on tree vigor and stand health. We found that stands that were both thinned and burned had the lowest proportion of low vigor ITCs, and that stands treated before the 2014-2016 drought had lower proportions of low vigor ITCs than stands treated more recently (2016). Upland stands had significantly higher proportions of low vigor trees than lowland stands. Maps identifying the low vigor ITCs would assist managers in identifying priority stands for treatment and marking trees for harvest or retention. These maps can be created using already available imagery and GIS software.
Purpose of Review
The successful application of thermal infrared (TIR) remote sensing in the agricultural domain, largely driven by the arrival of new platforms and sensors that substantially increased thermal data resolution and availability, has sparked interest in thermography as a tool for monitoring forest health. In this review, we take a step back to reflect on what physiological responses are reflected in leaf and canopy temperature and summarise research activities on TIR remote sensing of stress responses in forest environments, highlighting current methodological challenges, open questions, and promising opportunities.
Recent Findings
This systematic literature review showed that whilst the focus still remains on satellite imagery, Uncrewed Aerial Vehicles (UAVs) are playing an increasingly important role in testing the capabilities and sensitivity to stress onset at the individual tree level. To date, drought stress has been the focal point of research, largely due to its direct link to stomatal functioning at leaf level. Though, research into thermal responses to other stressors, e.g. pathogens, is also gaining momentum.
Summary
Disentangling stress-induced canopy temperature variations from environmental factors and structural influences remains the main challenge for broader application of TIR remote sensing. Further development and testing of approaches for thermal data analysis, including their applicability for different tree species and sensitivity under different climatic conditions, are required to establish how TIR remote sensing can best complement existing forest health monitoring approaches.
