Wildfire-mediated changes to forests have prompted numerous studies on post-fire forest recovery of coniferous forests. Given climate change, a growing body of work demonstrates that conifer ...regeneration in temperate and boreal forests is declining, a phenomenon often termed “regeneration failure.” However, the definition and parameters are numerous and variable. Characterization of drought also varies greatly, thus hindering the ability to compare results among areas. This review discusses new perspectives on conifer regeneration failure and places these studies into the context of drought and fire activity. We focus this review on three forest types where conifer regeneration failure is well documented: western boreal forests, cold mixed-conifer forests, and dry pine forests. To place the challenges to conifer tree regeneration in the context of regional climate trends, we present a novel regional analysis that summarizes drought conditions prior, during, and following the year of a large wildfire. We demonstrate the need to assess failure in the context of specific forest dynamics and well-defined metrics. For example, tree establishment may historically occur over longer periods, and current and future climate may exacerbate this and not promote pre-fire forest structure and composition. Many forests are undergoing rapid change and the type, magnitude, and causes of changes need to be compared among areas. As such, we should be cautious of quantifying “regeneration failure” and drought without providing spatial and temporal context.
Celotno besedilo
Dostopno za:
BF, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Biotic disturbances (BDs, for example, insects, pathogens, and wildlife herbivory) substantially affect boreal and temperate forest ecosystems globally. However, accurate impact assessments ...comprising larger spatial scales are lacking to date although these are critically needed given the expected disturbance intensification under a warming climate. Hence, our quantitative knowledge on current and future BD impacts, for example, on forest carbon (C) cycling, is strongly limited. We extended a dynamic global vegetation model to simulate ecosystem response to prescribed tree mortality and defoliation due to multiple biotic agents across United States forests during the period 1997–2015, and quantified the BD‐induced vegetation C loss, that is, C fluxes from live vegetation to dead organic matter pools. Annual disturbance fractions separated by BD type (tree mortality and defoliation) and agent (bark beetles, defoliator insects, other insects, pathogens, and other biotic agents) were calculated at 0.5° resolution from aerial‐surveyed data and applied within the model. Simulated BD‐induced C fluxes totaled 251.6 Mt C (annual mean: 13.2 Mt C year−1, SD ±7.3 Mt C year−1 between years) across the study domain, to which tree mortality contributed 95% and defoliation 5%. Among BD agents, bark beetles caused most C fluxes (61%), and total insect‐induced C fluxes were about five times larger compared to non‐insect agents, for example, pathogens and wildlife. Our findings further demonstrate that BD‐induced C cycle impacts (i) displayed high spatio‐temporal variability, (ii) were dominated by different agents across BD types and regions, and (iii) were comparable in magnitude to fire‐induced impacts. This study provides the first ecosystem model‐based assessment of BD‐induced impacts on forest C cycling at the continental scale and going beyond single agent‐host systems, thus allowing for comparisons across regions, BD types, and agents. Ultimately, a perspective on the potential and limitations of a more process‐based incorporation of multiple BDs in ecosystem models is offered.
Biotic disturbances (BDs) play a key role in shaping boreal and temperate forest ecosystems, and they are expected to intensify with a warming climate. We extended a dynamic global vegetation model, the LPJ‐GUESS framework, to simulate and quantify continental‐scale impacts of multiple BDs on forest carbon dynamics for the first time. Overall, tree mortality and defoliation due to insects, pathogens, and wildlife herbivory have caused significant vegetation carbon loss across United States forests during 1997–2015, a loss that is comparable in magnitude with carbon emissions from fire.
Digital terrain models (DTMs) and vegetation canopy height models (CHMs) are used in a wide range of earth and environmental sciences. An increasing number of CHM products are available from active, ...passive, and photogrammetric remotely sensed data; however, high-resolution (≤5 m), wall-to-wall CHMs for the arctic and northern boreal domains that are suitable for detailed spatial analysis are lacking. Recently, a 5-m spatial resolution pan-arctic digital surface model – the ArcticDEM – was created using automated stereopair analysis of high-resolution satellite data. The ArcticDEM is unprecedented in extent and spatial resolution, yet the product generally follows the uppermost surface elevation (i.e., representing a digital surface model, DSM) without regard to whether the surface is comprised of vegetation or bare-earth terrain. To address this limitation, we developed and tested an approach to map vegetation canopy height at a 5-m spatial resolution (hereafter called the local ArcticCHM), and then subtracted these estimated canopy heights from the ArcticDEM in order to create a 5-m resolution DTM (local ArcticDTM). We selected three pilot study areas (total 58 km2) across a north-south gradient in Alaska, representing a range of vegetation types and topographic conditions. We estimated and mapped canopy height using randomForest and imputation modeling approaches, with the ArcticDEM and high spatial resolution multispectral satellite data (WorldView-2) used as predictors. Airborne laser scanning (ALS) data was used for model calibration and independent validation. Canopy height was reliably predicted across the three study areas, with the best models ranging from root mean square errors (RMSE) 2.2 to 2.6 m and R2 ranging from 0.59 to 0.76 relative to ALS-based CHM reference data. Similarly, the RMSE between the new local ArcticDTM product and ALS-based DTM reference data was 45–68% less than similar comparisons with the ArcticDEM. Our results offer a means to extend these local ArcticDTM and CHM products to establish high-resolution products elsewhere in Alaska of high value for a wide range of earth and environmental sciences research investigations.
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•The ArcticDEM is a high-resolution digital surface model for northern latitudes.•The ArcticDEM generally follows the highest local surface, vegetation or terrain.•We developed methods that accurately quantify canopy height.•We used the canopy height to remove vegetation elevations from the ArcticDEM.•Our approach is portable to similar areas where lidar data is not available.
Characterizing pre- and post-fire fuels remains a key challenge for estimating biomass consumption and carbon emissions from wildfires. Airborne laser scanning (ALS) data have demonstrated ...effectiveness for estimating canopy, and to a lesser degree, surface fuel components at fine-scale (i.e., 30 m) across landscapes. Using pre- and post-fire ALS data and corresponding field data, this study estimated consumption of canopy fuel (ΔCF), understory fuel (ΔUF), total fuel (ΔTF), and canopy bulk density (ΔCBD) for the 2012 Pole Creek fire in Oregon, USA (10,760 ha), and portions of the 2011 Las Conchas fire in New Mexico, USA (4,934 ha). Additionally, the feasibility of predicting fuel consumption was tested using separate pre- and post-fire models (PrePost), models combining all pre- and post-fire data (Pooled), and models using all data from both fires (Global). Estimates of ΔTF were then compared to fire radiative energy (FRE, units: MJ) derived from Fire Radiative Power (FRP, units: MW) observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA Terra and Aqua satellites to mechanistically derive a biomass combustion coefficient (BCC, units: kg MJ−1). The PrePost and Pooled approaches yielded similar results at Las Conchas, but at Pole Creek insufficient pre-fire field data resulted in erroneous fuel consumption estimates outside the fire perimeter using the PrePost models. These results demonstrated that pre-fire field data were less important for these models than having field data which represent the full range of fuel conditions likely to exist across the landscape. Estimated total biomass consumed for the PrePost, Pooled, and Global models were 226 Gg, 224 Gg, and 224 Gg at Las Conchas, and 581 Gg, 713 Gg, and 552 Gg at Pole Creek. Comparisons between estimated ΔTF and FRE yielded an average BCC for both fires of 0.367 (s.d. ± 0.049) kg MJ−1 based on pixels with at least five MODIS observations. Both higher MODIS observations per pixel and accounting for canopy occlusion of FRE improved the relationship between ΔTF and MODIS-FRE. This study suggested a practical modelling approach for future efforts using only post-fire field observations and quantified a landscape-scale relationship between MODIS-derived FRE and fine-scale fuel consumption consistent with prior experiments.
•Multitemporal ALS quantifies consumption of canopy and understory fuel.•Capturing full range of forest fuels is important for accurate modelling.•ALS fuel models exhibit temporal and spatial transferability.•Link between biomass consumption and FRE is consistent with prior studies.
Abstract
One of the greatest challenges for land managers is to maintain a multitude of ecosystem services while reducing hazards posed by wildfires, insect outbreaks, and other disturbances ...accelerating due to climate change. In response to limited available resources and improved technical abilities, natural resource managers are increasingly using geospatial data to plan and evaluate their management actions. Large amounts of public resources are invested in research and development to improve geospatial datasets, yet there is limited knowledge about the specific data types and data characteristics that clients (e.g. land managers) prefer. Our overall objective was to investigate what geospatial data characteristics are preferred by natural resource professionals to monitor and manage forests and fuels across large landscapes. We performed an online survey and collected supplemental data at a subsequent workshop during the 2020 Operational Lidar Inventory meeting to investigate preferred data use and data characteristics of data users of the Pacific Northwest. Our online survey was completed by 69 respondents represented by managers and natural resource professionals from tribal/state, federal, academic, and industry/consulting entities. We found that metrics related to species composition, total biomass/timber volume, and vegetation height were the most preferred attributes, yet preference differed slightly by employment type. From the workshop we found that metric preferences depend upon which management priorities are central to the management application. There was preference for data with Landsat pixel-level (30 m) spatial resolution, annual temporal resolution, and at regional spatial extents. To maintain viable ecosystem services in the long term, it is important to understand the metrics and their data characteristics that are most useful. We conclude that our study is a useful way to understand (a) how to improve the data utility for the users (clients) and (b) the development and investment needs for the data developers and funders.
Forests are major components of the carbon cycle, and disturbances are important influences of forest carbon. Our objective was to contribute to the understanding of forest carbon cycling by ...quantifying the amount of carbon in trees killed by two disturbance types, fires and bark beetles, in the western United States in recent decades. We combined existing spatial data sets of forest biomass, burn severity, and beetle-caused tree mortality to estimate the amount of aboveground and belowground carbon in killed trees across the region. We found that during 1984-2010, fires killed trees that contained 5-11 Tg C year−1 and during 1997-2010, beetles killed trees that contained 2-24 Tg C year−1, with more trees killed since 2000 than in earlier periods. Over their periods of record, amounts of carbon in trees killed by fires and by beetle outbreaks were similar, and together these disturbances killed trees representing 9% of the total tree carbon in western forests, a similar amount to harvesting. Fires killed more trees in lower-elevation forest types such as Douglas-fir than higher-elevation forest types, whereas bark beetle outbreaks also killed trees in higher-elevation forest types such as lodgepole pine and Engelmann spruce. Over 15% of the carbon in lodgepole pine and spruce/fir forest types was in trees killed by beetle outbreaks; other forest types had 5-10% of the carbon in killed trees. Our results document the importance of these natural disturbances in the carbon budget of the western United States.
Greenhouse gas emissions have altered global climate significantly, increasing the frequency of drought, fire, and insect- and pathogen-related mortality in forests across the western United States. ...The accuracy of satellite-based estimates of canopy change has been limited by difficulties associated with discriminating overstory canopy from understory vegetation. To overcome this issue, we developed a method to quantify forest canopy cover using winter-season fractional snow covered area (FSCA) data from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) snow covered area and grain size (MODSCAG) algorithm. The method utilizes time series of FSCA data to identify images with continuous ground snow coverage and a snow-free overstory, effectively masking out the influence of understory vegetation. Using this method, we determined that MODSCAG-retrieved viewable gap fraction (VGF; i.e. fraction of pixel sub-canopy viewable area) was significantly correlated with an independent product of yearly crown mortality caused by mountain pine beetles derived from Landsat imagery at 25 high-mortality sites in northern Colorado (r−=0.96±0.03,p<0.03). Additionally, we determined the temporal lag between tree mortality and needlefall, showing that needlefall occurred an average of 2.6±1.2years after year of attack. The canopy change detection method described herein is the first to utilize snow cover to mask understory impacts on overstory detection. The method can be applied anywhere in the seasonal snow zone and therefore has wide applicability given that 30% of the global land surface is seasonally snow covered. In this regard, the approach addresses significant limitations of previously published methods of canopy change detection and has broad implications with regard to understanding forest mortality and the representation of disturbance within hydrologic, land surface, and climate models.
•MODIS estimates of fractional snow covered area quantify viewable gap fraction.•Forest mortality is detected by changes in viewable gap fraction.•Changes in viewable gap fraction lag spectral canopy changes by 2.6years.
Relationships between gross primary productivity (GPP) and the remotely sensed photochemical reflectance index (PRI) suggest that time series of foliar PRI may provide insight into climate change ...effects on carbon cycling. However, because a large fraction of carbon assimilated via GPP is quickly returned to the atmosphere via respiration, we ask a critical question—can PRI time series provide information about longer term gains in aboveground carbon stocks? Here we study the suitability of PRI time series to understand intra‐annual stem‐growth dynamics at one of the world's largest terrestrial carbon pools—the boreal forest. We hypothesized that PRI time series can be used to determine the onset (hypothesis 1) and cessation (hypothesis 2) of radial growth and enable tracking of intra‐annual tree growth dynamics (hypothesis 3). Tree‐level measurements were collected in 2018 and 2019 to link highly temporally resolved PRI observations unambiguously with information on daily radial tree growth collected via point dendrometers. We show that the seasonal onset of photosynthetic activity as determined by PRI time series was significantly earlier (p < .05) than the onset of radial tree growth determined from the point dendrometer time series which does not support our first hypothesis. In contrast, seasonal decline of photosynthetic activity and cessation of radial tree growth was not significantly different (p > .05) when derived from PRI and dendrometer time series, respectively, supporting our second hypothesis. Mixed‐effects modeling results supported our third hypothesis by showing that the PRI was a statistically significant (p < .0001) predictor of intra‐annual radial tree growth dynamics, and tracked these daily radial tree‐growth dynamics in remarkable detail with conditional and marginal coefficients of determination of 0.48 and 0.96 (for 2018) and 0.43 and 0.98 (for 2019), respectively. Our findings suggest that PRI could provide novel insights into nuances of carbon cycling dynamics by alleviating important uncertainties associated with intra‐annual vegetation response to climate change.
The photochemical reflectance index (PRI) is a promising remote sensing (RS) approach which is sensitive to gross plant carbon uptake. Presently, little is known about the sensitivity of PRI time‐series to radial tree growth because the fate of fixed carbon is complex. Analysis of a unique in situ dataset has allowed us to link highly temporally resolved PRI and radial tree growth time‐series unambiguously, indicating that PRI time‐series can track seasonal radial tree growth dynamics in remarkable detail. Hence, PRI could enable RS of carbon cycling dynamics to alleviate important uncertainties associated with vegetation response to warming.
The persistence of wildlife species in fire‐prone ecosystems is under increasing pressure from global change, including alterations in fire regimes caused by climate change. However, unburned islands ...might act to mitigate negative effects of fire on wildlife populations by providing habitat in which species can survive and recolonize burned areas. Nevertheless, the characteristics of unburned islands and their role as potential refugia for the postfire population dynamics of wildlife species remain poorly understood.
We used a newly developed unburned island database of the northwestern United States from 1984 to 2014 to assess the postfire response of the greater sage‐grouse (Centrocercus urophasianus), a large gallinaceous bird inhabiting the sagebrush ecosystems of North America, in which wildfires are common. Specifically, we tested whether prefire and postfire male attendance trends at mating locations (leks) differed between burned and unburned areas, and to what extent postfire habitat composition at multiple scales could explain such trends.
Using time‐series of male counts at leks together with spatially explicit fire history information, we modeled whether male attendance was negatively affected by fire events. Results revealed that burned leks often exhibit sustained decline in male attendance, whereas leks within unburned islands or >1.5 km away from fire perimeters tend to show stable or increasing trends.
Analyses of postfire habitat composition further revealed that sagebrush vegetation height within 0.8 km around leks, as well elevation within 0.8 km, 6.4 km, and 18 km around leks, had a positive effect on male attendance trends. Moreover, the proportion of the landscape with cheatgrass (Bromus tectorum) cover >8% had negative effects on male attendance trends within 0.8 km, 6.4 km, and 18 km of leks, respectively.
Synthesis and applications. Our results indicate that maintaining areas of unburned vegetation within and outside fire perimeters may be crucial for sustaining sage‐grouse populations following wildfire. The role of unburned islands as fire refugia requires more attention in wildlife management and conservation planning because their creation, protection, and maintenance may positively affect wildlife population dynamics in fire‐prone ecosystems.
Unburned islands of vegetation within fire perimeters might mitigate the negative effects of wildfire on wildlife populations in fire‐prone ecosystems, but they remain poorly understood. In this article, we researched and found out that unburned islands allow for the persistence of greater sage‐grouse after fire, thus acting as potential fire refugia. This is important for management because it suggests that creating, protecting, and enhancing unburned islands within fire perimeters may positively impact wildlife populations in these ecosystems.
•Estimated bark beetle-caused tree mortality in the western US from aerial surveys.•A range of results from different methods indicates some uncertainty.•Annual Westwide tree mortality continued to ...be high in recent years.•Multiple severe outbreaks in different locations contributed to mortality.
Bark beetle (Curculionidae, Scolytinae) outbreaks have been extensive and severe across the western United States in recent decades, and assessments of tree mortality are critical for documenting patterns and increasing understanding of drivers and impacts. Aerial surveys have produced a rich data set that includes damage severity, year and location of damage, and beetle and tree species. Here we update a data set of bark beetle-caused tree mortality for the western United States to include five additional years (now 1997–2018) and use these data to characterize recent outbreaks and compare with earlier tree mortality. We estimated “mortality area” (MA), the canopy area of beetle-killed trees and a more accurate representation of outbreak impacts than “affected area”, as well as the number of killed trees. Recently, the US Forest Service changed survey approaches, creating challenges for linking observations from the old and new approaches. We compared four methods to harmonize these approaches to produce consistent time series of tree mortality. General similarity of MA occurred in several methods; however, the range of results indicated some uncertainty. Based on limited analyses and a desire to be conservative in damage estimates, we suggest that the FHPR1-R4 method, which produced intermediate mortality among the methods, is most realistic for representing tree mortality from bark beetles. Using this recommended method, we found that bark beetles caused 4.3 Mha of MA and 3.8x109 killed trees when summed across space, time, and bark beetle/host combination (range among methods: 1.5–7.4 Mha and 1.2–6.3 x 109 killed trees). This total mortality area was 4.7% of forest area in the western United States; 28% of this mortality occurred in 2013–2018. Annual tree mortality remained high recently, with values comparable to earlier years, and was a result of combinations of outbreaks of different beetle species in different regions with different timing. Bark beetles continue to be agents of significant forest disturbance in the western United States. Given a range of mortality area results from the different methods, we encourage further evaluation of estimates using independent observations.
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