Aaron M. Sparks

Background: Multitemporal airborne laser scanning (ALS) data provide an avenue to quantify three-dimensional changes in forest structure, but fire-affected areas with both pre- and post-fire ALS data are rare. Satellite-derived active fire products provide a potential alternative dataset that could be used to estimate changes in forest structure and biomass given fire radiative energy (FRE) is proportional to the biomass consumption. Aims: We assessed the ability of pre- and post-fire ALS and active fire observations to characterize fire-induced forest structure changes. Methods: ALS data and FRE derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on the Terra and Aqua satellites were used to quantify fire-induced forest structural changes across a fire in central Oregon, US. Key results: Significant reductions in ALS-derived canopy density and canopy volume were observed over most of the fire extent and were positively related to FRE. Areas with greater mountain pine beetle tree mortality prior to the fire had greater FRE and canopy volume loss than unaffected areas. Conclusions: Results demonstrate the potential of both multitemporal ALS and FRE for assessing forest structure change and canopy consumption. Implications: Active fire products may be useful for estimating structure change in fire affected areas where multitemporal ALS data are not available.

Background: Under shifting fire regimes, forested ecosystems within the western US are predicted to experience a shorter time interval between wildland fire events, leading to increased fire exposure to younger tree cohorts. Consequently, the assessment of silvicultural approaches to help increase the resistance of these younger trees to wildland fires may help inform land management actions to promote growth and sustain healthy forests. Methods: We assessed the structural overshoot hypothesis in the context of vegetation fire severity by applying supplemental fertilizer to Pinus ponderosa saplings in advance of experimental fires. We explored three different treatments: no fertilizer, 7 weeks prior to fires, and 1 week prior to fire. We then used pyro-ecophysiology experiments to assess whether saplings subjected to fertilization treatments differed from untreated control groups across a range of fire intensities and assessed post-fire growth, physiology, tissue nutrient content, and mortality. Results: We demonstrated that the addition of nitrogen, either 1 week or 7 weeks prior to the fires, did not change the relationship between fire intensity and percentage mortality observed in the unfertilized fire treatments. Fertilization across all groups caused a decline in pre-fire leaf area and caused a more rapid decline of the change of chlorophyll fluorescence post-fire in the highest intensity treatment. Separability in the spectral indices were pronounced in the 7-week fertilization treatment, suggesting cellular changes occurred over this period. The 0.8 MJ m ⁻² FRE level that was associated with 100% mortality across all treatments was associated with increases in root aluminum concentrations. Conclusions: Pre-fire fertilization does not increase resistance to fire-induced mortality in ponderosa pine saplings, but this should be evaluated in other species with improved ability to effectively use supplemental nutrients. Further research is also warranted to assess how fertilization is altering the physiology properties of saplings and how this in turn impacts fire effects under fires of varying intensity. The increase in root aluminum concentration in the 0.8 MJ m ⁻² treatment may indicate an additional mechanism contributing to fire-induced tree mortality, but this should be reassessed in a variety of soil mediums. Further research is warranted to assess this mechanism and other soil chemistry processes and characteristics (e.g., pH) that may affect nutrient uptake and post-fire survival and recovery.

Background: In the dry montane forests of western North America, quaking aspen ( Populus tremuloides Michx.) is an iconic tree species, where it exists in vegetation communities of predominantly evergreen trees. While wildland fires can cause high mortality among conifer saplings and may kill the aboveground portions of aspen, this species is distinctive in its ability to resprout. Resprouting is a key physiological process that enables whole plant survival following fire through the production of new stems. However, there is limited pyro-ecophysiological knowledge of what levels of fire intensity induce top-kill, resprouting, and whole plant mortality in young trees. Quaking aspen is a prolific resprouting tree species that can resprout at all age classes, making it an ideal species to assess this information on saplings. Results: Here we present the findings of a pyro-ecophysiology experimental approach designed to assess the degree of top kill, fire-induced whole plant mortality, and physiological traits following experimental fires. We applied discrete levels of fire radiative energy (FRE, MJ m ⁻² ) using custom fuel beds of pine needles and woodchips, followed by monitoring the post-fire responses over a 42-week period. None of the FRE treatment levels resulted in whole plant mortality and all saplings that experienced top kill resprouted. The moderately high fire intensity groups (FRE levels: 2.0 MJ m ⁻² and 3.0 MJ m ⁻² ) produced a greater number of resprouts (+ 126%) than the lower intensity groups (FRE < 1.0 MJ m ⁻² ), but the highest fire intensity group (FRE level: 4.0 MJ m ⁻² ) produced a lower number of resprouts compared to the lower intensity group (+ 59%). Post-fire diameter growth of the main stem exceeded that of the unburned controls in the low fire intensity groups (FRE levels: 0.3 and 0.6 MJ m ⁻² ). Post-fire height growth exhibited FRE thresholds for the resprouts, with FRE doses above 1.0 MJ m ⁻² exhibiting notably higher heights (e.g., + 200% at week 8) compared to the resprouts associated with FRE doses 0.3 MJ m ⁻² and 0.6 MJ m ⁻² . In the main stem, the difference between chlorophyll fluorescence in top killed versus surviving crowns (at FRE = 1.0 MJ m ⁻² ) was associated with a notable decline (e.g., 75% at week 4). Conclusions: Our results provide predictive information that managers could use to make informed decisions on the potential impacts of fires on aspen saplings. Given aspen’s ability to regenerate following fires, whereas many other tree species do not, it may have a competitive advantage under more severe fires. At the landscape scale this could lead to a shift in tree community composition, especially considering the observed survival in the aspen saplings even at high fire intensity levels. Land managers seeking to restore aspen in decadent aspen forests should use surface fires of moderately high fire intensity (i.e., 2.0 MJ m ⁻² < FRE < 3.0 MJ m ⁻² ) to maximize production of suckers. Future fire-induced tree mortality model research should include a focus on improving data associated with small diameter trees.

Background: Climate change is expected to alter fire return intervals in cold and wet forests in the northwestern United States. This coupled with an expected rise in prescribed fires to restore healthy forests, disproportionately increases risk to saplings of tree species adapted to colder and wetter environments that have low fire resistance. To assess this potential impact, we evaluated the impacts of increasing fire intensity on Picea engelmannii and Thuja plicata sapling physiology, morphology, and mortality. This was achieved using established pyro-ecophysiology experiments where saplings were subjected to controlled surface fires across a range of fire intensities and post-fire growth, physiology and mortality were assessed up to 7 months post-fire. Results: In this study we demonstrate that the probability of mortality in the saplings of these two conifer species displays a sigmoidal increase with increasing fire intensity. At fire radiative energy dosage levels < 0.6 MJ m ⁻² , the observed mortality in both species was lower than predicted by existing crown scorch-based models due to their limited sensitivity at small diameters. Prior to sapling death, chlorophyll fluorescence transiently recovers before a rapid decline, though the timing varies by species and fire intensity dosage. A new general sapling mortality model derived from 7 conifer species is presented. Conclusions: Our results provide predictive tools that managers could use to make informed decisions on the potential impacts of fires on conifer saplings growing in cold and wet environments. Results from both species suggest that chlorophyll fluorescence temporal trends could serve as a potential early warning indicator of fire-induced tree mortality, however, future work should explore whether similar responses are observable using remote sensing data from solar-induced chlorophyll fluorescence and assess potential mechanisms underlying this signal. The general sapling mortality model presented in this paper appears to provide an improved method of predicting conifer sapling mortality over existing approaches, however, research is needed to develop coefficients to adjust the model with tree age and environmental factors. Further studies could also explore whether phenotypic plasticity is driving observed tree responses to fire from plants grown from similar environments.

Spectral indices are widely used to assess vegetation fire severity following wildland fires. Although essential, ground-based assessments of how such indices change due to varying fire intensities remain limited, especially with deciduous tree species that exhibit resprouting. In this paper, we evaluate the efficacy of detecting post-fire physiological change and top kill in quaking aspen (Populus tremuloides) saplings using differenced spectral indices. Saplings (n = 64) were burned under controlled conditions over a range of discrete fire intensity levels from 0 to 4.0 MJ m⁻², and reflectance was collected pre-fire and at six post-fire intervals up to 16 weeks. Ten spectral indices (CCI, CSI, MIRBI, NDVIL8, NBR, NBRL8, PRI, SAVI, SW-NIR_ratio, and SW-SW_ratio) were calculated, differenced from pre-fire, and related to the change in net photosynthesis and top kill. Fire intensity most strongly influenced the observed spectral changes at weeks 1–2 post-fire, especially for ΔCSI, ΔCCI, and ΔPRI. Pre- to post-fire change in net photosynthesis was strongly related (Tjur’s R² > 0.5) with ΔCCI, ΔCSI, ΔNBRL8, and the ΔSW–NIR ratio at one week post-fire. Of the spectral indices assessed, ΔCCI and ΔPRI were most effective at predicting top kill. This study illustrates the potential of spectral indices for monitoring vegetation fire severity in deciduous tree species.

Forest managers need stand-level forest inventories to make operational decisions and model growth and yield to inform long-term planning. However, few studies have quantified errors in field sampling- and airborne laser scanning (ALS)-derived inventories at the stand level, particularly in species-diverse and structurally diverse mixed conifer forests. In this study, we compared stand-level metrics derived from field cruise measurements of a forest-wide stratified sample of variable-radius plots, an ALS-derived area-based approach (ABA) trained and tested using an independent sample of fixed-area stem-mapped plots, and two ALS-derived individual tree approaches. Inventory volume estimates were validated using the gross volume of harvested logs from multi-stand harvest data, tracked by load and location and scaled at the processing mill. Results show that the ABA and individual tree approaches produced stand-level volume estimates with similar errors (−8 to 6%) to the cruise estimated volume (−16 to 6%) when compared with scaled volume. Across the entire forest, regression-based equivalence tests showed that merchantable and total stand volume estimates between the cruise and ALS-derived individual tree methods were more similar than between cruise and ABA methods, potentially due to underestimation of trees by both cruise and individual tree methods in some areas of the study area. Our results also highlight important differences between conventional cruise inventories and ALS-derived inventories, such as the spatial variability of within-stand attributes that ALS inventories provide. Overall, this study improves our understanding of the limitations and advantages of conventional and ALS-derived stand-level inventories in mixed conifer, structurally diverse forests.

Premise: In the face of novel fire regimes driven by anthropogenic changes to climate, ignitions, and fuels, understanding the evolution and present distribution of fire‐adapted traits is critical. Four common fire adaptations in conifers are thick bark, serotinous cones, seedling grass stage, and resprouting. Methods: We focused on these fire‐adapted traits and their abundance in North American conifers within a community phylogenetic context. Using ranges derived from numerous sources, we divided the continent into 20 conifer regions. We assessed regions for species richness, number of fire adaptations, number of taxa with each fire adaptation, and the percent composition of species with fire adaptations. We calculated regional phylogenetic dispersion metrics for each fire adaptation as well. Results: For almost every analysis, a north–south trend was evident and had strong spatial correspondence with fire intensity and frequency metrics of contemporary fire regimes. Species richness and nearly every measure of fire adaptation abundance, increases with decreasing latitude. Northern latitudes also lack resprouting and the grass stage. We identified three fire‐prone regions: California, Mexico and Central America, and the Gulf Coast. In terms of community composition, northern and mid‐latitude regions trend toward neutral assembly, while southern regions are phylogenetically clustered. In most regions, fire adaptations are phylogenetically clustered, indicating that even in neutrally assembled ecosystems, fire adaptations are restricted to a few clades. Conclusions: Given this information, we advocate tailoring management strategies toward present fire adaptations (or lack thereof), emphasizing strengthening fire resilience in fire‐adapted ecosystems and maintaining the integrity of fire refugia.

Background The increased interest in why and how trees die from fire has led to several syntheses of the potential mechanisms of fire-induced tree mortality. However, these generally neglect to consider experimental methods used to simulate fire behaviour conditions. Aims To describe, evaluate the appropriateness of and provide a historical timeline of the different approaches that have been used to simulate fire behaviour in fire-induced tree mortality studies. Methods We conducted a historical review of the different actual and fire proxy methods that have been used to further our understanding of fire-induced tree mortality. Key results Most studies that assess the mechanisms of fire-induced tree mortality in laboratory settings make use of fire proxies instead of real fires and use cut branches instead of live plants. Implications Further research should assess mechanisms of fire-induced tree mortality using live plants in paired combustion laboratory and landscape fire experiments.

On-the-ground sample-based forest inventory methods have been the standard practice for more than a century, however, remote sensing technologies such as airborne laser scanning (ALS) are providing wall-to-wall inventories based on individual tree measurements. In this study, we assess the accuracy of individual tree height, diameter, and volume derived from field-cruising measurements and three ALS data-derived methods in a 1.1 ha stand using direct measurements acquired on felled trees and log-scale volume measurements. Results show that although height derived from indirect conventional field measurements and ALS were statistically equivalent to felled tree height measurements, ALS measured heights had lower root mean square error (RMSE) and bias. Individual tree diameters modeled using a height-to-diameter-at-breast-height model derived from local forest inventory data and the software ForestView had moderate RMSE (8.3–8.5 cm) and bias (-3.0 – -0.3 cm). The ALS-based methods underdetected trees but accounted for 78%–91% of the field reference harvested merchantable volume and 71%–99% of the merchantable volume scaled at the mill. The results also illustrate challenges of using mill-scaled volume estimates as validation data and highlight the need for more research in this area. Overall, the results provide key insights to forest managers on accuracies associated with conventional field-derived and ALS-derived individual tree inventories. Study Implications: Forest inventory data provide critical information for operational decisions and forest product supply chain planning. Traditionally, forest inventories have used field sampling of stand conditions, which is time-intensive and cost-prohibitive to conduct at large spatial scales. Remote sensing technologies such as airborne laser scanning (ALS) provide wall-to-wall inventories based on individual tree measurements. This study advances our understanding of the accuracy of conventional field-derived and ALS-derived individual tree inventories by evaluating these inventories with felled tree and log scaling data. The results provide key insights to forest managers on errors associated with conventional field and ALS-derived individual tree measurements.