Geology topics

Kristen L. Shive

Publications and source records attributed to Kristen L. Shive.

5 recordsLinked to original sources

USGS2026

Estimating basal area change by tree size with Sentinel-2 imagery following four fires in California, USA

Background Failure to account for tree size when estimating burn severity may not accurately capture post-fire tree mortality and post-fire forest structure. Aims We explored whether basal area mortality by tree size class could be determined from remotely-sensed burn severity indices based solely on Sentinel-2 satellite imagery. Methods We used data collected in four large California wildfires to model the relationship between proportional basal area mortality and burn severity indices derived from Sentinel-2 imagery for three tree diameter class thresholds: small (15 to 30 cm), medium (30 to 50 cm) and large (>50 cm). Key results Our models showed that for a given burn severity index value, the proportion of mortality was greater overall in smaller trees, and that the proportion of mortality in large trees changed more slowly than that of smaller trees with changing burn severity index values. Conclusions We found that models that accounted for tree size can more precisely estimate changes in forest size structure than a similar model that did not account for tree size. Implications Explicitly accounting for tree size can improve estimates of post-fire forest structure, including for large trees which make up the bulk of stand biomass and post-fire seed sources.

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USGS2026

The state of the giant sequoias: Losses, risks, and opportunities

Background Giant sequoias, an iconic tree found only in a narrow band along the western slope of the Sierra Nevada of California, are facing unprecedented threats. Despite being adapted to fire and resistant to both drought and insect attack, the species has exhibited signs of vulnerability to these stressors in recent years, with the most substantial impacts coming from wildfire. Recently, several unprecedentedly large and severe fires have resulted in the deaths of many large trees and, in some cases, limited postfire regeneration. These impacts have led to heightened efforts by managers to implement treatments to protect those places that are still vulnerable. Here, we provide a comprehensive, range-wide assessment of the condition of the species, including an evaluation of the trends in wildfire and its effects on large giant sequoia mortality and the potential for local extirpation, treatment patterns, and vulnerability to future fire. Results From 2015 to 2024, 82% of giant sequoia grove area burned in wildfire, compared to 24% between 1910 and 2014. We conservatively estimate that 17.6% of all large giant sequoias have been killed by wildfire since 1984, with most dying in 2020 and 2021. Due to very severe wildfire, roughly 13% of the range is at some risk of local extirpation due to the loss of mature trees and limited predicted regeneration. Treatment activity has increased since 1995, with treatment type and amount varying by jurisdiction. The combination of treatment and wildfire has created a mosaic of resistance to severe fire, with 26% of grove area at high resistance, 38% at moderate resistance, 2% at low resistance, and 16% having no resistance. The remaining 18% are areas where mature giant sequoia forest was lost due to high severity fire. Conclusions Giant sequoias have experienced substantial losses due to recent wildfire, with large areas at risk of grove loss. However, most burned area had lower severity effects, creating opportunities to leverage wildfire as a treatment. When combined with areas of active management, these opportunities can promote giant sequoia conservation in a time of rapid change.

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USGS2025

The effectiveness of wildfire at meeting restoration goals across a fire severity gradient in the Sierra Nevada

As a consequence of both warming temperatures and over a century of fire suppression, wildfires in the historically frequent-fire forests of the western US have increased both in size and intensity, resulting in large patches of high severity fire that are well outside the historic range of variation. Postfire fuels research has often focused on such high severity patches because of the risk of both type conversion and repeated high severity fire. Yet a substantial portion of any given wildfire will likely still have burned at low to moderate severity. These areas generally retain live mature trees and surface fuels, suggesting that wildfire effects may be in keeping with some forest restoration goals. To better understand the range of postfire fuels conditions across severity classes and how well those conditions align with restoration targets, we sampled three wildfires in mixed conifer forests and giant sequoia groves of the southern Sierra Nevada. These wildfires appear to have met short-term restoration goals for surface fuel reduction, with burned areas having 79.5 % less fuels than unburned areas. Fine woody debris and litter and duff declined with severity, while coarse woody debris was more variable. Small tree density targets were roughly met after low and moderate severity fire, but large tree densities tended to be lower than restoration targets, possibly due to high levels of recent tree mortality. For long-term management, restoration plans set targets for the proportions of the landscape that should be in different fuel load categories, reflecting patterns shaped by many frequent and patchy fires. Observed post-wildfire surface fuels outside of groves were overwhelmingly in the lowest fuels category across severity classes, which is in keeping with short-term goals to reduce surface fuels but is not necessarily contributing to the heterogeneity desired at landscape scales. Surface fuels within giant sequoia groves were higher than those outside groves and therefore more closely matched long-term management targets for variation in fuel loads. However, for a highly valued species that has recently seen substantial losses to high severity fire, managers may find that these higher fuel loads are not desirable even in a landscape context. In summary, low and moderate severity wildfire appear to have beneficial effects in terms of meeting several management goals, however, the large amount of standing postfire fuels, the relative dearth of large trees, and the potential lack of postfire fuel heterogeneity may still pose potential management concerns.

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USGS2021

Effects of postfire climate and seed availability on postfire conifer regeneration

Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by increasingly hot and dry conditions. To better inform postfire management in low elevation forests of California, USA, we collected 5‐year postfire recovery data from 1,234 study plots in 19 wildfires that burned from 2004–2012 and 18 years of seed production data from 216 seed fall traps (1999–2017). We used this data in conjunction with spatially extensive estimates of climate, topography, forest composition, and burn severity to construct taxon‐specific, spatially explicit models of conifer regeneration that incorporate estimated climate conditions and seed availability during postfire recovery windows. We found that after accounting for other predictors both postfire and historical precipitation were strong predictors of regeneration, suggesting that both direct effects of postfire moisture conditions and biological inertia from historical climate may play a role in regeneration. Alternatively, postfire regeneration may simply be driven by postfire climate and apparent relationships with historical climate could be spurious. The estimated sensitivity of regeneration to postfire seed availability was strongest in firs and all conifers combined and weaker in pines. Seed production exhibited high temporal variability with seed production varying by over two orders of magnitude among years. Our models indicate that during droughts postfire conifer regeneration declines most substantially in low‐to‐moderate elevation forests. These findings enhance our mechanistic understanding of forecasted and historically documented shifts in the distribution of trees.

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USGS2017

Evidence of fuels management and fire weather influencing fire severity in an extreme fire event

Following changes in vegetation structure and pattern, along with a changing climate, large wildfire incidence has increased in forests throughout the western U.S. Given this increase there is great interest in whether fuels treatments and previous wildfire can alter fire severity patterns in large wildfires. We assessed the relative influence of previous fuels treatments (including wildfire), fire weather, vegetation and water balance on fire severity in the Rim Fire of 2013. We did this at three different spatial scales to investigate whether the influences on fire severity changed across scales. Both fuels treatments and previous low to moderate severity wildfire reduced the prevalence of high severity fire. In general, areas without recent fuels treatments and areas that previously burned at high severity tended to have a greater proportion of high severity fire in the Rim Fire. Areas treated with prescribed fire, especially when combined with thinning, had the lowest proportions of high severity. Proportion of the landscape burned at high severity was most strongly influenced by fire weather and proportional area previously treated for fuels or burned by low to moderate severity wildfire. The proportion treated needed to effectively reduce the amount of high fire severity fire varied by spatial scale of analysis, with smaller spatial scales requiring a greater proportion treated to see an effect on fire severity. When moderate and high severity fire encountered a previously treated area, fire severity was significantly reduced in the treated area relative to the adjacent untreated area. Our results show that fuels treatments and low to moderate severity wildfire can reduce fire severity in a subsequent wildfire, even when burning under fire growth conditions. These results serve as further evidence that both fuels treatments and lower severity wildfire can increase forest resilience.

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