Abstract:Background
An extreme drought from 2012–2016 and concurrent bark beetle outbreaks in California, USA resulted in widespread tree mortality. We followed changes in tree mortality, stand structure, and surface and canopy fuels over four years after the peak of mortality in Sierra mixed conifer and pinyon pine (Pinus monophylla) forests to examine patterns of mortality, needle retention after death, and snag fall across tree species. We then investigated how the tree mortality event affected surfa… Show more
“…The pulsed increase in fine fuel loads from the MPB outbreak seems to be stabilizing (Figure 5A), but the coarse wood load was still increasing 10 years post‐outbreak and was much higher in 2022 in the control and Fire treatment relative to the Mech and Mech+Fire (Figure 5B). Studies of beetle‐killed ponderosa pines in the Southwest US and California have indicated low persistence of standing snags beyond 10 years (Chambers & Mast, 2014; Reed et al, 2023), which aligns with our findings of only approximately 21% of dead ponderosa pine still standing in 2020. Therefore, while the main fuel pulse from the outbreak is likely over, we expect that coarse fuel loading in the control and Fire treatment will continue increasing in the near future as many of these remaining snags fall.…”
Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high‐severity wildfires in forests with historically frequent, low‐severity fire regimes are increasingly common, but long‐term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20‐year responses to thinning and prescribed burning treatments commonly used in dry, low‐elevation forests of the western United States from a long‐term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short‐term (<4 years) and mid‐term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5–10 years post‐treatment and describe 20‐year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB‐induced changes to forest structure and fuels increased the fire hazard 20 years post‐treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.
“…The pulsed increase in fine fuel loads from the MPB outbreak seems to be stabilizing (Figure 5A), but the coarse wood load was still increasing 10 years post‐outbreak and was much higher in 2022 in the control and Fire treatment relative to the Mech and Mech+Fire (Figure 5B). Studies of beetle‐killed ponderosa pines in the Southwest US and California have indicated low persistence of standing snags beyond 10 years (Chambers & Mast, 2014; Reed et al, 2023), which aligns with our findings of only approximately 21% of dead ponderosa pine still standing in 2020. Therefore, while the main fuel pulse from the outbreak is likely over, we expect that coarse fuel loading in the control and Fire treatment will continue increasing in the near future as many of these remaining snags fall.…”
Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high‐severity wildfires in forests with historically frequent, low‐severity fire regimes are increasingly common, but long‐term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20‐year responses to thinning and prescribed burning treatments commonly used in dry, low‐elevation forests of the western United States from a long‐term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short‐term (<4 years) and mid‐term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5–10 years post‐treatment and describe 20‐year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB‐induced changes to forest structure and fuels increased the fire hazard 20 years post‐treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.
“…The percentage of brown-stage dead trees against the number of all dead trees (Methods) indicates areas with recency of mortality, which is relevant for indicating a potential ongoing mortality event, such as a burgeoning bark-beetle outbreak. This indicator is also relevant to fire risk modelling and fire behaviour forecasting because brown-stage trees are more likely to increase the risk for crown fires as compared to grey-stage dead trees, due to the presence of flammable low-moisture content foliage 17 , 49 , 50 . Fig.…”
In recent years, large-scale tree mortality events linked to global change have occurred around the world. Current forest monitoring methods are crucial for identifying mortality hotspots, but systematic assessments of isolated or scattered dead trees over large areas are needed to reduce uncertainty on the actual extent of tree mortality. Here, we mapped individual dead trees in California using sub-meter resolution aerial photographs from 2020 and deep learning-based dead tree detection. We identified 91.4 million dead trees over 27.8 million hectares of vegetated areas (16.7-24.7% underestimation bias when compared to field data). Among these, a total of 19.5 million dead trees appeared isolated, and 60% of all dead trees occurred in small groups ( ≤ 3 dead trees within a 30 × 30 m grid), which is largely undetected by other state-level monitoring methods. The widespread mortality of individual trees impacts the carbon budget and sequestration capacity of California forests and can be considered a threat to forest health and a fuel source for future wildfires.
“…Compared with abiotic disturbances, biotic (e.g., insect pests or pathogens) disturbances are a longer process, and the impacts on forest ecosystems need to be validated through further studies [44]. Experimental plots and data on the intensity of different pest disturbances in the natural state are difficult to obtain; therefore, existing research on the disturbance of forest ecosystems with respect to biological factors (pests and pathogens) has focused on the aftermath of the large-scale forest mortality caused by pest disturbances [8,45,46], leading to a lack of studies on the disturbance of forests due to other biological factors and studies on forest disturbance processes in general. In this study, larch forests characterized by different levels of disturbance were selected as research objects, allowing for the further clarification of the soil changes occurring during forest death or decline.…”
Section: Soil Response To Bark Beetle Disturbancementioning
confidence: 99%
“…However, forests are currently being impacted by a wide range of disturbances globally, including changing wildfire regimes, biotic disturbances, and climate change [3][4][5], which have multiple impacts on biodiversity, ecosystem service function, and the carbon cycle [6]. For example, climate change-induced bark beetle outbreaks in countries such as Canada and the United States have led to widespread tree mortality [7,8]. Upon disturbance, the total amount of dead branches and Forests 2024, 15, 677 2 of 14 fallen logs on the forest floor increases, resulting in greater carbon inputs to the soil, thus affecting the nutrient and microbial composition of the soil [9].…”
Forests are affected by a wide range of disturbances globally, resulting in the decline or death of large areas of them. There is a lack of comparative studies on how soil properties change in forests that die under the influence of disturbances, especially considering different levels of disturbance. For this study, we took Larix olgensis—a major plantation forest species in northeast China—as the research object, one in which a large outbreak of bark beetle led to large-scale forest death, and set up fixed sample plots characterized by different disturbance intensities. We investigated the responses of soil physicochemical properties and microbial community compositions to different disturbance intensities through the determination of soil nutrient indices and high-throughput sequencing. The results show that there were significant differences (p < 0.05) in the effects of different disturbance intensities on soil physicochemical properties, where the soil moisture content, total nitrogen, total carbon, and total phosphorus in the control group were significantly higher than those in the disturbed groups. The soil pH was highest under low-intensity disturbance and the soil total potassium content was highest under high-intensity disturbance. At different disturbance intensities, the highest soil moisture content was found in the high-intensity group. Proteobacteria, Actinobacteria, Verrucomicrobia, Acidobacteria, Candidatus_Rokubacteria, Chloroflexi, Gemmatimonadetes, and Thaumarchaeota were the dominant populations with higher abundances; meanwhile, the relative abundance of Bacteroidetes, Tenericutes, and a tentatively unclassified fungus differed significantly (p < 0.05) across disturbance intensities. Among the dominant microbial populations, Acidobacteria showed a significant negative correlation with soil pH and a significant positive correlation with total potassium content, Thaumarchaeota showed significant positive correlations with soil moisture content and total nitrogen content, and Firmicutes and Gemmatimonadetes showed significant negative correlations with total carbon content in the soil. Furthermore, soil total nitrogen content was the key factor driving changes in microbial communities. The results of this study provide a scientific basis for the study of the long-term effects of tree mortality caused by insect pests on soil microbial communities and their response mechanisms, which is of great theoretical value for the establishment of scientific and effective methods for woodland restoration.
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