Drought Impacts and Compounding Mortality on Forest Trees in the Southern Sierra Nevada

Exploring the ecosystem services delivered by urban woody plants at Çankırı Castle

Urban green spaces are becoming more widely acknowledged for their capacity to offer a variety of ecosystem services, such as supporting, regulating, and cultural roles, all of which enhance the general sustainability of urban settings. Trees and vegetation have been added to many planned cities to improve air quality and slow down climate change, but little is known about the traits and patterns of urban nature and ecosystem services in smaller cities. Smaller cities like Çankırı, particularly the historically and environmentally significant Çankırı Castle, remain understudied despite their unique ecological and climatic conditions; its selection as a case study highlights the crucial role of urban vegetation in mitigating climate challenges and enhancing sustainability in semi-arid environments. The purpose of this study was to assess the woody plant species in Çankırı Castle, to identify 10 tree species with the highest importance values, to estimate the removal of pollutants (CO, O3, NO2, SO2, PM10, and PM2.5), to evaluate carbon storage, carbon sequestration, oxygen production, and avoided runoff using the i-Tree Eco model, and to reveal the relationships of diameter at breast height with these ecosystem services. The study showed that O₃ ranked first for pollution removal (31.2738 kg/yr) and second for economic value ($18.0735/yr), while PM₁₀ had the highest economic value for removal ($318.0080/yr) but ranked second in quantity (11.1354 kg/yr). The total carbon storage, sequestration, oxygen production, and avoided runoff of the 10 highest-ranking species were 71,300 kg/yr, 3,040 kg/yr, 8,048 kg/yr, and 32,370 mm/yr, respectively. This study highlights the key role of urban green spaces in providing essential ecosystem services, with species like Pinus sylvestris, Fraxinus excelsior, and Cedrus libani significantly contributing to carbon storage, sequestration, oxygen production, and runoff mitigation. However, species such as Pinus nigra and Prunus mahaleb, despite being abundant, have limited ecological contributions, emphasizing the importance of strategic species selection. The study also reveals that while diameter at breast height is crucial for carbon sequestration, factors like tree density and leaf area play significant roles in mitigating runoff, underscoring the need for a comprehensive approach to urban forestry.

Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event

Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought-related mortality derived from measurements of tree-ring growth (ring width index; RWI) and carbon isotope discrimination (∆¹³ C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012-2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆¹³ C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆¹³ C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine-dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management-particularly in drier regions-may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.

Combined drought and bark beetle attacks deplete non-structural carbohydrates and promote death of mature pine trees

How carbohydrate reserves in conifers respond to drought and bark beetle attacks are poorly understood. We investigated changes in carbohydrate reserves and carbon-dependent diterpene defences in ponderosa pine trees that were experimentally subjected to two levels of drought stress (via root trenching) and two types of biotic challenge treatments (pheromone-induced bark beetle attacks or inoculations with crushed beetles that include beetle-associated fungi) for two consecutive years. Our results showed that trenching did not influence carbohydrates, whereas both biotic challenges reduced amounts of starch and sugars of trees. However, only the combined trenched-bark beetle attacked trees depleted carbohydrates and died during the first year of attacks. While live trees contained higher carbohydrates than dying trees, amounts of constitutive and induced diterpenes produced did not vary between live and beetle-attacked dying trees, respectively. Based on these results we propose that reallocation of carbohydrates to diterpenes during the early stages of beetle attacks is limited in drought-stricken trees, and that the combination of biotic and abiotic stress leads to tree death. The process of tree death is subsequently aggravated by beetle girdling of phloem, occlusion of vascular tissue by bark beetle-vectored fungi, and potential exploitation of host carbohydrates by bark beetle symbionts as nutrients.

Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality

The recent Californian hot drought (2012-2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350 km of latitude and 1000 m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality.

Few generalizable patterns of tree-level mortality during extreme drought and concurrent bark beetle outbreaks

Tree mortality associated with drought and concurrent bark beetle outbreaks is expected to increase with further climate change. When these two types of disturbance occur in concert it complicates our ability to accurately predict future forest mortality. The recent extreme California USA drought and bark beetle outbreaks resulted in extensive tree mortality and provides a unique opportunity to examine questions of why some trees die while others survive these co-occurring disturbances. We use plot-level data combined with a three-proxy tree-level approach using radial growth, carbon isotopes, and resin duct metrics to evaluate 1) whether variability in stand structure, tree growth or size, carbon isotope discrimination, or defenses precede mortality, 2) how relationships between these proxies differ for surviving and now-dead trees, and 3) whether generalizable risk factors for tree mortality exist across pinyon pine (Pinus monophylla), ponderosa pine (P. ponderosa), white fir (Abies concolor), and incense cedar (Calocedrus decurrens) affected by the combination of drought and beetle outbreaks. We find that risk factors associated with mortality differ between species, and that few generalizable patterns exist when bark beetle outbreaks occur in concert with a particularly long, hot drought. We see evidence that both long-term differences in physiology and shorter-term beetle-related selection and variability in defenses influence mortality susceptibility for ponderosa pine, whereas beetle dynamics may play a more prominent role in mortality patterns for white fir and pinyon pine. In contrast, incense cedar mortality appears to be attributable to long-term effects of growth suppression. Risk factors that predispose some trees to drought and beetle-related mortality likely reflect species-specific strategies for dealing with these particular disturbance types. The combined influence of beetles and drought necessitates the consideration of multiple, species-specific risk factors to more accurately model forest mortality in the face of similar extreme events more likely under future climates.

Changing climate reallocates the carbon debt of frequent-fire forests

Ongoing climate change will alter the carbon carrying capacity of forests as they adjust to climatic extremes and changing disturbance regimes. In frequent-fire forests, increasing drought frequency and severity are already causing widespread tree mortality events, which can exacerbate the carbon debt that has developed as a result of fire exclusion. Forest management techniques that reduce tree density and surface fuels decrease the risk of high-severity wildfire and may also limit drought-induced mortality by reducing competition. We used a long-term thinning and burning experiment in a mixed-conifer forest to investigate the effects of the 2012-2015 California drought on forest carbon dynamics in each treatment, including the carbon emissions from a second-entry prescribed fire that followed the drought. We assessed differences in carbon stability and tree survival across treatments, expecting that both carbon stability and survival probability would increase with increasing treatment intensity (decreasing basal area). Additionally, we analyzed the effects of drought- mortality on second-entry burn emissions and compared emissions for the first- and second-entry burns. We found a non-linear relationship between treatment intensity and carbon stability, which was in part driven by varying relationships between individual tree growing space and survival across treatments. Drought mortality increased dead tree and surface fuel carbon in all treatments, which contributed to higher second-entry burn emissions for two of the three burn treatments when compared to the first burn. Our findings suggest that restoration treatments will not serve as a panacea for ongoing climate change and that the carbon debt of these forests will become increasingly unstable as the carbon carrying capacity adjusts to severe drought events. Managing the carbon debt with prescribed fire will help reduce the risk of additional mortality from wildfire, but at an increasing carbon cost for forest management.

Stronger influence of growth rate than severity of drought stress on mortality of large ponderosa pines during the 2012-2015 California drought

Forests in the western United States are being subject to more frequent and severe drought events as the climate warms. The 2012-2015 California drought is a recent example, whereby drought stress was exacerbated by a landscape-scale outbreak of western pine beetle (Dendroctonus brevicomis) and resulted in widespread mortality of dominant canopy species including ponderosa pine (Pinus ponderosa). In this study, we compared pairs of large surviving and beetle-killed ponderosa pines following the California drought in the southern Sierra Nevadas to evaluate physiological characteristics related to survival. Inter-annual growth rates and tree-ring stable isotopes (∆¹³C and δ¹⁸O) were utilized to compare severity of drought stress and climate sensitivity in ponderosa pines that survived and those that were killed by western pine beetle. Compared to beetle-killed trees, surviving trees had higher growth rates and grew in plots with lower ponderosa pine basal area. However, there were no detectable differences in tree-ring ∆¹³C, δ¹⁸O, or stable isotope sensitivity to drought-related meteorological variables. These results indicate that differences in severity of drought stress had little influence on local, inter-tree differences in growth rate and survival of large ponderosa pines during this drought event. Many previous studies have shown that large trees are more likely to be attacked and killed by bark beetles compared to small trees. Our results further suggest that among large ponderosa pines, those that were more resistant to drought stress and bark beetle attacks were in the upper echelon of growth rates among trees within a stand and across the landscape.

Bark Beetle Epidemics, Life Satisfaction, and Economic Well-Being

Evidence of increased biotic disturbances in forests due to climate change is accumulating, necessitating the development of new approaches for understanding the impacts of natural disturbances on human well-being. The recent Mountain Pine Beetle (MPB) outbreak in the western United States, which was historically unprecedented in scale, provides an opportunity for testing the adequacy of the life satisfaction approach (LSA) to estimate the impact of large-scale forest mortality on subjective well-being. Prior research in this region used the hedonic method (HM) to estimate the economic impacts of the MPB outbreak, and results are used here to evaluate the reasonableness of economic estimates based upon the LSA. While economic estimates based upon the LSA model do not appear to be unreasonable, several limitations in using the LSA for nonmarket valuations are discussed. New avenues for research that link the LSA with stated preference methods are discussed that appear likely to address major concerns with standard LSA models as used in nonmarket valuation.