Stable isotopes of tree rings reveal seasonal-to-decadal patterns during the emergence of a megadrought in the Southwestern US

Valley fever under a changing climate in the United States

This review summarizes studies on the relationships between climate change and Valley Fever (VF), also termed Coccidioidomycosis, a potentially fatal upper-respiratory fungal infection caused by the pathogenic fungi, C. immitis or C. posadasii. The intensified onset of climate change has caused frequencies and possibly intensities of natural hazard events like dust storms and drought to increase, which has been correlated with greater prevalence of VF. These events, followed by changes in patterns of precipitation, not only pick up dust and spread it throughout the air, but also boost the growth and spread of Coccidioides. In California alone, cases of VF have increased fivefold from 2001 to 2021, and are expected to continue to increase. From 1999 to 2019, there was an average of 200 deaths per year caused by VF in the United States. The number of deaths caused by VF fluctuates year to year, but because more infections are predicted to occur due to a changing climate, deaths are expected to rise; thus, the rising prevalence of the disease is becoming a larger focus of the scientific community and poses an increased threat to public health. By reviewing recent and past studies on Coccidioidomycosis and its relationships with climate factors, we categorize future impacts of this disease on the United States, and highlight areas that need more study. Factors affecting the incidence of VF, such as modes of dispersal and the optimum environment for Coccidioides growth, that could potentially increase its prevalence as weather patterns change are discussed and how the endemic regions could be affected are assessed. In general, regions of the United States, including California and Arizona, where VF is endemic, are expanding and incidences of VF are increasing in those areas. The surrounding southern states, including Nevada, New Mexico, Utah, and Texas, are experiencing similar changes. In addition, the entire endemic region of the United States is predicted to spread northward as drought is prolonged and temperatures steadily increase. The findings from the keyword search from eight databases indicate that more studies on VF and its relation to dust and climate are needed especially for endemic states like Nevada that are currently not adequately studied. Overall, results of this survey summarize mechanisms and climate factors that might drive spread of VF and describes trends of incidence of VF in endemic states and predicted likely trends that might occur under a changing climate. Through reviewing recent and past studies of Coccidioidomycosis and its relationships with climate factors, future impacts of this disease have been categorized and speculated on effects it might have on the United States. Better understanding of how climate factors affect VF as well as identifying regions that require more research could inform both environmental managers and medical professionals with the resources needed to make more accurate predictions, design better mitigation strategies, send timely warnings, and protect public health. Shortened version This review explores how climate change affects Valley Fever (VF), a dangerous fungal infection caused by C. immitis or C. posadasii. Climate change has increased natural hazard events such as dust storms and droughts, which have caused the spread of VF. Cases of the disease have increased fivefold between 2001 and 2021 in California alone, and it poses an increasing threat to public health. The review summarizes mechanisms that drive the spread of VF and highlights trends in endemic states under a changing climate. It recommends more studies on VF and its relation to dust and climate, especially for states like Nevada. Identifying regions that require more research can help make more accurate predictions, design better mitigation strategies, send timely warnings, and protect public health.

Intra-annual tree-ring δ18O and δ13C reveal a trade-off between isotopic source and humidity in moist environments

Tree-ring intra-annual stable isotopes (δ13C and δ18O) are powerful tools for revealing plant ecophysiological responses to climatic extremes. We analyzed interannual and fine-scale intra-annual variability of tree-ring δ13C and δ18O in Chinese red pine (Pinus massoniana) from southeastern China to explore environmental drivers and potential trade-offs between the main physiological controls. We show that wet season relative humidity (May-October RH) drove interannual variability of δ18O and intra-annual variability of tree-ring δ18O. It also drove intra-annual variability of tree-ring δ13C, whereas interannual variability was mainly controlled by February-May temperature and September-October RH. Furthermore, intra-annual tree-ring δ18O variability was larger during wet years compared with dry years, whereas δ13C variability was lower during wet years compared with dry years. As a result of these differences in intra-annual variability amplitude, process-based models (we used the Roden model for δ18O and the Farquhar model for δ13C) captured the intra-annual δ18O pattern better in wet years compared with dry years, whereas intra-annual δ13C pattern was better simulated in dry years compared with wet years. This result suggests a potential asymmetric bias in process-based models in capturing the interplay of the different mechanistic processes (i.e., isotopic source and leaf-level enrichment) operating in dry versus wet years. We therefore propose an intra-annual conceptual model considering a dynamic trade-off between the isotopic source and leaf-level enrichment in different tree-ring parts to understand how climate and ecophysiological processes drive intra-annual tree-ring stable isotopic variability under humid climate conditions.

Assessing Role of Drought Indices in Anticipating Pine Decline in the Sierra Nevada, CA

Tree mortality in Sierra Nevada’s 2012–2015 drought was unexpectedly excessive: ~152 million trees died. The relative performance of five drought indices (DIs: SPEI, AI, PDSI, scPDSI, and PHDI) was evaluated in the complex, upland terrain which supports the forest and supplies 60% of Californian water use. We tested the relative performance of DIs parameterized with on-site and modeled (PRISM) meteorology using streamflow (linear correlation), and modeled forest stand NDVI and tree basal area increment (BAI) with current and lagged year DI. For BAI, additional co-variates that could modify tree response to the environment were included (crown vigor, point-in-time rate of bole growth, and tree to tree competition). On-site and modeled parameterizations of DIs were strongly correlated (0.9), but modeled parameterizations overestimated water availability. Current year DIs were well correlated (0.7–0.9) with streamflow, with physics-based DIs performing better than pedologically-based DIs. DIs were poorly correlated (0.2–0.3) to forest stand NDVI in these variable-density, pine-dominated forests. Current and prior year DIs significantly predicted BAI but accounted for little of the variation in the model. In this ecosystem where trees shift seasonally between near-surface to regolithic water, DIs were poorly suited for anticipating the observed tree decline.

A Tree Ring Proxy Evaluation of Declining Causes in Pinus sylvestris L. and Pinus nigra J.F. Arnold in Northeastern Romania

Drought-induced dieback has been extensively studied in various forests habitats. We used a retrospective tree ring width (TRW), basal area increment (BAI), oxygen isotope ratios in tree ring cellulose (δ18OTR) and carbon isotope ratios in tree ring cellulose (δ13CTR) to assess causes in declining Pinus sylvestris L. and Pinus nigra J.F. Arnold. The climate data analysis indicates a significant increased trend occurred after 1980 in minimum, mean and maximum temperature and a reduced amount of precipitation compared to the 1920–1980-time scale. According to the Palmer Drought Severity Index, we found two extreme drought years (1946 and 2000) and three years with severe drought (1990, 2003 and 2012). One-way ANOVA indicated no significant difference between P. nigra and P sylvestris tree ring width, basal area increment, but a considerable difference between δ13CTR and δ18OTR. Basal area increment evaluated the climate-growth relationship most accurately, comparing to δ18OTR and δ13CTR, which explained the influences of environmental factors in tree rings formation. The δ13CTR was mainly negatively correlated with high temperatures from April-August current growing seasons. The negative correlation between δ13CTR and NDVI indices (June, August) shows a decreased carbon uptake induced by drought from summer to early autumn. The low δ18OTR signal was associated with a complex of factors, including the strong influence of heavy precipitation occurring in the growing season and a weak reaction of declined trees to resources. Species-specific responses to drought in 1990, 2003 and 2012 indicated P. sylvestris as more sensitive to drought whit higher demand for water supply in the optimal compared with P. nigra. Weak and unstable correlations in time with increasing/decreasing values in drought periods were obtained more accurately using δ18OTR compared to δ13CTR. The species-specific resilience response to drought years showed a weak resilience and resistance in P. sylvestris occurred more evident after the 2012 event compared to less sensitive P. nigra trees. Decision-makers can use presented results to reinforce specific management plans capable of protecting and changing local compositions where is the case with species more resistant to drouth.

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.

Rapid increases in shrubland and forest intrinsic water-use efficiency during an ongoing megadrought

Globally, intrinsic water-use efficiency (iWUE) has risen dramatically over the past century in concert with increases in atmospheric CO₂ concentration. This increase could be further accelerated by long-term drought events, such as the ongoing multidecadal "megadrought" in the American Southwest. However, direct measurements of iWUE in this region are rare and largely constrained to trees, which may bias estimates of iWUE trends toward more mesic, high elevation areas and neglect the responses of other key plant functional types such as shrubs that are dominant across much of the region. Here, we found evidence that iWUE is increasing in the Southwest at one of the fastest rates documented due to the recent drying trend. These increases were particularly large across three common shrub species, which had a greater iWUE sensitivity to aridity than Pinus ponderosa, a common tree species in the western United States. The sensitivity of both shrub and tree iWUE to variability in atmospheric aridity exceeded their sensitivity to increasing atmospheric [CO₂]. The shift to more water-efficient vegetation would be, all else being equal, a net positive for plant health. However, ongoing trends toward lower plant density, diminished growth, and increasing vegetation mortality across the Southwest indicate that this increase in iWUE is unlikely to offset the negative impacts of aridification.

Protein expression plasticity contributes to heat and drought tolerance of date palm

Climate change is increasing the frequency and intensity of warming and drought periods around the globe, currently representing a threat to many plant species. Understanding the resistance and resilience of plants to climate change is, therefore, urgently needed. As date palm (Phoenix dactylifera) evolved adaptation mechanisms to a xeric environment and can tolerate large diurnal and seasonal temperature fluctuations, we studied the protein expression changes in leaves, volatile organic compound emissions, and photosynthesis in response to variable growth temperatures and soil water deprivation. Plants were grown under controlled environmental conditions of simulated Saudi Arabian summer and winter climates challenged with drought stress. We show that date palm is able to counteract the harsh conditions of the Arabian Peninsula by adjusting the abundances of proteins related to the photosynthetic machinery, abiotic stress and secondary metabolism. Under summer climate and water deprivation, these adjustments included efficient protein expression response mediated by heat shock proteins and the antioxidant system to counteract reactive oxygen species formation. Proteins related to secondary metabolism were downregulated, except for the P. dactylifera isoprene synthase (PdIspS), which was strongly upregulated in response to summer climate and drought. This study reports, for the first time, the identification and functional characterization of the gene encoding for PdIspS, allowing future analysis of isoprene functions in date palm under extreme environments. Overall, the current study shows that reprogramming of the leaf protein profiles confers the date palm heat- and drought tolerance. We conclude that the protein plasticity of date palm is an important mechanism of molecular adaptation to environmental fluctuations.