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1. Climate change determines the sign of productivity trends in US forests

   https://doi.org/10.1073/pnas.2311132121  

   Hogan, J Aaron; Domke, Grant M; Zhu, Kai; Johnson, Daniel J; Lichstein, Jeremy W (January 2024, Proceedings of the National Academy of Sciences)

   Forests are integral to the global land carbon sink, which has sequestered ~30% of anthropogenic carbon emissions over recent decades. The persistence of this sink depends on the balance of positive drivers that increase ecosystem carbon storage—e.g., CO₂fertilization—and negative drivers that decrease it—e.g., intensifying disturbances. The net response of forest productivity to these drivers is uncertain due to the challenge of separating their effects from background disturbance–regrowth dynamics. We fit non-linear models to US forest inventory data (113,806 plot remeasurements in non-plantation forests from ~1999 to 2020) to quantify productivity trends while accounting for stand age, tree mortality, and harvest. Productivity trends were generally positive in the eastern United States, where climate change has been mild, and negative in the western United States, where climate change has been more severe. Productivity declines in the western United States cannot be explained by increased mortality or harvest; these declines likely reflect adverse climate-change impacts on tree growth. In the eastern United States, where data were available to partition biomass change into age-dependent and age-independent components, forest maturation and increasing productivity (likely due, at least in part, to CO₂fertilization) contributed roughly equally to biomass carbon sinks. Thus, adverse effects of climate change appear to overwhelm any positive drivers in the water-limited forests of the western United States, whereas forest maturation and positive responses to age-independent drivers contribute to eastern US carbon sinks. The future land carbon balance of forests will likely depend on the geographic extent of drought and heat stress. 

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2. Local groundwater decline exacerbates response of dryland riparian woodlands to climatic drought

   https://doi.org/10.1111/gcb.16376  

   Williams, Jared; Stella, John C.; Voelker, Steven L.; Lambert, Adam M.; Pelletier, Lissa M.; Drake, John E.; Friedman, Jonathan M.; Roberts, Dar A.; Singer, Michael Bliss (January 2022, Global Change Biology)

   Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro-isotopic analysis of radial growth and seasonal (semi-annual) carbon isotope discrimination (Δ13C) to investigate the response of riparian cottonwood stands to the unprecedented California-wide drought from 2012 to 2019, along the largest remaining free-flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ13C) during peak drought years. However, patterns of radial growth and Δ13C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ13C for individual trees, and higher inter-correlation of Δ13C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year−1 beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded 3 m. These findings indicate that drought-induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures. 

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3. No carbon storage in growth-limited trees in a semi-arid woodland

   https://doi.org/10.1038/s41467-023-37577-8  

   Thompson, R. Alexander; Adams, Henry D.; Breshears, David D.; Collins, Adam D.; Dickman, L. Turin; Grossiord, Charlotte; Manrique‐Alba, Àngela; Peltier, Drew M.; Ryan, Michael G.; Trowbridge, Amy M.; et al (December 2023, Nature Communications)

   Abstract Plant survival depends on a balance between carbon supply and demand. When carbon supply becomes limited, plants buffer demand by using stored carbohydrates (sugar and starch). During drought, NSCs (non-structural carbohydrates) may accumulate if growth stops before photosynthesis. This expectation is pervasive, yet few studies have combined simultaneous measurements of drought, photosynthesis, growth, and carbon storage to test this. Using a field experiment with mature trees in a semi-arid woodland, we show that growth and photosynthesis slow in parallel as$${\psi }_{{pd}}$$ ${\psi }_{pd}$declines, preventing carbon storage in two species of conifer (J. monospermaandP. edulis). During experimental drought, growth and photosynthesis were frequently co-limited. Our results point to an alternative perspective on how plants use carbon that views growth and photosynthesis as independent processes both regulated by water availability. 

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4. Drought, Psychosocial Stress, and Ecogeographical Patterning: Tibial growth and body shape in Samburu (Kenyan) pastoralist children

   https://doi.org/10.1002/ajpa.24529  

   Straight, B. (August 2022, American journal of physical anthropology)

   Objectives: This study of Samburu pastoralists (Kenya) employs a same-sex sibling design to test the hypothesis that exposure in utero to severe drought and maternal psychosocial stress negatively influence children’s growth and adiposity. As a comparison, we also hypothesized that regional climate contrasts would influence children’s growth and adiposity based on ecogeographical patterning. Materials and Methods: Anthropometric measurements were taken on Samburu children ages 1.8 – 9.6 years exposed to severe drought in utero and younger same-sex siblings (drought-exposed, n = 104; unexposed, n = 109) in two regions (highland, n = 128; lowland, n = 85). Mothers were interviewed to assess lifetime and pregnancy-timed stress. Results: Drought exposure associated to lower weight-for-age and higher adiposity. Drought did not associate to tibial growth on its own but the interaction between drought and region negatively associated to tibial growth in girls. Also, drought exposure and historically low rainfall associated to tibial growth in sensitivity models. A hotter climate positively associated to adiposity and tibial growth. Culturally specific stressors (being forced to work too hard, being denied food by male kin) associated to stature and tibial growth for age. Significant covariates for child outcomes included lifetime reported trauma, wife status, and livestock. Discussion: Children exposed in utero to severe drought, a hotter climate, and psychosocial stresss exhibited growth differences in our study. Our results demonstrate that climate change may deepen adverse health outcomes in populations already psychosocially and nutritionally stressed. Our results also highlight the value of ethnography to identifying meaningful stressors. 

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5. Rodent declines track regional climate variability in North American drylands

   https://doi.org/10.6073/pasta/f183393c8060f407f32d323491247f35  

   Rudgers, Jennifer A; Schooley, Robert L; Ernest, Morgan; Stapp, Paul (January 2021, Environmental Data Initiative)

   Regional long-term monitoring can enhance the detection of biodiversity declines associated with climate change, improving future projections by reducing reliance on space-for-time substitution and increasing scalability. Rodents are diverse and important consumers in drylands, which cover ~45% of Earth’s land surface and face increasingly drier and more variable climates. Here, we analyzed abundance data for 22 rodent species across grassland, shrubland, ecotone, and woodland habitats in the southwestern USA. We captured two time series: 1995-2006 and 2004-2013 that coincide with phases of the Pacific Decadal Oscillation (PDO), which influences drought in southwestern North America. Regionally, rodent species diversity declined 20-35%, with greater losses during the later time period. Abundance also declined regionally, but only during 2004-2013, with losses of ~5% of animals captured. During the first time series (PDO wet phase), plant productivity outranked climate variables as the best regional predictor of rodent abundance for 70% of taxa, whereas during the second period (dry phase), climate best explained rodent abundance for 60% of taxa. Temporal dynamics in rodent diversity and abundance differed spatially among habitats and sites, with the largest declines in woodlands and shrublands of central New Mexico and Colorado. Both habitat type and phase of the PDO modulated which species were winners or losers under increasing drought and amplified interannual variability in drought. Fewer taxa were significant winners (18%) than losers (30%) under drought, but the identities of winners and losers differed among habitats for 70% of taxa. Our results suggest that the sensitivities of rodent species to climate contributed to regional declines in diversity and abundance during 1995 - 2013. Whether these changes portend future declines in drought-sensitive consumers in the southwestern USA will depend on the climate during the next major phase of the PDO. 

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