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1. Changing climate response of Northeast Ohio white oaks, USA: Is it tree age or site age?

   Wiles, G; Wiesenberg, N; Pollock, M; Denes, C; Cooper, T; Smith, D; Wiles, M (February 2025, Dendrochronologia)

   White oak, a keystone species of the broadleaf forests of the North American Midwest, has a significant role in providing ecosystems services in a region experiencing warming and increasingly pluvial conditions. A one- hundred-year-old white oak stand in an arboretum, along with two second growth (~200-year-old) stands from Northeast Ohio have consistently responded positively to summer (June-July) precipitation over the past century, whereas four nearby old growth sites (>300 years old) have lost their moisture sensitivity since about the mid 1970s. This “fading drought signal,” which has been previously reported, appears to be more a result of the legacy of land use at the individual sites rather than tree age. The younger oak stands and their relative sustained drought sensitivity is also related to their history of recently attaining the canopy and similar responses associated with intervals of selective logging. All sites are strongly, negatively correlated with summer (June- July) maximum monthly temperatures and in general the maximum temperatures are negatively correlated with precipitation in those months. Future warming in the Midwest is projected to see increases in spring precipitation and likely decreases in late summer precipitation linked to a northward migration of the North American Westerly Jet. This projected decrease in summer precipitation coupled with an increase in maximum and min- imum summer temperatures in the coming decades would increase the moisture stress on these trees. Our ex- amination of these varying climate responses with respect to site characteristics and forest age can help future assessments of tree health and the forest’s ability to sequester carbon, as well as facilitate efforts to reconstruct climate by using a range of tree sites for intervals when sensitivity in old growth sites is lost. 

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2. Increasing plant water stress and decreasing summer streamflow in response to a warmer and wetter climate in seasonally snow‐covered forests

   https://doi.org/10.1002/eco.2256  

   Christensen, Lindsey; Adams, Hallie R.; Tai, Xiaonan; Barnard, Holly R.; Brooks, Paul D. (October 2020, Ecohydrology)

   Abstract Warming temperatures and precipitation changes are expected to alter water availability and increase drought stress in western North America, yet uncertainties remain in how concurrent changes in the amount and seasonality of precipitation interact with warming to affect hydrologic partitioning. We combined over a century of streamflow (Q) and climate observations with two decades of tree growth data and remotely sensed vegetation activity to quantify how temperature and precipitation interact to control hydrologic partitioning in the Front Range of Colorado, Boulder Creek Watershed. Temperature and precipitation significantly increased over the last five decades, with precipitation increasing primarily in winter (11.2 mm decade⁻¹) and temperature increasing primarily during the growing season (0.12°C decade⁻¹). In response to wetter winters and warmer summers, streamflow decreased −9.8 mm decade⁻¹, with largest declines occurring during summer and autumn baseflow (−8.4 mm decade⁻¹). Spring warming was associated with increases in episodic, short spring melt events, earlier and slower snowmelt and an increase in fraction of precipitation available to plants (catchment wetting or W). Warming during the growing season resulted in an increase in the fraction of W lost as evapotranspiration (ET), earlier and lower peaks in remotely sensed normalized difference vegetation index (NDVI) and lower tree ring width index (RWI). These analyses highlight that vegetation is becoming increasingly water limited even as increases in precipitation and slower melt increase plant water availability. Further, catchment‐derived metrics like the Horton Index (ET/W) provide insight in to how simultaneous changes in temperature, precipitation and melt impact vegetation across complex watersheds. 

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3. Soil moisture and micrometeorological differences across reference and thinned stands during extremes of precipitation, southern Cascade Range

   https://doi.org/10.3389/ffgc.2022.898998  

   Hardage, Kyle; Wheelock, Shawn J.; Gaffney, Rowan; O’Halloran, Theresa; Serpa, Benjamin; Grant, Gordon; Coppoletta, Michelle; Csank, Adam; Tague, Christina; Staudacher, Matthew; et al (October 2022, Frontiers in Forests and Global Change)

   Modern forest management generally relies on thinning treatments to reduce fuels and mitigate the threat of catastrophic wildfire. They have also been proposed as a tool to augment downstream flows by reducing evapotranspiration. Warming climates are causing many forests to transition from snow-dominated to rain-dominated precipitation regimes—in which water stores are depleted earlier in the summer. However, there are relatively few studies of these systems that directly measure the hydrologic impacts of such treatments during and following snow-free winters. This work compares the below-canopy meteorological and subsurface hydrologic differences between two thinning prescriptions and an unaltered Control during periods of extreme drought and near-record precipitation (with little snow). The field site was within a coniferous forest in the rain-snow transition zone of the southern Cascades, near the Sierra Nevada Range of California. Both thinning-prescriptions had a modest and predictable impact on below-canopy meteorology, which included their causing lower nighttime minimum temperatures in the critical summer months and higher wind speeds. Relative to the Control, both treatments affected soil moisture storage by delaying its annual decline and increasing its minimum value by the end of the season. The onset of soil moisture depletion was strongly tied to the magnitude of winter precipitation. In dry years, it began much earlier within the dense Control stand than in the treated ones, and, without snow, soil moisture was not replenished in the late spring. During high precipitation years, the storage capacity was topped off for all three stands, which resulted in similar timing of moisture decline across them, later in the season. The two thinning prescriptions increased stores through the height of summer (in wet and drought years). Finally, the basal area increment (BAI) of the remaining trees rose in both, suggesting they used the excess moisture to support rapid growth. 

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

   https://doi.org/10.1111/gcb.15973  

   Keen, Rachel_M; Voelker, Steven_L; Wang, S‐Y_Simon; Bentz, Barbara_J; Goulden, Michael_L; Dangerfield, Cody_R; Reed, Charlotte_C; Hood, Sharon_M; Csank, Adam_Z; Dawson, Todd_E; et al (November 2021, Global Change Biology)

   Abstract 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 (∆¹³C), 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 ∆¹³C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆¹³C 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. 

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5. Tree-Ring Evidence of Forest Management Moderating Drought Responses: Implications for Dry, Coniferous Forests in the Southwestern United States

   https://doi.org/https://doi.org/10.3389/ffgc.2020.00041  

   van Mantgem, P. (April 2020, Frontiers in forests and global change)

   null (Ed.)

   Drought, coupled with rising temperatures, is an emerging threat to many forest types across the globe. At least to a degree, we expect management actions that reduce competition (e.g., thinning, prescribed fire, or both) to improve growth of residual trees during drought. The influences of management actions and drought on individual tree growth may be measured with high precision using tree-rings. Here, we summarize tree-ring-based assessments of the effectiveness of thinning and prescribed fire as drought adaptation tools, with special consideration for how these findings might apply to dry coniferous forests in the southwestern United States. The existing literature suggests that thinning treatments generally improve individual tree growth responses to drought, though the literature specific to southwestern coniferous forests is sparse. Assessments from studies beyond the southwestern United States indicate treatment effectiveness varies by thinning intensity, timing of the drought relative to treatments, and individualistic species responses. Several large-scale studies appear to conflict on specifics of how site aridity influences sensitivity to drought following thinning. Prescribed fire effects in the absence of thinning has received much less attention in terms of subsequent drought response. There are limitations for using tree-ring data to estimate drought responses (e.g., difficulties scaling up observations to stand- and landscape-levels). However, tree-rings describe an important dimension of drought effects for individual trees, and when coupled with additional information, such as stable isotopes, aid our understanding of key physiological mechanisms that underlie forest drought response. 

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