Asymmetric effects of hydroclimate extremes on eastern US tree growth: Implications on current demographic shifts and climate variability.

Forests around the world are experiencing changes due to climate variability and human land use. How these changes interact and influence the vulnerability of forests are not well understood. In the eastern United States, well-documented anthropogenic disturbances and land-use decisions, such as logging and fire suppression, have influenced forest species assemblages, leading to a demographic shift from forests dominated by xeric species to those dominated by mesic species. Contemporarily, the climate has changed and is expected to continue to warm and produce higher evaporative demand, imposing stronger drought stress on forest communities. Here, we use an extensive network of tree-ring records from common hardwood species across ~100 sites and ~1300 trees in the eastern United States to examine the magnitude of growth response to both wet and dry climate extremes. We find that growth reductions during drought exceed the positive growth response to pluvials. Mesic species such as Liriodendron tulipifera and Acer saccharum, which are becoming more dominant, are more sensitive to drought than more xeric species, such as oaks (Quercus) and hickory (Carya), especially at moderate and extreme drought intensities. Although more extreme droughts produce a larger annual growth reduction, mild droughts resulted in the largest cumulative growth decreases due to their higher frequency. When using global climate model projections, all scenarios show drought frequency increasing substantially (3-9 times more likely) by 2100. Thus, the ongoing demographic shift toward more mesic species in the eastern United States combined with drier conditions results in larger drought-induced growth declines, suggesting that drought will have an even larger impact on aboveground carbon uptake in the future in the eastern United States.

The xylem of anisohydric Quercus alba L. is more vulnerable to embolism than isohydric codominants.

The coordination of plant leaf water potential (ΨL ) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem more conservatively regulate ΨL than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh. and Liriodendron tulipifera L., by synthesizing 1600 ΨL observations, 122 xylem embolism curves and xylem anatomical measurements across 10 forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated ΨL less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose ΨL regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought-prone future.

PSInet: a new global water potential network.

Given the pressing challenges posed by climate change, it is crucial to develop a deeper understanding of the impacts of escalating drought and heat stress on terrestrial ecosystems and the vital services they offer. Soil and plant water potential play a pivotal role in governing the dynamics of water within ecosystems and exert direct control over plant function and mortality risk during periods of ecological stress. However, existing observations of water potential suffer from significant limitations, including their sporadic and discontinuous nature, inconsistent representation of relevant spatio-temporal scales and numerous methodological challenges. These limitations hinder the comprehensive and synthetic research needed to enhance our conceptual understanding and predictive models of plant function and survival under limited moisture availability. In this article, we present PSInet (PSI-for the Greek letter Ψ used to denote water potential), a novel collaborative network of researchers and data, designed to bridge the current critical information gap in water potential data. The primary objectives of PSInet are as follows. (i) Establishing the first openly accessible global database for time series of plant and soil water potential measurements, while providing important linkages with other relevant observation networks. (ii) Fostering an inclusive and diverse collaborative environment for all scientists studying water potential in various stages of their careers. (iii) Standardizing methodologies, processing and interpretation of water potential data through the engagement of a global community of scientists, facilitated by the dissemination of standardized protocols, best practices and early career training opportunities. (iv) Facilitating the use of the PSInet database for synthesizing knowledge and addressing prominent gaps in our understanding of plants' physiological responses to various environmental stressors. The PSInet initiative is integral to meeting the fundamental research challenge of discerning which plant species will thrive and which will be vulnerable in a world undergoing rapid warming and increasing aridification.

Eastern US deciduous tree species respond dissimilarly to declining soil moisture but similarly to rising evaporative demand.

Hydraulic stress in plants occurs under conditions of low water availability (soil moisture; θ) and/or high atmospheric demand for water (vapor pressure deficit; D). Different species are adapted to respond to hydraulic stress by functioning along a continuum where, on one hand, they close stomata to maintain a constant leaf water potential (ΨL) (isohydric species), and on the other hand, they allow ΨL to decline (anisohydric species). Differences in water-use along this continuum are most notable during hydrologic stress, often characterized by low θ and high D; however, θ and D are often, but not necessarily, coupled at time scales of weeks or longer, and uncertainty remains about the sensitivity of different water-use strategies to these variables. We quantified the effects of both θ and D on canopy conductance (Gc) among widely distributed canopy-dominant species along the isohydric-anisohydric spectrum growing along a hydroclimatological gradient. Tree-level Gc was estimated using hourly sap flow observations from three sites in the eastern United States: a mesic forest in western North Carolina and two xeric forests in southern Indiana and Missouri. Each site experienced at least 1 year of substantial drought conditions. Our results suggest that sensitivity of Gc to θ varies across sites and species, with Gc sensitivity being greater in dry than in wet sites, and greater for isohydric compared with anisohydric species. However, once θ limitations are accounted for, sensitivity of Gc to D remains relatively constant across sites and species. While D limitations to Gc were similar across sites and species, ranging from 16 to 34% reductions, θ limitations to Gc ranged from 0 to 40%. The similarity in species sensitivity to D is encouraging from a modeling perspective, though it implies that substantial reduction to Gc will be experienced by all species in a future characterized by higher D.