Sandra Bucci

Water potential gradients drive water flow within and between soils and plants, and the internal plant water potential controls a wide range of physiological processes including photosynthesis, growth, and mortality. Notwithstanding this clear relevance for many critical aspects of ecosystem function, water potential data have historically been relatively inaccessible and unnetworked. The absence of a centralized repository for plant water potential time series limits our ability to integrate a wealth of ecophysiological information from other networks and from remote sensing. Closing this gap is necessary to address unresolved questions about plant responses to drought and heat stress, and to make confident predictions about plant and ecosystem function in a warming world. Here, we introduce the PSInet database -- a global collection of plant water potential time series from 285 datasets representing 523 species. We present the workflow that guided database development and evaluate its key features. Through a series of preliminary analyses, we then highlight the potential of the PSInet database for applications including: a) advancing plant water use strategy frameworks; b) disentangling the impacts of soil versus atmospheric drought stress; c) assessing the long-held assumption of pre-dawn equilibration of ecosystem water potential; d) understanding the risk of drought-driven mortality; and e) benchmarking remote-sensing data products and land-surface models.

Temperate ring‐porous tree species are more vulnerable to freezing‐induced embolism than their diffuse‐porous counterparts due to their large earlywood vessels. Although ring‐porous species can restore hydraulic function by forming earlywood vessels in early spring, they may face high risk of embolism during spring frosts. Here, we tested the hypothesis that ring‐porous temperate trees employ a strategy of enhanced freezing avoidance to protect their vulnerable earlywood vessels from the risk of freezing‐induced embolism. We quantified the degree of embolism in overwintering stems, ice nucleation temperature (INT _xylem ), water content (WC _xylem ), and non‐structural carbohydrate (NSC) concentrations in stem xylems of 20 temperate tree species (nine diffuse‐porous and 11 ring‐porous) grown in a common garden. Consistent with our hypothesis, ring‐porous species suffered severe embolism over the winter and exhibited stronger freezing avoidance during spring. While diffuse‐porous species rely on smaller vessels to resist freezing‐induced embolism, ring‐porous species showed stronger ability of freezing avoidance by lowering INT _xylem via reduced WC _xylem and higher NSC concentrations. This strategy likely protects their vulnerable, newly developed earlywood vessels from freezing‐induced hydraulic dysfunction. These findings reveal distinct strategies between temperate diffuse‐porous and ring‐porous species for coping with spring frost stress, providing insights into forest responses to climate change.

Vegetation activity in arid ecosystems is traditionally interpreted as a direct response to precipitation pulses. However, the extent to which vegetation dynamics are regulated by antecedent soil water storage and its interaction with atmospheric demand remains poorly understood. Here, we combined six years of near-surface GCC from PhenoCam, field NDVI, and satellite-derived vegetation indices to examine the ecohydrological controls of vegetation activity and phenological dynamics in a Patagonian steppe in southern Argentina. Vegetation activity and phenological metrics describing both timing and magnitude were more strongly associated with antecedent soil water storage (SWS) integrated across the 2-m soil profile than with precipitation alone. This suggests that vegetation responses are better explained by hydrological memory than by recent rainfall pulses. However, this control changed with atmospheric demand: under low vapor pressure deficit (VPD) vegetation activity was mainly associated with shallow SWS, whereas under high VPD, deeper SWS became the primary driver. Across phenological metrics, SWS showed a pervasive influence, while atmospheric variables exerted more selective but biologically critical effects. In addition, integrated canopy greenness was positively correlated with grass aboveground net primary productivity, supporting its use as a proxy for seasonal productivity. Our results demonstrate that vegetation activity and phenology in the Patagonian steppe are governed by the interaction between antecedent SWS and atmospheric demand. By identifying profile-scale water availability as a key ecohydrological control, this study highlights the need to incorporate both hydrological memory and atmospheric demand into monitoring and modeling approaches under increasing aridity.

Summary: A framework of traits and strategies for drought adaptation is critical for understanding the effects of climate change on natural and cultivated plant communities. The ‘growth potential – stress survival’ trade‐off, a key concept in ecology, underpins plant ecological strategies but lacks a time dimension that is crucial to plant responses to drought. We built a three‐phase physiological model incorporating both plant traits and time as a gradient of decreasing water availability, which allowed the identification of traits involved in maximizing growth potential (Phase I), growth/turgor maintenance during drought (Phase II – drought resistance), or survival after growth cessation (Phase III – drought survival). Modelling plant water use for annuals, perennials, resurrection, and succulent species revealed a trade‐off between water use in Phases I–II (water acquisition associated with tissue dehydration avoidance) and Phase III duration (water conservation associated with water storage capacity and/or tissue dehydration/desiccation tolerance). This trade‐off underpins a novel framework of plant water use economics among and within species. As growth potential and growth/turgor maintenance, that is, drought resistance, trade‐off with drought survival duration, a time‐informed framework considering the balance between productivity and drought resilience is required in plant growth models and in breeding efforts for plant drought adaptation.

Under the scenario of global warming, the response of carbon (C) fluxes of arid and semi-arid ecosystems, is still not well understood. A field warming experiment using open top chambers (OTCs) was conducted in a shrub-grass patagonian steppe to evaluate the effects on bare soil respiration (R_soil), and ecosystem respiration (R_eco), gross primary productivity (GPP) and net C exchange (NEE) during the growing season. Air (T_air) and soil (T_soil) temperature, and soil available phosphorus changed significantly while there were no changes in soil moisture, soil organic carbon, total soil nitrogen and root biomass, after one-year of treatment. Inside OTCs R_soil increased by 61%, with larger changes found during the daytime than during nighttime and tended to increase the temperature sensitivity (Q₁₀) from 1.49 to 1.58. Enhanced daytime NEE was observed inside OTCs mainly in the shrub Senecio filaginoides patches where C sequestration increased by 37% and GPP by 35% without effects on R_eco. In the grass Poa ligularis patches changes in C fluxes were not significant, but lower decrease in GPP than in R_eco tended to increase C uptake inside OTCs. This study reveals that future climate scenarios will lead to an enhanced C sequestration of vegetated patches but also to a higher bare soil CO₂ emission which may turn this ecosystem in a substantial C source if bare soil fraction increases due to global changes.

Changes in water and nitrogen availability can affect the structure and function of arid ecosystems. How these resources affect aboveground primary productivity (ANPP) remains far from clear. We examined the N and water limitation of ANPP from the species to the community level and the response of ANPP to annual precipitation in a Patagonian steppe. We conducted a 7‐year field experiment with water addition (+W), nitrogen addition (+N) and +NW. Destructive methods for grasses and allometric relationships for shrubs were used to assess ANPP and vegetation indices (NDVI and MSAVI2) to estimate community ANPP. An increase in ANPP of one grass species (Papposstipa humilis) and a decrease of the grass Poa ligularis under +N were observed. Some shrub species exhibited mortality under nitrogen addition. Nitrogen exerted a positive effect on grass ANPP and amplified the sensitivity of grass ANPP to annual precipitation. However, +N had not effects on the shrub ANPP and shrub ANPP‐precipitation relationship. Water addition by itself had no effect on ANPP for either shrubs or grasses. However, shrubs responded positively to an unusually wet year regardless of treatment and were also more sensitive to changes in annual precipitation than grasses. Total ANPP increased significantly in +N relative to the C and +W but without changes in the sensitivity to annual precipitation. The results suggest that the responses of grasses and shrubs to water inputs are driven by soil moisture redistribution and rooting depth and that grass and community ANPP are more limited by N than by water.

Changes in water and nitrogen availability can affect the structure and function of arid ecosystems. How these resources affect aboveground primary productivity (ANPP) remains far from clear. We examined the N and water limitation of ANPP from the species to the community level and the response of ANPP to annual precipitation in a Patagonian steppe. We conducted a 7-year field experiment with water addition (+W), nitrogen addition (+N), and +NW. Destructive methods for grasses and allometric relationships for shrubs were used to assess ANPP and vegetation indices (NDVI and MSAVI2) to estimate community ANPP. An increase in ANPP of one grass species ( Papposstipa humilis ) and a decrease of the grass Poa ligularis under +N were observed. Some shrubs species exhibited mortality under nitrogen addition. Nitrogen exerted a positive effect on grass ANPP and amplified the sensitivity of grass ANPP to annual precipitation. However, +N had not effects on the shrub ANPP and shrub ANPP-precipitation relationship. Water addition by itself had no effect on ANPP for either shrubs or grasses. However, shrubs responded positively to an unusually wet year regardless of treatment and were also more sensitive to changes in annual precipitation than grasses. Total ANPP increased significantly in +N relative to the C and +W, but without changes in the sensitivity to annual precipitation. The results suggest that the responses of grasses and shrubs to water inputs is driven by soil moisture redistribution and rooting depth and that grass and community ANPP is more limited by N than by water.

Stored water in inner tissues influences the plant water economy, which might be particularly relevant for trees facing increasing dry conditions due to climate change. We studied the water storages in the inner bark and the sapwood of Araucaria araucana. This species has an extremely thick inner bark and thus it can be used as a model system to assess the impact of internal water storage on plant water balance. Specifically, we analyzed the water circulation pathways in and out of the elastic water storages by using simultaneously frequency domain moisture sensors and dendrometers inserted in the inner bark and in the sapwood, and sap flow determinations during the dry season. The daily patterns of water content and expansion and contraction of the stem tissues were similar to the sap flow pattern. The whole-stem water content and diameter increased in the morning and decreased in the afternoon, contrary to typical pattern observed in most tree species. An osmotic gradient favoring the water influx from sapwood to inner bark was observed in the morning. There were no lags in the onset of sap flow between different stem heights at the time that recharge of reservoirs occurred. Sap flow at 6 m height was higher than basal sap flow in the afternoon, when the sapwood water content started to decline followed by the water content of the inner bark. Inner bark and sapwood contributed with 5 to 11% to total daily transpiration allowing the maintenance of high water potentials in the dry season. Our results suggest that the stored water in the stems, the atypical dynamic of recharge and discharge of water from reservoirs and the high tissue capacitance may make an important contribution to the survival of A. Araucana during drought periods by maintaining the water balance.