Biology subjects

Dawson, T. E.

Publications and source records attributed to Dawson, T. E..

3 recordsLinked to original sources

bioRxiv2025

Groundwater and phenology data reveal vulnerability of riparian trees to drought

The increasing frequency and magnitude of climatic extremes are altering water availability in dryland ecosystems globally. However, riparian vulnerability to hydroclimate whiplash remains poorly understood. Here, we examined how riparian trees respond to groundwater fluctuations and drought through their water use patterns and phenology. To this end, we combined time-series analysis of long-term, high-frequency groundwater monitoring and satellite imagery from a drought-prone and relatively pristine watershed in California (Chalone Creek, Pinnacles National Park). We found that trees by intermittent river reaches displayed consistent but depth-limited groundwater reliance, while those at perennial reaches primarily relied on groundwater during the dry season. Machine-learning models revealed that at intermittent sites groundwater depth predominantly controlled vegetation greenness, represented by Normalized Difference Vegetation Index (NDVI). In contrast, variation in photoperiod length dominated at perennial sites where water was more reliably available. During the severe 2020-2022 drought, all species experienced reduced greenness, but phenological responses differed by flow regime. While the start of season was delayed across all sites, trees at intermittent reaches exhibited substantially earlier end of season during drought, resulting in growing seasons shortened by as much as 28 days. These phenological shifts vastly exceed those documented across aridity classifications in global datasets from satellite observations, ground-based monitoring networks, and experimental precipitation manipulations. Although riparian trees in drylands have been shaped by exposure to drought over evolutionary timescales, our findings challenge the prevailing assumption that ecosystems regularly exposed to hydrological are more resilient to drought. Instead, we show that trees in intermittent systems may be operating close to critical groundwater thresholds, rendering them particularly vulnerable to increasingly long and severe droughts.

ecology↗

bioRxiv2024

Deep rhizospheres extend the nitrogen cycle meters below the base of soil into weathered bedrock

Nitrogen is the most limiting nutrient to forest productivity worldwide. Recently, it has been established that diverse ecosystems source a substantial fraction of their water from weathered bedrock, leading to questions about whether root-driven nitrogen cycling extends into weathered bedrock as well. In this study, we specifically examined nitrogen dynamics using specialized instrumentation distributed across a 16 m weathered bedrock vadose zone (WBVZ) underlying an old growth forest in northern California where the rhizosphere--composed of plant roots and their associated microbiome--extends meters into rock. We documented total dissolved nitrogen (TDN), dissolved organic carbon (DOC), inorganic N (ammonium and nitrate) and CO2 and O2 gasses every 1.5 m to 16 m depth for two years. We found that biologically available nitrogen in the weathered bedrock rhizosphere was comparable in concentration to temperate forest soils and primarily organic. TDN concentrations in the WBVZ exhibited distinct patterns with depth and were correlated with periods of increased whole-ecosystem metabolic activity as well as stream discharge, suggesting competing rhizosphere and leaching processes in the fate of TDN in the WBVZ. Carbon isotope composition of the DOC suggests that dissolved organic matter in the WBVZ is primarily derived from fresh plant sources. We conclude that N cycling in the WBVZ is driven by an active rhizosphere meters below the base of soil and represents an important and overlooked component of deeply rooted ecosystems that must be incorporated into future models and theory of ecosystem function.

ecology↗

bioRxiv2017

Water relations of Calycanthus flowers: hydraulic conductance, capacitance, and embolism resistance

Introduction Introduction Materials Methods Results Discussion Conclusions References Flowers are developmentally and morphologically complex structures whose primary function is to promote sexual reproduction (Specht & Bartlett 2009). Coevolution with animal pollinators has long been considered the primary selective agent responsible for the many, diverse forms apparent among angiosperm flowers (Sprengel 1793, 1996; Fenster et al. 2004). Yet, floral adaptations to pollinators may not be as frequent as commonly considered, and non-pollinator agents of selection, such as the resource costs of building and maintaining flowers, may also influence floral form and function (Herrera 1996; Strauss & Whittall 2006). Fo ...

ecology↗