Engineering topics

Das, Adrian J.

Publications and source records attributed to Das, Adrian J..

3 records

DOE OSTI2022

Climate Change Risks to Global Forest Health: Emergence of Unexpected Events of Elevated Tree Mortality Worldwide

Recent observations of elevated tree mortality following climate extremes, like heat and drought, raise concerns about climate change risks to global forest health. We currently lack both sufficient data and understanding to identify whether these observations represent a global trend toward increasing tree mortality. In this work, we document events of sudden and unexpected elevated tree mortality following heat and drought events in ecosystems that previously were considered tolerant or not at risk of exposure. These events underscore the fact that climate change may affect forests with unexpected force in the future. We use the events as examples to highlight current difficulties and challenges for realistically predicting such tree mortality events and the uncertainties about future forest condition. Advances in remote sensing technology and greater availably of high-resolution data, from both field assessments and satellites, are needed to improve both understanding and prediction of forest responses to future climate change.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2021

Continent-wide tree fecundity driven by indirect climate effects

Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here we find from a synthesis of tree species in North America that climate-condition interactions dominate responses through two pathways, i) effects of growth that depend on climate, and ii) effects of climate that depend on tree size. Because tree fecundity first increases and then declines with size, climate change that stimulates growth promotes a shift of small trees to more fecund sizes, but the opposite can be true for large sizes. Change the depresses growth also affects fecundity. We find a biogeographic divide, with these interactions reducing fecundity in the West and increasing it in the East. Continental-scale responses of these forests are thus driven largely by indirect effects, recommending management for climate change that considers multiple demographic rates.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Mortality predispositions of conifers across Western USA

Conifer mortality rates are increasing in North America, but the physiological mechanisms underlying this trend are still not well understood. We examined tree-ring based growth rates, stable carbon and oxygen isotope ratios (d13C and d18O, respectively) of dying and surviving conifers at eight old-growth forest sites across a strong moisture gradient in the western USA to retrospectively investigate mortality predispositions. Compared to surviving trees, lower growth of dying trees was detected at least one decade prior to mortality at seven of the eight sites. Intrinsic water-use efficiency (iWUE) increased over time in both surviving and dying trees, with a weaker increase in dying trees at five of the eight sites. The relationship between iWUE and d18O was much stronger and steeper for surviving trees, suggesting an important hydraulic role in mortality. Carbon starvation was a strong correlate of conifer mortality based on a conceptual model incorporating growth, d13C, and d18O. Our results provide a direct link of mortality to growth constraints in western USA conifers.

Wang, Wenzhi↗