Moffett, K. | bioRxiv Science

bioRxiv2023

Canary in the Forest? Tree mortality and canopy dieback of western redcedar linked to drier and warmer summer conditions

Tree mortality and partial canopy dieback are increasing in many forest ecosystems from unfavorable climate conditions. Examining how tree growth and mortality are affected by climate variability can help identify proximate causes of tree mortality and canopy dieback. We investigated anomalously high mortality rates and partial canopy dieback of western redcedar (Thuja plicata, WRC), a culturally, ecologically, and economically important species in the Pacific Northwest (USA), using tree-ring methods. We sampled trees in three tree status groups--no canopy dieback, partial canopy dieback, and trees that died (0-30 years ago)--from 11 sites in coastal (maritime climate) and interior (continental climate) populations of WRC trees. In our study, WRC tree mortality was portended by on average 4-5 years of declining radial growth. Warmer and drier climate conditions in May and June that extend the annual July-September dry season reduced radial growth in 9 of 11 sites (1975-2020). Defining drought events as warm, dry May-June climate, we found that WRC trees recovered radial growth to pre-drought rates within three years when post-drought climate conditions were average or cooler and wetter than average. However, radial growth recovery from drought was slower or absent when conditions were warmer and drier during the post-drought recovery period, which appeared to lead to the widespread mortality event across coastal populations. Annually resolved tree mortality in coastal populations predominately occurred in 2017-2018 (80% of sampled trees) and coincided with exceedingly hot temperatures and the longest regionally dry period for May to September (1970-2020). In interior populations, tree mortality was associated with warmer, drier conditions from August to September. Our findings forewarn that a warming climate and more frequent and severe seasonal droughts will likely increase the vulnerability of WRC to canopy dieback and mortality and possibly other drought-sensitive trees in one of the worlds largest carbon sinks.

ecology↗

bioRxiv2022

One thousand soils for molecular understanding of belowground carbon cycling

While significant progress has been made in understanding global carbon (C) cycling, the mechanisms regulating belowground C fluxes and storage are still uncertain. New molecular technologies have the power to elucidate these processes, yet we have no widespread standardized implementation of molecular techniques. To address this gap, we introduce the Molecular Observation Network (MONet), a decadal vision from the Environmental Molecular Sciences Laboratory (EMSL), to develop a national network for understanding the molecular composition, physical structure, and hydraulic and biological properties of soil and water. These data are essential for advancing the next generation of multiscale Earth systems models. In this paper, we discuss the 1000 Soils Pilot for MONet, including a description of standardized sampling materials and protocols and a use case to highlight the utility of molecular-level and microstructural measurements for assessing the impacts of wildfire on soil. While the 1000 Soils Pilot generated a plethora of data, we focus on assessments of soil organic matter (SOM) chemistry via Fourier-transform ion cyclotron resonance-mass spectrometry and microstructural properties via X-ray Computed Tomography to highlight the effects of recent fire history in forested ecosystems on belowground C cycling. We observed decreases in soil respiration, microbial biomass, and potential enzyme activity in soils with high frequency burns. Additionally, the nominal oxidation state of carbon in SOM increased with burn frequency in surface soils. This results in a quantifiable shift in the molecular signature of SOM and shows that wildfire may result in oxidation of SOM and structural changes to soil pore networks that persist into deeper soils.

ecology↗