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Abstract Reforestation of degraded riparian areas provides climate mitigation benefits through increased carbon (C) storage. However, the magnitude of this potential natural climate solution (NCS) remains uncertain across ecoregions. Few studies have evaluated riparian planting C sequestration and storage, particularly in highly productive wet riparian ecosystems. In recent decades, riparian reforestation has accelerated in the Pacific Northwest (PNW) of the United States, primarily aiming to restore ecosystem functions and associated benefits. Using these plantings as a ‘natural experiment’, we assessed C storage in woody vegetation (trees and shrubs) and soils across a chronosequence of PNW riparian reforestation sites. Our study evaluated changes in C storage with planting age and identified key covariates affecting C storage in plants and soils and their relationship with planting age across a ∼430 km latitudinal gradient in western Oregon, USA. We found that woody and soil C stocks increase with planting age, averaging 24% and 1% per year, respectively. Increases in tree C were strongly driven by increasing planting age and tree stem density. Understory C was weakly related to stand characteristics and geomorphic properties, and strongly related to planting age. Soil C gains were positively driven by precipitation. We find that riparian reforestation can result in increased C storage, with woody vegetation comprising most of the increase. Our results highlight the importance of including both trees and shrubs in plantings to realize C accumulation gains in the earlier years. Because C accumulation is gradual, yet compounding (i.e. 10+ and 15+ years for total C stocks to increase by 1.95, and 19.2 Mg C ha⁻¹, respectively), riparian reforestation projects implemented today could take over a decade to deliver high NCS benefits, emphasizing the urgency to implement these projects to limit the worst of climate change impacts. 

Abstract Riparian zones are a critical terrestrial‐aquatic ecotone. They play important roles in ecosystems including (1) harboring biodiversity, (2) influencing light and carbon fluxes to aquatic food webs, (3) maintaining water quality and streamflow, (4) enhancing aquatic habitat, (5) influencing greenhouse gas production, and (6) sequestering carbon. Defining what qualifies as a riparian zone is a first step to delineation. Many definitions of riparian boundaries focus on static attributes or a subset of potential functions without recognizing that they are spatially continuous, temporally dynamic, and multi‐dimensional. We emphasize that definitions should consider multiple ecological and biogeochemical functions and physical gradients, and explore how this approach influences spatial characterization of riparian zones. One or more of the following properties can guide riparian delineation: (1) distinct species, elevated biodiversity, or species with specific adaptations to flooding and inundation near streams relative to nearby upland areas; (2) unique vegetation structure directly influencing irradiance or organic material inputs to aquatic ecosystems; (3) hydrologic and geomorphic features or processes maintaining floodplains; (4) hydric soil properties that differ from the uplands; and/or (5) elevated retention of dissolved and suspended materials relative to adjacent uplands. Considering these properties for an operational and dynamic definition of riparian zones recognizes that riparian boundaries vary in space (e.g., variation of riparian corridor widths within or among watersheds) and time (e.g., responses to hydrological variance and climate change). Inclusive definitions addressing multiple riparian functions could facilitate attainment of research and management goals by linking properties of interest to specific outcomes. 

Abstract Increasing warming and drought severity are projected for the Pacific Northwest (PNW) and are expected to negatively impact species composition and ecosystem function. In this study, we test the hypothesis that the impact of climatic stress (i.e., experimental warming and drought) on PNW grasslands are mediated by interactions between plant functional diversity and soil biogeochemical processes, including symbiotic nitrogen (N) fixation in legumes and free‐living asymbiotic nitrogen fixation (ANF) by soil microorganisms. To test this hypothesis, we measured the response of plants and soils to three years of warming (+2.5°C) and drought (−40% precipitation) in field experiments replicated at three different sites across a ∼520‐km latitudinal gradient. We observed interactive effects of warming and drought on functional diversity and soil biogeochemical properties, including both positive and negative changes in ANF. Although direct measurements of symbiotic nitrogen fixation (SNF) rates were not conducted, the observed variations in ANF, in conjunction with changes in legume cover, suggest a compensatory mechanism that may offset reductions in SNF. Generally, high ANF rates coincided with low legume cover, suggesting a connection between shifts in species composition and N cycling. Our ANF estimates were performed using isotopically labeled dinitrogen (¹⁵N₂) in tandem with soil carbon (C), phosphorus (P) and iron (Fe), pH, and moisture content. Along the latitudinal drought severity gradient, ANF rates were correlated with changes in species composition and soil N, P, moisture, and pH levels. These results highlight the importance of soil‐plant‐atmosphere interactions in understanding the impacts of climatic stress on ecosystem composition and function. 

Abstract Plants and mycorrhizal fungi form mutualistic relationships that affect how resources flow between organisms and within ecosystems. Common mycorrhizal networks (CMNs) could facilitate preferential transfer of carbon and limiting nutrients, but this remains difficult to predict. Do CMNs favour fungal resource acquisition at the expense of plant resource demands (a fungi‐centric view), or are they passive channels through which plants regulate resource fluxes (a plant‐centric view)?We used stable isotope tracers (¹³CO₂and¹⁵NH₃), plant traits, and mycorrhizal DNA to quantify above‐ and below‐ground carbon and nitrogen transfer between 18 plant species along a 520‐km latitudinal gradient in the Pacific Northwest, USA.Plant functional type and tissue stoichiometry were the most important predictors of interspecific resource transfer. Of ‘donor’ plants, 98% were¹³C‐enriched, but we detected transfer in only 2% of ‘receiver’ plants. However, all donors were¹⁵N‐enriched and we detected transfer in 81% of receivers. Nitrogen was preferentially transferred to annuals (0.26 ± 0.50 mg N per g leaf mass) compared with perennials (0.13 ± 0.30 mg N per g leaf mass). This corresponded with tissue stoichiometry differences.SynthesisOur findings suggest that plants and fungi that are located closer together in space and with stronger demand for resources over time are more likely to receive larger amounts of those limiting resources. Read the freePlain Language Summaryfor this article on the Journal blog. 

Abstract Despite much interest in relationships among carbon and water in forests, few studies assess how carbon accumulation scales with water use in forested watersheds with varied histories. This study quantified tree growth, water use efficiency, and carbon‐water tradeoffs of young versus mature/old‐growth forest in three small (13–22 ha) watersheds in the H.J. Andrews Experimental Forest, Oregon, USA. To quantify and scale carbon‐water tradeoffs from trees to watersheds, tree‐ring records and greenness and wetness indices from remote sensing were combined with long‐term vegetation, climate, and streamflow data from young forest watersheds (trees ∼45 years of age) and from a mature/old‐growth forest watershed (trees 150–500 years of age). Biomass production was closely related to water use; water use efficiency (basal area increment per unit of evapotranspiration) was lower; and carbon‐water tradeoffs were steeper in young forest plantations compared with old‐growth forest for which the tree growth record begins in the 1850s. Greenness and wetness indices from Landsat imagery were not significant predictors of streamflow or tree growth over the period 1984 to 2017, and soil C and N did not differ significantly among watersheds. Multiple lines of evidence show that mature and old‐growth forest watersheds store and accumulate more carbon, are more drought resistant, and better sustain water availability compared to young forests. These results provide a basis for reconstructions and predictions that are potentially broadly applicable, because first‐order watersheds occupy 80%–90% of large river basins and study watersheds are representative of forest history in the Pacific Northwest region. 

Abstract Prescribed fire has been increasingly promoted to reduce wildfire risk and restore fire‐adapted ecosystems. Yet, the complexities of forest ecosystem dynamics in response to disturbances, climate change, and drought stress, combined with myriad social and policy barriers, have inhibited widespread implementation. Using the forest succession model LANDIS‐II, we investigated the likely impacts of increasing prescribed fire frequency and extent on wildfire severity and forest carbon storage at local and landscape scales. Specifically, we ask how much prescribed fire is required to maintain carbon storage and reduce the severity and extent of wildfires under divergent climate change scenarios? We simulated four prescribed fire scenarios (no prescribed fire, business‐as‐usual, moderate increase, and large increase) in the Siskiyou Mountains of northwest California and southwest Oregon. At the local site scale, prescribed fires lowered the severity of projected wildfires and maintained approximately the same level of ecosystem carbon storage when reapplied at a ~15‐year return interval for 50‐year simulations. Increased frequency and extent of prescribed fire decreased the likelihood of aboveground carbon combustion during wildfire events. However, at the landscape scale, prescribed fire did not decrease the projected severity and extent of wildfire, even when large increases (up to 10× the current levels) of prescribed fire were simulated. Prescribed fire was most effective at reducing wildfire severity under a climate change scenario with increased temperature and precipitation and on sites with north‐facing aspects and slopes greater than 30°. Our findings suggest that placement matters more than frequency and extent to estimate the effects of prescribed fire, and that prescribed fire alone would not be sufficient to reduce the risk of wildfire and promote carbon sequestration at regional scales in the Siskiyou Mountains. To improve feasibility, we propose targeting areas of high concern or value to decrease the risk of high‐severity fire and contribute to meeting climate mitigation and adaptation goals. Our results support strategic and targeted landscape prioritization of fire treatments to reduce wildfire severity and increase the pace and scale of forest restoration in areas of social and ecological importance, highlighting the challenges of using prescribed fire to lower wildfire risk. 

A new study of nutrient levels in soil cores supports oral Indigenous history, informing future estuary restoration efforts. 

Increasing drought pressure under anthropogenic climate change may jeopardize the potential of tropical forests to capture carbon in woody biomass and act as a long-term carbon dioxide sink. To evaluate this risk, we assessed drought impacts in 483 tree-ring chronologies from across the tropics and found an overall modest stem growth decline (2.5% with a 95% confidence interval of 2.2 to 2.7%) during the 10% driest years since 1930. Stem growth declines exceeded 10% in 25% of cases and were larger at hotter and drier sites and for gymnosperms compared with angiosperms. Growth declines generally did not outlast drought years and were partially mitigated by growth stimulation in wet years. Thus, pantropical forest carbon sequestration through stem growth has hitherto shown drought resilience that may, however, diminish under future climate change. 

Rapid climate change poses a fundamental challenge to seed sourcing in restoration. While local provenancing is a common practice in restoration, local seeds may not survive or persist under future climate conditions. Alternative provenancing strategies, such as climate‐adjusted provenancing, that mix local seeds with non‐local seeds aim to increase the buffering capacity of restored populations. We hypothesized that seeds sourced from warmer and drier sites have higher seedling performance under drought than seeds sourced from cooler and wetter sites. We conducted a common garden experiment in a Great Basin rangeland where more frequent, severe drought events are expected to increase in the future. We sourced Bottlebrush squirreltail (Elymus elymoides[Raf.] Swezey) seeds from six locations along an aridity gradient and sowed them under three rainfall scenarios: ambient, moderate drought, and severe drought. We found strong interannual variation in seedling recruitment. In 1 year, some provenances from warmer/drier sites had high emergence and subsequent seedling survival under moderate drought. In another, emergence was low across provenances and rainfall treatments. Two provenances that survived 2 years of moderate drought had divergent seedling traits. Specifically, one had a high germination temperature optimum and high water‐use efficiency, such that it likely avoided freezing and resisted drought, while another had a low germination temperature optimum and low water‐use efficiency, such that it likely tolerated freezing and escaped drought. We highlight that understanding these differences in recruitment and stress coping strategies across provenances is important for creating climate‐adaptive seed mixes in anticipation of future climate conditions.