1. Rising sea levels under climate change may have significant impacts on coastal vegetation dynamics, yet the response of coastal forest growth, gas exchange and survival to seawater intrusion remains poorly documented.2. We conducted a dendroecology study across six sites in western Washington, USA, to examine how tree growth, gas exchange (indexed by basal area increment (BAI) and wood δ 13 C respectively), and survival varies with seawater exposure through two approaches. First, tree core samples were collected at a site where seawater exposure started only 4 years prior to sampling, which allowed a cause-and-effect test of the impacts of seawater exposure on trees, and second, samples were collected at five additional sites where we compared downstream to upstream trees under current sea-level conditions. 3. At the seawater intrusion site, BAI and carbon isotope discrimination (Δ) decreased significantly (p < 0.01) in the year of intrusion (2014) and stayed unchanged thereafter. Four years later (2018), the percentage of recently standing dead trees in the forest was 73.0% of the basal area. Across the regional assessment, percentage of standing dead trees was significantly greater in downstream than upstream forests at five of the six sites (averaged 37.7 ± 11.0% and 4.3 ± 2.1% basal area for downstream and upstream, respectively). Growth was significantly lower (p < 0.01) at the downstream than upstream for five sites, and Δ was lower for all needle-leaf trees (three sites) on the downstream compared to the upstream, but no difference was observed between downstream and upstream for broad-leaf trees (three sites). Synthesis.Combined both the cause-and-effect manipulative study and the regional assessment demonstrate that seawater exposure drives reductions in growth, decreased Δ of needle-leaf trees, increased mortality and greater climate sensitivity, regardless of whether the seawater exposure is recent or long-term. K E Y W O R D Scoastal forests, sea-level rise, seawater, tree growth, tree mortality, tree-ring δ 13 C

Increasing sea levels associated with climate change threaten the survival of coastal forests, yet the mechanisms by which seawater exposure causes tree death remain poorly understood. Despite the potentially crucial role of nonstructural carbohydrate (NSC) reserves in tree survival, their dynamics in the process of death under seawater exposure are unknown. Here we monitored progressive tree mortality and associated NSC storage in Sitka-spruce (Picea sitchensis) trees dying under ecosystem-scale increases in seawater exposure in western Washington, USA. All trees exposed to seawater, due to monthly tidal intrusion, experienced declining crown foliage during the sampling period, and individuals with a lower percentage of live foliated crown (PLFC) died faster. Tree PLFC was strongly correlated with subsurface salinity and needle ion contents. Total NSC concentrations in trees declined remarkably with crown decline, and reached extremely low levels at tree death (2.4% and 1.6% in leaves and branches respectively, and 0.4% in stems and roots). Starch in all tissues was almost completely consumed while sugars remained at a homeostatic level in foliage. The decreasing NSC with closer proximity to death and near zero starch at death are evidence that carbon starvation occurred during Sitka-spruce mortality during seawater exposure. Our results highlight the importance of carbon storage as an indicator of tree mortality risks under seawater exposure.

The systematic response of coastal ecosystems to inundation and salinity exposure is fundamental to their ecology and biogeochemical function. Here we observe and model freshwater‐seawater interactions in a first‐order stream—floodplain system where tidal access was recently restored. Subsurface flow and transport modeling were used to quantify and better understand the interplay of processes, properties, and conditions that control water level and salinity in the floodplain to the tidal stream. Water levels in the stream were highly correlated with tidal forcing, which resulted in episodic inundation of the floodplain at quasi‐monthly frequency. The tidal stream is the only source of salinity to the floodplain, yet shallow groundwater salinity was considerably higher than average stream salinity. The low‐permeability clay floodplain soils limit lateral groundwater flow and transport, resulting in floodplain groundwater and salinity dynamics driven almost exclusively by infiltration during inundation events. As inundation occurs during high tide, estuarine waters reach the floodplain with minor attenuation in salinity from the stream's freshwater discharge. Infiltration and salinity exposure are topography controlled and regulated by ponding depth and duration, seasonal ground saturation, and depth to water table. The model suggests that floodplain salinity is currently in an early stage of transition from pre‐restoration freshwater conditions and will not reach equilibrium for ~20 years. These findings have broad relevance for understanding how and over what time scales coastal ecosystems will respond to increasing seawater exposure from sea level rise, ocean‐originating storms, and changes in natural and man‐made barriers.

Coastal terrestrial-aquatic interfaces (TAIs) are dynamic zones of biogeochemical cycling influenced by salinity gradients. However, there is significant heterogeneity in salinity influences on TAI soil biogeochemical function. This heterogeneity is perhaps related to unrecognized mechanisms associated with carbon (C) chemistry and microbial communities. To investigate this potential, we evaluated hypotheses associated with salinity-associated shifts in organic C thermodynamics; biochemical transformations; and nitrogen-, phosphorus-, and sulfur-containing heteroatom organic compounds in a first-order coastal watershed on the Olympic Peninsula of Washington, USA. In contrast to our hypotheses, thermodynamic favorability of water-soluble organic compounds in shallow soils decreased with increasing salinity (43-867 µS cm −1 ), as did the number of inferred biochemical transformations and total heteroatom content. These patterns indicate lower microbial activity at higher salinity that is potentially constrained by accumulation of less-favorable organic C. Furthermore, organic compounds appeared to be primarily marine-or algae-derived in forested floodplain soils with more lipid-like and protein-like compounds, relative to upland soils that had more lignin-, tannin-, and carbohydrate-like compounds. Based on a recent simulation-based study, we further hypothesized a relationship between C chemistry and the ecological assembly processes governing microbial community composition. Null

The aim of this study was to examine the magnitude of greenhouse gas (GHG) concentrations in tree stems of Pacific Northwest, USA coastal forests and evaluate various tree and site characteristics along river‐to‐sea gradients as possible drivers of tree stem GHG variation. We measured the concentration of CH4, CO2, and N2O during summer and winter in live and dead tree stems of five species from six coastal watersheds and related this to soil porewater GHG concentrations, porewater salinity, and tree characteristics. Overall, average pCO2 and pCH4 were elevated above atmospheric concentration, and average pN2O was slightly below atmospheric concentration. Stem pCO2 was higher in the summer than the winter and was higher in angiosperm trees compared to gymnosperm trees, whereas pCH4 was significantly higher in fresh upstream compared to salt‐influenced reaches. Stem pCH4 was also positively correlated with porewater pCH4 in contrast to other GHGs. The above results suggest that tree stem pCH4 in these coastal settings was primarily controlled by soil linkages, pCO2 was primarily regulated by tree physiology, and factors controlling pN2O remain unclear.