Constrained tree growth and gas exchange of seawater‐exposed forests in the Pacific Northwest, USA

Abstract

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

Floodplain Inundation and Salinization From a Recently Restored First‐Order Tidal Stream

“…The modeled total system water mass responds primarily to inundation events, similar to porewater turnover in tidal marshes, which are dominated by estuarine surface water exchange (Harvey & Odum, 1990). This similarity is consistent with the evolution of the Beaver Creek floodplain from a freshwater swamp to a marsh ecosystem as the floodplain becomes saline (Wang et al, 2019). During the largest infiltration events, the mass of infiltrating water is less than 1% of the overlying inundation water mass.…”

“…A 1 m digital elevation model (DEM) of the region (LiDAR Bare Earth DEM, 2010) was used without correction to delineate the 382 ha Beaver Creek catchment ( Figure 1). Most of the catchment is classified as upland forest (Sengupta et al, 2019;Wang et al, 2019). The catchment consists of several lower-elevation floodplains with a consistent slope break around 3.7 m elevation (all elevations presented herein are NAVD88 unless specified otherwise) ( Figure 1), accounting for~13% of the delineated watershed area (50 ha total).…”

“…Understanding the time scales of coastal ecosystem salinization is critical for determining the potential impact of these numerous drivers of inundation regime shifts on both coastal ecology and biogeochemical function. Coastal forests around the Pacific Northwest, for example, have been consistently declining in health with elevated tree mortality and declining growth rates over the last several decades due to press salinization, while events such as the Beaver Creek culvert removal can have the same effects in a year or less time (Wang et al, 2019). Similar observations of long-term coastal forest mortality due to SLR have been made on the U.S. East Coast (Kirwan & Gedan, 2019), and likewise, extreme events such as hurricanes can have similar effects as to what we observed after culvert removal (Cahoon et al, 2003).…”

“…Here we observe and model the variably saturated flow and salinity in a floodplain of a first-order tidal stream, Beaver Creek, on the coast of Washington State, USA, to ascertain the hydrologic mechanisms behind the observed salinization of floodplain groundwaters following seawater flow restoration in the tidal stream in 2014. The watershed was restored to full tidal effects after nearly a century of freshwater conditions (Washington Department of Fish and Wildlife, 2019), which has had a marked influence on tree health (Wang et al, 2019) and soil organic carbon composition (Sengupta et al, 2019). We postulate that topography is a key determinant of floodplain inundation and groundwater salinization and that the two-way exchange of water in the tidal river system is dominated by surface connectivity and flood events.…”

“…The mechanisms behind salinization of tidal river systems and the adjacent terrestrial environments may be unique from the comparatively well-studied freshwater wetland salinization (Ensign & Noe, 2018). Salinization impacts not only biogeochemical cycling (Ward et al, 2019) but also coastal freshwater ecosystem function, including tree health (Wang et al, 2019). Understanding the mechanisms behind seawater inundation and floodplain salinization in coastal ecosystems is therefore necessary to elucidate how such chronic salinity exposure arises, persists, and is incorporated into predictive modeling efforts needed for projecting future change.…”