Intrusion of Beach Stones and Saltwater into the Coastal Forest

Tom Okey wrote:

My son and I camped at French Beach Provincial Park on 24 March 2019, and we observed apparent encroachment of beach stones (pebbles/cobbles) into the coastal forest. We also observed considerable mortality of the trees on the landward side of this trail, and evidence that the vegetation and the berm along the seaward side of the trail had been greatly reduced by erosion and 'scouring' action of the pebbles/cobbles on this dynamic, high-energy beach.

I would classify French Beach as a semi-protected outer-coast beach that occasionally experiences high surf, and should thus be considered a high-energy beach. This section of the beach faces WSW and although protected from southern wave exposure by the Olympic Peninsula, and from direct northwest wave exposure by Vancouver Island, it is exposed to the open ocean directly to the west, and swell from the north or the south can otherwise wrap to hit this beach.

High energy is indicated by the pebble/cobble-composition of the beach. Alluvial sources of sediment from rivers also influence beach composition, but nearby rivers and streams are small and thus may not deliver large quantities of sand. The position of these beach stones on the coastal trail and into the coastal forest, in combination with a preponderance of apparently pebble-cobble-scoured trees, and a greatly-eroded 'forest-edge-berm' indicates that the beach has significantly encroached on the trail and the adjacent forest.

Sea level rise and increased intensity of storms---two climate change effects that increase coastal erosion---may explain much of this physical erosion and pebble/cobble encroachment, and some of the apparent salinity-related mortality of the trees on the landward side of the trail. However, a third climate change effect on southern Vancouver Island---declining precipitation and subsequent reduced seaward flow of fresh groundwater---may enable saltwater intrusion and thus explain the conspicuous mortality of trees at this location.

Streams percolating---i.e. springing---from the beach above the surf indicate that groundwater flows are unimpeded through this pebble/cobble substratum, which implies that reductions of freshwater runoff and seaward groundwater flow during recent droughts and dry periods could have enabled novel saltwater intrusion along this coastline. This could explain the significant mortality of trees we observed in this coastal forest, especially considering the apparent recent acceleration of pebble-cobble-scour that the vegetation and forest soils have been experiencing.

This is a different explanation than that of Earnshaw et al (2018) of Sitka Spruce (Picea sitchensis) die-off ~100 m from the present observation ~5 months earlier. I have reached out to these authors to attempt to reconcile these alternate hypotheses.

Literature cited

Earnshaw, Jacob, Elizabeth Graham and Robin Mulvey. 2018. Sitka spruce (Picea sitchensis) die-off French Beach. LEO Network (leonetwork.org). Accessed 23 August 2019.