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CSE soil change

Understanding long-term soil change in coastal sandy ecosystems

Comparing vegetation composition and biomass between the youngest and oldest dunes on northwestern Calvert Island, dramatic differences are evident.  Inland from West Beach, stabilized dunes less than 150 years old support a dense, vigorous Sitka spruce forest.  In contrast, the upper slopes of a ~ 10,000-year-old dune near Second Beach are occupied by an open, stunted woodland of lodgepole pine, western hemlock, yellow cedar, and western red cedar (Fig. 1).

Figure 1.  Contrasting vegetation types on (L) young and (R) old dunes, northwestern Calvert Island.  Photo credits:  J. Bryce, P. Sanborn.Caption

This pattern of apparent long-term decline in biomass and productivity resembles the phenomenon of ecosystem retrogression which has been documented in many parts of the world, from boreal to tropical, and on time scales ranging from thousands to millions of years (Wardle et al., 2004).  In these other settings, retrogression has been attributed to soil processes such as gradual leaching loss of phosphorus, as well as formation of cemented soil horizons that restrict root access and impede drainage (Vitousek et al., 2010; Coomes et al., 2013).

Evidence for both of these mechanisms can be seen in a soil profile at the crest of the oldest dune (Fig. 7).  The irregular reddish band in the upper mineral soil –sometimes split into up to three parallel bands – is a strongly cemented feature called a placic horizon.  It has a high concentration of iron oxide, and is sufficiently dense and cohesive that almost no roots penetrate it.  Total phosphorus (P) concentrations are quite low throughout this soil profile (Fig. 2).  To put these values into perspective, the concentrations found in the mineral horizons of this soil are at the low end of the range reported in a recent global compilation of P concentration data for bedrock (Porder and Ramachandran, 2013).  These low concentrations may reflect the chemical composition of local bedrock, which has particularly low P concentrations in samples collected in the Kwakshua Channel area (unpublished data, Geological Survey of Canada).

Figure 2.  Soil profile formed in ~ 10,000-year-old dune sand.  Total phosphorus (P) concentrations are indicated for selected soil horizons.  Blue knife handle is 11 cm long.

Total P concentrations only tell part of the story, and different forms of soil P may differ considerably in their biological availability.  For this reason, a major goal of the planned soils research in the Coastal Sandy Ecosystems program will be to characterize the speciation of soil P as it changes over time, using the chronosequence of soils available on Calvert Island.