Ecology of the Pacific Coastal Temperate Rainforest
The perhumid Pacific coastal temperate rainforest (PCTR) extends from central British Columbia to Yakutat, Alaska. The perhumid temperate rainforests of Alaska and Canada are differentiated from seasonal temperate rainforests further south by patterns and amounts of precipitation and associated lack of a fire regime. Due to the underlying continental geologic landforms, they are also much more closely linked with adjacent marine ecosystems. The PCTR includes the largest remaining extent of old-growth forests in North America, and is characterized by dense upland forests mixed with forested wetlands and coastal estuaries; and a mountainous terrain fragmented by glaciers, icefields, deep fjords, ocean channels, and thousands of small watersheds. The PCTR also hosts a variety of endemic species, and species distributions and ecological communities have been shaped by repeated glacial and inter-glacial cycles, island biogeography, and disturbance patterns associated with avalanches, windstorms, floods, and mass wasting events. The climate of the PCTR is driven by large frontal storms from the Gulf of Alaska, is highly variable year-to-year, and strongly influenced by the Pacific Decadal Oscillation and El Nino/La Nina. However, there is evidence that the region is becoming less influenced by these regional processes and more strongly impacted by the global trend of summer warming. Glacial runoff comprises about 50% of the runoff from the Alaskan portion of the PCTR and thus freshwater discharge in the region is inherently sensitive to changes in glacier volume and extent. In fact, rates of glacier mass loss in the PCTR are currently among the highest on Earth and are projected to increase with continued regional climate change.
Coastal margins are dynamic zones for the transfer of materials–water, carbon, and nutrients–between terrestrial and marine ecosystems. Coastal temperate rainforest (CTR) ecosystems such as those found along the Gulf of Alaska are characterized by physical and biogeochemical properties that contribute to substantial land-to-ocean materials fluxes including: steep topography, vast stores of organic carbon (OC) in peat-rich organic soils, glacially-derived nutrients, and high runoff rates. The PCTR exemplifies this type of coastal margin hotspot. Abundant rainfall and acidic forest litter promote enriched organic-mineral deposits in soils; these deep, saturated organic deposits, common in peatlands, sequester large amounts of stabilized organic carbon. The land-to-ocean flux of freshwater in the PCTR is also exceptionally high: in fact, the average annual freshwater discharge from PCTR rivers is about twice the annual discharge of the Mississippi River and more than 5 times the annual discharge of the Yukon River. This massive volume of freshwater supports the northward flow of the Alaska Coastal Current, and propagates warm, high-nutrient, low-salinity eddies far into the Gulf of Alaska. This freshwater flux also carries carbon and nutrients (such as nitrogen, phosphorous, and iron) from terrestrial to stream and nearshore marine ecosystems, and drives circulation patterns and nutrient exchange in fjords and estuaries. Phytoplankton capitalize on high nutrient levels in shallow estuaries and glacial fjords, providing seasonal nurseries for juvenile fish and an abundant food source for upper trophic levels.
Because average winter temperature at sea level hovers around 0ºC for much of the region, this ecosystem is likely to be highly sensitive to climate-driven shifts. Warming temperatures will increase glacier volume loss (and consequently runoff from watersheds with substantial glacial coverage); precipitation will increasingly fall as rain rather than snow; and seasonal snowcover will become infrequent except at higher elevations. All of these expected changes in abiotic drivers have important and largely unquantified implications for ecosystem functioning (eg., hydrologic cycle, terrestrial biogeochemical cycling, soil ecology, landcover and forest community dynamics, and nearshore biological oceanography) and ecosystem services (eg., timber production, fisheries, hydropower, and ecotourism opportunities) across the coastal margin of the PCTR.
The ACRC is involved in several projects exploring the intricate web of relationships that characterize this ecosystem so that we can better understand and manage the region’s resources. We work with partners across Alaska, British Columbia, Washington, and Oregon, and endeavor to create meaningful transboundary collaborations. Please check out the Coastal Rainforest Margins Research Network website for developing information.
Figure credit: K.Timm (Scenarios Network for Alaska and Arctic Planning, UAF) with contributions from Integration and Application Network, University of Maryland Center for Environmental Science, ian.umces.edu/imagelibrary/