Could Temperate Rainforests Survive Global Warming?

Douglas Island. Source: Joseph/Flickr
Douglas Island. Source: Joseph/Flickr

Nowadays, most of Europe’s temperate rainforests and the coast redwoods are disappearing as a result of over-harvesting. However, temperate rain-forests in Tongass and Great Bear (British Columbia) remain relatively intact. The Pacific Coastal Temperate Rainforest (PCTR) ecosystem stretches 4000 kilometers along coasts from northern California through Oregon, Washington and British Columbia to Alaska.

Known for its diversity, the PCTR region contains unscathed old-growth forests, substantial glaciers, wild fisheries, and human communities with economies based on natural resources and tourism. Studies show that global-scale warming will have stronger effects on the northern PCTR’s climate rather than regional and decadal time scale climate processes, including El Nino Southern Oscillation, Pacific Decadal Oscillation, and Arctic Oscillation. The northern PCTR is anticipated to undergo warming and receive less precipitation in snow in the coming decades.

Ecosystem linkages in the northern Pacific coastal temperate rainforest (PCTR). (Source: Shad O'Neel et al.)
Ecosystem linkages in the northern Pacific coastal temperate rainforest (PCTR). (Source: Shad O’Neel et al.)

Glacier coverage in northern PCTR is roughly 16% – there are 141 lake-terminating glaciers and 49 tidewater glaciers in the region. With so many glaciers, the region is particularly susceptible to ecosystem-level impacts of glacier change. Moreover, glacier mass loss rate in the region is anticipated to rise, with a total glacier volume loss of 7200 – 10000 cubic meters by the end of the twenty-first century. Glacial volume variability in northern PCTR involves prominent physical and chemical changes in hydrology, which should not be neglected because it serves as the primary source of freshwater to the Bering Sea. In addition, regional species diversity and species turnover could be influenced by glacier runoff.

On Benjamin Island. Source: Joseph/Flickr
On Benjamin Island. Source: Joseph/Flickr

A recent study published in BioScience discussed impacts of glacier volume change on surface water hydrology, biogeochemistry, coastal oceanography, and ecology. Total freshwater discharge from northern PCTR adds up to 870 cubic kilometers, half of which originates from glacier-covered area. More importantly, extremely short glacier-to-ocean stream length in the area, around 10 kilometers, allows rapid transfer of riverine substances, including sediment, nutrient, and organic matter, to estuaries and fjords. In other words, glacier change alters the terrestrial hydrologic cycle, which is significantly connected to near-shore marine ecosystem.

Freshwater runoff in ice-free basins and glacierized basins is primarily dependent on precipitation and surface energy balance respectively, because positive energy balance can result in glacier melting. The authors of the study, Shad O’ Neel et al, also found that glacierized watersheds tend to have higher annual freshwater discharge. The streamflow variability of glacierized basins varies in dissimilar patterns on different time scales. According to O’Neel and his colleagues, as glacier melt increases as a result of warmer temperature in the future, it will be more difficult to predict the variability of streamflow originated from glacierized basins.

Toward the Chilkats. Source: Joseph/Flickr
Toward the Chilkats. Source: Joseph/Flickr

In general, glacier ecosystems have strong impacts on biogeochemistry of downstream marine ecosystems through the release and cycling their nutrients, organic matter, and contaminants. The origin of organic matter in glacier ecosystem can be attributed to a variety of sources, including aerosol deposition, subglacial biological processes, as well as subglacial organic matter. In fact, glacier runoff in northern PCTR releases tremendous amount of organic matters. Those organic matters are primarily produced through microbial production and atmospheric deposition rather than plant detritus, which appears to be extremely bioavailable to heterotrophic organisms. In other words, glacier ecosystems are a significant source of organic matter in both riverine and near shore marine ecosystems. Unfortunately, melting mountain glaciers also bring pollutants to the near shore aquatic ecosystems, such as fossil fuel combustion byproducts and mercury. Moreover, downwelling caused by wind-forced Ekman transport has a consequence of nitrogen delivery to near shore region from off shore waters. In return, high flux of glacier runoff brings iron to off shore regions.

Kayak and Herbert Glacier. Source: Joseph/Flickr
Kayak and Herbert Glacier. Source: Joseph/Flickr

The coastal ocean circulation in northern PCTR is dominated by the counterclockwise Alaska Coastal Current, which transports heat, nutrients, and organisms northward to the Arctic. Freshwater runoff also plays an important role in affecting the vertical stratification of coastal water column. Specifically, coastal waters are well mixed and replenished with nutrients in winter. And they become stratified due to freshwater runoff in spring. Shad O’ Neel et al. pointed out that the impact of glaciers on physical oceanography of the northern PCTR is conspicuous in glacierized fjords. Cold and low-density freshwater discharge from seafloor glacier upwells to the surface while mixing with fjord water. As it rises to the surface, it leads to submarine melt of the ice cliff, and contributes to fresh overflow plume. Furthermore, nonlinear freshwater flow dynamics associated with deep-water calving fronts results in catastrophic glacier recession. As a result of a decrease in active tidewater glaciers in Alaska, the amount of fjords with glacier-driven circulation and tidewater glacier habitat also decline.

Dinner Time. Source: Kyle Breckenridge/Flickr
Dinner Time. Source: Kyle Breckenridge/Flickr

According to the study, glacier runoff generally leads to increase in regional species diversity and species turnover. For instance, “streams with moderate basin ice cover (5% – 30%) tend to have the highest macroinvertebrate taxonomic diversity, although macroinvertebrate abundance is generally low in these watersheds”, as described by the article. Glaciers also directly affect upper trophic level species in fjords, on which important fishery and tourism industries rely. Icebergs calved from glaciers serve as protection for predators, a habitat for harbor seals, and a resting space for seabirds.

In sum, the northern PCTR is economically significant to northern California, British Columbia, and Alaska due to diverse resources. Understanding both long-term and short-term glaciological variability is essential to decision-making and risk management processes. After all, glacier volume and extent variability is closely linked to surface-water hydrology, biogeochemistry, coastal oceanography, and ecology. Therefore, Shad O’Neel et al. suggested that “a holistic scientific approach should be undertaken to begin to resolve these uncertainties in ways that maximize utility to the resource management community and allow efficient and informed decision-making in an era of rapid ecosystem change”.

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Bob Binghamreply
April 16, 2015 at 12:04 AM

My understanding is that the changes are happening so fast that nature can not keep up. Trees in particular are very sensitive to temperature and rainfall and small changes can spell disaster.

April 22, 2015 at 03:04 PM
– In reply to: Bob Bingham

Many trees do have specific requirements for temperature and rainfall. In mountainous areas, some tree species can shift their ranges and survive. But others do face great difficulties.

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