Do ecosystem disturbances enhance forest carbon storage in Southeast Alaska?

Change spurs growth, even for forests. Disturbances such as landslides, wind, and wildfires are part of the ecosystem dynamics of a region. Over a short timescale, disturbances are known to decrease carbon stored as forest biomass, by burning and uprooting vegetation and disrupting growth. But across broad time and spatial scales, exposure to disturbances may actually increase carbon storage by plants.

A recent publication by CRMRN principal investigator Brian Buma and Thomas Thompson of the US Forest Service in PLOS ONE tackled the tangled relationship of carbon stocks and ecosystem disturbances in Southeast Alaska. Using nearly 1500 data points across the region from, Buma sought to answer the question: In ecosystems that experience infrequent disturbances, do wind and landslides enhance carbon storage over long timescales?

Buma studies the ecology of change. The study region, Southeast Alaska, makes an excellent site for this very question. With little variation in vegetation, climate, and absence of widespread forest fires, Buma could uniquely single out the long-term (multi-millenia) influence of disturbances in relation to tree density, size, and biomass. With little to no fire disturbance, the disturbance regime of this perhumid temperate rainforest is defined by infrequent wind and landslide or avalanche impacts.

“Disturbances are extremely rare here,” says Buma. In fact, the temperate rainforest of Southeast Alaska is one of the least disturbed forests on the continent, he says.

While most disturbance ecologists work in areas of rapid change and high disturbance levels, Buma was drawn to Southeast Alaska for its stable, unchanging forests. “Southeast provides the opposite extreme – a place that’s hardly ever disturbed. It’s a great place to test basic theoretical expectations of disturbance ecology that you can’t test anywhere else,” says Buma.

However, climate change is altering the disturbance dynamics of ecosystems, affecting how often and how severely different regions experience disturbances. To look ahead at how these changes will impact the carbon cycle and the amount of carbon stored in the temperate rainforest of this region, we need to look at the mechanisms connecting disturbances and carbon stored as biomass.

To do this, Buma created a model with factors from the slope, climate, and vegetation to wind patterns and susceptibility to landslides.

His findings suggest that areas with greater exposure to disturbances have higher carbon stocks. In infrequently disturbed ecosystems like the carbon-rich perhumid forest, this may indicate that disturbances actually increase the baseline carbon stock by increasing plant productivity. This is particularly true when considering landslides. When they weren’t present in the forest model, carbon biomass was 18-19% lower. Disturbances by wind had a lesser impact (~2% lower biomass) and varied spatially.

The full paper is available online at PLOS ONE.

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