Ecological dynamics

Closed terrace depressions have two associated ecological sites (XA232X01Y222 and XA232X01Y223; figure 7). Ecological site XA232X01Y222 has graminoid-dominant plant communities, and ecological site XA232X01Y223 has willow-dominant plant communities. The lowest part of the terrace depressions generally has a body of stagnant water. Bare mineral soils with sparse patches of various obligate wetland species are common in these areas (e.g., Hippuris vulgaris). Multiple graminoid-dominant community phases are adjacent to these bodies of water. These plant communities appear to be controlled largely by ponding and the persistence of a seasonal high water table. The soils in these areas commonly have sodic parent materials (For definitions, see Ogle and John 2010). The shrub-dominant plant communities associated with ecological site XA232X01Y223 generally are on the outer third or lip of the closed depressions (figure 7). The soils associated with shrubby willow communities tend to be drier than those associated with graminoid-dominant communities, generally lack sodic parent materials, and commonly have permafrost. The spatial dominance of each ecological site is presumed to be dynamic within these closed depressions and might be controlled by regional climatic variables.

Climatic Influences

This shrubby ecological site is thought to expand into closed depressions during warm and dry climatic periods, and the expansion can lead to permafrost aggradation (Briggs et al. 2014). Since many of the hydrologic inputs are tied to the local climate (e.g., precipitation), changes in climatic variables have a large impact on the associated water table and plant communities in these closed depressions. Large net evaporative losses from many years of low snowfall, warm annual temperatures, and low relative humidity have reduced the water table of closed depressions in the Yukon Flats Lowlands MLRA (Anderson et al. 2013). These warm and dry conditions are likely leading to tree and shrub colonization and community expansion. The opposite likely occurs during many consecutive years of colder and wetter conditions (see figure 8). Extended periods of warmer and drier conditions in the Yukon Flats Lowlands MLRA may be due in part to large-scale climatic regimes, such as the Pacific decadal oscillation. For more information, refer to the publications by Hartmann and Weller (2005) and Anderson et al. (2013).

Fire

When comparing all MLRAs of Interior Alaska, land in the Yukon Flats Lowlands MLRA burns most frequently (Begét et al. 2006). Within this MLRA, fire is considered a natural and common event that typically is unmanaged. Fire suppression generally occurs adjacent to villages or on allotments with known structures, both of which have a relatively limited acre footprint. From 2000 to 2015, 132 known fire events occurred on land in the Yukon Flats Lowlands MLRA and the burn perimeter of the fires totaled about 4.1 million acres (AICC 2016). Fire-related disturbances are highly patchy and can leave undisturbed areas within the burn perimeter. Ten of the fire events were attributed to human activities (affecting a total of 2,864 acres), but the majority were caused by lightning strikes (AICC 2016).

When a fire sweeps through areas associated with this ecological site, field data indicate that the majority of the aboveground willow and tree biomass is destroyed. The data also suggest that the fire consumes significant proportions of the organic mat, exposing bare mineral soil. Loss of the insulating layer results in a loss of permafrost in the soil profile. The postfire community is a mix of species that regenerate in place (subterranean root crowns for willow and rhizomes for graminoids) and/or from wind-dispersed seeds or spores that colonize the exposed mineral soil. A fire event in this ecological site will cause a successional transition to community 1.3.

Because the dominant vegetation (willow and graminoids) grows quickly and commonly regenerates after a fire event, minimal time is needed for postfire recovery back to the reference plant community (as compared to adjacent forested ecological sites). For an example from fire history perimeter data (AICC 2016), a site sampled in 2010 and one sampled in 2012 are believed to have burned as recently as 1988 and 1990. Over that 20-year period, the vegetation appeared to have fully recovered but permafrost was still undetected. Seven sites sampled from 2011 through 2014 are thought to have burned in 1969; five of the sites had permafrost. For this ecological site, field data suggest that full recovery of dynamic soil properties and vegetation typically takes 20 to 40 years. In comparison, it typically takes 100 to 150 years for a white spruce stand in Interior Alaska to mature (Chapin et al. 2006).

For this ecological site, the relationship between drainage class and fire-permafrost dynamics is complex and not fully understood. Field observations indicate that the depth to and presence of permafrost is impacted by fire. When areas associated with the reference plant community 1.1 are burned, loss of permafrost appears to improve drainage. The relationship among fire, permafrost, and drainage class for community 1.2, however, is poorly understood. Burned areas of community 1.2 may become wetter or drier. Future data collection could help to determine this relationship, which may lead to the creation of a unique wet, early-fire plant community.

Other Observations

While moose browse on willow was observed in each plant community, the severity of the browse is considered slight. Much of the willow was not browsed, and the majority that was browsed did not exhibit broomed architecture.