Integrating tall shrubs into an individual-based boreal forest gap model to capture vegetation dynamics at the tundra-taiga ecotone

Abstract:

As the climate of arctic-boreal regions changes, the ecotone between boreal forest and tundra is likely to be dynamic. The boreal component of the ecotone is typified by broadleaf and coniferous trees, whereas the tundra component is dominated by tall shrubs, dwarf shrubs, graminoids, mosses, and lichens. The two biomes come together both gradually and abruptly based on the complex environmental matrix of topography, soil characteristics, permafrost presence, light availability, temperature, moisture, and fire regimes. Using the boreal forest individual-based gap-dynamics model, SIBBORK-TTE, we can potentially simulate this shifting ecotone with relatively high spatial resolution; however, the intersection between boreal forest and tundra requires that shrub species and low-statured tundra plant types be integrated into the model that previously only included trees. Working within the domain of the NASA Arctic-Boreal Vulnerability Experiment (ABoVE), we integrated three tall shrub genera (Salix, Betula, and Alnus), into the SIBBORK model. Modeling shrub ramets within the tree canopy expands the simulation range capability of SIBBORK-TTE from boreal to boreal-transition sites, increasing the applicability of the model at the ecotone. The model was calibrated and tested for a number of key sites within the ABoVE domain of Alaska. The results highlight the importance of shrub-tree competitive dynamics in predicting treeline advancement and retreat within the ecotone. Because the dynamics of the latitudinal treeline are not only temperature driven, the integration of tall shrubs, and eventually all tundra plant functional types, will provide an important tool to accurately predict how the boreal forest and tundra interact at their ecotone.