Dynamics of plant functional types at the forest-tundra ecotone

Background/Question/Methods

Arctic and boreal ecosystems contain a suite of disparate plant functional types (PFTs) that includes evergreen and deciduous trees, evergreen and deciduous shrubs of varying morphologies (tall, low, dwarf, prostrate), sedges, grasses, forbs, mosses, and lichens.  In many circumpolar modeling efforts, these extremely different PFTS are frequently lumped into one or two broad categories that are not adequately precise for representing the response of vegetation to environmental changes; for example, the arctic tundra is still commonly parameterized as C₃ grassland, when grasses make up a very minor component of the plant community.  Some recent modeling studies have included a more detailed representation of these plant communities, with the assumptions that PFTs respond differentially to environmental changes, and that PFTs have widely varying effects on ecosystem functions, such as energy exchange, water cycling, and carbon cycling.  We briefly review PFT categorization in arctic tundra and boreal forest ecosystems, and then specifically address the necessity for understanding PFT dynamics at the forest-tundra ecotone, where vegetation responses to environmental change will likely have strong direct or indirect effects on important ecosystem properties, such as soil water, soil carbon, and permafrost. 

Results/Conclusions

The latitudinal forest-tundra ecotone is experiencing some dramatic changes in PFT composition that have been observed with remote sensing over the past 50 years.  One seemingly widespread change is the replacement of short-statured tundra (dominated by sedges, mosses and low/dwarf shrubs), with tall shrubs (e.g. alder, birch, willow).  This change in PFTs could amount to a three-fold increase in the height of the vegetation and a variety of changes in the functioning of these ecosystems.  Tall shrubs decrease albedo relative to upland sedge-dominated tundra, particularly in the spring when the shrubs extend above the snowpack.  They also decrease summer soil temperatures through shading and increase winter soil temperatures as a result of greater insulation (in part due to trapping of windblown snow beneath the shrub canopy).  Tall shrubs will also likely increase transpiration, leading to reduced soil moisture and potentially deeper water tables.  All of these functional differences will affect the soil thermal regime, with implications for permafrost and soil carbon.  Additionally, there may be short- and long-term changes to the understory PFT composition (e.g. mosses and lichens) and altered plant-herbivore interactions.  The case of the forest-tundra transition and other high-latitude vegetation changes illustrate the importance of PFT representation in high-latitude ecosystem and climate models.