We applied a well-tested mechanistic model, ecosys, to examine how different plant types (evergreens, graminoids, deciduous, moss, and lichen) across the boreal forests and Arctic tundra of Alaska will respond to projected 21st-century changes in climate and fire. We modeled, consistent with changes during the Holocene, that changes in 21st-century climate and fire will favor Alaskan deciduous plants, making them dominant in northern ecosystems. These changes occurred because of complex interactions between enhanced soil microbial activity early in succession and competition for light later in succession.
The expansion of deciduous plants in a warmer climate may result in several ecological and climatic feedbacks that affect the carbon cycle of northern ecosystems. For example, increases in surface litter input and lower litter lignin content results in positive feedbacks to more rapid microbial decomposition and nutrient cycling, changes seasonal phenology, and increases transpiration and thus summer longwave radiative forcing. Declines in herbaceous plant productivity may also affect the amount and distribution of summer forage, and thus change habitat for moose and other animals.
High-latitude regions have experienced the most rapid warming in recent decades and this trend is projected to continue over the 21st century. Fire is also projected to increase with warming. We show here, consistent with changes during the Holocene, that changes in 21st-century climate and fire are likely to alter vegetation composition of Alaskan boreal forests and tundra. We hypothesize that tradeoffs in competition for nutrients after fire in early succession and for light later in succession in a warmer climate will cause shifts in plant functional types. Consistent with observations, evergreen conifers were modeled to be the current dominant trees in Alaska. However, under future climate and fire, our study suggests the relative dominance of deciduous broadleaf trees nearly doubles, accounting for 58% of Alaska ecosystem net primary productivity (NPP) by 2100, with commensurate declines in contributions from evergreen conifer trees and herbaceous plants. The relative dominance of both deciduous and evergreen shrubs were shown to increase in much of the Arctic tundra, particularly in the Northern Slopes and Brooks Range, consistent with field experiments and observations indicating that climate warming will increase shrub cover in Arctic tundra. Post-fire deciduous plant growth under future climate was sustained from enhanced microbial nitrogen mineralization caused by warmer soils and deeper active layers. Expansion of deciduous trees and shrubs will affect the carbon cycle, surface energy fluxes, and ecosystem function, thereby affecting multiple feedbacks with the climate system.