Global and regional modelling of Arctic-boreal vegetation distribution andits sensitivity to altered forcing

Understanding the distribution and function of Arctic and boreal ecosystems under current conditions and their vulnerability to altered forcing is cru cial to our assessment of future global environmental change. Such efforts can be facilitated by the development and application of ecological models that simulate realistic patterns of vegetation change at high latitudes. Th is paper reviews three classes of ecological models that have been implemen ted to extrapolate vegetation information in space (e.g. across the Arctic and adjacent domains) and over historical and future periods (e.g. under al tered climate and other forcings). These are: (i) equilibrium biogeographic al models; (ii) frame-based transient ecosystem models, and (iii) dynamic g lobal vegetation models (DGVMs). The equilibrium response of high-latitude vegetation to scenarios of increased surface air temperatures projected by equilibrium biogeographical models is for tundra to be replaced by a northw ard shift of boreal woodland and forests. A frame-based model (ALFRESCO) in dicates the same directional changes, but illustrates how response time dep ends on rate of temperature increase and concomitant changes in moisture re gime and fire disturbance return period. Key disadvantages of the equilibri um models are that they do not simulate time-dependent responses of vegetat ion and the role of disturbance is omitted or highly generalized. Disadvant ages of the frame-based models are that vegetation type is modelled as a se t unit as opposed to an association of individually simulated plant functio nal types and that the role of ecosystem biogeochemistry in succession is n ot explicitly considered. DGVMs explicitly model disturbance (e.g. fire), o perate on plant functional types, and incorporate constraints of nutrient a vailability on biomass production in the simulation of vegetation dynamics. Under changing climate, DGVMs detail conversion of tundra to tree-dominate d boreal landscapes along with time-dependent responses of biomass, net pri mary production, and soil organic matter turnover-which all increase with w arming. Key improvements to DGVMs that are needed to portray behaviour of a rctic and boreal ecosystems adequately are the inclusion of anaerobic soil processes for inundated landscapes, permafrost dynamics, and moss-lichen la yer biogeochemistry, as well as broader explicit accounting of disturbance regimes (including insect outbreaks and land management). Transient simulat ion of these landscapes can be further tailored to high-latitude processes and issues by spatially interactive, gridded application of arctic/boreal f rame-based models and development of dynamic regional vegetation models (DR VMs) utilizing plant functional type schemes that capture the variety of hi gh-latitude environments.