Effects and Feedbacks of Windthrow/breaks in Boreal Forest Ecosystems

The increased frequency of severe storms (Leckebusch et al., 2007) as a result of ongoing climate change, results in a wide area damage events within boreal forest ecosystems. The damage occurs at exposed forest edges as well as inside forest stands creating the gaps. Once a windthrow/break gap occurs, it results in changes of surface albedo and microclimatological conditions and in increasing wind stress on remaining trees around the gap which in turn increase the risk of further wind damage. The self-induced growth of windthrow gap provides other positive as well as negative feedbacks to climate forcing at different spatial and temporal scales as shown in Vygodskaya et al., 2007, e.g. an increase of CO2 efflux (climate warming) and an increase of surface albedo (climate cooling). The present study characterizes the spatial variation of wind load and the changes in radiative regime (surface albedo) within the damaged forest stand. For description of wind field and load on trees the atmospheric boundary-layer two-equation closure model SCADIS based on transport equations for turbulent kinetic energy (E) and specific dissipation (omega) (E-omega model), which accounts for the flow dynamics within a plant canopy (Panferov and Sogachev, 2008; Sogachev and Panferov, 2006) was used. The radiative regime within the damaged forest is described by means of a three-dimensional radiation transfer model SPM3D (Panferov et al., 2005). A series of numerical experiments with circular and rectangular forest gaps with sizes from 3 to 75 tree heights, h, have been carried out for a modelled boreal forest. To evaluate the changes produced by gaps relatively to undisturbed forest all characteristics were normalized by their values for the latter. The results of the study show that the magnitude of wind load on trees surrounding the newly created forest gaps increases with gap size and is app. 7 times higher than the load on trees in an undisturbed forest. The gust component of wind load reaches its maximum at app. 20 tree heights, h, and remains almost constant for larger gaps, while the static load and radiation characteristics continue to change significantly with gap size. The study has also demonstrated that the spatial distribution of wind load and of short-wave radiation depends strongly on the gap size and on mutual distribution of gaps within the modelled domain. 1) Leckebusch, G. C., U. Ulbrich, L. Fröhlich, and J. G. Pinto (2007), Property loss potentials for European midlatitude storms in a changing climate, Geophys. Res. Lett., 34, L05703, doi:10.1029/2006GL027663; 2) Sogachev, A., and O. Panferov, 2006, Modification of two-equation models to account for plant drag, Boundary-Layer Meteorology 121:229-266; 3)Vygodskaya N.N., Groisman P.Ya., Tchebakova N.M., Kurbatova J.A., Panfyorov O., Parfenova E.I., Sogachev A.F., 2007, Ecosystems and climate interactions in the boreal zone of Northern Eurasia, Environ. Res. Lett. 2, 045033 (7pp) doi:10.1088/1748-9326/2/4/045033