Banded vegetation patterning in a subantarctic forest of Tierra del Fuego,as an outcome of the interaction between wind and tree growth

Banded patterns have been investigated in a Nothofagus betuloides primeval forest from Bahia del Buen Suceso, on the eastern edge of Tierra del Fuego island (Argentina). These forests grow on spodosols developed upon silicic shales, in a cold oceanic climate, with 5 degrees C mean annual temperature and 600 mm mean annual rainfall. Bands are oriented perpendicular to the p revailing wind direction, with older and dying trees in the windward edge a nd a seedling regrowth in the lee side of each band. Forest structure, spec ies composition and relevant soil properties were sampled in a wind-affecte d forest and in an undisturbed stand. In the former, samples were obtained in transects across the banding and along a hill-slope gradient. Results sh ow that wind causes about 50% reduction of stand basal area and of size of overstorey trees. Stand growth processes, such as self-thinning, basal area and height growth, and specific composition of the understorey, occur in a windward direction, as well as changes in soil properties such as C/N rati o and redox potential increase. Based on field observations, we have develo ped an hypothesis of how wind is able to generate this pattern. Its core is that bands develop when vulnerability of trees to wind damage increases wi th age and with lack of protection from older windward trees. In such condi tions, bands are the outcome of a tuning between tree growth rates and wind killing capacity. On the basis of this hypothesis, a simulation model, bas ed on the cellular automata approach, was constructed. Stimulated patterns that arise from heterogeneous forests with random age distributions match s uccessfully with those observed in nature. Increasing tree growth rates len d to longer wavelengths and higher wave propagation rates, while increasing wind killing potential leads to shorter wavelengths and lower propagation rates. This interpretation of banded patterning involves a resonance betwee n a directional disturbance and an oscillatory process, such as stand regen eration, growth and decay. (C) Elsevier, Paris.