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

Abstract

Abstract 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 °C mean annual temperature and 600 mm mean annual rainfall. Bands are oriented perpendicular to the prevailing wind direction, with older and dying trees in the windward edge and a seedling regrowth in the lee side of each band. Forest structure, species composition and relevant soil properties were sampled in a wind-affected forest and in an undisturbed stand. In the former, samples were obtained in transects across the banding and along a hill-slope gradient. Results show 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 ratio and redox potential increase. Based on field observations, we have developed 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 with age and with lack of protection from older windward trees. In such conditions, bands are the outcome of a tuning between tree growth rates and wind killing capacity. On the basis of this hypothesis, a simulation model, based on the cellular automata approach, was constructed. Simulated patterns that arise from heterogeneous forests with random age distributions match successfully with those observed in nature. Increasing tree growth rates lead 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 between a directional disturbance and an oscillatory process, such as stand regeneration, growth and decay.