Seasonal evapotranspiration over an invader vegetation (Pteridium aquilinum) in a degraded montane grassland using surface renewal

Cathedral Peak Research Catchments, uKhahlamba Drakensberg mountain range, South Africa. The focus was to determine the seasonal pattern of evapotranspiration (ET) over a bracken canopy, which is a product of degradation within the montane grasslands of the uKhahlamba Drakensberg mountains. Surface renewal (SR) was investigated as a viable alternative to eddy covariance for long-term estimation of ET within a remote high rainfall montane landscape. The SR dissipation method was used to observe ET over the bracken canopy for 23 months, across two consecutive years with variable seasonal rainfall timing and amounts. Besides ET, rainfall and energy flux components were observed. The SR dissipation method was shown to be adequate for the long-term estimation of evapotranspiration over a bracken canopy within a remote high summer rainfall montane landscape. The bracken canopy was found to have a distinct seasonal cycle. Autumn was most sensitive to rainfall amount and timing, as late annual rainfall during autumn decreases bracken ET compared to higher summer rainfall followed by a lower autumn rainfall. Comparing the evapotranspiration of bracken from this study to the evapotranspiration of natural grassland in previous studies, the largest difference in evapotranspiration between the canopies occurs in winter.