Using active optical sensing of biomass to investigate the effect of scattered trees on native perennial pastures

Trees scattered throughout paddocks have long been considered keystone structures in agricultural landscapes, however little is known about how they actually influence pasture growth. Assessing pasture biomass with sufficient spatial detail requires a cost and time-effective sensor. In this study we test whether an active optical sensor, utilising red and near-infrared wavebands to derive the normalised difference vegetation index (NDVI), can be used as a surrogate measure of photosynthetically active biomass (PAB) associated with native perennial grasses around scattered trees in grazing paddocks. Pasture cuts were acquired to create and test NDVI-biomass calibrations in the vicinity of 12 scattered trees across a range of species and soil parent materials. Observed regressions beneath the tree between NDVI and PAB were statistically significant although R² were generally less than 0.33. We believe that this result is largely due to the variable composition of the native pastures (live and dead fraction, physical structure). Overall, no significant difference was found in PAB with distance from the tree, meaning that under the tree there was similar pasture biomass to the open paddock.