The effects of phenology on indirect measures of aboveground biomass in annual grasses

Remote sensing is increasingly being used to quantify vegetation biomass across large areas, often with algorithms based on calibrated relationships between biomass and indices such as the normalized difference vegetation index (NDVI). To improve capacity to evaluate grassland dynamics over time, we examined the influence of phenological changes on NDVI-biomass relationships in annual grasses. Our findings support the use of NDVI throughout early growth and the beginning stages of canopy maturation, but suggest caution for later stages. In contrast, measurements of fractional photosynthetically active radiation (fAPAR) absorbed by the canopy and leaf area index (LAI) served as good season-long surrogates for canopy biomass. Canopies reached maximum biomass approximately 40days after maximum greenness, with biomass increasing by approximately 20% during senescence. For multi-year studies of management impact (i) avoid using seasonal comparisons from dates much after the point of maximum greenness or (ii) consider non-NDVI-based approaches.