Constraining Rooting Depths in Tropical Rainforests Using Satellite Data and Ecosystem Modeling

Accurate parameterization of rooting depth is difficult but important for capturing the spatio-temporal dynamics of carbon, water and energy cycles. In this study, we adopted a new approach to constraint rooting depth in terrestrial ecosystem models over Amazon based on a combination of satellite-based analysis (diagnostic analysis) and terrestrial ecosystem modeling (prognostic analysis). We used VEGETATION (VGT) Enhanced Vegetation Index (EVI) data as a satellite-based vegetation index data and Biome-BGC as a terrestrial ecosystem model. We simulated seasonal variations in Gross Primary Production (GPP) with different rooting depths (1m, 5m, and 10m), and discussed how rooting depth affects seasonal GPP variations and which rooting depth simulates GPP consistent with satellite-derived EVI. EVI seasonal variations are roughly similar to solar radiation variations. In June-July-August, EVI takes its minimum over the study area except for east-central Amazon, which corresponds to a radiation minimum. Then, it peaks in September-October-November, maximum radiation season. Contrasting to seasonal pattern in precipitation, EVI takes its minimum in March-April-May and JJA and maximum in SON in most regions. Seasonal variations in simulated GPP shows strong contrast depending on rooting depth settings. GPP variations under one meter rooting depth are consistent with EVI seasonal variations only in humid tropical regions, where water is not limiting. Five meter rooting depth roughly simulated EVI variations, however, was not consistent with EVI in regions with severe dry seasons. Only GPP under 10 meter rooting depth was consistent with EVI, which peaks in September to November season (late dry season over the most of Amazon basin). Our study showed that (1) satellite-based observation gave constraints on rooting depth of the tropical ecosystem modeling and it should be at least deeper than 5 meters and (2) GPP of tropical forests in normal years seem to respond primarily to changes in solar radiation.