Abstract
Despite its perennial canopy, the Amazonian tropical evergreen forest shows significant canopy growth seasonality, which has been represented by optical satellite-based observations. In this paper, a new Microwave Temperature–Vegetation Drought Index (MTVDI) based on Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sensors was used to capture the canopy seasonality from 2003 to 2010 in comparison with four climatic dryness indicators (Palmer Drought Severity Index (PDSI), Climatological Water Deficit (CWD), Terrestrial Water Storage (TWS), Vapor Pressure Deficit (VPD)) and two photosynthesis proxies (Enhanced Vegetation Index (EVI) and Solar-Induced chlorophyll Fluorescence (SIF)), respectively. Our results suggest that the MTVDI shows opposite seasonal variability with two photosynthesis proxies and performs better than the four climatic dryness indicators in reflecting the canopy photosynthesis seasonality of tropical forests in the Amazon. Besides, the MTVDI captures wet regions that show green-up during the dry season with mean annual precipitation higher than 2000 mm per year. The MTVDI provides a new way for monitoring the canopy seasonality of tropical forests from microwave signals.
Highlights
The Amazon rainforest, holding 50% of tropical forests carbon stocks [1], accounting for about 15% of global terrestrial photosynthesis [2], is commonly known as an important and continuous part of Earth system functioning [3]
From the AMSR-E 23.8 GHz Microwave Polarization Difference Index (MPDI) and, constructed Microwave Temperature–Vegetation Drought Index (MTVDI) based on Ts and Microwave Normalized Difference Vegetation Index (MNDVI) derived from microwave signals
In the MTVDI (Ts-MNDVI) triangle space, the wet edge means that the vegetation growth is not limited by water due to sufficient soil water supply and the surface evapotranspiration is equal to the potential evapotranspiration, while the dry edge represents that the effectiveness of soil moisture is low and the surface evapotranspiration is small
Summary
The Amazon rainforest, holding 50% of tropical forests carbon stocks [1], accounting for about 15% of global terrestrial photosynthesis [2], is commonly known as an important and continuous part of Earth system functioning [3]. More investigations from other types of satellite signals independent of optical sensors are needed to verify such a dry-season green-up phenomenon across Amazonian tropical evergreen forests. To date, few microwave remote sensing studies are focused on detecting the dry-season green-up of tropical evergreen forests. Vegetation Drought Index (TVDI) [28] and brightness temperature data (Tb) of the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) microwave sensor It utilized the advantage of TVDI, which combines land surface temperature (Ts) with Normalized Difference Vegetation Index (NDVI) when monitoring the vegetation drought condition and overcame the shortage of optical remote sensing, which is greatly influenced by atmospheric conditions. From the AMSR-E 23.8 GHz Microwave Polarization Difference Index (MPDI) and, constructed MTVDI based on Ts and MNDVI derived from microwave signals. We aggregated daily Tb data into monthly by using the average method
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