Abstract
Around the world, the increasing drought, which is exacerbated by climate change, has significant impacts on vegetation carbon assimilation. Identifying how short-term climate anomalies influence vegetation productivity in a timely and accurate manner at the satellite scale is crucial to monitoring drought. Satellite solar-induced chlorophyll fluorescence (SIF) has recently been reported as a direct proxy of actual vegetation photosynthesis and has more advantages than traditional vegetation indices (e.g., the Normalized Difference Vegetation Index, NDVI and the Enhanced Vegetation Index, EVI) in monitoring vegetation vitality. This study aims to evaluate the feasibility of SIF in interpreting drought effects on vegetation productivity in Victoria, Australia, where heat stress and drought are often reported. Drought-induced variations in SIF and absorbed photosynthetically active radiation (APAR) estimations based on NDVI and EVI were investigated and validated against results indicated by gross primary production (GPP). We first compared drought responses of GPP and vegetation proxies (SIF and APAR) during the 2009 drought event, considering potential biome-dependency. Results showed that SIF exhibited more consistent declines with GPP losses induced by drought than did APAR estimations during the 2009 drought period in space and time, where APAR had obvious lagged responses compared with SIF, especially in evergreen broadleaf forest land. We then estimated the sensitivities of the aforementioned variables to meteorology anomalies using the ARx model, where memory effects were considered, and compared the correlations of GPP anomaly with the anomalies of vegetation proxies during a relatively long period (2007–2013). Compared with APAR, GPP and SIF are more sensitive to temperature anomalies for the general Victoria region. For crop land, GPP and vegetation proxies showed similar sensitivities to temperature and water availability. For evergreen broadleaf forest land, SIF anomaly was explained better by meteorology anomalies than APAR anomalies. GPP anomaly showed a stronger linear relationship with SIF anomaly than with APAR anomalies, especially for evergreen broadleaf forest land. We showed that SIF might be a promising tool for effectively evaluating short-term drought impacts on vegetation productivity, especially in drought-vulnerable areas, such as Victoria.
Highlights
Droughts have significant adverse impacts on vegetation carbon assimilation by increasing vegetation mortality rate and changing the species composition of local ecosystems [1,2,3]
Regarding evergreen broadleaf forest land (Figure 3c), APARNDVI and APAREVI exhibited positive relative anomalies in February 2009, while solar-induced chlorophyll fluorescence (SIF) decreased by approximately 11.3% and gross primary production (GPP) decreased by approximately 8.5%
These results indicate that the relative GPP anomaly had a more consistent change with the relative SIF anomaly than with the relative APARNDVI and APAREVI anomalies, especially for evergreen broadleaf forest land
Summary
Droughts have significant adverse impacts on vegetation carbon assimilation by increasing vegetation mortality rate and changing the species composition of local ecosystems [1,2,3]. Australia is vulnerable to drought, as evident by the drought events in the 1980s, droughts in Victoria [4], 1990s Queensland drought [5], as well as the ‘Millennium drought’ between 2001 and 2009 [6]. In 2009, southeastern Australia suffered high temperatures, in excess of 40◦C, for several days in February [7]. Australia’s high vulnerability to drought threatens regional carbon balance in the terrestrial ecosystem. An accurate assessment of the impacts of drought on vegetation activity is crucial for understanding the response of terrestrial plants to climate anomalies, especially for drought-vulnerable regions, such as Australia
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