The other side of drought: What is the effect of hydrological environment on Amazon forest "green-up" and drought responses?
Abstract
Drought strongly influences ecosystem dynamics and forest function and hence, carbon cycle feedbacks to climate. Tropical forests could be particularly influential, but tropical forest drought response is uncertain, with intriguing puzzles that remain unresolved. For example, ground-based observations in Amazonia show increased tree mortality and biomass loss following drought while satellite-based studies show some regions green-up during large scale droughts of 2005 and El Nino 2015/2016. Here we propose that different kinds of forest response in the diverse hydrological environments across Amazonia may explain mechanisms of drought response and reconcile the seeming contradiction between observations from ground and space. We used water table depth (WTD) as a remote sensing/modeling derived proxy of hydrological environment and water availability, and investigated its influence on tropical photosynthetic dynamics during droughts in 2005, 2010 and 2015/2016 by analyzing two satellite-based measures of forest photosynthetic properties (enhanced vegetation index, EVI, corrected for sun-sensor geometry, and solar-induced chlorophyll fluorescence, SIF). Results show that the magnitude of drought response depends on WTD: the most positive photosynthetic deviations from mean responses were observed in forests with the shallowest water tables. As water tables deepen, photosynthetic response initially decreases until it reaches a minimum at a water table depth of 10-12m, after which responses increase again. These results suggest that both shallow water table forests (SWTF) and deep water table forests (DWTF) can tolerate drought to a degree. We hypothesize different mechanisms for this bimodal response: that photosynthesis of SWTF can be sustained or even benefit from drought due to higher access of otherwise waterlogged roots to oxygen during drought, while maintaining access to water. On the other hand, DWTF, which according to recent work in forests near Manaus appear to be dominated by trees with higher embolism resistance, may be more adapted to drought conditions. We suggest that hydrological environments (as indicated by water table depth) are an important control on water availability and thus key to predicting the overall vulnerability of forests to drought.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B24E..03C
- Keywords:
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- 0410 Biodiversity;
- BIOGEOSCIENCES;
- 0476 Plant ecology;
- BIOGEOSCIENCES;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE;
- 1616 Climate variability;
- GLOBAL CHANGE