Trees as sensors of pre-eruptive change

Can plants on volcanoes tell us when or where elevated CO₂ gradients reflect subsurface change? As volcanic systems move from repose to unrest, variations in continuous surface degassing on vegetated volcanic flanks may reflect deep magmatic changes. Capturing these earliest indicators of change possibly indicating intrusions into the 12-8km depth range, where first gas exsolution is dominated by CO₂ and helium, is difficult in the tropics: hundreds of square kilometers of often densely vegetated terrain make ground surveys difficult and space-borne techniques are too insensitive and coarse to systematically capture and track such emission changes (Schwandner et al, 2017, SCIENCE eaam5782).

To break this time-space-sensitivity conundrum of elusive early eruption precursors, we use the plant cover to our advantage. Plants exposed to mild excess of CO₂ (not in tree kill zones or thermal features) may experience a "beneficial" effect from having more "food" (CO₂) available, creating short- and long-term measurable effects. Photosynthetic rates may increase - leading to earlier stomatal closure and due to consequentially reduced evapotranspiration, temporary heat stress- measurable from space (e.g., NASA's ECOSTRESS mission). Some plants also build this excess CO₂ into extra biomass, measurable by remote sensing (Cawse-Nicholson et al. 2018 Biogeosciences 15:7403). This naturally isotopically labelled excess biomass creates a time-resolved archive of past exposure through both stable (e.g., Bogue et al. 2019, Biogeosciences 16:1343) and radiogenic carbon isotopes (Lewicki et al. 2014 EPSL 390:52). This effect may trace run-ups to past eruptions, complementary to the sedimentary record. Short-term near-real-time increased photosynthetic activity may help identify times and areas of increased exposure to excess CO₂ (Bogue et al, 2019).

The challenge is now to learn how to read these remote sensing and in-situ signals accurately enough to be useful for detecting, tracking and quantifying degassing behavior and to discern confounding factors. A more comprehensive observation system approach is needed to enable future space-borne missions like NASA's SBG (Surface Biology and Geology) to use trees as proxy detectors of Enhanced Levels of Emissions in Volcanically Active Tropical Ecosystems (ELEVATE).