Trees Talk Tremor—Wood Anatomy and δ13C Content Reveal Contrasting Tree-Growth Responses to Earthquakes

Large earthquakes can increase the amount of water feeding stream flows, raise groundwater levels, and thus grant plant roots more access to water in water-limited environments. Here, we quantify growth and photosynthetic responses of Pinus radiata plantations to the Maule Mw 8.8 earthquake in geometrically simple headwater catchments of Chile's Coastal Range. To this end, we combine high-resolution wood anatomic (lumen area) and biogeochemical (δ¹³C of wood cellulose) proxies of daily to weekly tree growth sampled from trees on valley bottoms and close to ridge lines. We find that, immediately after the earthquake, at least two out of six tree trees on the valley floor had enlarged lumen area and lowered δ¹³C, while trees along the hillslope ridge had a reverse trend. Our findings favor a control of soil water on this response, largely consistent with models that predict how enhanced postseismic vertical soil permeability causes groundwater levels to rise on valley floors, but fall along the ridges. Statistical analysis with non-parametric boosted regression trees reveals that streamflow discharge gained importance for photosynthetic activity on the ridges, but lost importance on the valley floor after the earthquake. We conclude that earthquakes may stimulate ecohydrological conditions favoring tree growth over days to weeks by triggering stomatal opening. The weak and short-lived signals that we identified, however, imply that such responses are only valid under water-limited, rather than energy-limited tree, growth. Hence, dendrochronological studies targeted at annual resolution may overlook some earthquake effects on tree vitality.