Insensitivity of Tree-Ring Growth to Temperature and Precipitation Sharpens the Puzzle of Enhanced Pre-Eruption NDVI on Mt. Etna (Italy)

Abstract

On Mt. Etna (Italy), an enhanced Normalized Difference in Vegetation Index (NDVI) signature was detected in the summers of 2001 and 2002 along a distinct line where, in November 2002, a flank eruption subsequently occurred. These observations suggest that pre-eruptive volcanic activity may have enhanced photosynthesis along the future eruptive fissure. If a direct relation between NDVI and future volcanic eruptions could be established, it would provide a straightforward and low-cost method for early detection of upcoming eruptions. However, it is unclear if, or to what extent, the observed enhancement of NDVI can be attributed to volcanic activity prior to the subsequent eruption. We consequently aimed at determining whether an increase in ambient temperature or additional water availability owing to the rise of magma and degassing of water vapour prior to the eruption could have increased photosynthesis of Mt. Etna's trees. Using dendro-climatic analyses we quantified the sensitivity of tree ring widths to temperature and precipitation at high elevation stands on Mt. Etna. Our findings suggest that tree growth at high elevation on Mt. Etna is weakly influenced by climate, and that neither an increase in water availability nor an increase in temperature induced by pre-eruptive activity is a plausible mechanism for enhanced photosynthesis before the 2002/2003 flank eruption. Our findings thus imply that other, yet unknown, factors must be sought as causes of the pre-eruption enhancement of NDVI on Mt. Etna.

Response to Referee #2

“…Elevation, and particularly the related moisture regime, in the eastern Mediterranean region is the main driver of tree-ring growth patterns in a multispecies conifer network comprised of P. nigra, P. sylvestris and P. pinea L. specimens (Touchan et al, 2016). A dipole pattern in tree-ring growth variability is reported for Mediterranean pines ranging from Spain to Turkey, with higher sensitivity to summer drought in the east than in the west, and with higher sensitivity to early summer temperature in the west (Seim et al, 2015). Strong correlations between autumn-to-summer precipitation and tree-ring growth and between summer drought and tree-ring growth have been reported for sites (mainly of conifers) in northern Africawestern Mediterranean, with trees in Morocco also responding to the North Atlantic Oscillation index (Touchan et al, 2017).…”

Spatial Upscaling of Tree-Ring-Based Forest Response to Drought with Satellite Data

“…Following the results from Huang et al [7], the time-scale of the SPEI with the strongest correlation with RWI is a duration of 11 months ending in July in the SWUS. Therefore, we used the SPEI between the previous September and July of the subject year (11-month time-scale in July) to reveal the relationship between forest growth and drought [7,25].…”

“…If a regional indicator of tree growth can be developed as a reliable surrogate of tree-ring width, it would allow the reverse process: upscaling of tree-ring data to a regional level [23,24]. Furthermore, to the extent that tree growth is a surrogate for climate conditions [9,23,25], it would allow another technique for evaluating prior climate conditions at regional, and perhaps global, scales.…”

Plant responses to volcanically elevated CO<sub>2</sub> in two Costa Rican forests

“…Additionally, none have been conducted in the tropics (Pinkard et al, 2010). A series of studies in Yellowstone National Park (USA) used its widespread volcanic hydrothermal CO 2 emissions for the same purpose, though it is not in the tropics (Sharma and Williams, 2009;Tercek et al, 2008). Yellowstone was particularly suitable for this type of study due to its protected status as a national park and because the large areas of CO 2 emissions made control points more available (Sharma and Williams, 2009;Tercek et al, 2008).…”

“…A series of studies in Yellowstone National Park (USA) used its widespread volcanic hydrothermal CO 2 emissions for the same purpose, though it is not in the tropics (Sharma and Williams, 2009;Tercek et al, 2008). Yellowstone was particularly suitable for this type of study due to its protected status as a national park and because the large areas of CO 2 emissions made control points more available (Sharma and Williams, 2009;Tercek et al, 2008). These studies reported changes in rubisco, an enzyme central to CO 2 fixation, and sugar production in leaves, similar to results from FACE experiments, suggesting that volcanically influenced areas like Yellowstone have untapped potential for studying the longterm effects of elevated CO 2 on plants.…”