The History Damaged Tree Rings Might Tell Saturday, March 17 We spend our last night in the city of Antigua, about 25 kilometers from the capital, Guatemala City. Towering over the city is the volcano named Agua, and the volcanoes Fuego and Acatenango lurk nearby as well. In the 16th century, a mudflow from Agua destroyed the previous capital and caused the seat of colonial government to be moved to Antigua. Pacaya, also near Antigua and quite active, showered Guatemala City with ash in 2010. Large tropical eruptions can have consequences far beyond their immediate region, too. The particles an eruption sends flying into the atmosphere can actually change the temperature of the planet and influence atmospheric circulation. Interestingly, one way we know about how the climate responds to volcanic eruptions is through the use of tree rings, like the ones I'm investigating here in Guatemala. There are typically two ways we can identify the fingerprint of a volcanic eruption in the annual rings of trees. One of the most common is called a frost ring. In those years in which a volcanic eruption during the growing season causes a sudden drop in temperature, the growing cells of the tree can be damaged. These cells look big and burst and distended, and are often surrounded by resin, and reflect the damage that sudden freezing temperatures can do to growing tissues. Frost rings are often found in trees at high elevations or high latitudes, where tree growth is limited by temperature. While frost events may not always be associated with volcanoes, their widespread presence in the same year for multiple trees at multiple sites can be a strong line of evidence for a large, climatically important eruption in that year. Even when the cooling from an eruption is not abrupt enough to cause the formation of a frost ring, the reduction in temperature can cause narrow or low-density rings to form simultaneously over large parts of the world. In combination with ice cores from the Northern and Southern Hemispheres, which contain the actual chemical signature of the eruptions themselves, the precise annual dating of tree rings provides evidence for the timing and consequences of large volcanic eruptions. So, do the trees from the high mountains of Guatemala show evidence of any local volcanic eruptions? We're just starting to examine this question, but there are some tantalizing hints that perhaps they do. In some trees, we've identified cellular damage in the rings that could be frost rings. We need to collect more samples to be sure, though, particularly from tree-line environments like the one we visited on this trip. The samples from this expedition might help answer this question, since they are from the highest and probably the most temperature-sensitive trees we've yet collected in Guatemala. Tomorrow, we'll drive over the pass to Guatemala City and the airport, and return home. We've seen some exciting new landscapes and we've got new questions to ask, as well as new samples that we hope will help answer them. Matthew and I intend to be back here, with Diego and Edwin, in the future. There are still a lot of question marks scribbled on our maps, and we're still in search of the oldest trees.