Tree-mendously accurate?

This post may seem rather contradictory after the last couple, but I found a very recent paper that actually gives very robust evidence for drought, with the location and dating accuracy in check. Stahle et al. (2011) have discovered two Montezuma baldcypress trees in Mexico (Barranca de Amealco, Queretaro and Los Peroles, San Luis Potosi) that provide millennium-long, annually-resolved palaeoclimatic records (the first exactly dated records for Mesoamerica might I add). Not only do they reconstruct past rainfall, but they can also be used to date archaeological sites to a highly accurate level, especially in comparison to the proxies such as speleothems and sediment cores that have been constrained by age estimates.

The Queretaro record shows that the most severe drought of the past 1,200 years occurred between AD 897 and AD 922, which extended into the central Mexican highlands. Both records confirm the Terminal Classic series of droughts (Figure 1), indicating that they were centred at AD 810 and 860 – the same as the dates given by the Cariaco Basin core.  I have drawn up a table (below) showing all the key dates for severe dry periods (provided by the studies mentioned in this blog) to see how they all compare with this new reconstruction and to make it less confusing:

Location of proxy	Proxy used	Peak drought conditions
Queretaro and San Luis   Potosi, Mexico	Tree rings	AD 810, 860, 897-922
Macal Chasm cave, western Belize	Speleothem (luminescence, colour, δ18O and δ13C)	AD 754-798, 871 and 893-922
Lake Chichancanab, Yucatan, Mexico	Lake sediments (δ18O, δ13C and gypsum/calcite ratios)	AD 922
Cariaco Basin, northern Venezuelan coast	Laminated marine sediments (titanium content)	AD 810, 860 and 910

Did drought affect other cultures?

I know at the beginning I said I would briefly look at the other Mesoamerican cultures, but there has been so much on the Maya I haven’t had a chance yet. This paper actually documents the rise and fall of other civilisations throughout Mesoamerica including the Aztecs and Toltecs. So...did drought affect them? Before I answer this question, have a look at the map below to see what parts of Mesoamerica they inhabited:

Map of Mesoamerica showing the areas dominated by the Maya, Toltec and Aztec
Source: http://www.erin.utoronto.ca/~w3env100y/env/ENV100/hum/map.htm

Toltecs

The Toltecs dominated central Mexico during the early Post-Classic era; since they lived in the highlands where freezing conditions in the autumn shortened the growing season, early growing season drought would have affected them severely. The tree-ring reconstruction shows that a 19-year drought prevailed from AD 1149 to 1167, which may have easily caused famine. There is a possibility that drought played a role in their demise by pushing the Chichimeca population to migrate to the Toltec state, causing instability and eventual abandonment.

Aztecs

The collapse of the Aztecs is less complex than that of the Maya, as much of the population was decimated with the arrival of the Spanish in 1521, who used weapons unfamiliar to them and introduced new diseases and infections that they were not immune to. A drought during the Colonial era may have also contributed to the dramatic depopulation, weakening their society and making them more vulnerable. The new reconstruction indicates that it was the worst Mesoamerican drought since AD 771, lasting from 1378-1404 (as you can see in figure 1); despite this their collapse has ultimately been attributed to the Spanish conquest (McAnany and Negron, 2010). 


References:

McAnany, P.A. and T.G. Negron (2010) 'Bellicose rulers and climatological peril?' In: McAnany, P.A. and N. Yoffee (eds) Questioning Collapse, Cambridge: Cambridge University Press, 142-175