TREE RINGS, AGRICULTURE AND THE GERMANIC TRIBES

            The relationship between tree rings and agriculture is not a direct relationship, but one can be established through the indirectly.  There have been studies that have found correlation between agriculture and tree ring indices in semiarid southwest USA (Burns, 1983).  This theory is based upon biological uniformitarianism.  If trees are varying growth rates due to changes in climate and changes in climate are affecting agriculture in modern times, it should be a feasible argument to suggest that this could be applied in the past.  Burns (1983) examined dry-farmed corn and beans and found a direct correlation with the tree-ring chronologies of the same area.

Theoretically, trees will respond to certain changes in weather conditions in a relatively predictable fashion.  These responses to changes in weather conditions will also in turn affect the regional agriculture.  So, there is a direct relationship between certain species of tree-ring growth and agricultural product.  In other words, tree-ring chronologies can identify periods that may be more or less productive agriculturally.  In some cases, especially in long drought events, both the trees and the agricultural crops will produce smaller amounts of biomass.  This will be reflected in smaller tree rings and lower yields in crops. 

Unfortunately, some studies have suggested that individual European oak chronologies are not always accurate paleoclimate indicators, unlike pine or spruce (Schweingruber, 1988).  However, this obstacle can be overcome through the utilization of a crossdatable network of oak chronologies that cover a larger area.  This network will often reduce the non-climate signal contained in the local oak chronologies (Briffa et al., 1986, Schweingruber, 1988, Esper et al., 2002).  This is one of the main reasons that several chronologies have been examined in this study.

However, even with the inference of tree ring revealing past climate and soil moisture availability, there are other factors in agricultural production.  For example, a crop reaction to adverse climate may vary by soil types.  It is also necessary to determine what the response of agricultural crops will be to a climate change and ascertain if the span of this change will cause an ephemeral reaction or a permanent effect (Parry, 1981) .  The lack of rainfall can be detrimental to the food stores for a variety of reasons, aside from just poor agricultural productivity and potential loss of crops.

Some crops that have been subjected to water stresses are more likely to suffer damage from insects (Flohn and Fantechi, 1984) .  Many farm livestock have been shown to be vulnerable to certain disease and pests when weakened by drought, or lack of soil moisture (Flohn and Fantechi, 1984) .  Studies have shown that if the annual rainfall decreases from 20 inches to 10 inches then the area of the Russian Steppes that would normally support up to 60 sheep per acre would fall to only one sheep per acre (Huntington, 1907) .  A similar study was done in Wales, United Kingdom that showed this same change in precipitation would drop the sustainability of the sheep from 600 per grazing acre to 10 sheep per grazing acre (Huntington, 1907) .

In areas that have decreased rainfall and grow cereal crops, as in Germania, it may actually benefit the winter and spring crops if the soil moisture is maintained (Flohn and Fantechi, 1984) .  That is one of the reasons why this study defines a below mean period as any period that has at least four consecutive years below the 21-year mean of the tree ring chronology, because this length of time should begin to effect the agriculture of the region.  The soil moisture availability would surely decrease over this length of time and would most likely be reflected in the tree ring chronologies that are compiled from predominantly river valley trees, which would be dependant on soil moisture availability.

Even though there has been correlation found between some crops and tree rings (Burns, 1983), there is still difficulty linking tree rings to agriculture uniformly around the earth.  The challenge with directly linking agriculture to tree rings is the problem of linking any two types of flora to the same climatological changes.  Different plant species could have distinctly different responses to climate change.  The best that can be done is to imply that drought events that affect trees would have affected a wider variety of plant life, which could have easily included the agricultural crops of the Germanic tribes.  The tree ring record supports a positive relationship between documented agriculture abundance in both AD 357 and 359 as discussed earlier, and it also shows a relationship in documented drought periods as in AD 348 and 301 discussed in chapter five.  These relationships, although they are somewhat indirect, are still significant in the evaluation of the historical events of 3^rd and 4^th century AD.