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Fire History Research -  Blodgett Forest Research Station, Georgetown, CA
Brandon M. Collins, Staff Researcher, Stephens Lab, UC Berkeley  brandonmcollins@hotmail.com

The objective of this research is to create a fire history for the UC Berkeley research forest, Blodgett Forest. This involves assigning dates to individual fires recorded within the growth rings, called fire scars. The method for assigning dates is called cross dating. Cross dating involves comparing ring width sequences of unknown dates, to a set of ring width sequences of known dates. The set of ring width sequences of known dates is called a master tree ring chronology. In many areas, master tree ring chronologies have already been developed. In our case, at Blodgett Forest, we found it necessary to create our own master tree ring chronology. We chose to do this because most of the established master tree ring chronologies in areas near Blodgett Forest were for pines. Most of our samples with fire scars are from incense cedar (Calocedrus decurrens), and we haven't found good correlation in ring width sequences between the pines and Calocedrus decurrens.

Master Tree Ring Chronology

We selected a drier site, with shallow soils, to collect cores for the master tree ring chronology. The reasoning behind selecting this type of site is that the trees would tend to show more of a climatic signal in the growth rings, as compared to trees on a better site. We took 2 cores, opposite each other and parallel to the contour, from each of 24 incense cedars. We then measured the ring widths for all the cores and compiled the measrurements using a program called COFECHA. We then made sure all the cores matched up appropriately so they could be entered into another program called ARSTAN. This program uses a series of statistical tools to develop the indicies for the ring widths that make up the chronology.

We made a comparison between historic precipitation and the tree ring chronology to see how well the master tree ring chronology tracked the actual climatic fluctuations. We have precipitation data for Blodgett Forest that begins in 1963, so 1963-1999 is the extent of this comparison. A precipitation index was created by standardizing each yearly value of precipitation by the mean of all the values from 1963-1999. The master tree ring chronology has already been standardized by ARSTAN, but we divided this by 1000 to produce values of precipitation and the chronology on the same scale.

The figure above shows that the trend in the chronology somewhat tracks the trend in precipitation, with, what appears to be, a time lag. This time lag probably reflects the growth pattern of Calocedrus decurrens. A year with below average precipitation would result in decreased growth one or two years after this event. Some of the more obvious years where this trend is observed are 1966, 1977, and 1989.

Fire Scars

We are cutting samples of fire scars in clusters. We have 6 cluster, consisting of approximately 10 samples each, spread out over the 4400 acres of Blodgett Forest. Once we cross date these samples we will be able to say, with some certainty, when the major fire events occurred in the last 350 years. And, we will be able to describe the fire return interval, for that area, in the last 350 years. We will hopefully be able to see if the fire return interval changed with a number of variables, such as: topography, distance to riparian areas, and time.

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