Annual growth ring patterns in Brachystegia spiciformis reveal influence of precipitation on tree growth

The availability of exactly dated tree-ring chronologies is limited in tropical regions. However, these chronologies could contribute widely to studies of the influence of natural and human-induced factors on tropical forests. We examine the potential for building a chronology based on three sites in the miombo woodland of western Zambia. Brachystegia spiciformis Benth., a dominant species from this vegetation type, is used. Response of the chronology to several climatic factors is examined. All specimens showed very clear growth rings, and cross-dating between radii of a tree was successful for all trees. Site chronologies could be constructed after cross-dating of growth ring series of individual trees. The mean growth ring curves of the three sites were significantly similar, allowing for the construction of a regional chronology. Correlation function analysis between the tree-ring chronology and regional climatic variables revealed that climate at the core of the rainy season, in December and January, has an explicit influence on tree growth. Where precipitation and relative humidity in these months influence tree growth positively, temperature correlates in a negative way. Some 20 percent of the variance in the B. spiciformis tree-ring chronology is accounted for by wet season rainfall. The successful cross-dating and correlation between a tree-ring chronology and climate demonstrated in this study indicate annual ring formation in B. spiciformis trees and sensitivity to climatic conditions.