Tropical Tree Rings and Environmental Change

The photograph on the cover of this issue of Southeastern Geographer is of a South Florida slash pine (Pinus elliottii var. densa; hereafter slash pine) cross-section from the Florida Keys that was prepared for tree-ring analysis. Slash pine is the foundation species and sole canopy tree of the endangered pine rockland ecosystem in the United States. This species also is the southernmost native conifer in the United States. As such, the cover photograph represents the equatorward progression of dendrochronology into tropical and subtropical regions. Dendrochronology is the science of using the annual growth rings in trees, dated to their exact calendar year of formation, to study patterns of processes that occur in the physical, natural, and cultural sciences. During the life of a tree, these biological organisms remain stationary and linked to the environment, making them effective indicators of local, regional, and global environmental changes. Like most living organisms, the growth rate of a tree is sensitive to both natural events and human activities. In most locations, environmental conditions (such as precipitation and temperature) during a given year will either be favorable or unfavorable for tree growth, resulting in ring widths that vary from year to year. Because trees are recorders of the environment, this pattern of wide and narrow growth rings can serve as an indicator to monitor environmental processes in most regions around the world.

Many tree species located outside of the tropics form a single and explicit growth ring each year in response to seasonal changes in some aspect of the environment, like precipitation, temperature, or solar radiation. These seasonal fluctuations cause tree growth to slow toward the end of the growing season, and finally stop in response to threshold levels of the environmental variable most important for tree growth. The majority of tree species located in tropical to subtropical latitudes do not experience strong seasonality in temperature or precipitation. These conditions prohibit trees from entering into a dormant state and therefore they fail to form distinct, annual growth bands. Yet, as the science of dendrochronology advances equatorward, scientists continue to find more tree species that produce annual rings (e.g., Worbes 2003; Speer et al. 2004; Brienen and Zuidema 2006; Buckley et al. 2007; Harley et al. 2012; Trouet et al. 2012). These rare occurrences are valuable because they provide researchers with an opportunity to acquire an annual record of environmental change from the tropics. Tree-ring records from the tropics are important because this region is the major recipient of direct insolation on Earth, and shifts in oceanic and atmospheric circulations, such as the El Niño-Southern Oscillation, have broad impacts on climate outside the tropics.

Tropical dendrochronology is one of the frontiers of tree-ring science, not because it exists as a fledgling subfield of dendrochronology—in fact, scientists have long recognized the dendrochronological potential of tropical tree species (reviewed by e.g., Stahle et al. 1999; Worbes 2002; Chrubini 2003; Wils et al. 2011)—but because of the scientific potential that still exists with the tropics being a global hotbed of biodiversity. Yet, at first glance, the term tropical dendrochronology seems inherently oxymoronic, given the scarcity of tropical trees that produce a single and explicit growth ring each year. However, with the advancement of science and technology, new methods and techniques for the analysis of tropical tree rings continue to emerge. On the forefront of such analyses is isotope dendrochronology. This subfield explores ways in which the use of oxygen, carbon, and hydrogen isotopes embedded within tree rings can provide information on linkages between atmospheric, oceanic, and terrestrial ecosystem processes. Other methods, like x-ray densiometry and reflected light image analysis (e.g., blue intensity), can be used to reveal past climatic or environmental change when applied to species that exhibit little ring width variation or inexplicit annual banding.

As the number of tree-ring studies increase around the world, especially in the tropics and subtropics, so does the ability to construct networks of tree-ring data. Broad networks of tree-ring time series can be used to address environmental questions over time and space at hemispheric and...