Seeing the Forest for the Trees

Andrea Lloyd (Middlebury College) studies trees in boreal forests across the circumpolar north in an effort to understand how a warming climate influences tree growth. Boreal forests, also called taiga, are northern hemisphere, high-latitude forests of coniferous trees. Boreal forest represents the world’s largest biome, covering most of northern North America. As climate warms, boreal forests are expanding within and into the Arctic. The potential effects of this expansion remain poorly understood.

Lloyd uses information derived from tree rings to understand the relationship between tree growth and climate. Tree rings, or growth rings, result from changes in xylem cells during the year. (Xylem is the water-transporting vascular tissue of plants, of which wood is constructed). In conifers, cells formed early in the summer are larger and appear lighter in color, whereas those produced later in the summer are smaller and appear darker. The alternating cell size results in annual rings visible in cross section or in tree cores. From tree-ring patterns that form over often hundreds and even (in rare cases of very long-lived conifers like bristlecone pines of Nevada) thousands of years, scientists can understand how tree growth changes with climate, and can also use tree rings to describe how climate itself has changed through time. Lloyd will include her data in models which predict the future relationship between tree growth and a changing climate, which is important for understanding land change in the circumpolar north.

“In the last few decades, tree growth has either become uncorrelated with temperature, or has become negatively correlated with temperature (so warm years = slow growth). This has been termed the ‘divergence problem’ because the trend in tree growth is diverging from the trend in temperature,” Lloyd explained about her U.S. National Science Foundation-funded 2007-2009 project in a recent email. “Our study was attempting to (a) determine how widespread this problem was by re-analyzing [more than] 200 archived data sets collected by other researchers and (b) determine the degree to which patterns of change in tree growth indicated by tree rings matched those identified by satellite-derived indices. We also had the goal of filling geographical holes in the existing tree-ring data—there was a dearth of archived data from Siberia or north-central Canada.”

The pattern in which warmer years are associated with a reduction in plant growth is called “browning.” Lloyd writes that in many areas of the boreal forest, browning has been happening for several decades. Whether the phenomenon is due to heat, drought or pollution, or something else, is unclear.

Although Lloyd’s team chose field sites based on geographic holes in the existing International Tree-Ring Data Bank, ease of logistics ultimately determined locations they visited. Travel in the Canadian Northwest Territories was relatively easy and the team put into field sites by float plane. Travel in Siberia proved a bit more difficult. Near Yakutsk, they travelled by car, by foot near Zhigansk, and by boat on the Lena and Kolyma Rivers.

The study focused on the distribution and productivity of several tree species in Siberia, including Dahurian larch, Siberian spruce, and Scots pine and in Canada’s Northwest Territories, Black and White spruce and Jack pine.

“We core them using an increment borer, which is a non-destructive method for extracting a thin cylinder of wood, about the diameter of a pencil, from a tree. We store the cores in straws, and hand-carry them home,” Lloyd wrote. “[In the lab] we sand the tree cores until we can see the rings, and then measure the rings and do a variety of statistical analyses to determine what the relationship is between tree growth and climate. Given the importance of the boreal forest for the global carbon cycle, understanding controls over tree growth is an important research goal.”—Marcy Davis