Size Does Matter When It Comes to Saving Forests
Faced with tight quarters, Noah made the right choice: Instead of filling the ark entirely with a multitude of cockatiels or red pandas, he gathered up two of all the animals. Modern conservationists face a similar quandary, trying to figure out how best to preserve as broad a swath of life's diversity as possible, given limited resources. The trouble, said ecologist Christian Marks of The Nature Conservancy, is that lacking divine guidance, it’s not easy to tell where conservation efforts will do the most good.
Marks was part of a team of scientists who turned up what he thinks could be a handy shortcut for conservationists working to preserve forests that slow climate change by storing carbon dioxide and that provide habitat for wildlife. In a new study published in the journal Ecology Letters, the team showed that the height of trees in forests in the United States is strongly correlated with how many tree species those forests contain—basically, the taller the trees, the more kinds of trees. “Tree height becomes a really good predictor,” Marks said.
The team’s findings could help conservationists use satellite or laser imaging data to assess which areas of a forest are most worth fighting for at extremely fine scales, even acre by acre. “You might have a really diverse forest with really tall trees, and then a quarter mile away, on a wind-exposed dry ridge, you don’t have a diverse forest,” he said.
The basic reason that tree height and forest diversity are tied, said Helene Muller-Landau, a staff scientist at the Smithsonian Tropical Research Institute in Panama and a coauthor of the study, is that “trees are always playing game theory”—each is constantly trying to get more light, water, and nutrients than its neighbors. Height is costly, though, and stressful environments put the brakes on the inter-tree competition. Too little water, too much water, salty water, heat, cold, and wind all affect tree height, Marks said. Collectively, these stressors can be described as an area’s “harshness.” In the past, scientists have struggled to connect area harshness to species richness, Muller-Landau said. The arguments often go in circles: “How do you know if an area is easy for species?” she asked. “Well, because there’s a lot of species there.” But tree height seems to provide a way out of the loop.
The team pulled data from some 80,000 U.S. Forest Service plots scattered across the eastern and western regions of the country. Foresters had measured both tree height and species composition in 6,458-square-foot sections. In both those small plots and bigger, 360-square-mile grids, maximum tree height was a good predictor of tree species diversity. Less stress, said Muller-Landau, means a broader range of niches for species, which means more competition and more height.
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Some ecologists think that might be putting it too simply. Thomas Givnish, a plant ecologist at the University of Wisconsin–Madison who was not involved in the study, pointed out that in the Appalachian Great Smoky Mountains, the areas with the greatest number of tree species are not the tall pine forests. Rather the most diverse forests were in an area with medium-height broadleaf trees and conifers.
Similarly, the towering redwood groves of California’s North Coast are relatively homogeneous compared with forests 100 miles inland. Despite what he called some obvious exceptions to the pattern, Givnish said the team’s findings could be useful—perhaps as a way to improve biomass estimates, or even species composition, in places where the mix of trees is known across a broad area but not at finer scales. “With regionally averaged heights, you’re bound to get insights into the composition of forests,” Givnish said.
There may be forests where it doesn’t apply, but Muller-Landau thinks the connection between tree height and diversity can likely be taken as a broad rule of thumb. Markku Larjavaara agrees. A forest ecologist at the University of Helsinki who studies tree height, he said it’s easy to see the patterns in many of the forests he’s visited, in Finland as well as Central America and Southeast Asia. In many cases, he said, greater tree diversity could point to diversity of other species: “For those directly dependent on trees, the buffet table is much longer, with more things to eat.”
Connecting tree height and diversity is also a way to make use of existing conservation tools, Marks said. Ecologists are increasingly mapping tree height using data from both satellites and airplanes equipped with light detection and ranging equipment, which can detect the height of swaths of forest, tree by tree, making it useful for detecting drought stress and spotting invasive species, and also for calculating carbon storage.
The burgeoning carbon credit market has driven a lot of forest preservation over the last decade, Larjavaara said, which he calls another happy overlap with the study—“Tall trees are the ones with carbon,” he said. It’s a bonus if those same tall patches have the most diversity. “Nature lovers like me, we like to preserve plants and trees for their intrinsic value, not for their carbon,” Larjavaara said. “But carbon is where the money is.”