I love trees. Photographic proof exists that I’ve hugged a wide variety of species. But, even though I love forests, I have to admit that not every ecosystem needs trees. In some ecosystems, like grasslands, natural and prescribed fires keep trees from taking over the grass’s territory. In the vast, typically tree-less sagebrush steppe of the Great Basin, scientists worry that increasing density of small pine and juniper trees are providing more fuel, and therefore, higher risk for severe wildfires.
Trees now threaten alpine meadows in the pacific northwest, according to research published in the journal Landscape Ecology. Researchers from Oregon State University and the U.S. Forest Service tracked tree invasions into a high elevation meadow in the Cascade Range for the past half a century. They found tree cover grew from just 8 percent of the meadow in 1950 to 35 percent in 2007, a 3-fold increase. The invading trees in this habitat were mountain hemlock and Pacific silver fir.
The researchers blame climate change for creating longer summers and less winter snow, both conditions that favor tree establishment in typical meadow communities. However, they also found that the patterns of tree invasion varied depending on local topography and microhabitat, as well as the broader regional climate trends. In short, it’s changing, but it’s not a one size fits all pattern.
Why is this bad? Many climate models suggest that alpine plant communities will all shift upslope as conditions become warmer and growing seasons lengthen. As this study suggests, the treeline may be able to advance successfully, but the meadow community may not be so mobile. Successfully ability for a species to shift its range depends on a host of factors, including dispersal mechanisms and the width of its niche. The higher elevations may be too steep, too exposed, or, since previously only sparsely vegetated, may not have enough soil or nutrients to sustain the current diversity of the plant community. For some high elevation communities, there just might not be enough mountain left to provide the microclimate they have adapted to.
Luckily, the Oregon researchers don’t suggest that trees are going to take over all the meadows. They report low tree invasion in the wetter meadow areas, and other micro-habitat factors, like the limitations of dispersal, seem contribute to keeping tree cover low in some places. They predict that alpine meadow habitat will persist, but perhaps at a smaller extent, broken up by areas where the trees are taking over.
With all the worry about deforestation across the globe, it seems counter intuitive to worry about tree invasion as well. But alpine communities are like islands, home to isolated collections of plants that you don’t find many other places, stranded on each high peak. Alpine species are tough, the only ones who can survive on steep slopes covered with snow most of the year, but they are simultaneously fragile, trapped with a very narrow niche when conditions are changing.
I climb mountains for the views, certainly, but also for the plants you can’t find anywhere else. It must be a hard life, covered by snow almost all year, but they make up for it with showy summer colors, celebrating their brief time in the sun. Ironically, warmer weather and less snow won’t make these high altitude plant’s lives easier, but instead, encourages other species, like these mountain hemlock trees to move up slope and changing up the neighborhood.