As the climate warms, forests encroach on meadowland – ‘Meadows’ biodiversity value is much larger than the area they occupy’
By Carrie Madren
12 February 2013 (Scientific American) – In perhaps the slowest invasion in history, mountain meadows in the Pacific Northwest—where hikers and backpackers revel in breath-taking scenery—are gradually giving way to hemlocks, Pacific silver firs and other conifers. In these high-elevation, subalpine meadows of Jefferson Park in the central Cascade Range in Oregon, snow typically covers the meadows until July or August and returns again in November or December—too short a growing season for most trees to take root. But with a warming climate, snow has begun melting earlier and growing seasons have lengthened; that extra time with little or no snow cover has given trees a boost. As a result, tree occupation rose from 8 percent in 1950 to 35 percent in 2008, reports a U.S. Department of Agriculture Forest Service–funded study published last October in Landscape Ecology. At a time when so many forests are threatened, aren’t more trees something to celebrate? Not necessarily, say the authors of the new study. These tall trees block light that meadow grasses, shrubs and wildflowers need to survive. Once trees become established, the surrounding seed banks of native grasses tend to fade away. The meadows’ “biodiversity value is much larger than the amount of area they occupy,” explains lead author Harold S. J. Zald, postdoctoral research associate at Oregon State University, who hatched the idea for the study while backpacking in the Cascade Range. The researchers do not yet know which plant or animal species would be endangered. The scientists did find one bright spot: depressions in the landscape carved out by glaciers held deeper snow that lasted longer through the summer. Such indentations might hold important reservoirs of meadow species even as global temperatures rise, Zald says. [more]
Where Few Trees Have Gone Before via Apocadocs
2 November 2012 (OPB) – Ask researcher Harold Zald why he’s concerned about shrinking alpine meadows and he gives two reasons. First, he says, meadows are home to a suite of species that aren’t found anywhere else, and it’s important to preserve this pocket of biodiversity. But Zald is also an avid hiker; he says the high country in the mountains is part of what makes the West, and Oregon, great: “You feel like you’re on top of the world. You have this gorgeous view, and these wildflowers. And you have space.” Zald, a post-doc researcher at Oregon State University, recently published a study documenting climate change in Jefferson Park, a 333 acre meadow in the central Oregon Cascades. He says grasses and wildflowers thrived in the meadow for about 400 years. But in 1950 trees started to sprout, and as the trees matured, they started blocking the sun. “Alpine aster, black sedge or certain lupines, they can’t survive in a shaded environment under a forest canopy,” Zald said. If the snowpack melts just a week or two earlier each year, it can allow trees to take root, and other light-loving meadow species to die out. The study suggests meadow species may be able to hang on in some places, even during a period of warming climate. Zald says features of mountain topography- like the shade cast by a ridge line- can have a powerful effect on how long snowpack lingers. Some particularly cold areas will remain unsuitable for trees, providing refugia where meadow species could persist. Zald says his study of Jefferson Park adds to an established body of research, which began with studies on Mt Ranier in the 1970s, connecting climate change to shrinking mountain meadows in the west. He says in spite of the research, he finds minimal public awareness of the threats to meadows. “We focus on older forests, we focus on salmon, and endangered species in areas that are more accessible. But there are a lot of important vegetation types like these meadows, that have really high biodiversity and conservation value, that have simply received much less attention.”
Mountain Meadows Dwindling In The Pacific Northwest
ABSTRACT: Tree invasions have been documented throughout Northern Hemisphere high elevation meadows, as well as globally in many grass and forb-dominated ecosystems. Tree invasions are often associated with large-scale changes in climate or disturbance regimes, but are fundamentally driven by regeneration processes influenced by interactions between climatic, topographic, and biotic factors at multiple spatial scales. The purpose of this research was to quantify spatiotemporal patterns of meadow invasion; and how climate, larger landforms, topography, and overstory trees have interactively influenced tree invasion. We combined airborne Light Detection and Ranging (LiDAR) characterizations of landforms, topography, and overstory vegetation with historical climate, field measurements of snow depth, tree abundance, and tree ages to reconstruct spatial and temporal patterns of tree invasion over five decades in a subalpine meadow complex in the Oregon Cascade Range, USA. Proportion of meadow occupied by trees increased from 8 % in 1950 to 35 % in 2007. Larger landforms, topography, and tree canopies interactively mediated regional climatic controls of tree invasion by modifying depth and persistence of snow pack, while tree canopies also influenced seed source availability. Landscape context played an important role mediating snow depth and tree invasion; on glacial landforms tree invasion was negatively associated with spring snowfall, but on debris flows tree invasion was not associated with snow fall. The importance of snow, uncertain climate change impacts on snow, and mediation of snow by interacting and context dependent factors in complex mountain terrain poses substantial hurdles for understanding how these ecotones may respond to future climate conditions.
