Reading drought from space: how Yellowstone’s habitats respond to dry years

Yellowstone has deep winter snow, yet it still burns: huge wildfires came in 1988 and 2016, both extremely dry years. Park and USGS scientists have used satellites to measure how plants respond to dry and wet years, and found that the wettest habitats are the ones that suffer most when drought arrives. The work appeared in Yellowstone Science.

A mountain meadow in full bloom. NPS photo.

What is being measured

Plants turn sunlight into stored energy, and new plant growth, called primary production, is the first link in the food chain and also fuel for fire. Ecological drought, defined only recently for wildlands, can be tracked by how vegetation responds badly to drought and well to conditions that offset it. Measuring production on the ground by clipping and weighing plants takes much time, so the Greater Yellowstone Inventory and Monitoring Network complements it with NASA's MODIS satellite sensor, which measures reflected visible and near-infrared light, and links the results to vegetation types, soils and climate.

In Yellowstone sunshine and carbon dioxide are generally plentiful, but soil nutrients vary with bedrock and soil type. Lodgepole pine does well on the low-fertility sandy, rocky soils of the rhyolite plateaus; subalpine fir, Douglas-fir and Engelmann spruce grow on richer soils that hold more water. By comparing places with similar soils, the team left climate and disturbance as the two main influences on production.

The method

For each habitat, they summed monthly greenness values (the normalized difference vegetation index, from red and near-infrared reflectance seen by the MODIS sensor on the Terra satellite) for each year from 2000 to 2016 as a stand-in for annual production, then calculated each year's difference from average. They compared those swings with plant water use (evapotranspiration) and unmet water need (deficit, a measure of drought stress) from a water-balance model. Two numbers describe each habitat:

• Sensitivity, the change in greenness for a change in water use or deficit.
• The climate pivot point, the water balance at which production tips above or below average, which places the habitat on a moisture gradient.

A pivot point is the evapotranspiration amount where production rises or falls relative to its long-term average; the line's slope is sensitivity. Each dot is one of 16 years for a single habitat map unit. NPS figure, adapted from Munson 2013.

What they found

• Wet meadows and alpine habitats responded most to water deficit, becoming much less green as drought stress rose. Sagebrush, dry meadow and wet meadow responded most, in a positive direction, to more evapotranspiration.
• Dry meadows and sagebrush sit in the driest parts of the park, with higher deficit pivot points, so they are more drought resistant. They also have higher evapotranspiration pivot points, meaning they need more water to stay above average. Wet meadows, moist meadows and moist subalpine fir stayed productive at lower evapotranspiration.
• Opposites react. Wetter habitats respond most strongly to dry years. They are normally wet during the growing season, so they do not gain from extra water, but a small rise in drought stress hurts them badly. Dry-adapted vegetation did respond to wet conditions, but less dramatically.

The reason is a tradeoff in plant traits: dry habitats favor small leaves and more roots, which resist drought, while wet ones favor large leafy growth that crowds out slower competitors but suffers in dry spells.

Habitats marked with a point were sensitive to deficit (a) or to evapotranspiration (b). NPS figure.

A look at Tower Junction

On the northern range, moist meadow, Douglas-fir and sagebrush share a climate but grow on different soils and slopes, so they feel drought differently. At Tower Junction annual water deficit has risen by about 2.4 inches (60 mm) since 1980. Deficit passed the sagebrush pivot point only twice since 1980, in 1988 and 2016, both extreme fire years, but passed the Douglas-fir pivot 20 times and the moist meadow pivot 24 times. Plants can recover when good years follow, but persistent stress above a pivot point will eventually cut production and the energy and nutrients passed to animals. Moist meadows showed a stronger net negative response than sagebrush or Douglas-fir, so the authors suggest checking on the ground whether the view from space is an early warning.

Left: habitats near Tower Junction. Right: deficit trend at Tower Junction against each habitat's pivot point. NPS figure.

Why it matters

The method can show where and when habitats may become vulnerable to drought and climate change anywhere in the park, and so where to look harder for change or try to reduce unwanted change. Not all change is bad, nor is all of it caused by climate; an area invaded by cheatgrass, for instance, may have extra drought stress and deserve priority. The authors say the effect of drought can be dampened or amplified by slope, aspect and soil water holding capacity, and that linking satellite findings to ground monitoring will give greater insight.

Sources

• National Park Service, Yellowstone Science 27(1), "Patterns of Primary Production & Ecological Drought in Yellowstone" (https://www.nps.gov/articles/patterns-of-primary-production-ecological-drought-in-yellowstone.htm), by David P. Thoma (NPS Inventory and Monitoring Program), Seth M. Munson (USGS Southwest Biological Science Center), Ann W. Rodman, Roy Renkin, Heidi M. Anderson and Stefanie D. Wacker; rewritten in hubnx's own words.