Reading Climate Change in Satellite Images of Colorado National Monument’s Vegetation

When juniper trees in southern Utah turned orange in fall 2019, National Park Service staff noticed the landscape was “the wrong color”: the trees were dying in one of the driest periods in 1,800 years. A study at Colorado National Monument uses satellite images and climate data to ask whether the same could happen nearby, and which plants are most at risk.

Colorado National Monument (NPS photo).

The question

The region’s dark-green pinyon-juniper communities normally contrast sharply with red-orange rock. Another tree-killing drought hit southern Utah parks in 2002–2003, and together the two droughts killed trees that had survived for centuries. That raised questions: should managers expect more? Could trees on the northern Colorado Plateau be affected? Which other vegetation might change, and when?

Juniper death in southern Utah in 2019 prompted the question of whether it could happen elsewhere on the northern Colorado Plateau (NPS/D. Witwicki).

Long-term monitoring records the past but also offers a window onto the future. Linking satellite observations of vegetation with climate data shows how climate affects plant production (growth) and phenology (timing of growth), and how sensitive each plant community is to weather and climate. Scientists with the Northern Colorado Plateau Network used satellite imagery at parks on the plateau, including Colorado National Monument, and the report is by D. Thoma (2022).

What the satellites saw, 2000–2019

Images show when growth begins in spring (“greenup”) and when plants turn brown at season’s end (“browndown”), and they also measure production. The team tracked nine vegetation types: annual exotic grassland, perennial exotic grassland, juniper woodland, pinyon-juniper woodland, riparian woodland, dry sagebrush shrubland, dry shrubland, mixed montane shrubland and wet shrubland.

Vegetation types analyzed at Colorado National Monument (NPS).

• Phenology. Greenup was starting earlier for almost all (7 of 9) types. Peak growth shifted later in the season for almost all types, and the growing season lengthened for all groups except perennial exotic grassland.
• Production. Total production rose for almost all (7 of 9) types. The two exotic grasslands had small, statistically insignificant increases.

Precipitation increased slightly over the period, by 3–11 mm per year depending on location, while air temperature stayed stable. The period began in the midst of a long-term drought. Over the longer view, since 1980 the monument has seen strong cycles of high precipitation mid-decade and low precipitation at decade’s end.

Precipitation rose slightly during the study (NPS photo).

What drives growth

• Precipitation was the climate variable most closely tied to production.
• Past years linger. Average soil moisture over the current and previous two years was the best indicator of this year’s growth, probably because roots, stems, leaves and seeds from earlier years are the biological legacy plants draw on.
• Spring start. A combination of the date of complete snowmelt, actual evapotranspiration and temperature most influenced when the growing season began.

Climate sensitivity is the rate of change in vegetation per unit change in a climate variable. Types ranked from most to least sensitive to precipitation: annual exotic grassland, dry shrubland, wet shrubland, mixed montane shrubland, riparian woodland, dry sagebrush, juniper woodland, pinyon-juniper woodland. Annual exotic grassland changed fastest, which matters because fire-prone cheatgrass dominates this community just outside the park boundary. In wet years, managers should watch for cheatgrass spreading faster than native plants, since fire-prone invaders could threaten pinyon-juniper forests that have not burned in more than a millennium. Higher, wetter pinyon-juniper communities respond more slowly, because deep roots and slow growth buffer them, to a point.

Ranked from most to least drought-tolerant by their water-deficit pivot points: dry shrubland, wet shrubland, annual exotic grassland, riparian woodland, dry sagebrush shrubland, pinyon-juniper woodland, juniper woodland and mixed montane shrubland. Perennial exotic grassland showed no significant relationship with deficit.

A drier future

All projections trend toward more aridity. The pivot point is how much drought stress a type can tolerate and still persist; water deficit above it limits growth and survival. Under the worst-case (warm-and-dry) scenario, deficit will exceed the pivot point in most years by 2030, which suggests a shift to more drought-tolerant vegetation. Under the best-case (warm-and-wet) scenario, deficit stays near its historical range, but several drought years in a row could still overwhelm less tolerant types such as juniper woodland. The juniper dieback seen by park managers in southern Utah was caused by acute drought stress, surprising given junipers’ low sensitivity to year-to-year precipitation changes. For trees that live 800 years or more, the authors write, that is not far off, and the time to plan is now.

Under best-case and worst-case scenarios, Colorado National Monument is projected to become more arid (NPS).

Using the results

The findings feed Climate Smart Conservation, a roadmap that helps managers decide whether to resist, accept or direct vegetation change. They can also pick the best timing and species mix for restoration projects, and answer questions such as which soils best buffer drought, whether forests will feel drought stress before shrublands, and whether animal species of concern depend on climate-sensitive vegetation.

Sources

• National Park Service, Helping Managers Plan for Climate Change with Remote Sensing at Colorado National Monument, summarizing D. Thoma (2022), Northern Colorado Plateau Network; rewritten in hubnx's own words.