Why the junipers turned orange: satellites and climate at Capitol Reef

In fall 2019, National Park Service staff in southern Utah saw something odd: part of the landscape was the wrong color. The dark-green pinyon-juniper woodlands there normally stand out against the red-orange rock of canyon country, but now some of the junipers were orange too.

In 2019 the death of juniper trees in southern Utah raised the question of whether the same could happen elsewhere on the northern Colorado Plateau. Photo: NPS/D. Witwicki.

The trees were dying, in one of the driest periods in 1,800 years, according to a 2022 tree-ring study. An earlier tree-killing drought had hit southern Utah parks in 2002–2003. Together the two droughts killed trees that had stood for centuries. Park managers were left asking whether to expect more of the same, whether pinyon and juniper elsewhere could be hit, which other vegetation might change, and when and where, and what could be done.

Using the past to see ahead

Long-term monitoring records what has happened, and it also offers a window into the future. If scientists know which vegetation is changing, where it grows, and which parts of the climate go with the change, they can use climate projections to judge what may change and when. Linking satellite observations of vegetation condition to climate data over time shows how climate affects plant production (growth) and phenology (the timing of growth), and how sensitive each plant community is to changes in weather.

Scientists of the Northern Colorado Plateau Network applied this to several parks, including Capitol Reef National Park. They studied satellite imagery of 19 vegetation groups in and near Capitol Reef from 2000 to 2019: blackbrush, bristlecone pine, disturbed areas, Douglas-fir, dry wash, geology, C3/C4 grassland, juniper, pinyon-juniper, ponderosa pine, quaking aspen, riparian, dry and mesic sagebrush, dry, mixed montane and wet shrubland, sparsely vegetated areas and water.

The vegetation groups analyzed at Capitol Reef National Park for trends in production, phenology and climate relations. Image from the National Park Service’s page.

What the satellites showed

• Timing. By the end of the study the growing season began earlier in all groups except quaking aspen and mesic sagebrush. It ended later in most groups, but earlier in juniper, quaking aspen, mesic sagebrush and bristlecone pine. The season lengthened in every group except aspen and mesic sagebrush.
• Growth. Production fell in sparsely vegetated areas, dry sagebrush and wet shrubland on several measures. Growing-season production rose in all the forest groups (bristlecone pine, Douglas-fir, juniper, pinyon-juniper, ponderosa pine) and in riparian, mesic sagebrush, mixed montane shrubland, geology and water groups.
• Rain. The study began in the middle of a long drought, and precipitation then rose slightly. Gains in vegetation growth in most of the study area are explained by that rise from drought conditions. The southwestern U.S. has been in an extended drought over the past 20–30 years, though wetter periods occur within it. A cycle of peaks in the middle of each decade since 1980 could, if it continues, help time restoration work that needs years with little drought stress.

Precipitation increased slightly during the study, which began in a long-term drought. Photo: NPS/Ariel Solomon.

What drives the changes

Annual precipitation and annual production were generally among the most strongly linked variables, which suggests that more rain may have caused the increase in growth. The climate variable most closely tied to production was three years of precipitation. Growing degree days and precipitation set the start of the growing season.

Climate sensitivity is the rate of change in vegetation per unit change in a climate variable, and understanding it is critical to knowing how vulnerable a type is to climate change. Sensitive types respond faster or more strongly, and the same change in rain can affect different types, or one type on different soils, differently. The study ranked the groups two ways:

If the landscape dries, the area suitable for drought-tolerant types will likely grow and the area for less tolerant types will likely shrink.

What lies ahead

The researchers say they cannot know exactly how change will play out at Capitol Reef, but change is coming. A study of the dieback found that it came from acute drought stress, a surprise given how little juniper normally responds to year-to-year changes in precipitation. Projections point to a drier future under both best-case and worst-case scenarios.

Under the best case (warm and wet), yearly soil moisture stays near its historic range, but several years below the “pivot point” (how much drought stress a vegetation type can tolerate and still persist) may exceed what ponderosa pine can adapt to, so it could transition through acute drought or fire. Under the worst case (warm and dry), soil moisture is lower in most years, stress is longer and more intense, transitions will likely come sooner, and the vegetation may become more dry-adapted. Either way, the time to start planning is now.

The findings can feed Climate Smart Conservation, a planning approach that helps managers decide whether to resist, accept or direct vegetation change. They can also pick the best windows for restoration, which is expensive and slow, by adjusting species mix and timing to the conditions the plants are likely to face. And they can help answer other questions: where the most and least sensitive types are, whether forests will feel drought before shrublands, which soils best buffer drought, whether climate after a fire or drought will support the old vegetation, and whether animals of concern depend on sensitive types.

Sources

• National Park Service, Northern Colorado Plateau Network, Helping Managers Plan for Climate Change with Remote Sensing at Capitol Reef National Park, summarizing D. Thoma (2023), Landscape phenology, vegetation condition, and relations with climate at Capitol Reef National Park, 2000–2019; rewritten in hubnx's own words.