Vegetation Production and Phenology at Curecanti National Recreation Area

The Blue Mesa Reservoir at Curecanti National Recreation Area, with rugged mesas rising in the background.

NPS / Amy Washuta

In fall 2019, National Park Service staff in southern Utah noticed something strange about the landscape. Some of it was the wrong color. The region’s dark-green pinyon-juniper communities usually contrast sharply with the red-orange rocks of canyon country. But now, some of the juniper trees were also orange.

In 2019, the death of juniper trees in southern Utah led to questions about whether the same could happen elsewhere on the northern Colorado Plateau.

NPS / Dana Witwicki

They later learned the trees were dying—during one of the driest periods in 1,800 years (Gangopadhyay et al. 2022). Another tree-killing drought had occurred in southern Utah parks in 2002–2003. In combination, these two droughts killed trees that had survived for centuries.

This raised some troubling questions. Should park managers expect more of the same in the future? Could pinyon and juniper trees in other areas be affected? Which other vegetation types might change—and when and where? And what, if anything, could be done about it?

If we had a crystal ball, how might our approach to management change?

We know long-term monitoring creates a record of the past. But it also provides a window into the future. Tracking which kinds of vegetation are changing, where they grow, and which aspects of climate are associated with the changes can help us understand what, when, where, and why future changes may occur, based on climate projections.

Linking satellite observations of vegetation condition with climate data over time can reveal how climate influences plant production (growth) and phenology (timing of growth). It can also tell us how sensitive different plant communities are to changes in weather and climate. With information like that, we won’t need a fortune teller.

To see how this might work, Northern Colorado Plateau Network scientists used satellite imagery to analyze vegetation condition over time at parks on the northern Colorado Plateau, including Curecanti National Recreation Area (NRA), and explored the relationships between climate and vegetation using new tools for assessing climate effects. At Curecanti, dramatic transitions in vegetation occur with increasing elevation, from lush riparian vegetation in canyons and lowland semi-arid shrublands surrounding Blue Mesa Reservoir to high desert vegetation with pockets of fir, quaking aspen, and spruce on mesa tops. Knowing which of the wide range of 16 vegetation assemblages found in Curecanti are more or less sensitive to climate change can help managers understand what to expect over the next few decades and plan for the changes coming their way.

Phenology and Production

Satellite images collected over time show us when plant growth begins in spring (“greenup”)—because the landscape turns green. At the end of the season, it senesces and turns brown (“browndown”). Linking the timing of greenup and browndown to weather provides insight about how weather affects phenology.

NCPN alliance groups analyzed for trends in production, phenology, and climate relations at Curecanti National Recreation Area.

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We can use those same satellite images to measure plant production. In combination, phenology and production tell us a lot about plant relationships with climate. And because plants are the biological engines of parks, this analysis tells us a lot about the impacts of climate change on park ecosystems.

In this study, we used satellite imagery from remote sensing to track phenology and production across 16 vegetation alliance groups in and near Curecanti National Recreation Area from 2000 to 2019.

Phenology

By the end of the study period, the growing season was starting 4.7 days later on average across all alliance groups and ending 7.8 days later. The length of the growing season was 3.1 days longer on average, and the day of peak growth was 7.7 days later.

Production

• Annual trends in vegetation production between 2000 and 2019 increased in 98% of the study area and decreased in just 2% of the study area.
• The greatest area of increased production occurred in the Mesic Sagebrush and Dry Sagebrush alliance groups, representing 55% of the study area.
• While decreased production was noted in small areas of many alliance groups, the largest areas of decrease occurred in the Quaking Aspen (310 ha) and Douglas-fir (206 ha) alliance groups.

Climate Conditions

We used a water-balance model to determine how different aspects of climate and site characteristics affected vegetation phenology and production.

From the time this study began in 2000 (during a very dry period) to its end in 2019, growing season primary production increased in most vegetation alliance groups in and near Curecanti NRA. This increase coincided with a slight increase in precipitation over the same stretch of time. However, there were also dry years during the study, and dry periods that lasted for several years, both of which resulted in substantial declines in vegetation cover. These declines largely rebounded by the time the study ended in 2019.

Taking the long view, an interesting decadal oscillation was evident: peaks in precipitation in the middle of each decade since 1980 translated to similar peaks in other water-balance variables like soil moisture and evapotranspiration. If the periodicity of these cycles continues in the future, it could serve as a useful guide for timing restoration efforts that require substantial planning efforts, long-term investments, and multiple years of minimal drought stress for germination and establishment.

Climate conditions at Curecanti NRA varied over the course of the study.

NPS / Amy Washuta

Relationships Between Climate and Vegetation

With this information about phenology, production, and climate in hand, we explored which climate variables were the most important drivers of phenology and production, which vegetation types were most sensitive to changes in water availability, and which types were most resistant to drought.

Climate drivers of production and phenology

• At the park scale, annual growing-season production was most strongly correlated with annual actual evapotranspiration from the current and previous two years, suggesting that increases in annual precipitation may have caused most of the increase in production over the study period. This was also generally true at the alliance-group scale.
• The Juniper, Pinyon-Juniper, and Quaking Aspen alliance groups required the highest average values of annual actual evapotranspiration to maintain production, whereas the Sparsely Vegetated, Wet Meadow, and Riparian alliance groups required the lowest average values of annual actual evapotranspiration to maintain production.
• The primary determinants of the start of the growing season were growing degree days, the date of snowmelt, and precipitation.

Vegetation sensitivity to climate

The way vegetation at Curecanti NRA responds to various climate drivers reflects the different plant communities and the site conditions where they grow. The same amount of change in actual evapotranspiration over the water year, for example, may cause different responses from different vegetation types, or even in the same vegetation type growing in different soils.

The rate of change in vegetation condition per unit change in a climate variable is known as its climate sensitivity. More sensitive vegetation types respond more quickly or more strongly to changes in climate conditions. Understanding a vegetation type’s climate sensitivity is critical to understanding its vulnerability to future climate change.

The following graph helps us understand how different vegetation types at Curecanti respond to changes in actual evapotranspiration. Each line represents a different area in the park, and the steepness of the line tells us how sensitive the area is to changes in evapotranspiration. Steeper lines, like the ones in the dashed black oval, represent areas that are more sensitive. In other words, these areas will experience more rapid changes in production when there is a shift in actual evapotranspiration. Lines that are less steep, like the ones in the solid black oval, represent areas that are less sensitive. Shifts in actual evapotranspiration will lead to slower changes in production in these areas. Since actual evapotranspiration is an indication of water use by plants, this chart also tells us which alliance types use the most water and—when water is not available—how much production will decline.

Changes in production in response to evapotranspiration in Curecanti NRA. The long pink line with a negative slope corresponds to an area along the shoreline of Blue Mesa Reservoir, where fluctuating water levels in the reservoir (rather than changes in evapotranspiration) control production.

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The vegetation types at Curecanti NRA, ordered from MOST to LEAST sensitive to changes in actual evapotranspiration:

• Sparsely Vegetated
• Disturbed
• Mixed Montane Shrubland
• Quaking Aspen
• Douglas-fir
• Mesic Sagebrush
• Dry Shrubland
• Juniper
• C3 Grassland
• Wet Meadow
• Dry Sagebrush
• Pinyon-Juniper
• Riparian

Drought tolerance

Understanding drought tolerance helps park managers determine which vegetation types may persist or perish if water stress increases due to climate change. Vegetation that requires less water is more drought-tolerant.

If the landscape gets drier, then the amount of area suitable for drought-tolerant species will likely increase. Under a drier scenario, there will likely be less area available for drought-sensitive species.

If we look at the graph again, we can also identify which vegetation types at Curecanti are most drought-tolerant. Each line on the graph crosses the x-axis (the bold black horizontal line at 0 change in production) at a specific point, which represents the amount of actual evapotranspiration needed for the vegetation in that area to switch from below-average to above-average production. Lines that cross the x-axis further to the left indicate that the vegetation switches to above-average production with less water, meaning these areas are more drought-tolerant.

Vegetation types at Curecanti NRA, ordered from MOST to LEAST drought-tolerant:

• Sparsely Vegetated
• Wet Meadow
• Riparian
• Dry Sagebrush
• Mixed Montane Shrubland
• Douglas-fir
• C3 Grassland
• Disturbed
• Mesic Sagebrush
• Dry Shrubland
• Quaking Aspen
• Pinyon-Juniper
• Juniper

A riparian zone along a creek that flows into Blue Mesa Reservoir at Curecanti NRA. Vegetation in the Riparian alliance group is more drought tolerant than most other vegetation alliance groups in Curecanti.

NPS / Amy Washuta

What Lies Ahead for Curecanti National Recreation Area?

Potential vegetation transitions

Though our science-based findings are more reliable than the musings of a fortune teller, they have limits. We can’t know exactly how vegetation change will play out at Curecanti NRA. But change is coming. The cause of juniper mortality seen by park managers in other parks in southern Utah in 2018 was acute drought stress (Kannenberg 2021)—a surprising finding, given the relatively dry locations where juniper trees grow in those parks. Recent conditions that caused die-offs in other southwestern parks may be in the future for Curecanti National Recreation Area. A range of scenarios indicate a trend toward more aridity in the future (see the following figure).

Under best-case and worst-case scenarios, Curecanti National Recreation Area is projected to become more arid in the future.

NPS

In the figure, “annual water deficit” represents the drought stress experienced by the vegetation at a single Dry Sagebrush polygon in Curecanti NRA. The historical water deficit “pivot point” – the dashed black line – represents how much drought stress this vegetation polygon can tolerate and still persist over time. Water deficit levels that exceed the pivot point (in this case, 312 mm) indicate an unmet water need that restricts plant growth and survival. When combined with climate projections of increased aridity, the vegetation sensitivity to water deficit reported in this study can help guide the time and place for management actions at Curecanti NRA.

Under the warm-and-wet (best-case) scenario, annual water deficit will only fall below the historical water deficit pivot point occasionally—meaning that even in the best-case scenario, in most years the water deficit pivot point will be exceeded and water availability will be insufficient to meet vegetation needs in this polygon. This will likely result in transition to a different vegetation assemblage due to drought stress, forest disease, or fire.

Under a worst-case (warm-and-dry) scenario, water deficit will exceed the best-case scenario in even more years. This will result in more intense and prolonged drought stress that will likely make transitions happen sooner. It may also result in more dry-adapted vegetation composition than in the best-case scenario. Either way, the time to start planning for change is now.

Informed planning

Managers can use information from studies like this one in a powerful planning framework. Planning for a Changing Climate is a roadmap for managing natural resources as the climate changes. Ultimately, this process helps managers decide whether to resist, accept, or direct vegetation change and identify management options for each approach. One of its key inputs is a resource’s sensitivity to climate change, which this study provides. The report associated with this web article offers a step-by-step application of this kind of information for climate adaptation planning.

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Information in this article was summarized from Landscape phenology, vegetation condition, and relations with climate at Curecanti National Recreation Area, 2000–2019, by D. Thoma (2025).

Other cited material includes:

Gangopadhyay, S., C.A. Woodhouse, G.J. McCabe, C.C. Routson, and D.M. Meko. 2022. Tree rings reveal unmatched 2nd century drought in the Colorado River Basin. Geophysical Research Letters, June 9. https://doi.org/10.1029/2022GL098781.

Kannenberg, S.A., A.W. Driscoll, D. Malesky, and W.R.L. Anderegg. 2021. Rapid and surprising dieback of Utah juniper in the southwestern USA due to acute drought stress. Forest Ecology and Management 480 (January 15):118639. https://doi.org/10.1016/j.foreco.2020.118639.