Vegetation Production and Phenology at Natural Bridges National Monument

A natural stone bridge spans a deep canyon at Natural Bridges National Monument, with stratified rock walls, patches of green vegetation, and a bright blue sky with scattered clouds highlighting the rugged desert landscape.

NPS/Amy Washuta

What We Wanted to Know

At a Glance

• Growing season production increased in all vegetation alliance groups (groups of similar plant communities) between 2000 and 2019.
• Actual evapotranspiration (AET) and soil moisture were the climate variables most strongly correlated with growing season production.
• Disturbed areas used more water than other alliance groups.
• Averaged across all alliance groups, the growing season was starting 26.1 days earlier, the end of season was occurring 6.3 days later, and the growing season was 37.2 days longer.

Scientists with the Northern Colorado Plateau Network (NCPN) analyzed 20 years of satellite imagery and climate data to examine how vegetation at Natural Bridges National Monument (NM) responds to weather and climate, with the goal of understanding which vegetation types are most likely to change, where in the park those changes occur, and why, recognizing that not all vegetation responds to climate in the same way. Vegetation response to long-term climate patterns reveals which plant communities are resilient—and which are more likely to shift as the climate changes. The results offer a foundation for anticipating future change and preparing for shifts in vegetation composition and productivity.

What We Did

Researchers analyzed Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery (imagery used to monitor Earth's surface), alongside daily climate data, to evaluate long-term trends in vegetation productivity (how much plant growth occurs over time) and phenology (the timing of growth) from 2000 to 2019. Using polygons mapped to four vegetation alliance groups, they assessed how vegetation production and phenology changed over time and how sensitive each group was to different aspects of climate (Figure 1).

Figure 1. NCPN alliance groups analyzed for trends in production, phenology, and climate relations at Natural Bridges National Monument. Areas not colored within the study boundary are alliance polygons smaller than the minimum target area of 8.3 hectares.

NPS/David Thoma

The study calculated pivot points (the climate value where plant growth shifts from low to high) and responses (change in plant growth from climate shifts). These two measures of plant traits help characterize each vegetation group’s drought tolerance and climate sensitivity. And since climate can be evaluated in different ways, the study tested vegetation response to multiple variables, including precipitation, soil moisture, actual evapotranspiration (water loss through evaporation and plant use), and water deficit.

What We Learned

Although the study began during a dry period, multiple indicators suggest annual trends in growing season vegetation production increased over time for all alliance groups at Natural Bridges NM between 2000 and 2019.

Actual evapotranspiration and soil moisture were climate variables most strongly correlated with growing season production, which suggests that increases in annual precipitation that resulted in higher soil moisture and plant water use may have caused the increase in growing season production (Figure 2). Disturbed areas (places altered by humans or natural forces) used more water than other alliance groups, but sensitivity to actual evapotranspiration was similar among alliance groups.

Figure 2. Growing-season pivot points and responses for vegetation types at Natural Bridges relative to water-year soil moisture; steeper slopes indicate greater sensitivity, and farther-left positions indicate lower drought tolerance.

NPS/David Thoma

Drought tolerance varied across the monument. At Natural Bridges NM, the least drought-tolerant alliance group was Disturbed (i.e., the one with the highest actual evapotranspiration pivot point). The other three alliance groups were more drought tolerant but overlapped in their tolerance (pivot point standard errors overlap). Of these, the most drought-tolerant alliance group was Ponderosa Pine, which includes Littleleaf Mountain-mahogany Bedrock Cliff & Canyon Wooded Scrub vegetation types. The next most drought tolerant alliance group was Riparian.

Understanding vegetation response is key to assessing vegetation vulnerability assessments. The importance of response is related to the need to understand which vegetation groups may respond sooner or more strongly to a given amount of climate exposure, and response indicates where change is likely to happen first and fastest as the climate shifts. At Natural Bridges NM, there was considerable overlap in response among the four alliance groups, but Disturbed areas were among both the most and least sensitive. The least sensitive group was Pinyon-Juniper, followed by Ponderosa Pine and Riparian alliance groups.

Phenology metrics revealed notable changes in the timing of growth. Averaged across all alliance groups, the start of growth advanced 26.1 days earlier, and the end of growth occurred 6.3 days later. Collectively, these changes resulted in a growing season that was on average 37.2 days longer by the end of the study. Precipitation, actual evapotranspiration, and growing degree days were identified as the most important predictors of start of growing season.

What We Recommend

Park managers can use the results of this study—and the tools developed from it—to interpret vegetation response in real time, anticipate future conditions, and inform both near-term decisions and long-term conservation goals:

Use pivot points to interpret current conditions: Cumulative actual evapotranspiration tracked in real-time can be compared to vegetation-specific pivot points. This “now-cast” approach allows managers to estimate whether current-year vegetation production is likely to be above or below average (without the complexity of analyzing satellite imagery), helping guide timely decisions during the growing season.

Monitor high-sensitivity and high-response areas: Riparian and some of the Disturbed areas responded rapidly when water was abundant. In years when precipitation is higher than normal, vegetation in Disturbed areas could become a management concern if species in those areas are undesirable. Monitoring these areas can help managers anticipate change and prioritize adaptive actions.

Incorporate legacy effects in planning: Because vegetation production often reflects conditions from previous years, restoration planning and invasive-species strategies could factor in multi-year precipitation and soil moisture trends.

Prioritize high-value or sensitive areas: High value vegetation that is important as wildlife habitat, forage, or even visitor enjoyment, such as shade trees near campgrounds and picnic areas, may warrant higher management concern if it is very climate sensitive. Although climate sensitivity varied among alliance groups at Natural Bridges NM, continued monitoring remains important to detect early signs of change in high-value or vulnerable vegetation types due to secondary effects from climate or weather that promote forest disease and drought conducive to wildfire.

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Information in this article was summarized from Landscape phenology, vegetation condition, and relations with climate at Natural Bridges National Monument, 2000–2019 by D. Thoma (2025). Content was edited and formatted for the web by E. Rendleman.