Vegetation Production and Phenology at Arches National Park, 2000–2019

Tower Arch, located in the Klondike Bluffs area of Arches National Park.

NPS/A.W. Biel

What We Wanted to Know

Scientists with the Northern Colorado Plateau Network used two decades of satellite imagery and climate data to examine how vegetation at Arches National Park (NP) responds to weather and climate. The goal was to understand 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. In a semiarid region where precipitation is variable, vegetation response to long-term moisture patterns reveals which plant communities are resilient—and which are more likely to shift as the climate changes. These findings offer a data-rich foundation for anticipating future conditions in one of the nation’s most iconic red rock landscapes.

At-a-Glance

• Growing season production increased in most vegetation groups from 2000 to 2019.
• Wet Shrubland, Riparian, and Annual Exotic Grassland were the most drought-tolerant.
• Wet Shrubland, Annual Exotic Grassland, and C3/C4 Grassland were most sensitive to precipitation.
• The growing season lengthened with green-up arriving nearly 11.9 days earlier and senescence (go dormant) occurring over 10.2 days later.

What We Did

Researchers analyzed MODIS (Moderate Resolution Imaging Spectroradiometer) satellite imagery alongside daily climate data to evaluate long-term trends in vegetation productivity and the timing of growth (phenology) from 2000 to 2019. Using polygons mapped to 13 vegetation alliance groups (categories of plant communities with similar species and ecological traits)—such as Pinyon-Juniper, Blackbrush, grasslands, and shrublands—they assessed how vegetation production changed over time and how sensitive each group was to different aspects of climate.

What’s phenology?

Phenology is the study of seasonal biological events—like when plants sprout leaves, flower, or go dormant—and how these events are influenced by climate and weather.

The study calculated “pivot points” (the value of a climate variable where vegetation teeters between below- and above-average condition) and “responses” (amount vegetation production changes in response to changes in a climate variable). 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, we studied how vegetation responded to different measurements of climate such as precipitation, soil moisture, actual evapotranspiration, and water deficit.

What We Learned

Although the study began during a dry period, growing season vegetation production at Arches NP generally increased, especially in Blackbrush, Dry Wash, Geology, Pinyon-Juniper, Dry Sagebrush, Dry Shrubland, Wet Shrubland, Sparsely Vegetated alliance groups. Most alliance groups (categories of plant communities with similar species and ecological traits) experienced steady recovery after dry spells, with a peak in productivity in 2014 and drought-related dips in 2002–2003, 2012, and 2018.

The response of vegetation to climate varied widely among groups and by climate variable. Precipitation and soil moisture were the dominant drivers of productivity but there was more variability in response to soil moisture than to precipitation (Figures 1 and 2). These findings suggest that vegetation type, soil properties, and climate interact to determine vegetation response. Taking a longer view, we found that legacy effects (conditions in prior years) also influence current-year growth. Most polygons responded to three years of precipitation, confirming that multi-year water availability—not just single wet or dry years—determines vegetation condition and production that over the long-term shape the landscape-scale vegetation assemblages and patterns.

Figure 1. Plot of pivot points and responses for 13 polygon types relative to water-year precipitation. Steeper lines indicate greater sensitivity; lines shifted left indicate greater drought tolerance.

NPS/D. Thoma

What are legacy effects?

Legacy effects occur when past climate conditions—like several dry years—continue to influence plant growth today. This means vegetation health often reflects multi-year trends, not just the current season.

Figure 2. Pivot points and responses for 13 polygon types relative to water-year soil moisture. Steeper lines indicate greater sensitivity; lines shifted left indicate greater drought tolerance.

NPS/D. Thoma

Drought tolerance also differed among vegetation types. Wet Shrubland and Riparian communities performed well even in low-precipitation years, likely due to their location in water-accumulating lowlands. On the other hand, Blackbrush, Geology, and C3/C4 Grassland were among the least drought tolerant. This suggests that these vegetation types may be most dramatically affected by drought.

Landscape phenology derived from satellite observations showed dramatic changes in growing season length across all vegetation groups. By the end of the study, the growing season started nearly two weeks earlier and ended more than a week later than in 2000. This expansion, driven by warming and possibly elevated CO₂, signals long-term ecological change that could affect pollinators and wildlife forage.

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

Real-time climate data—such as soil moisture, precipitation, or evapotranspiration—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 drought or high-heat events.

Monitor high-sensitivity and high-response areas

Alliance groups like Wet Shrubland and Annual Exotic Grassland respond strongly to precipitation changes and may serve as early indicators of shifting ecosystem conditions. Tracking these groups can provide early warning of ecological change.

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 visitor enjoyment, such as shade trees near campgrounds and picnic areas, may warrant higher management concern if it is very climate sensitive. Vegetation types with low drought tolerance—such as Blackbrush, Geology, C3/C4 Grassland—may need focused monitoring for early detection of undesirable change due to climate or other disturbance events.

Information in this article was summarized from Landscape Phenology, Vegetation Condition, and Relations with Climate at Arches National Park, 2000–2019 by D. Thoma. Content was edited and formatted for the web by E. Rendleman.

Tags: arches national park, ncpn, inventory and monitoring division, northern colorado plateau network, phenology, satellite observation, vegetation monitoring, remote sensing, phenology monitoring, long-term monitoring, monitoring, arch, northern colorado plateau, inventory and monitoring program