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

Zion National Park features towering sandstone cliffs and deeply carved canyons that define its iconic desert landscape.

NPS/A. Washuta

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 Zion National Park 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 park that spans a wide elevation range, these dynamics reveal which plant communities are resilient and which are vulnerable to drought or warming. The results offer a foundation for anticipating future change and preparing for shifts in vegetation composition and productivity.

At-a-Glance

• Vegetation productivity generally increased from 2000 to 2019.
• Precipitation and soil moisture were key drivers of productivity.
• The growing season was longer, starting later and ending later.
• Sparsely Vegetated, Blackbrush, and C3/C4 Grasslands were among the most drought-tolerant; Aspen and C3 Grasslands were less so.

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 21 vegetation alliance groups—such as Pinyon-Juniper, Blackbrush, grasslands, and Sparsely Vegetated—they assessed how vegetation production and phenology 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, the study tested vegetation response to multiple variables, including precipitation, soil moisture, actual evapotranspiration, and water deficit.

What We Learned

Although the study began during a dry period, growing season vegetation productivity increased in most areas of the park. C3 Grassland, Exotic Perennial Grassland, and Wet Meadow had the greatest increases, while overall growing season length expanded slightly due to later green-up and later senescence. Productivity trends were generally stable, but some areas showed declines due to drought or fire, especially in the Pinyon-Juniper alliance group.

The best indicators of production were water-year precipitation and soil moisture, and the most sensitive alliance groups included Blackbrush, Sparsely Vegetated, and C3/C4 Grassland (Figures 1 and 2). Drought tolerance varied widely across the park, often influenced by elevation, soils, and vegetation traits. Quaking Aspen had the highest water needs, while Riparian and drought-tolerant shrublands required less water. Vegetation production often depended on conditions in previous years, or “legacy effects,” highlighting the importance of multi-year precipitation trends.

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 1. Pivot points and responses for 21 polygon types relative to water-year precipitation. Steeper lines indicate greater sensitivity; lines shifted left indicate greater drought tolerance (see full report, section 3.8.4).

NPS/D. Thoma

Figure 2. Pivot points and responses for 21 polygon types relative to water-year soil moisture. Steeper lines indicate greater sensitivity; lines shifted left indicate greater drought tolerance (see full report, section 3.8.4).

NPS/D. Thoma

Phenology metrics revealed subtle but widespread changes in the timing of growth. Most vegetation groups experienced a later start and later end to the season, resulting in a slightly longer growing period overall. The timing of peak growth was also delayed—likely a response to shifting snowmelt, warming, and precipitation patterns.

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 findings into vulnerability assessments

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, picnic areas, and overlooks, may warrant higher management concern if it is very climate sensitive. Vegetation types with highly variable or outlier responses—such as Ponderosa Pine, Disturbed, and Sparsely Vegetated areas—may need focused monitoring to better understand their trajectories and detect early signs of change in high-value or vulnerable vegetation types due to climate or other disturbance events.

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

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