How Changing Water Availability Could Reshape Plants in US National Parks

To a plant, a place that got 5 inches more rain than normal but was 3°F warmer than normal could end up feeling drier than normal. That is the starting point for a National Park Service (NPS) Inventory & Monitoring Division article on how changing water availability is shifting the plants in national parks.

A patch of dying junipers with the Abajo Mountains behind, Cedar Mesa, Utah, 2019. In the two years before the photo, water deficit was higher than at any time since 1980. NPS/Dana Witwicki.

Why temperature and rain are not enough

People experience climate mostly as temperature and precipitation. Plants use water absorbed by their roots from the soil, and not all the water that falls ends up available to them: some runs off the surface, some drains below the roots and some evaporates, especially as temperatures rise. Plants grow where their traits suit the climate, so when the climate changes, vegetation changes. Shifts in which plants grow where can change the animals in a park and how visitors experience the landscape.

Climate also works at different scales. Within one park, topography, soil type and other factors mean different areas feel the same climate differently, which has made regional projections hard for managers to use. A new dataset from the NPS, the U.S. Geological Survey and partners may help.

Reading climate through water balance

The dataset is a pre-calculated grid. It uses a water-balance model to account for how temperature and precipitation interact over time and how soil and other site properties change the result. The model calculates two measures:

Weather stations give measurements at only a handful of places in a park, often one, and usually at park headquarters. A gridded dataset interpolates the station data across a grid like a checkerboard laid over the landscape, adjusting for elevation. The model then combines the weather grids with soil properties and hillslope aspect. The result is available for every square kilometer of every NPS unit in the continental US.

What 1980 to 2019 looked like

Across the continental US, the West had higher WD and lower AET than the East, meaning drier conditions for plants, and the East showed the opposite. WD was lower in the North than in the South. The most desert-like conditions, with low AET and high WD, were in southern California, Texas and Arizona, and the wettest, with low WD and high AET, were in Florida.

Spatial patterns in climatic water deficit and actual evapotranspiration, with the scatterplot colors keyed to the map.

Trends over time mirrored each other. Water deficit rose in the West and fell in the East, AET often rose in the East and fell in the West, and the middle longitudes were mixed.

Change in annual total WD (2A) and AET (2B) estimated for 1980 to 2019.

WD changed more than AET. Some high-elevation areas in the West, such as parts of Wyoming, Colorado and the Sierra Nevada, had large relative increases in AET, meaning wetting. Some of the greatest relative decreases in AET, meaning drying, were in the deserts of southern California, which were becoming even more arid.

Normalized change (percent of the historical mean per decade) in total annual WD (3A) and AET (3B), 1980 to 2019.

Across the national park system

The same trends held for NPS units in the continental US: the wettest parks (Eastern Temperate Forests) got wetter and the driest (North American Deserts) got drier. In the West the main driver of AET and WD trends was rising temperature, which climbed more steeply than in the East while precipitation was flat or falling. In the East precipitation played the larger role.

Combined change in WD and AET ("climate space") for all NPS units in the continental US, from means for 1980 to 1999 and 2000 to 2019, one vector per park. Any direction of movement favors some species over others.

Inside one park

Sequoia and Kings Canyon National Parks cover about 1,500 square kilometers and straddle two ecoregions, the Northwestern Forested Mountains and Mediterranean California. Their vegetation runs from chaparral to montane forest, giant sequoia groves and high alpine communities, and the study found a broad range of WD and AET along an elevation gradient of 500 to 4,300 meters.

Sequoia and Kings Canyon National Parks. NPS photo.

What it means for plants and managers

AET is tightly linked to plant growth, and WD is a good indicator of drought stress. When drought exceeds what a species can tolerate, it may decline. A drying West points to species better suited to dry conditions, sometimes non-natives, replacing current ones, while rising AET elsewhere can favor species that depend on abundant moisture and fast growth. Change can be gradual, as in eastern forests, or abrupt and driven by disturbance, as at western treelines. In the Southwest, rising aridity suggests more drought-tolerant species may have a competitive edge.

The findings show how the same temperature and precipitation can have different effects over short distances. At the park scale, local variability produces a range of responses. Managers can use the dataset to see how plant communities might shift under different climate scenarios, and the vulnerability of vegetation helps decide whether to resist, accept or direct change.

Long-term monitoring by the NPS Inventory & Monitoring Division captures changes in vegetation. Here, staff of the Northern Colorado Plateau Network repeat measurements on plants that were alive during earlier visits.

Sources

• National Park Service, Inventory & Monitoring Division, "Changing Patterns of Water Availability May Change Vegetation Composition in US National Parks" (https://www.nps.gov/articles/000/ncpn_water_availability.htm), summarizing Tercek MT, Thoma D, Gross JE, Sherrill K, Kagone S and Senay G, "Historical changes in plant water use and need in the continental United States," PLOS ONE, September 2, 2021; rewritten in hubnx's own words. The source gives 1981 to 2019 for one figure and 1980 to 2019 elsewhere; the range is given as 1980 to 2019 here.