Historical changes in plant water use and need in the continental United States

Abstract

A robust method for characterizing the biophysical environment of terrestrial vegetation uses the relationship between Actual Evapotranspiration (AET) and Climatic Water Deficit (CWD). These variables are usually estimated from a water balance model rather than measured directly and are often more representative of ecologically-significant changes than temperature or precipitation. We evaluate trends and spatial patterns in AET and CWD in the Continental United States (CONUS) during 1980-2019 using a gridded water balance model. The western US had linear regression slopes indicating increasing CWD and decreasing AET (drying), while the eastern US had generally opposite trends. When limits to plant performance characterized by AET and CWD are exceeded, vegetation assemblages change. Widespread increases in aridity throughout the west portends shifts in the distribution of plants limited by available moisture. A detailed look at Sequoia National Park illustrates the high degree of fine-scale spatial variability that exists across elevation and topographical gradients. Where such topographical and climatic diversity exists, appropriate use of our gridded data will require sub-setting to an appropriate area and analyzing according to categories of interest such as vegetation communities or across obvious physical gradients. Recent studies have successfully applied similar water balance models to fire risk and forest structure in both western and eastern U.S. forests, arid-land spring discharge, amphibian colonization and persistence in wetlands, whitebark pine mortality and establishment, and the distribution of arid-land grass species and landscape scale vegetation condition. Our gridded dataset is available free for public use. Our findings illustrate how a simple water balance model can identify important trends and patterns at site to regional scales. However, at finer scales, environmental heterogeneity is driving a range of responses that may not be simply characterized by a single trend.

Fig 1. Geographic patterns in Actual Evapotranspiration (AET) and Climatic Water Deficit (CWD).

LEFT: Scatterplot of 1981–2019 average annual CWD vs. 1981–2019 average annual AET for all locations in CONUS. The shading/intensity of colors indicates point density. Colors indicate equal-area zones in the AET-CWD bivariate plot; these are depicted on the map in the right panel. RIGHT: Map of CONUS showing the geographic locations of colored areas in the scatter plot. The AET vs CWD relationship has a well-established link to the distribution of dominant vegetation types (e.g., Stephenson [9]).

Fig 2. Comparison of EPA Ecoregions vs. modeled Actual Evapotranspiration (AET) and Climatic Water Deficit (CWD).

LEFT: Model pixels west of the 100^th meridian categorized by their geographic location within EPA Level I ecoregions (https://www.epa.gov/eco-research/ecoregions; [51]) and plotted in bivariate space: 1981–2019 average annual CWD vs. 1981–2019 average annual AET. For clarity of presentation, kernel density estimators (KDEs) were used to calculate outlines for the cluster of points in each ecoregion. Right: Model pixels east of the 100^th meridian, categorized according to EPA ecoregions, and plotted in the same way. Marginal plots (top row, LEFT and RIGHT) show the probability density functions of AET and CWD for each ecoregion. Points from the Marine West Coast ecoregion were too sparse to create a KDE polygon, but individual points from this region appear in Fig 8.

Fig 3. Change in annual total Climatic Water Deficit (CWD; mm/year) estimated for the period 1980–2019.

Positive slopes indicate increasing CWD and consequently drier conditions.

Fig 4. Normalized change (percent of historical mean per decade) in total annual Climatic Water Deficit (CWD), calculated as (regression slope/1980–2019 mean CWD) * 10.

Regression slopes appear in Fig 3.

Fig 5. Change in annual total Actual Evapotranspiration (AET) expressed as slopes (mm/year) for the period 1980–2019.

Positive regression slopes indicates increasing AET.

Fig 6. Normalized change (percent of historical mean per decade) in total annual Actual Evapotranspiration (AET), calculated as (regression slope/1980–2019 mean AET) * 10.

Regression slopes appear in Fig 5.

Fig 7. Combined (bivariate) change in total annual Actual Evapotranspiration (AET) and total annual Climatic Water Deficit (CWD) calculated from 1980–1999 and 2000–2019 means.

TOP: Direction of change (increasing or decreasing for each parameter). MIDDLE: Intensity of change (mm), calculated as the Euclidean Distance (length of vector in AET:CWD space) between the two period means. BOTTOM: Illustration of combined direction and intensity of change, Colors in the bottom panel are the same as shown in the legend for the top panel, with the intensity (brightness) of each pixel determined by the shading in the middle panel.

Fig 8. Combined change in Actual Evapotranspiration and Climatic Water Deficit for all CONUS National Park Service Units calculated from 1980–1999 and 2000–2019 means.

One centroid point for each park was selected, creating one vector for each park. Parks were categorized (different colors) by EPA Level 1 Ecoregion.

Fig 9. Change in bivariate climate space for Sequoia National Park.

Arrows = combined change in Actual Evapotranspiration (AET) and Climatic Water Deficit (CWD) from 1980–1999 to 2000–2019 (means). Black dashed line = 1:1 demonstrating equal change in AET and CWD for comparing vector directions. Vector colors (green shades) indicate elevation. Points (squares, pluses, triangles) are located at the starting point of every vector and indicate amount of temperature change between the two time periods, with colors of the points indicating the amount of change in average total annual precipitation between the two time periods. Insets show the strong correlations between elevation and average temperature/precipitation during the entire study period.