Impact of Juniper Trees on Local Water Budgets

By Robert K. Lyons, M. Keith Owens and Chris J. Alejandro

The impact of juniper trees on the local and regional hydrologic budget is hotly debated because water demands from rangelands are increasing.

The density and aerial cover of Ashe juniper (Juniperus ashei Buchholz), also know as blueberry cedar, in central Texas has increased over the last 200 years. Originally limited to rocky outcrops or areas of low fuel availability, Ashe juniper now covers almost 6.7 million acres on the Edwards Plateau.

Juniper trees (Juniperus sp.) are ideally suited for intercepting and retaining precipitation. The scale-like leaf structure and the large leaf area combine to hold a significant amount of water in the canopy.

Water demands from this area have increased due to agricultural irrigation and municipal growth. Aggressive shrub and tree control has been suggested as the solution to providing more water for aquifer recharge, although the feasibility of this suggested solution has not been demonstrated at a regional or landscape scale.

It is crucial to understand the water use of juniper trees and the physical impact of juniper trees on water availability, so for three years we have studied these questions. Our objectives were to:

1. Determine how rainfall is partitioned within juniper trees over a wide geographic region.

2. Determine how rainfall intensity alters the patterns of rainfall partitioning. This study focused on individual trees rather than on a closed forest canopy.

From Uvalde to San Marcos

Ten study sites were selected over a 174-mile range from Uvalde in the western portion of the Edwards Plateau of Central Texas to San Marcos in the eastern portion.

Long-term precipitation ranges from 24 inches on the western sites to 35 inches on the eastern sites.

Shallow soils, less than six inches, at all sites were underlain with a karst geology (porous limestone containing deep fissures and sinkholes and characterized by underground caves and streams). The highly fractured limestone in this area allows rapid water movement when rainfall reaches the soil surface.

At each site, two Ashe juniper trees were selected for instrumentation. These trees were representative of the site, were within about 100 feet of each other and did not overlap canopies with other trees.

We divide rainfall into four parts to explain how the physical presence of a tree or shrub affects the fate of rainfall.

Throughfall is the portion of the bulk rainfall that either 1) falls directly through the canopy and litter of a tree or shrub or 2) is initially captured by the canopy but falls from the canopy to mineral soil.

Stemflow is the portion of the bulk rainfall which is initially captured by the leaves and stems of a shrub or tree canopy, then makes its way to the main stems and eventually to mineral soil at the base of the plant.

Litter moisture is the portion of the bulk rainfall which is captured by the litter layer beneath a plant and evaporated to the atmosphere.

Canopy interception is the portion of the precipitation that is captured by the leaves and stems of the plant and evaporated to the atmosphere.

Each tree was instrumented to collect rainfall, throughfall, stemflow and litter moisture on an hourly interval with an electronic datalogger.

Rainfall above the canopy, which we refer to as bulk rainfall, was measured to the closest 0.01 inch using a tipping bucket rain gauge. Because canopy interception cannot be measured directly, it was estimated by subtraction.

During the three-year study, data were collected from more than 2,700 rainfall events, at all 10 sites.

If there was a one hour gap between recorded rainfall, these were considered separate rainfall events. Bulk rainfall was partitioned to canopy interception, evaporation, soil litter interception and soil water on a percentage basis.

Results

The average tree at all 10 sites was 18 feet tall (range of 12.5 to 25 feet) and had a canopy area of 230 square feet (range of 87 to 689 square feet).

Sixty percent of the storms at all the sites were less than 0.1 inch. Although these storms were numerous, they contributed only 5.4 percent of the total rainfall at each site.  Storms of more than 2.5 inches were less numerous, accounting for only 2.7 percent of the total number of storms, but they contributed more than 27 percent of the total rainfall.

Low-intensity storms were defined as storms yielding less than 0.5 inch of rain over a 24-hour period. These storms were numerous, but contributed little moisture to the soil surface. Most of the precipitation from storms less than 0.1 inch was either intercepted by the canopy (96 percent) or the litter layer (two percent) leaving only two percent of the bulk rainfall to reach the soil surface beneath the juniper trees.

At the highest rainfall levels in these low-intensity storms, at least 15 percent of the bulk rainfall was intercepted by the tree canopy. The litter layer became saturated at fairly low levels of rain and absorbed about five percent of the bulk rainfall, leaving about 80 percent of the bulk rainfall reaching the soil surface.

During low-intensity rainfall events, most of the initial rainfall was intercepted by the canopy and the litter layer. For example, during a 0.5-inch storm that lasted for 29 hours, for the first 16 hours of the storm, canopy interception and litter interception were the dominant factors. After 0.3 inch of rain accumulated (at hour 17), throughfall became the dominant factor in partitioning rainfall.

Overall stemflow was negligible in low-intensity storms. More than 60 percent of the rain received during a typical low-intensity storm was intercepted by either the tree canopy or the litter layer.

High-intensity storms can deposit more than one inch of rain in a very short time. As storm size increased, the proportion of water intercepted by the canopy and lost to evaporation decreased. Approximately 50 percent direct throughfall did not occur until at least 0.4 inch of rain occurred. At this time, about 43 percent of the rain was intercepted by the canopy, 5.6 percent was intercepted by the litter and two percent occurred as stemflow.

The remaining 50 percent directly reached the soil surface. At the highest rainfall levels, more than 80 percent of the rain directly reached the soil surface as throughfall, nearly 5.6 percent was intercepted by the litter layer, four percent occurred as stemflow and 10 percent was intercepted by the canopy. Interception by the litter layer peaked quickly and remained constant after saturation.

The hourly pattern of rainfall within high-intensity events dictates how rainfall is partitioned within tree canopies. For example, in one particular 2.7-inch storm — which began with a light rain during a 16-hour period — periods of low rainfall typically had high interception and low throughfall.

During the first 0.3 inch of the storm, most of the rainfall was captured by either the canopy or the litter (up to hour), but after that, throughfall was the dominant factor.

Certain hours within the storm had high-intensity rainfall — hours six to eight, and hours 11 to 13 – and had greater throughfall than other hours in the storm.

Stemflow seemed to lag behind the rainfall by about one hour. Cumulative partitioning demonstrated that only about 30 percent of the bulk rainfall received during a mixed-intensity storm is intercepted by the tree canopy or litter layer.  This particular storm started rather gently with only 0.3 inch over a three-hour period, but more intense storms behaved differently.

During a 2.9-inch rainfall event over a 15-hour period, the storm produced more than 0.3 inch of rain in the first hour. The canopy and litter were quickly saturated and throughfall was dominant early in the storm. Stemflow still lagged behind the precipitation, but was an important factor. During a one-hour interval (hour five to six), about 1.1 inches of rain fell, but very little of this rain was intercepted and retained in the canopy. Significant stemflow also occurred during this hour.

Cumulative partitioning de-monstrated that only about 15 percent of the rain received during a typical high-intensity storm is intercepted by either the tree canopy or the litter layer. Overall, these events have a greater proportion of throughfall than either low- or mixed-intensity events.

Comparison with other studies

Shrubs in semiarid systems have been reported to intercept from 13 to 40 percent of bulk rainfall, deciduous trees from nine to 20 percent, and coniferous trees from 20 to 48 percent. When growing in the same environment, conifers typically exhibit a higher interception than broad-leaf plants.

We found that Ashe juniper canopy and litter intercepted about 40 percent of the total bulk precipitation combined over all 10 study sites and all intensities of rainfall during a three-year period.

This proportion is much lower than the 79 percent interception reported by Thurow and Hester (1997). The large discrepancy between this study and the Thurow and Hester study results from different definitions of litter.

We measured interception only by the coarse litter fraction which was typically only 0.2 to 2.4 inches thick and amounted to an average of about 112 pounds per tree. Hester (1996) measured interception by the organic soil layer, about 10 inches thick, and recorded litter biomass greater than 101,000 pounds per acre.

We excluded the 2.4- to 10-inch depth because plant roots were prevalent and water use from this layer would be largely impacted by transpiration.

In more semiarid environments, stemflow may account for as little as .06 percent in a pine/oak forest to as much as 45 percent in some shrubs. A thorough review of woody plants shows an average of 8.2 percent of bulk precipitation can be accounted for by considering stemflow, although there is great variability between plant species. This average is slightly greater than the five percent we observed.

In Ashe juniper canopies, all of the precipitation of a 0.1-inch storm is held in the canopy and only 50 percent of a 0.4-inch storm reaches the soil surface. Water held in the canopy is lost to evaporation, although there is some possibility that the water may be absorbed by the plant.

The most significant difference between storm intensities was in the pattern of stemflow. Small storms did not generate stemflow and there was a one-hour lag between precipitation and stemflow during high-intensity storms. Stemflow would also continue for about one hour after precipitation had stopped with high intensity storms.

Canopy impact on local water budget

To calculate the impact of juniper trees on the local hydrological budget at each of the 10 research sites, we created a simple model combining average tree size, frequency of rainfall events and equations describing relationships, canopy interception, litter interception, throughfall, and stemflow to rainfall.

These estimates are based on solitary trees, although as tree density increases the canopies may influence one another to some extent. The model includes a range from 20 percent canopy cover, which would be an open savanna, to 100 percent canopy cover which represents a juniper dominated site, commonly called a cedar break.

When juniper cover was low (20 percent), the amount of water lost to canopy and litter interception averaged 2.4 inches per year, regardless of the site. This similarity in interception at this amount of juniper cover makes sense because the types of storms and the amount of rainfall should not affect canopy or litter interception when tree cover is low.

As tree cover increased from 20 percent to 100 percent, the amount of water lost to interception increased to an average of 12.6 inches per year. The site which received the most precipitation had the greatest amount of water lost to interception (15.4 inches per year). At drier sites, or at sites with little litter under the trees, interception averaged 10.6 inches per year with a closed juniper canopy.

Implications

We monitored interception and rainfall partitioning in individual Ashe juniper canopies at 10 sites over a three-year period. Averaged over all 10 sites and 2,700 total rain events, about 35 percent of the bulk rainfall falling on juniper trees was intercepted by the tree canopy, five percent was intercepted by the coarse litter and duff beneath the tree, 55 percent reached the ground surface as direct and released throughfall, and five percent was redirected to the base of the tree as stemflow.

Small rainfall amounts (less than 0.1 inch) were entirely captured by the canopy and evaporated to the atmosphere, contributing nothing to soil water under juniper trees. Low intensity rainfall (e.g. 0.5 inch over a 19-hour period), which could conceivably benefit the local plant community, was largely intercepted by the tree canopy (more than 60 percent interception).

High-intensity rainfall was less influenced by juniper canopies. At high intensities (e.g. more than 2.75 inches over a 15-hour period) only 20 percent of the bulk precipitation was intercepted by the canopy and litter.  

Canopy and litter interception effectively reduced the beneath-canopy precipitation from 24 to 14 inches in the western region and from 35 to 21 inches in the eastern region, about a 40 percent reduction for both regions.