‘umbrella effect’ /‘canopy interception’



Ran Pauker
In the south of the Israel many rain events are less than 20 mm. Such amounts cannot saturate evergreen foliage and will not wet the ground beneath these trees. The absorption capacity of a tree is a function of its foliage surface area and density and its effects can easily be seen as areas of dry soil, pavement or road in otherwise wet conditions. This creates a phenomenon I call the ‘umbrella effect’ (also termed ‘canopy interception’ in the literature).
Figure 1 illustrates the ‘umbrella effect’ in a prevailing wind. Downwind of the tree is effectively sheltered from the rain while upwind rain can reach the ground below the canopy. The zone of effect will, of course, be dependent on canopy spread, wind speed and direction. Plants growing within the umbrella effect zone will suffer from water stress. From a gardening point of view it would be easy to assume that this water stress is a result of tree and surface vegetation root competition for soil water. However, plants growing upwind do not suffer such water stress.
We examined this issue at Green Point (average rainfall – 250 mm per annum) over a number of years. Our study site was a north-south avenue of Tipuana (Tipuana tipu) trees. The depth of soil wetting, after the winter/spring rains, was on average 50 cm less on the east (downwind) side of the avenue than on the western upwind side. We estimate that this results in a loss of between 60 and 70 mm of rain on the affected side. To compensate for this shortfall additional spring irrigation is needed.
• Example 1. A 10 m tall pine tree (Pinus halepensis) was given 100 mm of ‘rain’ for three consecutive days. In the zone of influence or ‘umbrella’ the soil wetting front reached 20 cm depth. Beyond this, the wetting front reached 80 – 90 cm. In this case the tree and its ‘umbrella effect’ captured 75% of the rain.
• Example 2. Under a Cypress tree (Cupressus sempervirens) that had received 360 mm of seasonal rain the area under the 'umbrella' was still dry (sampling date 23/02/92) with no detectable wetting front (see Figure 2).
The conclusion is that in areas that receive less than 500 mm of rain annually it is preferable to plant deciduous trees to overcome this phenomenon, en if you don't want to plant understory vegetation. In situations like this it will not be necessary to add more water because the intercepting foliage is absent in winter. Additionally, it is desirable to plant deciduous trees, as during the cooler winter months, we prefer to sit or walk in the sun, or partial shade.
It should be emphasized that this issue has not been thoroughly tested experimentally, but the few examples that we have observed show that a substantial amount (25% – 100%) of the annual precipitation can be lost to the tree and its ‘umbrella effect’. This is, of course, dependent on the density of the foliage of a particular tree during the rainy season.
Figure 2: umbrella effect/canopy interception
Figure 1:canopy interception/umbrella effect

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