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Determining When Trees Are Drought Stressed

Unlike most annual crops, the roots of long-lived trees can penetrate through soils to great depth to reach water (Figure 1).  If we can’t tell how deeply roots penetrate, how do we determine when trees run out of water?  One way is to monitor the moisture status of leaves and twigs, because these are connected through the sapwood in branches, stems, and roots to water deep underground. Water in the tree’s conducting system is under tension; when a molecule evaporates from pores in the leaves, another molecule moves upward to maintain the integrity of the water column.  The tension on the column during a typical clear day is often equivalent to a tension of 15 atmospheres (14.7 pounds per square inch) or more. At night, when dew accumulates on the foliage and there is no transpiration, the tension on the water column falls to as low at 1 atmosphere when water is readily available to the roots. Similar values would be recorded during a foggy day. As water is depleted from the soil, tension in tree’s vascular system increases, sometimes exceeding values of 40 atmospheres. Such high values result in breakage of the water column that can, if maintained for long, kill trees.

Fig. 1. In western Australia, the roots of one species of eucalyptus extends downward 28 m (92 feet). Photo by Keith Smettem, University of Western Australia

Collecting twigs from tall trees at night is a dangerous job.  A shotgun loaded with buckshot is handy for sampling, although ones shoulder suffers (Fig. 2).  Once a twig is severed, the tension is released on the water column.  To determine how much tension the water column was under, the twig is inserted through a rubber stopper into a pressurized chamber, and the value recorded that is necessary to force water upward to the cut surface (Fig. 3).  Before the effects of climate change became obvious, forest vegetation in arid parts of the western U.S. separated along defined water-stress gradient with native oak woodland at one extreme and subalpine forests at the other (Waring and Cleary 1967).  

Fig. 2. The smile soon disappears.     


Fig. 3. Severed twig being inserted into cap of a pressurized chamber.         

Since 2000, much of the forests in the western U.S. has experienced prolonged drought. Process-based forest growth models have been used to predict areas where trees might be expected to succumb to drought (Fig. 4) and aircraft carrying sophisticated remote sensing gear have confirmed the cumulative effects of sustained drought (Fig. 5).  With the combination of modeling and ever-improving remote sensing tools, we have reached the stage where danger from wild fires can be predicted with increasing accuracy.  The extent that damage from wildfires can be reduced by thinning stands of trees and through fuel reduction will soon be analyzed using these methods.

Fig. 4. Areas in red indicates places in northern California where forests were predicted to be under sustained drought (Waring & Coops, 2016). 


Fig. 5.  Airborne surveys with remote sensing instruments are able to validate (and improve) model predictions (Asner et al., 2015).



Asner, G.P., P.G. Brodrick, C.B. Anderson, N. Vaughn, D.E. Knapp, and R.E. Martin. Progressive forest canopy water loss during the 2012-2015 California drought. Proc. Nat. Academy Sci. Dec. 28, 2015: E 249-E255.

Waring, R.H., Coops, N.C. 2016. Predicting large wildfires across western North America by modeling seasonal variation in soil water balance. Climatic Change 135:325–339.

Waring, R.H. and B.D. Cleary.  1967.  Plant moisture stress:  Evaluation by pressure bomb.  Science 155:1248-1254.