The present drought in California is the most prolonged in nearly two thousand years. In a study of tree ring width of fallen giant sequoias (Sequoadendron giganteum) in the central region of the Sierra, extending from Yosemite to Sequoia National Park, Hughes and Brown (Climate Dynamics 6: 161-167, 1991) found evidence for an average of about five drought years per century, with a maximum of twelve droughts in one century, over the past two millennia. Tree ring width that is less than 10% of average has been shown to correlate with extreme drought. The present drought is now in its fourth year. This, combined with the presence of higher than normal annual temperatures, has raised concerns about the potential for damage to the giant sequoias.
Drought stress has caused an increase in mortality in many conifers across the West, with ponderosa pine, sugar pine, white pine and incense cedar being major examples. In a recent study (Bennett et al. Nature Plants. 1: 1-5, 2015), the authors concluded that taller trees suffer most during drought in forests worldwide.
Vulnerability to drought stress increases with tree height because tall trees have to lift water to a greater height against the pull of gravity and therefore face greater hydraulic challenges. Large trees with crowns in or above the canopy are exposed to higher solar radiation than those in the understory. This may become a liability during drought, when lower water availability and higher evaporative demand make it more difficult for canopy-top leaves to regulate leaf temperature. Also, increasing drought stress can make trees more susceptible to insect attacks, particularly by bark beetles. However, unlike most conifers, giant sequoias are not usually subject to insect attack.
Thus, the evidence shows that tall trees are particularly susceptible to drought and abnormally warm temperatures. Less clear, however, is the effect on the giant sequoias themselves, partly due to the fact that few studies have included them. Experienced observers have noted some localized pockets of limb die-back in a few giant sequoia groves, but there are, as yet, no reports of unusual giant sequoia mortality.
We do not know the reason for their apparent ability to survive major droughts. A possibility is that the vertical extent of their root system may be much greater than previously described. One researcher, Todd Dawson of UC Berkeley, observed that a small landslide in the southern Sierra Nevada exposed giant sequoia roots about 70 feet below ground level. If this is more generally the case, it would give giant sequoia roots a reach into the water table much deeper than that of many other species.
As of the fall of 2015, the present severe drought and abnormally high temperatures have not been found to have a lethal effect on the giant sequoias. The reasons for this are not known. These magnificent trees have survived many severe climate changes over the millennia. We do not yet know how well they will survive this present assault.
I wish to thank Wayne Harrison for the information that he provided for the preparation of this article.
By Peter Ralston