Tasmanian Blue Gum: Side Effects of the Drought, by Jake Sigg

This is reprinted from the September 2015 Westside Observer and was written while a political battle was still raging.  That is now past, but some of the dangers posed by these overcrowded and stressed trees will be with us for some time. Fire has not happened yet, but we must be constantly aware of its possibility.

Most of the California eucalyptus groves, including the Tasmanian blue gum (Eucalyptus globulus)--the tree synonymous with the word eucalyptus in the Bay Area--were planted in the late 19th century, meaning the older trees are around 130 years old. That may not seem old in a state boasting coastal redwoods, giant sequoias, and bristlecone pines, whose ages are in the thousands of years, but it is impressive enough, especially for a tree of such enormous proportions, majesty, and strength. For those and other reasons they command the affection of large numbers of people, including the writer*. Two separate and overlapping problems have arrived that must be dealt with.

One subject is how to manage the lands, and there is a draft management plan due to come out of environmental review soon that is part of a process to address these problems. We will go into this aspect in the October issue of this newspaper. For this article, the focus will be on the more immediate concerns resulting from four dry years: the potential for out-of-control fire, and the danger of falling trees and limbs. There are extensive plantations of blue gums scattered over the city, concentrated in groves of thousands of trees (there are >11,000 on Mt Davidson alone, and the city total must exceed 50,000 in areas without irrigation). To hikers accustomed to mucky trails with streams running down them in mid-summer (exclusive to the west sides of the peaks) they must think talk of fire is laughable. But to those working in them (I volunteer with the San Francisco Recreation and Park Department's Natural Resources Division, as well as with the Sutro Stewards on Mt Sutro) it is a subject to be taken seriously. Even in wet years there are dry periods, and clearing ivy or blackberry reveals large quantities of organic matter--bark, leaves, branches, seed pods, whole limbs, whole trees, plus the buildup from ivy and blackberry debris. Once while working on Mt Sutro the ground gave way beneath me and I plunged into such debris up to my armpits. Don't be fooled by all that green ivy and blackberry; all that debris can burn. And what was just described was before the current four-year dry spell.

The tree's vulnerability to drought has caught us by surprise, so we have no management experience to guide us--and no maintenance or capital funds to pay for what is certain to be very expensive. Up to now rainfall was fairly reliable, and summer's copious fog drip met the tree's water needs in our cool climate, simulating the year-round rain the tree gets in its native Tasmania. It's a tough tree and its endurance in a four-year dry spell is remarkable. However, even a casual look at the groves reveals all is not well when winter rains are deficient: dying tops, thinning and premature shedding of leaves, and epicormic shoots along the main stem all signal plants in crisis. This became apparent in the spring of 2014, and has worsened since. If we get the hot days we usually get in autumn and/or dry winds from the north that sometimes occur in October/November (think Oakland, 1991) the condition could become dangerous indeed. One hard fact: long strips of blue gum's shedding bark have been documented carrying fire 12 miles. That could set fires across the city, leapfrogging from, say, Glen Canyon to the groves in McLaren Park, Bayview Hill, and Yerba Buena Island, each consisting of several thousand stressed blue gums--and even carrying the fire to the East Bay. Worst-case scenarios do happen, and this mere possibility is too awful to contemplate. From the perspective of fire, we are entering the most dangerous time of year--and worse, the state's beleaguered firefighters may not be able to assist our firefighting personnel.

The dense knots of foliage are epicormic shoots. Even if heavy rain comes soon, it is too late to save these trees. Just four years ago, these mature eucalyptus trees were so dense there was no sky visible through their canopy. On Johnstone, Mount Sutro's East Ridge, August 2015. Photo: Craig Dawson

Moving to another problem--the danger of falling tree limbs--it helps to understand the blue gum's response to water stress. When a mature tree loses a large limb, or when a tree is cut down, or injured by fire or freezing, it responds by producing epicormic shoots with blue-green juvenile leaves (as opposed to the long, sickle-shaped adult leaves). This is a survival adaptation, and it responds to water stress the same way: when the capillary columns transferring water to the treetop become broken, it responds by producing epicormic shoots lower down the trunk or limbs. It is a desperate measure to keep the water column moving--once the column taking water and nutrients from the ground upwards is broken, it cannot be reestablished and the tree dies from that point on up. If the tree doesn't die it still creates a problem, described in the next paragraphs.

Some background on the physiology of woody plants is helpful here. Limbs originate at the center of a plant stem or other branch. Both the main stem and the branch expand in diameter as successive layers of xylem are laid down annually. That means that when a limb becomes really large it is securely anchored deep in the main stem or trunk. It is like a tapered, deeply embedded bolt or screw. A limb produced in an epicormic shoot doesn't have this deep anchoring; it is weakly attached only in the outermost layer. Every year it survives the sprouted limb gets bigger and heavier. Although the attachment also gets slightly stronger, it does so only proportionate to the limb getting heavier, so the young limb, lacking deep anchoring, is forever subject to tearing off and falling. The attachment is weak throughout its life and can never be beyond danger of sudden failure. Ask an arborist about the difficulties of pruning large blue gums.

The situations on Mt Sutro and Mt Davidson are nearly identical, and their western sides receive copious fog drip. Photos reflect the conditions to the immediate east of the fog-drip zone. The epicormic shoots clothing main stems and branches, when ascending high up the stem, could theoretically keep a tree alive for a while if substantial rain comes soon. However, the shoots are weakly attached and are forever in danger of breaking off, so they become a permanent safety hazard, and the tree never resumes a normal tree shape. And the prognosis after several years' stress has weakened the root system and initiated core decay is poor. Photo: Craig Dawson

Many of these attachments don't survive even the first year; they are vulnerable to even a gust of wind. Look at the trees bearing these new shoots. Some of the shoots have broken already and are just hanging on the trunk; others will tear off in winter storms. Some of the limbs may live several years and become large enough they could come loose and fall on someone below. There is no way around it: they will be hazards as long as they live.

At time of writing, news reports are rife with speculation that the coming rain season will be wet, perhaps very wet. If we are lucky and get rain before a catastrophic fire, and if the rain season is heavy, that will bring temporary relief, but not an end to worries, as much damage has already been done. When a tree's limbs die there is a corresponding death of its roots, meaning the tree's stability is compromised and it is subject to toppling. And you still have all those weakly-attached sprouts that will forever be unsafe to walk under.

What to do? Getting the public to agree on a plan of action, even for its own safety, is difficult. There are always those who don't want any tree cut down anywhere, anytime, for any reason, and they are not amenable to reasoned argument. Government becomes hamstrung and finds itself unable or unwilling to act. Add that much of the public distrusts government, is not fully aware of the novel danger, and is confused regarding what to do, and what you get is not a recipe for quick action to protect life and property. Our normal means of address--the political process--is not likely to be effective when the body politic is uncertain and divided.

All we have left is to spread information--and to hope; hope that the worst doesn't happen and we are allowed a little more time to muddle our way through. But as Peanuts' Linus says "Hoping to goodness is not sound theology". Better to remind elected representatives that these lands are in the middle of the third densest city in the U.S. and management should weigh heavily on the side of protecting life and property.

* I wrote a tribute to the Tasmanian blue gum in the February 2014 issue of Miraloma Life