The great Tasmanian blue gum and its management on Mt Sutro,

By Jake Sigg
The Tasmanian blue gum, Eucalyptus globulus, is surely one of the world’s great trees, but in recent decades it has become the center of controversy in California.  This controversy—most of it uninformed—has unfortunately obscured the tree’s merits and its proper use.  The controversy has little to do with the tree itself—an amazingly beautiful and successful creation of nature—but with improper siting, being often planted in inappropriate areas.  Although Mt Sutro was one of those inappropriate areas—putting a tree plantation on top of a priceless wildflower/grassland—the damage was done long ago, and the plantation will remain, perhaps to the end of time.  But it needs management, which the University of California has undertaken for the first time.  Unfortunately, public acceptance of management involves understanding of basic biological facts—understanding that is in short supply in today’s urban world.  My mantra is that we live in a biologically illiterate society, and this fact makes land management problematic.

There is no sound reason for controversy about the University’s plan to thin the eucalyptus plantation, because it should please both the tree’s lovers and its detractors.  All “forests” (in this case, not a forest but a plantation) must be managed, whether by nature or by man.  Nature does it easily, naturally, without effort.  For humans it is fraught with difficulty.  There is the added problem of getting understanding and agreement from the human community.  Adolf Sutro’s plantation is no longer robust, in part because decades of prodigious self-sowing means that there are too many trees competing for too little space and other resources, something that wouldn’t happen in a natural forest.  The result is overcrowding, forcing the trees to expend most of their energy in growing upward to reach the light.  Most of them are devoid of limbs or foliage for the first 50’
or more.  This seldom occurs in natural conditions, and the Tasmanian forests have a totally different character, being much richer and diverse.  Further, the now-changed Sutro environment creates conditions for the invasion and proliferation of Himalayan blackberry, and English, Algerian, and Cape ivies.  They form smothering blankets and impenetrable thickets that deprive access to both humans and animals; only the volunteer work of the Sutro Stewards has penetrated into these thickets, but large areas are still monotonous and uninviting due to the lack of diversity and the paucity of wildlife occasioned by the few-species understory.  The blackberry and ivies, incidentally, also prevent the trees from regenerating, as seeds can’t germinate in the ivy/blackberry thickets.  It is ironical that the blue gums carry the seeds of their own destruction, and only humans can assure their continuance here.

Those who object to the fire hazard-reduction plan must realize that management is not an option.  The laissez-faire of the past century has come to an end, and management has become a compelling necessity.  The possibility of a major fire—even a General Alarm fire—is too great and the University should not be asked to take this risk.

I have an additional concern not mentioned by the University:  the possibility of landslides.  Why do I have the temerity to posit this, when it is a geotechnical matter for which I have no training?  Simple:  nature will always assert the angle of repose.  Count on it.  With rock of integrity, such as granite, it may take millions of years to achieve the angle; with softer rock or rock lacking integrity, such as chert, it takes much less—like decades.  Chert is a multi-layered rock, its layers alternating with mudstones and other constituents and riddled with fractures.  Landslides are common in the band of chert traversing San Francisco from Bayview Hill to west of the Golden Gate Bridge.  These slides are occasioned by removing toes of slopes for roadcuts or to build houses.  One of the slides, disconcertingly, is on Medical Center Way (MCW) off Parnassus.  One area to be thinned is directly above the whole medical center complex, including the stem-cell research building now being built.  Can incalculable tons of mud and trees bury the stem cell building?  In the wet season I don’t take my chances and I avoid my usual route up MCW while going to my monthly work parties with the Sutro Stewards.  Should I mention earthquakes?

Adding to the likelihood of a slide is the presence of approximately 200 mature blue gums, each weighing several tons.  This is weight the land has never had to support in millions of years; remove the previously-existing angle of repose when you cut the slope in two places (a hairpin turn doubles the road back, which doubles the risk) and, well, you see the picture.  And there’s that previous slide just a hundred yards further down the road, in case you have doubts.

Jake Sigg is a member of the California Natural plant society and the “dean” of the San Francisco native plant community