Wind and Trees

The public comment period for the Draft Environmental Impact Report (DEIR) for UCSF’s plans to destroy 90% of the forest and its understory on 46 acres of the Mount Sutro Open Space Reserve ended on March 19, 2013.  We studied that document carefully to evaluate its accuracy and form our own opinion of the impact this project will have on the environment.  Because Mount Sutro is a very windy environment, we paid particular attention to the influence of the wind for the consequences of UCSF’s proposed plans to destroy most of the forest.  In the process, we learned something about the interaction between wind and trees that we would like to share with our readers.

The wind on Mount Sutro

Mount Sutro is a 900+ foot hill that is directly exposed to the wind from the west, coming off the ocean.  Steep slopes accelerate the wind as it moves uphill.  On the leeward side of a steep hill the wind breaks into turbulent gusts.  This is an oversimplification of the movement of wind over a hill because in a complex topography such as Mount Sutro, the movement of the wind is as complex as the topography.  For example, Mount Sutro is penetrated by a number of steep canyons that funnel the wind as it moves uphill.   

Anyone who has visited the UCSF campus on the top of the hill knows that it is a cold, miserable, windy place much of time.  But if you walk in the dense forest on Mount Sutro, you are often unaware of the wind because the trees are shielding you from the wind. 

UCSF plans to destroy its windbreak by reducing the density of the forest from 740 per trees acre to only about 15-50 trees per acre.  The campus and the neighborhoods on the leeward side of Mount Sutro are going to be subjected to a great deal more wind.  They will also experience more fog which is now being “caught” by the tall trees and condensed as moisture to the forest floor.  That fog is now going to flow freely from the ocean to the neighborhoods on the leeward side of Mount Sutro.

The consequences for the few trees that remain

UCSF would like the public to believe that it doesn’t intend to destroy the entire forest.  However, that is the likely consequence of destroying 90% of the trees on 46 acres because trees develop their defenses against the wind in a specific environment with a specific amount of wind and they often fail when they are exposed to more wind than they are adapted to.

Wndthrow caused by adjacent clearcut, Britain.  Creative Commons
Wndthrow caused by adjacent clearcut, Britain. Creative Commons

UCSF’s own written plans for this project acknowledge that thinning the forest will increase the likelihood of the remaining trees failing:  “Individual trees that suddenly become more exposed to high winds are also more likely to fall.  For this reason, any thinning of the forest that is considered must not be so extensive that it will subject remaining trees to increased windfall.”  (1, page 15) Ignoring its own advice, UCSF proposes to destroy over 90% of the trees on 46 acres of the reserve.

The DEIR also acknowledges that the trees remaining after the forest is thinned will be vulnerable to windthrow for some unknown period of time during which they adjust to the changed environment.  The DEIR suggests that it is possible to mitigate for this potential for windthrow by monitoring the remaining trees to identify potential hazards.  In other words, the DEIR claims that it is possible to accurately identify trees that might fall before they fall. 

This is a fiction.  If it were indeed possible to accurately predict that a tree will fall, we wouldn’t read reports of thousands of trees falling all over the country every year.  Over 5,400 tree failures were reported to the University of California’s “California Tree Failure Report Program” in 2012.  Since reporting is voluntary, we assume that is an underestimate of all tree failures in California in 2012.  (Oaks (Quercus) were the most frequently reported genus to have failed in 2012:  22.7% of 5,415 reported tree failures were oaks.   Failures of eucalyptus were nearly half that (11.90%).)  

Any reputable arborist will tell you that evaluation of trees for potential hazards is an art, not a science.  That is, it is a subjective judgment and this is reflected in the wide numerical range used to rate trees for potential hazards.  When an arborist agrees to a contract to conduct such an evaluation, he/she usually does so with a liability caveat, making it clear that he/she cannot accept legal responsibility for trees that fail which haven’t been identified as hazardous by their evaluation. 

For these reasons, the mitigation offered by the DEIR looks like a trap.  If the evaluation is applied conservatively, the ultimate destruction of the entire forest seems likely.  In other words, the few trees that remain will be declared hazardous and destroyed.  Since those who demand this project have made it perfectly clear that they want the entire forest destroyed, that seems the likely scenario.  If, on the other hand, the evaluation is not applied conservatively, unpredicted tree failures are likely.  In either case, the ultimate outcome is a forest with fewer trees than projected by the DEIR. 

In a consultation with Professor Joseph McBride of UC Berkeley, we were provided with two specific examples to illustrate this trap.  Professor McBride evaluated two extreme windthrow events in the San Francisco Presidio and Sea Ranch.  This study is cited by the Sutro DEIR. (2)  Professor McBride told us that of the 6,000 trees that failed in the Presidio in an extreme weather event in 1993, most would not have been identified in advance as being vulnerable to windthrow.  Healthy, structurally sound trees fail in extreme weather events.  Conversely Professor McBride told us of an evaluation of all trees on the Berkeley campus in 1976 that judged about 3% of the trees as hazardous for which removal was recommended.  Shortly after the evaluation was conducted, UC went through a period of budgetary constraints (much like the one UC is having presently) which prevented the removal of the trees judged to be hazardous.  Over 35 years later, about 80% of those trees are still standing.  In other words, trees judged healthy by professional arborists sometimes fail and trees judged hazardous often do not fail. 

On April 7, 2013, the Bay Area experienced high winds that demonstrated both our windy environment and the consequences for our trees.  Winds of 75 miles per hour were recorded in San Francisco.  At the San Francisco airport, on the eastern (leeward) side of the City, winds of 35 miles per hour or more were recorded for 21 consecutive hours, an unusually sustained high wind.  Both the strength of the wind and its duration caused many trees to fail.  In San Francisco, 75 fallen trees were reported to the Department of Public Works.  Here’s a brief article in the San Francisco Chronicle about this destructive wind, including photos of some of the many trees that fell.

 How wind affects the health of trees

The DEIR would like the public to believe that the thinned forest will be capable of growing sufficiently to compensate for the loss of the existing capability to sequester carbon and recoup the loss of much of the existing stored carbon because the remaining trees will be released from competition.  One of the reasons why this is wishful thinking is that the trees that remain will be subjected to a great deal more wind and that wind is going to reduce the trees’ ability to grow:

“As the magnitude of the stress (windspeed) increases, so do the resulting strains, resulting in a cascade of physiological strain responses.  The physiological responses range from rapid changes in transpiration and photosynthesis at the foliar level, to reduced translocation, callose formation and ethylene production in the phloem and cambial zone.  Long-term developmental and structural changes occur in canopy architecture and biomechnical properties of the xylem. “(3)

This same article explains that the canopy of a tree that is subjected to a great deal of wind tends to be narrower than one subjected to less wind and its leaves are smaller, which is one of the reasons why photosynthesis and transpiration are suppressed in a windy environment.

We turn to Joe McBride’s wind study of the Presidio (4) for a specific, local example that illustrates these general principles.  This is what Professor McBride observed at the Presidio: 

“Wind at the Presidio affects tree growth, form, and mortality.  Exposure to winds in excess of 5 mph usually results in the closure of the stomata to prevent the desiccation of the foliage (Kozlowski and Palhardy, 1997) Photosynthesis is thereby stopped during periods of moderate to high wind exposure resulting in a reduction in tree growth…Eucalyptus showed the greatest reduction in growth with trees at the windward edge being only 46 percent as tall as trees on the leeward side.” (4, page 6)

The plans to destroy 90% of the trees on 46 acres of Mount Sutro will subject the few trees that remain to a great deal more wind.  The growth of the few trees that remain will be significantly retarded by the wind.  The claim of the DEIR that those trees will grow significantly larger when released from competition from their neighbors is fallacious because it does not take into account that the trees will be subjected to significantly more wind.

 Why, oh why?

We cannot imagine why UCSF wants to destroy most of its forest.  These are a few of the most mysterious questions that we cannot answer:

  • Why does UCSF want to subject its students, its patients, and its staff to more wind?  Why does it want to subject its neighbors to more wind and fog? 
  • Does UCSF really believe that destroying 90% of the forest on 46 acres of the Sutro Reserve will not result in the destruction of the entire forest? 
  • Does UCSF really believe that the few trees that remain will grow so large and so fast as to compensate for the loss of the ability of the forest to sequester carbon?

We are speaking of a world-class scientific institution.  Could it really be so ignorant?  Or is there some ulterior motive that is not visible to us?  Conspiracy theories abound in the public comments that have been submitted.  We cannot verify any of those theories, so we won’t repeat them.  We actually prefer to believe the latter explanation, because the thought of such an important scientific institution being so ignorant of scientific facts is too painful to contemplate.


(1)    “Mount Sutro Open Space Reserve Management Plan,” EDAW (consultant to UCSF), 2001

(2)    McBride and Leffingwell, “Assessing windthrow potential in urban forests of coastal California,” Society for American Forests Newsletter, 2006

(3)    F. W. Telewski, “Wind induced physiological and development responses in trees,” in Wind and Trees, edited by MP Coutts and J Grace, Cambridge University Press, 1995

(4)    Joe R. McBride, “Presidio of San Francisco, Wind Study, First Phase,”  circa 2002

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