In defense of a dense forest

In the fifteen years that we have debated with native plant advocates about their desire to destroy our non-native urban forest, the arguments they use to justify their plans have changed many times in response to our push-back against their plans.  In the latest round of argument about plans for the Sutro Forest in San Francisco, the justification devolved to the claim that “thinning” the forest would improve its health and reduce fuel loads.  We will set aside the claim about fuel loads in this post because we have published many articles to address the bogus claim that the existing Sutro Forest is a fire hazard.  Although we don’t agree that “thinning” is an accurate description of a project that intends to destroy 90% of the trees on 75% of the Sutro Forest, we will set that issue aside for the moment as well.

In this post we will address the claim that “thinning” a forest to less than 50 trees per acre—with trees a minimum of 30 feet apart–will improve forest health.   We have reported that the few trees that remain will be subjected to a great deal of wind which will suppress their growth.  The few remaining trees will also be vulnerable to windthrow because they will be subjected to more wind than they are adapted to.  Trees develop their defenses against the wind based on the amount of wind they experience during early stages of their growth.

A dense forest is a healthy forest

As we often do on Million Trees, we will challenge the conventional wisdom that a “forest” with few trees is healthier than a dense forest, based on the assumption that the trees are released from competition with their neighbors for available resources.  We will report on a new argument for the advantages of dense forest for optimal forest health, i.e., that the forest is essentially a community which functions best when it is densely populated:

“It is the evolutionary nature of a tree to be part of a forest or plant community.  Trees do not grow as lone individuals under natural conditions.  This principle of cooperation referred to by biologists as mutualism appears to have governed organisms from their beginnings.

“Despite the universal gregarious nature of trees, they are almost always discussed and depicted as solitary specimens.  Children’s books, technical publications, and literature on gardening only illustrate and discuss the atypical form of a tree—the symmetrical, low-branched, open grown form.  All of our knowledge about trees is colored by this cultural archetype.  We celebrate the singular specimen.

“Observe trees growing on a natural woodland site.  The tall erect trunks of closely spaced forest trees and branch configurations shaped to admit light are two of the more obvious adaptive responses of trees to forest conditions.  Each layer of the forest contains examples of this kind of adaptive geometry.  The result is that trees can grow very well in dense forest conditions, and in fact are uniquely suited to what we regard as close spacing.  For example, it is not unusual to find northeastern forests growing at densities of 400 trees per acre.  This is equivalent to trees ten feet apart in both directions.  Much higher and lower densities also occur naturally.  It is significant that trees can adapt to such a wide range of conditions.  A group of Maple trees growing five feet apart is just as healthy or at least better able to survive than a single tree growing in an open meadow.  The slower growth rate of trees growing close together is part of their adaptive response and does not indicate that they are less healthy than faster growing trees.

“The popularized open grown individual tree has an adapted form that is not as sturdy as the forest shaped tree.  The tree needs the lower, more spreading branching as protection for its trunk and roots.  Its faster growth rate actually produces less sturdy wood.

“To retain their vitality when growing close together, trees adjust their form and growth rate.  In this way, they are able to share the more limited amount of sunlight and root space.  Trees of the same species do not kill each other off in a fight for survival.  This more dramatic image has greater popular appeal but limited accuracy.  In the natural process of forest succession, a certain species will dominate all other trees for a given soil type and climate.  This is a long term process that occurs because less tolerant trees tend to grow on a site first and would not occur if the climax species were planted first.  It is true that in natural repropagation of a clear area, a superabundance of seedlings is often produced, and later thinned by competitive survival of the stronger individuals.  This process is limited for the most part to the seedling and early stages of forest succession, and is a lesser factor to later development of the forest when the adaptive process assumes a more important role than competition…

“Through observation of trees growing in natural habitats, a designer can conclude that there is no biological basis for keeping trees far apart, since they grow at every possible spacing” (1) (emphasis added)

An analogy to human society

The community of trees in a forest reminds us of a civilized human society in which individuals cooperate for the collective benefit of society.  Like a healthy forest, a civilized human society is one in which cooperation trumps competition.  It doesn’t seem to be a coincidence that native plant advocates consider competition more powerful than cooperation both in a forest of trees and in human society.  Many native plant advocates seem to share a dark outlook about both human society and our healthy urban forest.

Update:  We are pleased to report that UCSF announced on Thursday, November 21, 2013, a significant revision of their plans.  Only trees 6″ in diameter will be destroyed, * which means fewer trees will be destroyed than originally planned.  Only the perimeter of the forest will be thinned to reduce perceived fire hazards to the surrounding residential neighborhood.  They do not plan to use herbicides.  The details of the revised plan and the timeline for its approval and implementation are available on the Save Sutro website.  

*Update:  UCSF has corrected this information in response to an inquiry from the Save Sutro webmaster:  “The correct information is that the proposed Hazard Reduction Measures recommend removing trees with a stem diameter of less than 10” in the North and South project areas, and to remove trees with a stem diameter less than 6” in the West project area.”

******************************

(1)    H.F. Arnold, Trees in Urban Design, 1993, pgs 49-50