CRYING WOLF WITH BEETLES

In the Sutro Stewards blog last month, Craig Dawson (who is its Executive Director) wrote a post claiming that the forest was in dire straits, infected with funguses and beetles: specifically, Anthracnose, armillaria, phytopthora, wood decay fungi, the snout beetle and the tortoise beetle. It concluded: “The bottom line is that we cannot expect much of the declining forest to recover from the condition it is currently found in, rather we can expect further widespread die-off. The dying trees will quickly pose a significant hazard within a year or two as we have already witnessed.”

It sounded alarming.

We sent the link to the article to a number of experts. None of them thought it was particularly serious. (One academic ecologist called it  “…pure twaddle…” ) Nor did they agree with its conclusion that the forest would therefore decline.

• “The diseases and insects mentioned in the Sutro report could be found in any forest…” (from a certified arborist and plant pathologist)
• “The description of common conditions of eucalypt trees on the part of Mr Dawson’s piece seems to me solid as such—a description—but unconvincing as an argument that pretends to show some state of pathological emergency in Sutro…” (from an environmental science professor)
• “This is amateur plant pathology at its best….” (from an urban forester)
• “…faith-based botany…” (from an urban forester)
• “This is certainly not the first time I have seen someone want to use a disease threat as a roundabout way to get some politically inconvenient trees removed.” (from an academic plant pathologist)

 THE SPECIFICS

Some commented specifically on the individual fungi/ beetles. We also investigated ourselves, using the UC Davis website.

• Anthracnose: “anthracnose is found on the leaves of many plants…” [In San Francisco] “sycamore leaves are filled with anthracnose…” (We would also note the UC Davis website says, “In California, anthracnose rarely causes permanent damage to plants except for elm trees.”)
• Armillaria: “…definitely all over the place in the coast ranges and is even rampant in Golden Gate Park.” (This does not indicate a dire disease requiring intervention, especially tree-felling.)
• Phytopthora: We could find no references to phytophthora in eucalyptus in California.
• Wood decay fungi: “..these are mostly associated with older trees. The pictures represent Trametes versicolor – mostly found on dead wood, very rarely on living trees; Laetiporus gilbertsolnii – common on living Eucalyptus and oaks…” (Again, there’s no indication that these are reason for alarm.)
• Eucalyptus snout beetle:  These beetles feed on eucalyptus leaves. According to UC Davis’s website, “Eucalyptus snout beetle is controlled biologically by Anaphes nitens, an introduced parasitic wasp. No further control is necessary.”
• Eucalyptus tortoise beetle: Also a leaf feeder, these beetles don’t usually kill trees. From the UC Davis website: “Unsightly, tattered leaves are usually just an annoyance that does not appear to threaten eucalyptus survival or health.” Since some tattered leaves in a forest setting are quite natural, we don’t think this is a problem.

Following a recent walk through Sutro Forest, Dr McBride (Professor Emeritus, UC Berkeley) noted that the forest looked healthy and thriving, with no evidence of the feared decline. He pointed out that in a naturalized setting like this one, we should expect some number of trees to do poorly or even die, as the forest “self-thins.”  Furthermore, he said, without fungi and other creatures as part of the forest ecosystem, we’d be up to our eyeballs in dead logs.

We have to say that in our years of frequent walks in the forest, in all weather and at all times of the year, most of these fungi and beetles are rare. Rare enough, in fact, that when we see fungi or mushrooms (the fruiting body of some fungi) we take pictures. We found a few leaves with evidence of tortoise beetles (semi-circular “bites” from the leaves), but they were few and far between. So far, we have not been able to find leaves showing the elongated perforations made by snout beetles.

We asked about hollow trees. Dr McBride said that unless the remaining wood is less than 30% of the diameter, hollows in trees did not weaken them. “A tube is structurally one of the strongest forms,” he said. The life of a tree is in its outer layers. The center of a tree essentially provides structure. (And – hollow trees are great wildlife habitat.)

WHAT ABOUT THAT CANOPY?

The Sutro Stewards article also includes a picture of a stand of trees with a defoliated canopy, implying that is typical of the forest. It is not. This picture at the start of this article, taken in June 2014, is actually much more representative of the conditions in Sutro Forest. (Here’s a picture of the forest taken from Twin Peaks.)

The stand portrayed in the article does exist. It is on the lower part of the East Ridge – right above an area where UCSF has removed a lot of trees and understory as part of their “fire hazard” action in August 2013. This has made the forest there much drier and less able to retain moisture – particularly since this is on a steep slope near the edge of the forest. Dr McBride considers that the trees’ intergrafted root system may also have been damaged during the work, making the stand much more vulnerable. However, the trees do seem to be recovering, currently with epicormic growth.

CAUTION: DON’T MESS WITH THE FOREST

But rather than indicating that the forest is diseased and trees should be removed, it suggests much more caution. The removal of smaller trees and understory and damage to root systems can stress trees, reducing the moisture available and increasing wind damage. Instead of making the remaining trees more healthy by “releasing” them, it can make them less healthy – as we see on the lower part of the East Ridge. Similar impacts are visible in Glen Canyon, where a lot of clearing has been going on – exacerbated by pesticide use.

Furthermore, with the normal fungi present, and with the usual damp conditions in this cloud forest environment, chopping down trees doesn’t help reduce fungi, it only spreads it around.