Wednesday, December 7, 2011

Frank Ogawa Plaza Flooding Threatens Historic Oak Tree

By Nate Seltenrich
Wed, Dec 7, 2011 at 4:56 PM

Update 12/8 10:57 a.m.: I visited the tree this morning to take some photos of the flooding, which you'll find posted below. In one, you'll see a sign reading "PLEASE KEEP OUT: Help us keep our oak tree healthy" — ironic considering the potential damage the excess water could be causing to the tree.

A common rule of thumb is that the diameter of the root system of a mature tree is approximately two to four times the diameter of its crown. This means that the oak's roots extend well beyond the concrete barrier where this sign is posted, and are likewise susceptible to water damage throughout that entire area. (As the site also notes, "improper watering can injure roots, increasing stress and susceptibility to disease and insects. ... Once a root system is severely affected, the tree usually must be removed.")

This morning I also witnessed a Public Works employee, perhaps prompted by our calls to Sue Piper and Karen Boyd yesterday or the below story, checking the sprinkler valve in the tree well to ensure it was closed tight, testing the soil moisture with a narrow rod, and taking a series of photos around the base of the tree. You'll see him in the first photo in a bright orange vest. Stay tuned to find if this results in any changes to the city's excessive watering of Frank Ogawa Plaza.

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The Express received a letter this afternoon from a reader concerned that the ongoing flooding of Frank Ogawa Plaza, which dates to before November 19, when this photo was taken*, threatens the survival of the plaza's iconic coast live oak. Even today the area surrounding the tree well is soaked, as is much of the rest of the plaza.

I called Mayor Quan's spokeswoman, Sue Piper, to see if she could tell us what was going on. But she said she had no idea, other than that the city is trying to regrow the plaza lawn after the Occupy Oakland encampment killed much of it, and referred me to Public Works about a potential leak. I've yet to hear back from them, but the argument doesn't make sense: For one thing, a leak could potentially explain the flooding of the lower-elevation tree well, but it wouldn't account for the incredible amount of water that has pooled elsewhere in the plaza. Likewise, inundating soil with water is not the way to grow a lawn; overwatering to this degree, especially on the brink of the rainy season, will do more to prohibit new growth than encourage it.

Others, such as the poster of the above photo and our letter writer, Oakland resident Molly Batchelder, offer a different explanation: that the overwatering of the plaza is designed to keep Occupy Oakland campers from returning to the plaza. However, as both are smart to point out, this practice could well kill the huge oak at the heart of the plaza — an oak that symbolizes the City of Oakland and reflects its omnipresent oak-tree logo in the flesh.

We'll update you when we hear more from the city, and provide updated photos tomorrow. But for now, read Batchelder's letter, which explains why the excess water is so harmful to the coast live oak.

It is unconscionable that those involved in deciding to flood the plaza would sacrifice the health of this symbolic tree in order to limit public access to a public park and to suppress freedom of expression.

Saturated soil conditions are deadly to California Coast Live Oak trees (Quercus agrifolia). This is an evergreen species that requires good drainage. Unlike deciduous trees that can handle seasonal flooding by dropping leaves during the wet season, evergreen trees cannot. Flooding the surrounding soil eliminates oxygen which leads to the inability of roots to uptake water and nutrients. Such soil conditions also provide a hospitable environment for pathogenic fungi, which can infest roots and persist to bring about an earlier demise for the tree.

Though an exploratory investigation into the location of the oak roots has not been undertaken, anyone who believes that roots only exist within the circular planter is wrong. It should be noted, however, that the tree is planted at a lower elevation than the flooded area above, and therefore water is more likely to move inside of the cement wall and possibly inundate the roots within.

The value of the tree is likely more than $200,000 and possibly invaluable to the City as it cannot be replaced.

The over irrigation should cease immediately and the soil profile should be examined for signs of anaerobic conditions. The tree deserves a full investigation to determine where the roots are, what the level of soil compaction is, as well as what nutritional supplementation is required so that health mitigation treatments can be appropriately prescribed to ensure the tree can remain for another 200+ years.

This is the City’s most valuable tree and we do hope it is provided more respect than what is currently being shown.

References:

Costello, Laurence R., Bruce W. Hagen, and Katherine Jones. Oaks in the Urban Landscape: Selection, Care, and Preservation. The Regents of the University of California Agriculture and Natural Resources, 2011.

Council of Tree and Landscape Appraisers. Guide for Plant Appraisal, 9th Edition. International Society of Arboriculture, 2000.

*Note: While it did rain in Oakland on November 19, it wasn't until evening, after this photo was taken.

Correction: An earlier version of this story suggested that the soil surface inside the tree well was soaked Wednesday, as opposed to the area immediately surrounding it. Both pose a threat to the tree and its roots, which are not confined to the concrete tree well.

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