20 Sep 2008

Good Tree Bad Tree

This long article discusses water issues and ends with...
Mesquites have deep, deep taproots, wells in miniature that can provide them with water even during severe drought, thereby lending them an edge over such competitors as grasses. And they’ve been spreading — because of changes in wildfire regimes, livestock grazing or climate change or because woody plants are generally benefited by higher levels of atmospheric carbon dioxide. No one’s entirely sure why.

Mesquite trees enrich the soil under them, and they furnish good wildlife food and habitat, but in an era of climate change, another service they provide may prove to be more important: They’re long-lived and effective at sequestering carbon. If residents of the arid Southwest decide to get serious about doing what they can to limit concentrations of carbon dioxide in the atmosphere, encouraging the growth of mesquite trees wouldn’t be a bad way to go.

Except for this rub: A mesquite woodland uses a lot of groundwater — more than twice as much as the grasslands that are native to the same terrain.
So the trees are good as carbon sinks (good tree!) but bad as water consumers (bad tree!). I suppose that we should just be happy to know this, but you can be sure that someone will choose ONE aspect (good OR bad) to emphasize when proposing a "mesquite" policy.

A few years ago, I met a forestry guy who said we should cut down trees to increase the water supply. This policy sounded a little suspicious to me at the time, but now I see his point. In places where "no fire" policies were in place for years, dense tree cover should be thinned. OTOH, such an idea is no excuse to log, say, an old growth forest at steady-state densities.

Bottom Line: It's fine to reverse our own dumb forest policies, but let's not undo Nature's policies.

hattip to DW

1 comment:

  1. Yeah, I'd have to have my arm twisted real hard to support logging old-growth forests unsustainably (or maybe even sustainably, if staying hands-off might allow them to come back beyond their current footprint).

    But the point is interesting about mesquite, and about no-fire forest policies that cause the proliferation of water-intensive underbrush.

    Funny, I'd like to hear Peter Gleick weight in on forest water-use efficiency the way he has on ag water-use efficiency. Would he attack status-quo forest policies the way he took on the ag sector? I would think there could be a TON of water at stake, the national forests being as big as they are...


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