Saturday, February 25, 2017

Nitrogen Deficiency?

Last week we learned that sword ferns typically live hundreds of years, and now this week, another surprising possibility turns up.

The Andrews Experimental Forest is 3000 acres of old-growth in the Oregon Cascades.  It’s been intensively studied by all manner of scientists since the early 70’s.   The Hidden Forest describes it well.   One breakthrough came early when female undergraduates, rock climbers all,  taught the old foresters how to climb hundreds of feet into the canopy.  It seems there’s a whole world up there, sort of like a coral reef 200-300 high and out of sight.

A long-haired biologist, Bill Denison, fresh from Quaker Swarthmore, moved to Corvallis in 1968, opened a draft resistance counseling center in town, thereby gaining the eternal enmity of his department chair, and also took on the problem of nitrogen flow in the 3000 acre old-growth Andrews Forest.  He was completely puzzled.  The big trees and all the plants need lots of nitrogen (after carbon from C02 and water from rain, nitrogen is the next most important element).  There was no obvious source.  

When a northwestern forest is young, red alder trees come into colonize bare ground.  Like beans and peas, they have mutualistic bacteria that attach to their roots.  In exchange for the glucose the host plants make, the bacteria extract nitrogen from the air.  A fair trade.   Lots of nitrogen accumulates in the soil from the alders and their symbionts, enough (it seems) to keep the forest healthy for a few decades after the red alders die, replaced by firs, cedars, maples and hemlocks.  Then a new source must be found. But which one?  Nobody knew.

Eventually Bill Denison noticed that a lettuce-leafed lichen (Lobaria oregana) grew way up in the canopy, in huge amounts, and steadily rained old bits down onto the forest floor.  He did the chemistry, did the upper canopy exploration (soon they got a huge construction crane installed), and found out that most of the nitrogen in the forest came from this one profuse lichen.  It had the biochemical means to fix nitrogen from the air into compounds plants can use.  These lichens live up high, almost exclusively in douglas firs. The mystery was solved.

Our original die-off ground zero at Seward Park has no doug firs.  It looks like they all succumbed to some root disease many decades ago.  Just maples, cedars, and a few hemlocks remain, and perhaps 10k sq ft of mostly bare ground, where the ferns used to form a lush understory..   All the ferns are now dead - except for some stragglers, all of which grow right next to the big cedars.   I keep thinking that these two things (no doug firs, the surviving stragglers only next to cedars) were a clue, that they were part of the story.  I just couldn’t see where they fit.

Re-reading The Hidden Forest  on Sunday night, reminded of Bill Denison and his work, my hypothesis - completely speculative at this point - fell into place.  The Seward Park ferns might be sick and dead partly because they had grown weak from a lack of nitrogen.  There are diseases ready to invade all the time; usually the plants can fight them off.  But in a weakened state, they succumbed.  The disease, whatever it is, got a foothold, and now spreads.

I next recalled something I have heard a few times from Seattle Parks ecologists - that the urban forests all have a “regen” problem.   That is, they don’t grow as many new plants and trees as their far-from-the-city counterparts do.   There is something missing, it seems.   The lichen (lobaria) is apparently sensitive to air pollution, which may affect their ability to thrive in our urban forests.

Maybe a nitrogen deficit connects all these things.

Using a soil sampling kit from Tim Billo, I just now ran 10 samples.  Four are from a sampling of cores collected at ground zero.  Four are from a healthy north-facing slope where sword ferns dominate, at the peninsula's north tip, right behind the old brick bathrooms.  The last two are from planters (two separate planters) containing ornamental plants right outside my Rainier Avenue studio apartment.  These plants were installed less than 12 months ago.

All of the Seward Park soil samples, and a sample from one of the planters, appear to have levels of nitrogen below detection thresholds.  The remaining sample, from the other planter box, has very high nitrogen levels.    This last reading suggests that my assay techniques may have been adequate.  If the results are reproducible - right now, that's a big "if" - then the nitrogen deficiency hypothesis may be worth pursuing.