David;
My definition of depauperation is that the site is made "poor" or much
less able to grow a new stand at the same rate as the previous stand. If
I start off with $20 in my pocket, and spend $2 (or maybe $3 here in
Seattle) on a mocha, I don't consider myself depaupered. If it costs $18
and I have $2 left, then I'd feel depaupered. And if I found another $20
along the road, I'd feel better off that I did initially, definitely not
depaupered.
No, I don't agree that lots more nutrients are onsite in an intact
old-growth Douglas-fir ecosystem compared to a clearcut on the same
site. Positively, absolutely, without a question, the vast majority of
most nutrients are still there following the clearcut, with the
exception of carbon, which will be about half removed...and remember
that carbon is not taken up from the soil by trees, its taken up from
the atmosphere.
I'll speak only for Douglas-fir forests that I'm aware have been
studied. People tend to be focused on the part of the forest that is
aboveground, not on what is in the ground or in the soil. A great deal
of biomass will be removed by clearcutting an old-growth stand, but only
about half of the total, and probably less than half. A very small
fraction of the ecosystem nutrients will be removed, partly because they
aren't in the trees to begin with (they're in the soil), but also
because the nutrients aren't in the material being removed (the stem) in
as high concentration as in other plant parts. I'd appreciate more data
if you have it, since this is a subject of discussion in my courses on
forest soils.
It is also a very common misconception that an old-growth forest
ecosystem always has lots more biomass and nutrients than a
second-growth stand. They often don't.
One of my undergraduate students in Forest Soils did an analysis of this
a few years ago for Douglas-fir in the Pacific Northwest. It is
important to note that these results are not comparing the same forests,
and there are likely to be many exceptions in absolute terms, including
clearcuts that contain lots more biomass and nutrients than old-growth
forest ecosystems and vice versa. The student also did not find good
soil estimates for the particular 450-year-old forest soil for which
tree estimates are available (these data are rare), so used another site
for soil estimates.
The estimates for an 450-year-old old-growth stand (Grier et al., 1974)
was as follows:
part biomass N P K Ca
--------------------------------------------
foliage 8900 75 20 70 93
branches 48500 50 10 50 243
stems 472600 190 12 125 284
roots 74000 60 5 18 97
above-ground 530000 315 42 245 620
total tree 604000 375 47 263 717
soil + ff 494000 10043 3902 1625 3836
total 1098000 10418 3949 1888 4553
--------------------------------------------
% in soil+roots 52% 97% 99% 87% 86%
The estimates for a 55-year-old stand (Binkley et al., 1992) was as
follows:
part biomass N P K Ca
--------------------------------------------
foliage 17100 210 30 125 94
branches 50500 88 13 45 103
stems 490000 165 53 62 119
understory 1100 13 2 0 0
aboveground 558700 476 98 233 316
forest floor 26000 243 24 26 156
soil 0-0.9 m 468000 9800 3878 1599 3680
total 1052700 10519 4000 1858 4152
--------------------------------------------
% in soil+roots 47% 95% 98% 85% 92%
Interesting with this assessment is that the aboveground biomass in this
55-year-old Douglas-fir stand is estimated to be a little higher
(558,700 kg/ha vs 530,000 kg/ha) than the aboveground biomass in the
450-year-old Douglas-fir stand. However, I wouldn't conclude that if
this particular 55-year-old stand was grown for an additional 395 years
that the biomass would be lower than it is at age 55. Its certainly
possible that it will lose biomass, but the difference may be due to
stocking, or to inherent site quality. However, this particular
old-growth stand does seem to have less biomass aboveground when the
studies were done.
The final evaluation of harvest scenarios for biomass and nutrient
removal show that if the bole-only is removed from the
450-year-old-stand, that most of the biomass and nutrients will still be
there. Removals are as follows:
percent removed in harvest
harvest -------------------------------
age scenario biomass N P K Ca
---------------------------------------------------------
450-y bole + bark 43.0 1.8 0.3 6.6 6.2
55-y bole + bark 46.5 1.6 1.3 3.3 2.9
This represents less than half of the biomass, and less than 10% (most
less than 3%) of the nutrients in all cases. However, biomass is not a
tree nutrient. If we translate it to carbon, which is a tree nutrient,
more than half of the carbon is also still there. However, the nutrient
C is taken up from the atmosphere by foliage, not by roots, and will be
restored to the ecosystem as forest growth takes place. A subject of
contention right now is how long that restoration would require. I have
models, but no definitive answers.
There is just no conclusive evidence that removal of carbon itself will
result in degradation of the site in terms of future forest growth.
There is some evidence that growth can be reduced with harvests because
of the removal of relatively small amounts of limiting nutrients (such
as N in the Pacific Northwest). However, there is only one study that
has followed the growth of the next stand and it shows that all of that
productivity can be regained by replacing the N lost through the harvest
removals by forest fertilization.
Please do not consider that I think a clearcut is like an old-growth
forest stand in other respects (i.e. habitat for certain species of
animals). I certainly do not. In this discussion, I'm focusing on the
specific organic matter (biomass) and nutrient contents that were the
subject of the original discussion. I feel we need to manage some of our
forest land intensively but renewably primarily to provide wood so that
we can have other forests off-limits to intensive logging or logging at
all. I feel we also need to manage some of our forests less-intensively
to provide multiple values without the inputs required by intensive
management.
Would you now agree that I have proved my point that clearcutting alone
will not depauperate these sites of nutrients for the growth of the
next stand, at least for these examples? Some nutrients are removed,
yes, but a small amount of the total. The proof is in the growth of
the next stand, and I have never seen a clearcut depauper a site of
nutrients such that it will not grow a new forest. Two exceptions
to this are if following the clearcut:
1) there is massive soil erosion, and/or
2) there is a strong wildfire that consumes the bulk of nitrogen
and organic matter in the soil.
Weed competition, root diseases and frost are also common causes of
lack of adequate tree regeneration and survival.
Sincerely yours, Rob Harrison
******************************************************
Robert B. Harrison
Dept. of Ecosystem Science 206-685-7463 voice
& Conservation 206-685-3091 fax
Box 352100
Univ. of Washington
Seattle WA 98195-2100
mailto:RobH@u.washington.edu
http://weber.u.washington.edu/~robh/Faculty/RobCV.html
******************************************************
DavidOrr wrote:
> Well, I suppose that your definition of depauperation differs from mine.
> Would you agree that an old growth Doug Fir forest has lots more nutrients and
> biomass onsite compared to a clearcut on the same site? If so, then I have
> proven my point. If you disagree, then I would appreciate an explanation of
> where I'm wrong.
>
> Sincerely,
> David Orr
Mail converted by
MHonArc 1.1.0