Understanding Forest Ecology: Fire, Water, and Bark Beetles


*birds chirping* Currently in the Sierra Nevada’s of
California we have a massive tree die off going on. The last projection was
over 100 million trees have been killed over the last year, year and a half.
What’s triggered that? What’s different now? Why are we seeing something that we haven’t
seen before? So, the way I look at this, is you need to step back and try to figure out
how the ecosystem works, how it’s tied together. One of the things we’ve really found
over the last 10 or 15 years, in the Sierras but is also pretty true for much of the drier forests in the West, is that there are two big things
that really drive these forests, that really influence them; and that’s
fire and water. The availability of water really influences the size of trees; how
many trees you can have concentrated in a particular place, their local density;
as well as the type of tree that’s there. The other half of that equation is fire.
How does fire factor into this? Fire really controls what’s going on in the
understory, and the understory is the future of the forest. The trees are
regenerating down there, the shrubs are down they’re growing and competing with
the trees, and it’s the fire that comes through, historically used to come
through every 10 to 15 years, which would thin the forest out, kill some of the
trees, kill some of the shrubs, and kind of determine who succeeds and who
eventually grows up into the overstory. There is definitely a role for
mechanical thinning and there’s very, very much a role for prescribed
and managed fire in these systems as well. Fire is actually, under the right type of
conditions, much cheaper to use and it can be used in a much more extensive,
larger area and is very, very effective. There’s an interplay between the
climate and stand density, and what we’re seeing right now,
really, with regard to the heavy mortality related to this this
current drought, is not all that surprising. We’ve really done an inadequate
job of density management. These are things we understand. We understand
that forests, if the density is managed properly, either through activity that we do or through
natural means, can be very resilient. One of the things that’s really driving the stress in the forests that we’ve currently got
is the absence of fire, which has created all sorts of small trees growing into the
forest, increasing the density of the forest, and because of that increase in
density, there’s a real high demand on the limited resources within the
ecosystem, which is water. The drought sets the event for the trees to then
be killed by bark beetles. So there’s an interaction going on between the bark
beetle and the drought, with the drought facilitating the bark beetle by
reducing the tree’s vigor. Fortunately this year in California,
we’ve had a really abundant snowfall, a really abundant rain precipitation event,
which I think a lot of people believe is going to ease this problem in terms of
the trees dying in the Sierra Nevada. It’s unclear whether that’s really
going to be the case. There’s only two ways to go
really to address it. One is to take the approach that nature will
handle it, stands will regenerate naturally. And to the extent that there are still remaining
trees in the area, they probably will. However, what we might see is what’s called
a type change. So from, say a mixed conifer forest, which was heavy to pine,
to a mixed conifer forest, which hardly has any pine is now dominated by
other species like Incense Cedar or Douglas Fir. The other option is to plant.
We know how to plant trees and we know how to get plants to regenerate.
We can put Ponderosa Pine back on the landscape if we want to do it that way. One of the benefits,
if you will, of a massive mortality event, like our recent drought kills,
is that if there is a genetic basis for adaptation by individual trees,
either to dry or to wet, which is highly likely because our trees are so
genetically diverse, then a mortality event has a much more rapid impact on
the adaptation, evolution of the trees. Those trees that stay alive are, if they have a
genetic basis to more drought-adapted conditions, they are the ones that then become
the future parents for the next forest. It’s not simple forestry;
it’s not the old system where you would leave the trees evenly spaced.
It takes a little bit more thought to figure out where you’re going to remove
clumps of trees and where you’re going to leave other ones.
One way you can judge or get some measure of how effective your management is,
is how many acres of forests are fuels reduced on each year mechanically
or with prescribed fire vs. how many historically had reduced fuels on them
from fire when it was left unimpeded to burn through the landscape.
Well, on Forest Service land in the Sierra Nevada, that was almost half a million acres
a year. Thinning and prescribed fire are about thirty-five thousand
acres a year, so you can see that there’s a gigantic imbalance there. We really
need to increase our current pace and scale by about ten times. To do that
we’re really going to have to start talking about different ways of thinking
about this problem, some institutional changes; ways in which
we can actually start doing treatments or putting fire out there on thousands
of acres at a time. That’s really one of the fundamental problems is we really
need to change the pace and scale at which we’re working at.