Martin,
I have been working on a project that measured microclimate in cut
openings ranging from .08-.2 ha in size. We noticed surface
temperatures above litter at the north ends of the largest openings
exceeding 70C on sunny days when the litter was dry. Although we did
not measure in clearcuts, it seems reasonable to think that
temperatures near the surface could be hotter, due to less free-air
convection, as you mentioned in your post.
Nevertheless, at 2 m above the surface, air temperatures were somewhat
marginally warmer than in the surrounding stand (means 1.5C higher,
maximums ~5C higher) (manuscript in preparation). We have noticed
strong air movement from the surrounding stand into the openings on
hot days, which probably mitigates surface heating. I would guess
that a larger clearcut, with more heated surface area and less of a
source of nearby cool air, would attain higher air temperatures
(though maybe slightly cooler surface temperatures). A very large
clearcut may develop enough convective air movement to move and mix
warm air from the surface more rapidly, so temperatures at 2 m may be
somewhat cooler.
See R. Geiger's book "The Climate Near the Ground" (1965 english
translation from the book in German) where a case study is presented
(p. 352) suggesting that air temperature increases with opening size
up to a opening diameter:tree height ratio of ~2 then apparently drops
off somewhat with larger openings.
Hope this helps,
Andrew Gray
Forest Science Lab
Corvallis, Oregon, USA
graya@ccmail.orst.edu.us
______________________________ Reply Separator _________________________________
Subject: Heat in small clear-cuts
Author: Martin =?ISO-8859-1?Q?B=E9land?= <Martin.Beland@uqat.uquebec.ca> at
Internet_Gateway
Date: 11/12/97 11:35 AM
Dear list members,
I have a problem for you.
I am working on natural regeneration of jack pine from logging slash
left on site in the boreal forests of the clay belt of northwestern
Québec, Canada. High temperatures reached close to the ground in summer
are important for this method to work because they can favor cone
opening and seed dispersion. Someone recently suggested to me that small
clear-cuts could heat up more rapidly in the spring and possibly reach
higher temperatures in summer because the neibouring forest protects the
boundary layer of warm air close to the ground from the wind. On the
other hand, there is a common belief that forested areas have a more
narrow range of temperatures than open areas. Considering this, one
might think that small clear-cuts should experience a smaller range of
temperature variation than large clear-cuts, thus possibly lower maximum
temperatures.
All this seems contradictory to me. I have installed field experiments
with jack pine natural regeneration in small clear-cuts and in large
clear-cuts. However, the design of the experiments doesn't allow me to
infer any valuable conclusions about the specific impact of clear-cut
size on maximum temperatures in the soil-air boundary layer.
I would appreciate any hint or reference that would help me to sort this
all out. What would be the most pertinent or relevent hypotheses one
could make about this problem?
Thanks,
Martin Béland, M. Sc. Env.
Student reasearcher
Unité de recherche et de développement forestiers de
l'Abitibi-Témiscamingue (URDFAT)
Université du Québec en Abitibi-Témiscamingue (UQAT)
(819)762-0971 #2362
fax (819)797-4727
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