Íslenskar landbúnaðarrannsóknir - 01.03.1980, Side 24

22 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
TOTAL DRY BIOMASS (g/m2) TOTAL DRY BIOMASS (g/m2)
Figure 5. The IR/red radiance rados for the six experimental trials. (A) 1300 on 13 07 76, (B) 1400 on 13 07
76, (C) 1500on 1307 76, (D) 0815 on 14 07 76, (E) lOOOon 1407 76, and (F) 1330 on 1407 76afterclipping.
The transgenerated variables are
formed from the red and IR radiances and
therefore combine the properties of the red
and IR radiances with respect to total dry
biomass. This is evident when one consid-
ers the asymptotic properties of these
three transgenerations with respect to to-
tal dry biomass. It appears, from visual
inspection of the plotted data, that
asymptotic responses from the IR/red
ratio, the IR-red difference, and the veg-
etation index occur at~450 to 550 g/m2.
Unfortunately, more experimental plots
associated with higher levels of total dry
biomass were not included in this experi-
ment and therefore accurate estimates of
the amounts of total dry biomass as-
sociated with the IR asymptotic response
can not be given. They can be inferred
and, based upon other work of the senior
author, are thought to occur in the~450 to
550 g/m2 region. Plots containing total
dry biomass from predominately green
canopies over and beyond this amount
cannot be effectively estamated using this
method. As more dead vegetation ac-
cumulates in the gross coropy, the re-
lationship between the various ratio var-
iables and total dry biomass become more
linear (Tucker 1977).
Practical Jield. use of the spectral method in lce-
land
It is not enough to simply report en-
couraging r2 values derived from regres-
sion analyses. The questions must be
asked whether the spectral method of
biomass estimation holds any promise in a
country such as Iceland, will it prove to be
practical in a field situation, and does it
offer any improvements over existing
methods of biomass estimation?
Several constraints or iimitations of this
method are apparent and include:
(1) The method only works for green