Agriculture Reference
In-Depth Information
different significantly. However, if we discount periods in the seasonal climate
unfavorable for photosynthetic activity, then the rate of leaf production in the
system is not appreciably different between regions (Fig.
3.6
).
This conceptually simple adjustment in gauging the longevity of leaves has
interesting implications for estimating the photosynthetic production at the eco-
system level. Gross primary production can be expressed as the product of leaf
biomass and average photosynthetic capacity rate over the favorable season:
(3.8)
PkBA d
=
·
·
·
mean
where
P
is gross primary production (g m
−2
year
−1
),
B
is leaf biomass (g m
−2
),
A
mean
is the average maximum photosynthetic rate (
A
max
) over the favorable season, and
d
is the duration (s year
−1
) of the favorable season and
k
is a constant. The duration
can be partitioned into duration within a day (mean labor time,
m
h day
−1
) and
duration within a year (days in which plants can carry out photosynthesis within a
year, the favorable period length,
f
days year
−1
).
(3.9)
dmf
= ·
Here we incorporate functional leaf longevity
L
f
into (3.8), multiply the right-hand
side of (3.8) by (
L
f
/
L
f
= 1), and substitute (3.9) into (3.8) to obtain:
( )
(3.10)
P kBL A m L f
=
/
·
·
·
f
mean
f
The first term in (3.10),
B
/
L
f
, is the rate of daily leaf production; this is not so appre-
ciably different among forests (Fig.
3.6
). The next term,
A
mean
m
·
L
f
, is the lifetime
photosynthetic gain by a single leaf. Thus, gross primary production (GPP) of a plant
community potentially can be expressed simply as the product of only three terms:
(
) (
)
GPP
=
Life time gain by a leaf
×
×
daily leaf production rate
favorable period length
(3.11)
(
)
If lifetime photosynthetic gain for individual leaves can be taken as a constant
across species, then gross primary production could be determined simply by the
length of favorable period (
f
). These rather remarkable, if speculative, possibilities
are not without support in published data. Kira (1969) summarized the gross
primary production data of forests in the world and concluded that gross primary
production can be explained by the leaf area index (LAI) and the length of growing
season (Kira 1970). Here, LAI is the total leaf area per unit land area of the forest
and is equivalent to the product of leaf biomass and specific leaf area (SLA: m
2
g
−1
).
The length of the growing season is the favorable period length (
f
). When we plot
the relationship between gross primary production and favorable period length
using Kira's data, we obtain a significant relationship (see Fig.
3.7
), suggesting the
strong contribution of
f
in determining gross primary production. Whether or not
these possibilities are sustained by further work, it is clear that the functional link-
ages between leaf longevity and ecosystem productivity merit close investigation.
