Agriculture Reference
In-Depth Information
Leaf longevity is an integral part of a quintet of highly intercorrelated and
functionally interdependent traits that organize the function of leaves as photo-
synthetic organs: photosynthetic capacity,
A
max
; leaf mass per unit area, LMA;
foliar nitrogen content,
N
; and leaf dry matter content, LDMC (Wright et al.
2004; Shipley et al. 2006). Photosynthetic capacity, a direct measure of foliar
function, is the natural focal element in the quintet. Leaf longevity, LMA, and
foliar
N
initially drew attention as correlates of photosynthetic capacity and
only later were recognized as part of a unified set of traits characterizing overall
variation in leaf function: the “leaf economic spectrum” (Wright et al. 2004).
Leaf dry matter content subsequently was identified as a little-studied trait that
in fact underpinned the relationships among
A
max
, LMA, foliar
N
, and leaf lon-
gevity (Shipley et al. 2006). Considering the innumerable characteristics of
leaves, including some that figure in theories of leaf longevity, what makes
these the cardinal traits central in defining trends in variation of leaf function?
There are basically two reasons these five traits have primacy. First, all these
characteristics bear on the costs of leaf construction and the photosynthetic
functions that repay those costs over the life of the leaf. Second, these traits
show a wider and ecologically more consistent range of interspecific variation
than other characteristics of leaves.
Take leaf construction cost as an example of a foliar trait that one might well
expect to be an important element in any quantification of leaf function given its
central place in theories of leaf longevity. In fact, the cost of leaf construction per
unit mass, which is what we can most readily measure, is a trait that turns out to
be relatively invariant across both evergreen and deciduous species from a wide
variety of ecosystems; hence, it is not particularly useful in interspecific compari-
sons of leaf function. Griffin (1994) reviewed leaf construction costs from 87
studies, which ranged from 1.08 to 2.09 g g
−1
and averaged 1.54 g g
−1
. Reviewing
162 studies, Villar and Merino (2001) reported very similar results: an average of
1.52 g g
−1
and a range from 1.08 to 1.92 g g
−1
. The difference in leaf construction
costs between evergreen and deciduous habits within plant families is not signifi-
cant (Villar et al. 2006). One might instead consider that something as simple as
variation in the total area of the leaf could affect a broad range of variation in leaf
longevity despite the narrow range of leaf construction costs, but this is unlikely
because it is the areal rate of photosynthesis that determines the rate of recovery
of costs. We must look instead to one of the cardinal traits to make sense of this
situation, to LMA. By using LMA, we can convert our measured leaf construction
cost (
c
) per unit leaf weight to an estimate of the construction cost of leaves per
unit area (
C
) :
Cc
= · LMA
(6.1)
As
c
varies at most twofold whereas LMA varies tenfold or more (Wright et al.
2004), the interspecific variation of leaf architecture reflected in LMA clearly will
have more influence on the time required for recovery of the cost of construction
than simply the costs of the differing materials composing the leaf tissues. This
concept helps illustrate why LMA is among the cardinal traits defining the principal
