Agriculture Reference
In-Depth Information
80.0
80.0
70.0
70.0
60.0
60.0
50.0
50.0
40.0
40.0
30.0
30.0
20.0
20.0
10.0
10.0
0.0
0.0
1.0
10.0
100.0
1.0
10.0
100.0
Leaf longevity, months
leaf longevity, months
Fig. 10.5
There is no apparent relationship between leaf longevity and either NRE or PRE, although
the available data are sparse and not entirely consistent (Wright et al. 2004; Yuan and Chen 2009)
in evergreen compared to deciduous woody species (Yuan and Chen 2009), which
presumably has more to do with the functional ecology of the two foliar habits than
with site-dependent differences in nitrogen and phosphorus availability. Considering
the carbon costs of acquiring the nitrogen and phosphorus to construct a leaf
(Givnish 2002), we might expect that interspecific variation in NRE and PRE
would be related to leaf longevity. In principle, recovery of nitrogen or phosphorus
from senescing leaves might be less costly than acquiring these resources de novo
from the soil, and hence could reduce the total carbon cost of leaf construction.
What little evidence there is, however, suggests there is no relationship between
leaf longevity and either NRE or PRE (Fig.
10.5
). Reich et al. (1992) did, however,
report a significant negative relationship between the absolute amount of resorbed
nitrogen and leaf longevity: the greater the amount of resorbed nitrogen, the shorter
the leaf longevity. This result and the broad range of resorption efficiencies across
species suggest that at least in some instances resorption may act to reduce the
effective cost of leaf construction and thus might act to reduce leaf longevity.
Resolving this possibility will require more studies of nitrogen and phosphorus
availability at sites where NRE and PRE are determined.
In this regard, it is noteworthy that longer-lived leaves generally have lower
nitrogen concentration (Wright et al. 2004) and hence decompose more slowly
(Parton et al. 2007). Because leaffall and decomposition comprise a critical path-
way connecting the production and decomposing functions of ecosystems (Thomas
and Sadras 2001), a positive feedback may generally exist between the availability
of soil resources and the frequency-weighted mean leaf longevity of ecosystems.
The quality and quantity of materials in fallen leaves will affect their fragmentation
and decomposition (Grime et al. 1996). If the decomposition rate of the fallen
leaves is rapid, organic matter can be decomposed to inorganic material quickly and
absorbed by plant roots. Able to absorb abundant nutrients, plants could then grow
vigorously, elongating shoots and shedding relatively short-lived leaves that are
subject to more rapid decomposition. Conversely, longer-lived leaves are difficult
