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Leaf life span (number of growth seasons)
Fig. 10.1 Relationship between leaf longevity and mean retention time of biomass (MRTB) in
various ecosystems. (From Mediavilla and Escudero 2003b)
biomass in canopies (Fig. 10.1 ). The ratio of leaf biomass and leaf litter produc-
tion gives an estimate of mean retention time of leaves in the canopy, and these
data in turn can be used to estimate leaf longevity (cf. Chap. 3). More importantly,
the voluminous data gathered by ecosystem ecologists on the quality of fallen
foliage has considerable bearing on theory for leaf longevity. Foliar nitrogen in
particular is not only an important determinant of decomposition and nutrient
cycling at the ecosystem level (Cornwell et al. 2008) but also a critical correlate
of leaf longevity and photosynthetic function (Wright et al. 2004). Construction
of leaves requires investment of nitrogen that can only be acquired from the
environment at some carbon cost to the plant (Givnish 2002), which contributes
to the total carbon cost of leaf construction that under current theory (Kikuzawa
1991, 1995b, 1996) must be recouped over the lifetime of the leaf. If nitrogen and
also phosphorus can be resorbed from senescing leaves and recycled into new
leaves, then leaf construction costs may be less than if these foliar resources had
to be acquired de novo in the environment.
Nutrient Resorption and Leaf Longevity
Resorption of nutrients is a normal part of the leaf senescence process, but full
recovery of critical and often relatively scarce nutrients such as nitrogen and phos-
phorus apparently is not possible. Killingbeck (2004) distinguishes potential
resorption and realized resorption. Potential resorption considers all biochemically
resorbable nutrients, those not so refractory as to be mobilizable only at exceed-
ingly high metabolic costs. Potential resorption is considered a fixed value specific
to each plant species and is thought to be phylogenetically dependent. In terms of
realized resorption, not all nutrients that potentially could be resorbed in fact will
be retranslocated from senescing leaves, so realized resorption typically is less than
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