Agriculture Reference
In-Depth Information
(Vincent 2006). The onset of full photosynthetic function would be the most logical
starting point from which to estimate leaf longevity, but this is not practical in broad
comparative studies because of species-specific variation in the relation between foliar
development and foliar function (Niinemets and Sack 2004). We generally resort to
recording a phenophase consistent with records in phenological networks (Koch et al.
2007; Morisette et al. 2009) that is associated with a late stage of foliar development,
such as expansion and flattening of the leaf blade in broadleaf deciduous trees
(Kikuzawa 1978). Similar uncertainties are involved in scoring the timing of leaffall.
Senescence of fully formed leaves is generally more drawn out than budburst and early
leaf development and hence is less amenable to timing precisely (Worrall 1999). Leaf
abscission, which might offer an unambiguous terminal event, is often preceded by
significant declines in photosynthetic capacity as leaves change color during senes-
cence (Diemer et al. 1992; Hensel et al. 1993), and some trees retain dead leaves
(marcesence: Abadia et al. 1996). Any scoring system based on changing color or even
abscission also can be disrupted by a stress event such as an early freeze that abruptly
kills leaves outright regardless of their degree of senescence or development of their
abscission layer. We review here the common methods for estimating leaf longevity,
touching on ways to minimize uncertainty associated with scoring leaf emergence and
leaffall when that is possible for a given method.
Box 3.1
Heterophylly
(continued)
