Quantifying Leaf Longevity

Leaf longevity and “leaf lifespan” are sometimes used as equivalent terms, and at other times “leaf longevity” designates the potential longevity of leaves and “leaf lifespan” their realized longevity. To keep things simple, we here consistently refer only to leaf longevity, qualifying the context as may be necessary. With an emphasis on times when a leaf can carry out its photosynthetic function, we define leaf longevity as the period from the emergence to the fall of a leaf. Because leaf development is a continuous process, a reasonably consistent operational definition of leaf appearance and leaffall is necessary. It is impractical to include the period of leaf initiation and early development before budburst in estimations of leaf longevity, and in any case these earliest stages in leaf development are not directly relevant to photosynthetic function (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).