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and Kikuzawa 2004, 2006). Deciding general criteria for defining an unfavorable
period caused by drought is no less straightforward than for winter cold. The differ-
ent phenological adaptations of species can affect variation in the degree of unfavor-
able conditions for photosynthesis even among co-occurring species. For example,
an unfavorable period resulting from drought in Australia has been defined as the
occurrence of at least 3 consecutive months with less than 25 (or 50) mm precipita-
tion (Eamus and Prior 2001). Eamus et al. (1999b) compared photosynthetic rates
throughout a year for some tree species in a seasonal tropical forest subject to an
unfavorable dry season under this criterion. Two evergreen species (
Eucalyptus tet-
rodonta,
Eucalyptus miniata
) showed relatively stable photosynthetic rates with only
modest declines in the dry season. In contrast, decline in the photosynthetic rate of
leaves retained during the dry season on semideciduous
Erythrophleum chloros-
tachys
was greater, and in fully drought deciduous species such as
Cochlospermum
fraseri
and
Terminalia ferdinandiana
was zero because they are leafless. Despite
these complications, in principle it makes sense to discount leaf longevity for peri-
ods unfavorable to photosynthetic activity.
Available data suggest there also are some unappreciated and potentially useful
linkages between functional leaf longevity and gross primary production at the
ecosystem level (Kikuzawa and Lechowicz 2006); this is apparent in the relation-
ship between the standing biomass of foliage and foliage longevity estimated as
the inverse of leaf turnover in diverse seasonal and aseasonal forests (Fig.
3.6
).
Considering the traditional definition of leaf longevity without regard to favorable
or unfavorable conditions for photosynthesis, then leaf production rates (the slope
of this relationship) in forests from seasonal and aseasonal climates appear to
Fig. 3.6
Evidence that functional leaf longevity can provide a clearer relationship to ecosystem
function than leaf longevity uncorrected for time unsuitable for photosynthetic activity (Kikuzawa
and Lechowicz 2006).
Left
: Relationship of standing leaf biomass and leaf longevity in diverse
forests; the slopes differ significantly between aseasonal and seasonal forests.
Right
: The differ-
ence in slopes is no longer significant when functional leaf longevity is considered.
Closed circles
,
aseasonal forest;
open circles
, seasonal forest
