Agriculture Reference
In-Depth Information
competition for seedlings (see
Chapters 4
,
5
and
6
), and this likely has always been
a factor selecting for fire-independent reproduction.
Undoubtedly the proportion of landscape favoring obligate resprouters has
shifted over geological timescales, but there is little about their current life history
that suggests they are maladapted to contemporary landscapes or dependent on
other taxa for facilitating recruitment. The strategy of not delaying reproduction
to postfire conditions isn't surprising given that obligate resprouters are most
abundant on sites less conducive to seedling recruitment. Considering that land-
scape heterogeneity leads to heterogeneous fire regimes, it is to be expected that
evolution would select for, and communities assemble, diverse life history solu-
tions to fire, including both fire-independent and fire-dependent recruitment
strategies. Coexistence of evergreen fire-independent recruiters with evergreen
fire-dependent recruiters in the same fire-prone environment should be thought
of in the same light as winter annuals and summer annuals in summer-drought
MTC environments. Neither of these annuals are better adapted than another,
but rather both are subdividing resources in ways that promote coexistence
(Chiariello
1989
).
One way in which resources are subdivided between these two life history types
is in different strategies of drought tolerance, which has implications for character
syndromes associated with these recruitment strategies (Keeley
1998
; Paula &
Pausas
2006
,
2011
; Pratt
et al.
2007
,
2008
; Saura-Mas & Lloret
2007
). Fire-
independent recruiters handle drought by avoiding it whereas fire-dependent
recruiters have evolved greater tolerance to soil drought stress.
Physiologically,
drought avoiders
have anisohydric behavior in which daytime
leaf water potential closely tracks soil water availability. These plants maintain
suitable water potentials by accessing soil moisture sources with deep roots or by
limiting growth to periods when shallow soil moisture is available. Evergreen
species may become dormant but at the metabolic cost of leaf maintenance under
conditions where photosynthetic carbon gain is not significant. Demographically,
this makes seedling recruitment precarious on postfire sites due to the extended
maturation period required to develop an adequate root system. Therefore, they
have specialized reproduction on more mesic sites under the shrub canopy or
appropriate gaps generated between fires. In other words, drought avoidance is
not a viable strategy for seedlings on open burned sites because it is dependent on
deep roots.
Drought tolerators
have isohydric behavior and are able to maintain physio-
logical activity and growth under conditions of low water availability by main-
taining favorable gradients of water potential to their tissues through
morphological, physiological and/or biochemical traits. Such tolerance has the
advantage of allowing metabolic activity at levels of low water potential (high
water stress) that would not be possible in drought tolerators. This contributes to
their ability to establish well on severely drought-prone sites, allowing seedlings to
capitalize on the availability of resources after fire. Where fires are highly predict-
able they have specialized reproduction to a single postfire pulse of recruitment.
