Agriculture Reference
In-Depth Information
(1) What factors have led to selection for postfire seeding vs. recruitment
independent of fire (evolutionary step 1 in
Fig. 9.5
)?
(2) What factors have led to selection against resprouting as a postfire
regeneration mode (step 2 in
Fig. 9.5
)?
Fire-independent Recruitment
Postfire obligate resprouters, with fire-independent seedling recruitment, are char-
acteristic of many dominant species in MTC shrublands of the Mediterranean
Basin, California and Chile. It is of lesser importance in South Africa and minor
importance in Australian shrublands (see
Table 3.1
). The lesser importance of this
functional type in the latter two regions appears to be tied to low soil fertility as it
does increase markedly in Australian forests on more fertile soils (French &
Westoby
1992
; Chalwell & Ladd
2005
). This functional type represents a number
of lineages with parallel evolution in different MTC regions. For example, obli-
gate resprouters include many of the same genera in both northern hemisphere
MTC regions and some of the same families, such as Anacardiaceae, in both
hemispheres. Almost all have seedling recruitment that is independent of fire and a
surprising number have similar bird-dispersed fruits (
Fig. 9.6
).
All of these obligate resprouters with fire-independent seedling recruitment
appear to have originated in the early or middle Tertiary and have persisted
seemingly unchanged to the present (see
Chapter 10
). They are a class of func-
tional types that Herrera (
1992
) considered as relictual taxa that failed to adapt to
contemporary fire-prone MTC conditions. In his view they represent evolutionary
inertia that are present today merely by chance avoidance of random extinctions.
Building on this idea, Valiente-Banuet
et al.
(
2006
) have suggested that their
reproductive mode of fire-independent recruitment has persisted through seem-
ingly impossible changes in climate by using “modern” arid-adapted taxa as nurse
plants for recruitment.
These papers need serious scrutiny because of their assumptions about the lack
of arid sites and thus lack of fire during much of the Tertiary, and their assump-
tion that one can accurately determine time of origin with the fossil record. When
dealing with arid land plants this is a very dangerous assumption (see
Chapter 10
).
The idea that fire-independent recruitment is dependent on other taxa for facilita-
tion is supported by a database that is based on expert opinion rather than hard
data (Valiente-Banuet
et al.
2006
). Their study only demonstrates that these
species can recruit into vegetation dominated by so-called Quaternary taxa, not
that they are dependent on such sites for reproduction. In the California chapar-
ral, these fire-independent recruiters not only are not dependent on other taxa for
reproduction but they in fact reproduce best on sites dominated by these presumed
relict species, and sites dominated by presumed modern species are far less favored
(Keeley
1992c
).
The perspective presented in both Herrera (
1992
) and Valiente-Banuet
et al.
(
2006
) is shaded by the mistaken view that seasonal climates, and fire-prone
landscapes, are a new phenomenon associated with Quaternary climate changes.
