Agriculture Reference
In-Depth Information
(a)
(b)
(c)
o oo
o
o
o
o
o
o
o
300
s
s
s
300
300
o
s
s s s
s
o o
o o
o
o
o
s
s
s
s s
s
s
s
o
o
s
o
s
s
sss
200
200
200
o
o
o
o
s
s
s
o
o
s s
s
o
o
o
o
s
s
o o
o
s
s
o
o
o
o o
o
s
s s
o
o
s
s
s
s
s
o
s
o
o o o
o
o o
o
ss
s
s s
100
100
100
s
s s
s
s
s
s s
s s
s s
o o
o
o
o
o
o o o
oo
o
o
s s
s
s
s s s
s
s
o
o
o
0
5 0 5 0
0
5 0 5 0
25
0
5 0 5 0
Fire cycles
Fig. 9.11
Modeled competition between facultative seeder (s) and obligate seeder (o) life
histories under different parameters of (a) equal seed production for both life histories and
20% postfire mortality of resprouts in “s,” (b) three times greater seed production for “o”
and 20% mortality of resprouts in “s” doubled every fifth fire cycle, and (c) two times
greater seed production in “o” and 20% mortality of “s.” (From Keeley 1986.)
obligate seeder seedling recruitment were important, the most sensitive component
was the seedling survival of obligate seeders and the subsequent successful estab-
lishment as a mature reproductive adult. Thus, factors contributing to seedling
success, such as adaptation to arid sites enhanced by greater numbers of sexual
generations or allocation of resources to growth over storage (
Fig. 9.5
), may be
critical to the evolution of obligate seeding.
Simulating competition between facultative and obligate seeding species pre-
dicted that an obligate seeder taxon could drive a facultative seeder to extinction
with three times greater successful seedling recruitment for the former and 20%
resprout mortality each fire cycle (doubled to 40% every fifth cycle) (
Fig. 9.11
).
The model predicts that as resprouting success declines there is a proportional
decrease in the seedling recruitment differential required to drive the evolution of
obligate seeding. These predictions are consistent with empirical studies that
suggest trade-offs in seedling recruitment differentials and resprouter mortality.
In postfire California chaparral, relative to the parent population size, a
Ceanothus
obligate seeder had 1.8 times greater seedling recruitment than the facultative
seeder and this resprouter experienced 67% mortality of resprouts, whereas dif-
ferent trade-offs were evident in a congeneric pair of
Arctostaphylos
; the resprouter
had only 40% mortality but the obligate seeder had 9.8 times greater seedling
recruitment (Keeley
1977
).
Comparisons in other MTC regions have generally found similar patterns
of seedling recruitment differentials between obligate seeders and facultative
seeders (Thomas & Davis
1989
; Hansen
et al.
1991
), but most have not reported
