Agriculture Reference
In-Depth Information
seed output by obligate seeders over facultative seeders (Keeley
1977
; Enright
et al.
2007
).Thus, ecological models need to address the cost:benefit ratio
between resprouting and seeding in order to find a solution to why selection
has worked to generate the non-resprouting mode.
(3) In considering trade-offs, however, it has not been widely appreciated that the
costs of resprouting may not lie entirely in diversion of resources at the stage
of seed production. Diverting resources during seedling growth toward the
production of adventitious buds and for starch storage to lignotubers could
reduce seedling survivorship under very arid conditions and thus select for the
non-resprouting mode with a greater capacity for deep roots and taller shoots.
Indeed, greater seedling survivorship is commonly observed in obligate
seeders over facultative seeders (Keeley & Zedler
1978
; Thomas & Davis
1989
; Lloret
et al.
1999b
). Comparisons of starch storage in roots consistently
show much greater concentrations in juvenile resprouters than in seeders in
Australian, South African and Californian shrubs (Pate
et al.
1990
; Bell &
Ojeda
1999
; Verdaguer & Ojeda
2002b
; Knox & Clarke
2005
; Schwilk &
Ackerly
2005
). However, it should be noted that such postulated differences
in allocation patterns and growth rates are not universally observed in
resprouter and non-resprouter species (Chew & Bonser
2009
).
Trade-offs in resource allocation at the seedling stage have the potential for
explaining the pattern of increasing obligate seeders in MTC summer-drought
conditions. On fertile mesic sites under summer-rain conditions stiff competi-
tion with grasses greatly favors resprouting seedlings over obligate seeder
seedlings (Clarke & Knox
2009
). Level of starch resources diverted to ligno-
tubers has a demonstrable effect on resprouting following top removal (Wal-
ters
et al.
2005
), and this allocation strategy could be very effective under
conditions of intense seedling herbivory. These different patterns of internal
resource allocation might be accentuated on very oligotrophic sites such as in
South African fynbos, where Ojeda (
1998
) hypothesized that diversion of
resources for lignotubers takes away from resources needed to develop mycor-
rhizal associations in seedlings. In his view, this cost is sufficient to favor non-
resprouting seeders under summer-drought conditions, thus explaining the
depauperate representation of resprouting in the MTC part of South Africa,
and their much greater representation in summer-rain climates.
(4) Another hypothesized cost associated with resprouting is that it enforces a multi-
stemmed growth form incapable of competing with single-stemmed non-resprou-
ters in height (Midgley
1996
;Kruger
et al.
1997
). Although multistemmed
resprouting shrubs often do not attain the maximum heights of single-stemmed
non-resprouting shrubs, there is nothing about the resprouting mode that prevents
selection for a monopodial growth form if there was always a competitive height
disadvantage to being multi-stemmed (e.g.
Fig. 9.9
). Also, the fact that the tallest
tree in the world (
Sequoia sempervirens
) is a resprouter doesn't help that case.
Additionally, the selective factors determining height are complex and involve
trade-offs in photosynthetic rates and shade tolerance. In some cases obligate
