Agriculture Reference
In-Depth Information
100
50
0
0.2
1.0
0.6
Length of Favorable Period (f)
(year)
Latitude
Fig. 9.4
Simulation of changes in the percentages of deciduous (
open
) and evergreen (
shaded
)
species at different length of favorable period (year) within a year (
f
).
Dotted bar
indicates the
evergreen habit but shorter leaf longevity than 1 year
whose foliar habit is evergreen or wintergreen on King Christian Island at 77°50¢N,
ten species were summergreen in Greenland at 72°50¢N. Similarly, seven evergreen
species whose leaf longevity is longer than 2 years on King Christian Island were
reported to be wintergreen with leaf longevity shorter than 2 years in Greenland
(Bell and Bliss 1977).
Because favorable period length often shortens from the Equator to higher
latitudes, the trend on length of the favorable period shown in Fig.
9.5
might also
be taken to reflect changes in percentages of deciduous and evergreen habits from
the Equator to the poles. This possibility is appealing because the bimodal distribution
of evergreenness on the latitudinal gradient has long puzzled ecologists (Chabot
and Hicks 1982), but is this a fair interpretation of the simulations? Taking winter
cold as an example of a constraint on photosynthetic production, there is some
intuitive basis for interpreting the simulations in this way (cf. Fig.
9.3
). When the
unfavorable period is short, it can be advantageous to use overwintered leaves in
the next spring by paying maintenance costs in winter rather than shedding old
leaves at the end of summer and producing new leaves in spring (see Fig.
9.3a
).
When winter becomes longer, the cumulative maintenance costs of maintaining
foliage overwinter increases, so that shedding leaves before the onset of the
unfavorable period and producing new leaves with high photosynthetic capacity in
the next season becomes advantageous (see Fig.
9.3b
). When the unfavorable
period length becomes still longer, it may be difficult for the leaves to pay back
