328
T.G.A. Green and O.L. Lange
rainforest show atmosphere CO2 inside the forest to be 11 % above ambient
outside the forest, while, amongst the lichen and bryophyte layer, it was 23
and 55% higher, respectively (Tarnawski et al. 1994). Also, initial studies of
Antarctic mosses have shown even higher CO 2 levels, up to 1500 IllC02 1-1
air in the upper layer of moss turfs (Tarnawski et al. 1992). It is clear,
although from a limited literature, that bryophytes appear to be strongly
COz-limited at ambient CO2 levels of around 3501l1C021-1 air and that
saturation may not occur until around 2OO01l1C021-1 air (Silvola 1985;
Coxson and Mackey 1990; Adamson et al. 1990; pers. unpubl. data). If
natural CO2 levels are higher than normal amongst bryophytes then this will
have three major effects. First, actual A in the field will be greater than the
values reported in the literature measured at 3501l1C021-1 air. A good
example would be the Antarctic bryophytes Bryum argenteum and B.
antarcticum (correct name Pottia heimii) , that have much lower A at
3501l1C021-1 air than at higher CO2 levels (Rastorfer 1970; Green 1981).
Second, optimal temperatures for A will also increase with increased CO 2
concentration in the same manner that it would increase with PFD at levels
below saturation (Bannister 1976). In general, reported temperature optima
for A in bryophytes appear to be lower than for other C3 plants, although it
must be remembered that the relatively high dark respiration rates,
especially at higher temperatures, will also act to lower these optima in both
bryophytes and lichens (Larcher 1984). Third, increased CO2 will lead to
higher PFD being required for A saturation (Green et al. 1991). The
situation for lichens is more complex since the response of A to CO 2
concentration depends strongly on thallus water content. Some lichens are
close to CO2 saturation at the normal ambient CO 2 levels of 350 1l1C02 1-1
air and optimal thallus water content. At supraoptimal thallus water
content, lichens are also substantially COz-limited.
16.3 Plant Morphology and Photosynthesis
Both lichens and bryophytes show a wide range of growth forms.
Bryophytes are particularly diverse with ten defined life forms described
from the solid thalli of the Metzgeriales to the thin leafy Bryales and the
complex tissues of the Polytrichales and Marchantiales (Magdefrau 1982).
Lichens have traditionally been divided into a smaller number of forms;
foliose, fruticose, and crustose, which may overlap, and also there is the
complication of heteromerous and homoiomerous photobiont arrangement
(Hawksworth and Hill 1984). An attempt is made here to define lichens and
bryophytes in terms of the presentation of their photosynthetic tissue. This
approach results in two basic photosynthetic forms for bryophytes and one
for lichens (Fig. 16.2). The approach is simplistic but takes into account
T.G.A. Green and O.L. Lange
rainforest show atmosphere CO2 inside the forest to be 11 % above ambient
outside the forest, while, amongst the lichen and bryophyte layer, it was 23
and 55% higher, respectively (Tarnawski et al. 1994). Also, initial studies of
Antarctic mosses have shown even higher CO 2 levels, up to 1500 IllC02 1-1
air in the upper layer of moss turfs (Tarnawski et al. 1992). It is clear,
although from a limited literature, that bryophytes appear to be strongly
COz-limited at ambient CO2 levels of around 3501l1C021-1 air and that
saturation may not occur until around 2OO01l1C021-1 air (Silvola 1985;
Coxson and Mackey 1990; Adamson et al. 1990; pers. unpubl. data). If
natural CO2 levels are higher than normal amongst bryophytes then this will
have three major effects. First, actual A in the field will be greater than the
values reported in the literature measured at 3501l1C021-1 air. A good
example would be the Antarctic bryophytes Bryum argenteum and B.
antarcticum (correct name Pottia heimii) , that have much lower A at
3501l1C021-1 air than at higher CO2 levels (Rastorfer 1970; Green 1981).
Second, optimal temperatures for A will also increase with increased CO 2
concentration in the same manner that it would increase with PFD at levels
below saturation (Bannister 1976). In general, reported temperature optima
for A in bryophytes appear to be lower than for other C3 plants, although it
must be remembered that the relatively high dark respiration rates,
especially at higher temperatures, will also act to lower these optima in both
bryophytes and lichens (Larcher 1984). Third, increased CO2 will lead to
higher PFD being required for A saturation (Green et al. 1991). The
situation for lichens is more complex since the response of A to CO 2
concentration depends strongly on thallus water content. Some lichens are
close to CO2 saturation at the normal ambient CO 2 levels of 350 1l1C02 1-1
air and optimal thallus water content. At supraoptimal thallus water
content, lichens are also substantially COz-limited.
16.3 Plant Morphology and Photosynthesis
Both lichens and bryophytes show a wide range of growth forms.
Bryophytes are particularly diverse with ten defined life forms described
from the solid thalli of the Metzgeriales to the thin leafy Bryales and the
complex tissues of the Polytrichales and Marchantiales (Magdefrau 1982).
Lichens have traditionally been divided into a smaller number of forms;
foliose, fruticose, and crustose, which may overlap, and also there is the
complication of heteromerous and homoiomerous photobiont arrangement
(Hawksworth and Hill 1984). An attempt is made here to define lichens and
bryophytes in terms of the presentation of their photosynthetic tissue. This
approach results in two basic photosynthetic forms for bryophytes and one
for lichens (Fig. 16.2). The approach is simplistic but takes into account