Gas Exchange and Growth
159
Demetriades-Shah et al. (1992) called the attention to some misuses of
the short-term correlation between plant productivity and intercepted light
especially in the case of remote sensing, but contributes very little to the
understanding of its relevance in physiological terms because, as mentioned
above, there are no physiological reasons to expect that short-term c is
related with Rw.
8.7 Phenology and Rates of Growth and Photosynthesis
Phenology and plant life-form often modify the relationship between growth
and gas exchange. For example, ruderals and annual plants usually have
high metabolic and growth rates. In normal conditions, this insures that
the maximum number of viable propagules is produced during the favorable part of the year. However, competition and environmental stress
may drastically change this simplistic generalization. For example, in the
Mediterranean environment, C3 annuals do best if they can grow and photosynthesize in the relatively low temperatures of the winter (when water
is plentiful) even if these do not correspond to optimal temperatures. In
woody perennials, fast growth rates have to be sacrificed to large partitioning coefficients to support perennial biomass. Usually, plants with greater
longevity have slower growth rates because resistance of perennial biomass
to decay (herbivory, disease, or saprophytic fungi) and to mechanical stress,
result in additional respiratory costs and a high carbon concentration of the
biomass (low Cf) related to the accumulation of lignin and other phenolic
compounds. It is a low F and Y G that will contribute to lower growth rates
in long living woody perennials in comparison with herbaceous. The same
is often true for the comparison between fast- and slow-growing trees and
shrubs. The growth characteristics and physiology of different plant lifeforms was reviewed comprehensively by Schulze (1982) and therefore I will
focus on generalities.
It is often assumed that evergreens always have lower photosynthetic
capacity than deciduous trees and most herbaceous plants. In a classical
study, Schulze et al. (1977) showed that the evergreen Picea abies in central
Germany compensated for the lower photosynthetic capacity of its needles
with a longer photosynthetic season and larger foliage mass, when compared
with the deciduous co-occurring Fagus sylvatica in terms of annual carbon
budget. However, this generalization should not be taken too far. For
example, Mediterranean evergreens often have photosynthesis rates of the
same magnitude or greater than temperate zone deciduous woody plants
(Korner et al. 1979). On the other hand, evergreens may differ a great deal
in terms of leaf turnover, and that may change the photosynthetic capacity
of the canopy.
159
Demetriades-Shah et al. (1992) called the attention to some misuses of
the short-term correlation between plant productivity and intercepted light
especially in the case of remote sensing, but contributes very little to the
understanding of its relevance in physiological terms because, as mentioned
above, there are no physiological reasons to expect that short-term c is
related with Rw.
8.7 Phenology and Rates of Growth and Photosynthesis
Phenology and plant life-form often modify the relationship between growth
and gas exchange. For example, ruderals and annual plants usually have
high metabolic and growth rates. In normal conditions, this insures that
the maximum number of viable propagules is produced during the favorable part of the year. However, competition and environmental stress
may drastically change this simplistic generalization. For example, in the
Mediterranean environment, C3 annuals do best if they can grow and photosynthesize in the relatively low temperatures of the winter (when water
is plentiful) even if these do not correspond to optimal temperatures. In
woody perennials, fast growth rates have to be sacrificed to large partitioning coefficients to support perennial biomass. Usually, plants with greater
longevity have slower growth rates because resistance of perennial biomass
to decay (herbivory, disease, or saprophytic fungi) and to mechanical stress,
result in additional respiratory costs and a high carbon concentration of the
biomass (low Cf) related to the accumulation of lignin and other phenolic
compounds. It is a low F and Y G that will contribute to lower growth rates
in long living woody perennials in comparison with herbaceous. The same
is often true for the comparison between fast- and slow-growing trees and
shrubs. The growth characteristics and physiology of different plant lifeforms was reviewed comprehensively by Schulze (1982) and therefore I will
focus on generalities.
It is often assumed that evergreens always have lower photosynthetic
capacity than deciduous trees and most herbaceous plants. In a classical
study, Schulze et al. (1977) showed that the evergreen Picea abies in central
Germany compensated for the lower photosynthetic capacity of its needles
with a longer photosynthetic season and larger foliage mass, when compared
with the deciduous co-occurring Fagus sylvatica in terms of annual carbon
budget. However, this generalization should not be taken too far. For
example, Mediterranean evergreens often have photosynthesis rates of the
same magnitude or greater than temperate zone deciduous woody plants
(Korner et al. 1979). On the other hand, evergreens may differ a great deal
in terms of leaf turnover, and that may change the photosynthetic capacity
of the canopy.