The architecture and photosynthesis of tropical and temperate plant communities

Abstract

This project investigates the growth, development and photosynthetic rate of temperate (C3) and tropical (C4) plant communities growing under conditions of optimum moisture and nutrients in nursery type swards.Community growth was measured by dry weight harvesting whilst instantaneous photosynthetic rate was determined by measuring the carbon dioxide uptake by means of a field chamber and infra-red gas analyzer. The validity of field chamber estimates of photosynthesis is checked against dry weight assessments of growth. Agreement between replicates is variable but falls within reasonable bounds. The growth, development and leaf age structure of temperate and tropical communities is traced from leaf emergence following germination to developmental stages well past closed canopy conditions. The dispersion of foliage in space is examined in detail and the effect of foliage dispersion on photosynthesis discussed. Methods for estimating plant efficiency are proposed. A literature survey of (a) leaf photosynthetic rates and efficiency levels and (b) plant community growth rates and efficiency levels is presented. Tropical communities are shown to display no diurnal trend in photosynthetic efficiency levels while temperate plant communities exhibit marked photosynthetic efficiency depressions at high levels of solar insolation. Light saturation of temperate plant communities is found.Photosynthetic light response curves for individual foliage elements together with foliage architecture and instantaneous solar radiation levels at the field site are incorporated into models which provide theoretical predictions of instantaneous photosynthetic rate. It is shown that detailed descriptions of foliage architecture are not necessary for adequate simulations of community photosynthesis in tropical species. These species have response curves approaching the linear condition in the range of operative light intensities for community photosynthesis. Accurate simulations of community photosynthesis may be obtained by representing community architecture by a single horizontal sheet of foliage. The photosynthetic light response curve for individual foliage elements for temperate species is markedly non-linear and as a consequence detailed geometrical descriptions of community architecture are necessary for accurate simulations of community photosynthesis for these plant communities. The necessity of obtaining photosynthetic light response curves that are representative of leaf activity under field conditions is illustrated. The effect of changes in foliage area index (Φ), foliage angle (ā), and latitude (L) on photosynthetic rate is investigated by simulating community photosynthesis. Community photosynthetic rate for high $ is always greater than for low Φ communities. Steeply inclined foliage (ā = 65°) results in higher photosynthetic rates than less steeply inclined foliage (ā = 20°) at the high solar intensitieis around mid-day, provided the communities are located at low L and possess high Φ. Plant coiranunities located at high L in summer have higher photosynthetic rates in early morning and late afternoon than identical communities at low L. The magnitude of the error involved in photosynthetic simulations when the effect of leaf age is not incorporated into the model is assessed. This error is significant although far less than the magnitude of the change in photosynthetic activity of individual foliage elements.