An e-publication by the World Agroforestry Centre
AN INTRODUCTION TO AGROFORESTRY
11.2. Plant productivity
Plant productivity, i.e., the amount of growth that can be attained by a plant within a given period of time, is a function of the net rate of photosynthesis (PN), which is the difference between gross photosynthesis (PG) and respiration (R):
Respiration involves the oxidation (or breakdown) of complex substances such as sugars and fats. The general reaction is:
Photosynthesis and respiration are, in many ways, similar but opposing reactions. Respiration uses energy from photosynthesis. Photosynthesis results in increased dry weight due to CO2 uptake, while respiration results in the release of CO2, and therefore reduction of dry weight (Table 11.2). Both processes are essential. The simple carbohydrates formed by photosynthesis are transformed by respiration to the structural, storage, and metabolic substances required for plant growth and development. Under optimal conditions, respiration accounts for about a 33% loss or reduction of photosynthates.
In crop physiology, the concept of Leaf Area Index (LAI) is widely used in growth analysis. LAI is the ratio of the leaf area (one side only) of the plant to the ground area. Productivity of crop canopies is usually expressed by the term Crop Growth Rate (CGR), which is dry matter accumulation per unit of land area per unit of time. It is usually expressed as gm-2 (land area) day-1. Since leaf surfaces are the primary photosynthetic organs, crop growth is also sometimes expressed as net assimilation rate (NAR), which is the dry matter accumulation per unit of leaf area per unit of time, usually expressed as gm-2 (leaf area) day-1. The NAR is a measure of the average net CO2 exchange rate per unit of leaf area in the plant canopy; therefore NAR x LAI = CGR.
In agriculturally advanced areas, photosynthetic efficiencies (meaning the efficiency of converting solar energy into photosynthates, in terms of equivalent energy units) of only 2-2.5% are obtained. On a global basis, efficiencies of less than 1% are very common (San Pietro, 1967). For high-intensity, multiple cropping systems involving three crops per year and total crop duration of up to 340 days per year, Nair et al. (1973) reported photosynthetic efficiencies ranging from 1.7% to 2.38% in northern India (29°N, 79°E, and 240 m altitude). Extremely high short-term productivities have been reported from some natural grassland ecosystems. For example, above-ground net primary productivity (ANPP) as high as 40 gm-2 day-1 ( = 146t ha-1 yr-1), with values consistently > 20 gm-2 day-1, have been recorded during the wet season from the Serengeti ecosystem of Tanzania; these are higher than for any other managed or natural grasslands in the world (Sinclair and Norton-Griffiths, 1979). In forestry systems, mean net primary productivity values of 10-35 and 10-25 t ha-1 yr-1 have been reported for tropical rain forest and tropical seasonal forest, respectively (Jordan, 1985). These values, however, are influenced by a number of factors such as sampling error, choice of sites, and species composition of the system; therefore, great caution should be exercised in using these values of productivity as feasible goals. Nevertheless, they give some indication of the potential that could be achieved. Field measurements of such photosynthetic efficiency or productivity figures are not yet available for agroforestry systems. Young's (1989) calculations, presented in Chapter 16, give 20 t dry matter per hectare per year as a conservative estimate of productivity in humid lowland agroforestry systems. Considering that roots constitute roughly 33% of total photosynthate, 20 t ha-1 yr-1 of above-ground dry matter would represent 30 t ha-1 yr-1 of total dry matter production, a figure comparable to those of most high-input agricultural systems. It seems reasonable to surmise that the productivity of agroforestry systems is comparable to, if not better than, that of high-input agricultural systems.
However, such comparisons of total productivity have some limitations. In practical terms, it is the economically useful fraction of total productivity that is more meaningful than total productivity per se. Harvest Index is a term that has been used to denote this fraction: