ABSTRACT
Primary productivity, or the production of organic compounds by plants through the photosynthetic process, is fundamental to the functioning of ecosystems. Populations of plants have the potential continually to increase the level of productivity at any site, through increasing their individual size (vegetative growth) or the size of the population, provided that environmental conditions are not limiting. Controls on the level of productivity are normally set by either the availability of resources needed for growth, or the existence of other limiting conditions (such as temperature) that impinge upon the physiology of the growth processes. In the presence of abundant resources and the absence of other constraints on growth, short-term rates of productivity among plants are set by the energy conversion efficiency of the photosynthetic process and the architectural efficiency of plants in establishing a light-intercepting canopy. However, over annual time periods, other limits to productivity normally appear. In temperate areas, climate generally sets the limits to annual productivity by way of low temperatures during winter months. In the tropics, the absence of a comparable temperature limitation to annual productivity means that this should exceed that of temperate areas by a factor comparable to the relative length of the growing period (i.e. ~×3). However, estimates of net primary productivity for tropical and temperate biomes suggest considerable overlap in values. Whittaker and Likens (1975) estimate net primary productivity in tropical forests to range from 1,000 to 3,500 g.m• 2.yr• 1, while those of temperate forests range from 600 to 2,500 g.m• 2.yr• 1. For tropical and temperate grasslands, the same authors estimate respective ranges of 200-2,000 and 200-1,500 g.m• 2.yr• 1.
