NPP changes during ecosystem development

As ecosystems develop during primary or secondary succession (see Concept 17.2), NPP changes as the abundance of plants and associated leaf area index, the ratio of photosynthetic to nonphotosynthetic tissue, and plant species composition all change.

Most ecosystems have their highest NPP at mid-successional stages. Several factors contribute to this pattern, including the tendency for the proportion of photosynthetic tissues, plant diversity, and nutrient supply to be highest at mid- successional stages. In forest ecosystems, the leaf area index and the photosynthetic rates of leaves decrease in old-growth stands, lowering GPP and thus NPP. In some grasslands, such as the tallgrass prairies of the central United States, the accumulation of dead leaves near the ground surface and the development of a closed upper canopy of leaves decrease light availability to short plants, lowering the photosynthetic carbon gain of the ecosystem. However, the decrease in NPP over time is far less pronounced in grasslands than in forest ecosystems. Although NPP may decrease in late successional stages, lowering the uptake of CO₂ from the atmosphere, these old-growth ecosystems contain large pools of stored carbon and nutrients and provide habitat for late successional animal species.