Because of extensive human activity, the Earth is currently undergoing profound changes in the global environment. Climate warming is the most discussed aspect of global change, but changes in the atmospheric CO2 content, widespread enhancement in nutrient availability, changes in land cover, and the invasion of exotic species into new habitats are also profoundly altering the productive potential of the planet. Rising CO2 is widely noted to stimulate C3 photosynthesis due to the suppression of photorespiration and enhancement of WUE. For sustained photosynthetic enhancement, however, sufficient mineral nutrients must be present in the soil or else the plant will reduce the amount of photosynthetic enzymes, and hence photosyn-thetic rate, following prolonged exposure to elevated CO2. Because of this, most natural ecosystems should respond to a doubling of atmospheric CO2 with at most modest (5-25%) enhancements of NPP.
Increasing temperature, and increasing deposition of mineral nutrients should interact with rising CO2 to enhance NPP in the coming decades. Through industrial and automobile pollution, and widespread use of fertilizers, humans currently release more reduced nitrogen into the environment than all biological processes. Anthropogenic nitrogen drifts far from the application points to enrich natural soils and water, increasing potential NPP. Not all species benefit; instead, a handful of rapidly growing plants and algae crowd out most other species, reducing biodiversity and simplifying the ecosystem.
With climate warming, NPP is primarily increasing because of longer growing seasons. Springs have advanced by 2-4 weeks over much of the temperate and boreal zones in the past century, and autumns have been delayed. Warming will largely be concentrated at high latitudes, and big enhancements in NPP can be expected in these areas, particularly as more productive species adapted to warmer conditions migrate north. Warming of the cold, high-latitudes soils and associated drying of the peatlands will accelerate decomposition, releasing large amounts of sequestered carbon to the biosphere as CO2. High-latitude systems will also be prone to burning in a warmer world. Consequently, NEP at high latitudes could be negative despite the enhanced NPP.
In the future, exotic weedy species will do well, as they are well adapted to exploit warmer climates, nutrient-enriched soils, and higher CO2 conditions. Once established, many of the weedy species alter critical ecological controls in a system, radically changing the vegetation cover and ecosystem properties. Weedy C3 and C4 grasses have already captured vast landscapes through their ability to enhance fire. In western North America, the invasive C3 species cheat grass (Bromus tectorum) accelerates fire cycles that kill the native steppe vegetation, allowing cheat grass to form dense, low-diversity stands with half the NPP and NEP of the original vegetation. In a similar manner, invasive C4 grasses in the tropics have replaced large tracts of native rainforest. To predict NPP patterns in the future, it will be necessary to account for the spread of these and other invasive species that have high potential to alter ecosystem dynamics.
The single greatest factor determining the future of autotrophy will be the demand humans place upon the primary production of the planet. In the early 1990s, humans were estimated to consume 40% of the global primary productivity, either directly through food and fiber consumption, or indirectly by diverting land and water to our purposes. Today, the fraction of global NPP consumed by humans is probably well over 50%. With continued growth of the population and economic activity, the demand for NPP will further increase. The diversion of NPP to humanity is likely the single greatest threat to the future ecology of the planet, for without the energy and biomass provided by primary production, an ecosystem cannot function, and wild species cannot persist. In order to prevent humans from monopolizing NPP flow, a major challenge for future ecologists will be to convince the public to ensure NPP is set aside for natural organisms. Natural species are vital to the biosphere because they use solar energy to regenerate the atmosphere, cleanse water, and stabilize the climate. Without a recognition of this essential role, people could support business as usual economic and political decisions, allowing exploitation of NPP to grow beyond levels required to sustain the global ecosystem. If this happens, then the life-support system of the planet could be at risk of collapse.
See also-. Abiotic and Biotic Diversity in the Biosphere; Biomass, Gross Production, and Net Production; Global Change Impacts on the Biosphere; Plant Demography; Plant Ecology.
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