11.2: What might stimulate net primary production?

Terrestrial ecosystems are dependent on the sun's energy to support growth and maintenance. Plants are primary producers that, through photosynthesis, manufacture organic molecules such as carbohydrates and lipids from raw inorganic materials (CO2, water, mineral nutrients). Primary productivity is thus a fundamental determinant of both the structure and functioning of terrestrial biomes. The energy and carbon of primary production supplies consumers, including humans, with the necessary fuel to support their metabolism while providing essential carbon compounds that form the bricks and mortar of living cells. In addition to solar radiation, the main abiotic factors that affect rates of photosynthesis and NPP are water, temperature, carbon dioxide concentration, and nutrients. Globally, there is broad equilibrium relationship between NPP, temperature and precipitation that is strongly impacted by nutrient limitations and, hence, knowledge of the global distribution of NPP is important for understanding vegetation dynamics in biomes, patterns of biodiversity, potential agricultural yield, and predicting global climatic changes.

Figure \(\PageIndex{1}\): Erosion on hillsides. Extreme topsoil erosion in Iceland. An extreme example of near total loss of topsoil having huge impacts on NPP. In Iceland, thick Andosoils have been removed by the forces of wind and water leaving shallow and poor soils with limited vegetation cover.
Source: CC-BY Zinneke. Wikimedia Commons.

Some of the key factors that lead to long-term reductions in NPP are associated with various forms of land degradation. This includes soil erosion (wind, water), nutrient depletion, salinisation, soil compaction and crusting, topsoil losses and nutrient depletion, alterations of vegetation composition and structure, and water depletion. In fact, most types of soil disturbance can have devastating impacts on a region’s productivity. One of the most extreme examples of soil loss’s impacts on productivity is Iceland. Icelandic agriculture is primarily based on sheep farming, dairy and poultry production. Due to numerous factors, but especially poor grazing management and wood harvesting, the entire topsoil has been lost over vast areas of the island. Numerous biogeochemical processes affect the carbon balance of terrestrial biomes, including photosynthesis, plant respiration, microbial respiration, leaching losses, erosion, herbivory, fire, and rates of rock weathering. Human appropriation of NPP and modification of the Earth's surface over the past several centuries has altered many of these processes. Net biome production (NBP), which applies to changes in carbon stocks over large spatial areas and long time periods, is helpful to describe changes in carbon balances after losses due to natural or anthropogenic disturbances. NBP balances carbon emissions with non-respiratory losses such as fire, deforestation, insect infestation, droughts, agricultural harvests, and ecosystem respiration fluxes with NPP, and indicates the carbon source-sink strength and possible positive feedback flux to atmospheric CO2 concentrations. Climate change and human-induced degradation may well result in an increased or decreased NBP. Global environmental change is rapidly altering the dynamics of terrestrial biomes. This has major consequences for the functioning and structure of the Earth system, including the provision of ecosystem services. Long-term satellite measurements have identified a widespread greening of the Earth. The four key drivers of this change are the fertilisation effects of atmospheric CO2 (this explains 70 % of the observed greening trend), nitrogen deposition (9 %), climate change (8 %) and land cover change (4 %). Global CO2 concentrations have risen from about 280 ppm at the start of the industrial revolution to about 406 ppm in 2017.

Elevated carbon dioxide concentrations have numerous effects on plants, such as acting as a fertiliser that stimulates increases in photosynthesis. Elevated CO2 concentrations also tend to reduce water loss in plants, which may be more important than the direct effect of increased photosynthesis rates due to global trends in changing aridity. There is evidence that the CO2 fertilisation effect can change plant-species mix (such as enhancing woody plant growth over grass growth), lower the carbon-to-nitrogen ratio of plants (making grazing and browse material less palatable), and evoke long-term evolutionary responses. It is therefore likely that increased NPP due to increased CO2 concentrations may be changing useful ecosystem services in some areas, despite the higher plant productivity. However, generalisations are difficult to make because there are many feedbacks and interactions with other variables, such as temperature, nutrients, water availability, and plant-plant competition.
The direct CO2 effect on plants should be most strongly expressed in warm, arid environments where water is the dominant limitation to vegetation growth and where land degradation is widespread. Indeed, it has been shown that the 14 % increase in atmospheric CO2 (covering the period 1982–2010) led to a 5-10 % increase in green foliage in warm, arid environments. Can global increases in NPP mask the impacts of degradation? Long-term change in NPP is potentially a useful indicator of land degradation, but interpreting short-term changes in NPP as degradation can be misleading because it can be a reflection of climatic fluctuations. Scientists have attempted to use rain use efficiency (NPP per unit of water) to better understand degradation tends, which have been most successful over long time periods. Also, in combination with climate impact, anthropogenic land use can increase NPP but mask other forms of degradation. Highly productive cultivation systems may increase NPP but affect other ecosystem services, such as water and nutrient supply. Grazing pressure can cause a species change to, for example, woody shrubs showing an increase in NPP but reduce palatability and biodiversity. Hence, it is important to include NPP changes in land degradation assessment but these changes can only be interpreted correctly when considering the dynamics of other ecosystem services and socio-economic situations.

Biomass production is the most important process of the biosphere. It directly impacts many ecosystem services, such as the global carbon cycle, which in turn affects the water cycle and climate. In fact, many ecosystem services are positively correlated with net primary production (NPP), including food production, climate regulation, purification of water, maintenance of nutrients, healthy soils, carbon sinks, biodiversity, and aesthetic landscape function. NPP dictates the amount of carbon synthesised within an ecosystem, which is ultimately available to consumers, including humans. In fact, associated with increased population growth over the last millennium, a disproportionate amount of the world’s NPP is now consumed by humans.

Figure \(\PageIndex{1}\): A flux tower in Kruger National Park South Africa, measures the “breathing” of an African savanna ecosystem. During summer days, CO2 is taken up by the vegetation leading to an increase in overall biomass. Some of this biomass is lost to herbivory and microbial decay, but most is lost to the frequent fires that are an integral component of the ecosystem. An important question in the context of climate change is whether this system is in long-term equilibrium, or if the carbon store is increasing or decreasing. The flux tower helps scientists understand the dynamics of the system, and trends in carbon dynamics, as well as being a useful tool in calibrating satellite-based NPP products.
Source: photo from and text based on: https://www.csir.co.za/eddy-covariance-flux-towers

Humans have major impacts on NPP through the use of irrigation and fertilisers. It is the loss of NPP through actions such as increased soil erosion, deforestation and soil salinisation that forms the basis for many forms of land degradation. There are also forms of degradation where NPP may stay constant, or even increase, but where important ecosystem services change. Examples include plant species, compositional changes in response to grazing pressure where palatable grasses are replaced by less palatable ones, or in some cases where palatable grasses are replaced by unpalatable woody shrubs. In such circumstances the grazing capacity of the rangeland may be greatly reduced, biodiversity can be lost, but carbon sequestration and other regulating ecosystem services may be maintained or even enhanced. CO2 fertilisation is having an impact on Leaf Area Index (LAI) (and hence NPP) over vast areas of the Earth. 70 % of the observed impacts on LAI to CO2 fertilisation effects, with factors such as nitrogen deposition, climate change-induced rainfall and temperature as well as land-cover change being responsible for the remaining observed changes.

Source

European Commission, Joint Research Center, World Atlas of Desertification, Limits to Sustainability, Net Primary Production. Accessed December 2023 https://wad.jrc.ec.europa.eu/primaryproduction