Boreal Forests and Temperate Grasslands
Two of the most expansive terrestrial regions on Earth are the boreal forests
in North America and Europe, and grasslands found on most continents. Terrestrial
regions are an important component in the carbon cycle, as we talked about earlier.
This section is designed to give the reader a background on these expansive
regions, and a comparison of their productivity levels. We also included a study
done to show how climate change and increasing anthropogenic carbon-dioxide
emissions can affect these regions, as well as tropical ecosystems.
The northern boreal ecoregion accounts for about one third of this planet's total forest area. In North America, the boreal ecoregion extends from Alaska to Newfoundland, 412 million hectares (Hicke et al., 2003), bordering the tundra to the north and touching the Great Lakes to the south. By far the most dominant tree species are conifers which are well-adapted to the harsh climate, and thin, acidic soils. Black and white spruce is characteristic species of this region along with Tamarack, Jack Pine and Balsam Fir. There are also deciduous trees which are at times mixed in among the conifers, especially in more southern areas - they may include White Birch and Poplars (Sierra Club website).
Current CO2 emissions have been on an increase and pose a problem to all ecosystems. In particular, the boreal forest is thought to have a strong influence on the net CO2 exchange with the atmosphere (Amiro et al., 2002). Although it is not one of the most productive ecosystems, recent studies have shown the boreal forest to be one of the largest carbon sinks. Deciduous and mixed forests, like the boreal forest, have the highest capability of absorbing carbon per unit surface area in Canada (Liu et al., 2002). Coniferous forests are only half as productive as deciduous forests; however, coniferous forests contribute the greatest proportion to North America’s annual NPP, 42%, because of their large coverage area (Liu et al., 2002).
Typically, terrestrial ecosystems exchange CO2 rapidly with the atmosphere, in which CO2 is removed from the atmosphere via photosynthesis and stored in organic matter (Falkowski et al., 2000). As we have previously explained, as CO2 levels increase there is an increase in the Greenhouse Effect, but potential increases in the ability of forests to act as carbon sinks could help to balance the increased emissions. However, as CO2 levels increase, the saturation function of ecosystems decreases, causing all terrestrial plants to become less of a sink for CO2 (Falkowski et al., 2000). One would think that this would then increase NPP, because if less carbon is being stored in the soil, then there must be due to an increase in photosynthesis (NPP). However, NPP may level off at only 10- 20% above current rates at an atmospheric CO2 concentration of 550 to 650 ppmv, or double pre-industrial concentrations (Falkowski et al., 2000). When saturation occurs in both the terrestrial and aquatic systems, and NPP levels off there will be an increase in carbon dioxide in the atmosphere.
The carbon cycle is a closed circuit system in which the amount of carbon on
Earth stays on Earth; it is not released through the atmosphere into space.
Carbon is continuously cycled and is stored in the oceans, terrestrial regions,
the atmosphere, or fossil fuels. Carbon storage in fossil fuels is a slow process
that is unable to keep up with the current emission rates, and is not considered
in our analysis for this reason. Since the other three systems are able to absorb
carbon at a faster rate, these will be the most effected by the saturation and
NPP level limit. With the increase in carbon in the atmosphere, there will also
be an increase in temperature, since carbon is one of the main greenhouse gases.
Combined effects of higher CO2 concentrations, higher temperatures, and changes
in disturbance and soil moisture regimes lead to considerable uncertainty about
the ability of terrestrial ecosystems to mitigate against rising CO2 in the
coming decades (Falkowski et al., 2000) Recent studies on long-term soil warming
in a boreal forest contradict the idea that the projected rise in temperature
is likely to lead to forests that are now carbon sinks becoming carbon sources
(Falkowski et al., 2000). The study of carbon flux, primary productivity and
how climate change effects these cycles will enable researchers to better understand
how these systems and their changes affect us.
Grasslands are an important part of life on Earth in terms of primary productivity. They cover about 14% of the Earth’s surface and account for 2% of global phytomass and 17% of global productivity (Esser, 1992). The Greening Earth Society, an organization dedicated to dismantling the notion of human activity affecting the climate, insists that an increase in CO2 would benefit primary productivity on Earth (GES, 2004). However, several additional variables lead to a strikingly different conclusion.
The productivity of grasslands is more heavily dependent on precipitation and temperature than on the amount of Carbon in the atmosphere (Esser, 1992). While an increase in Carbon could feasibly bolster productivity in grasslands, changes expected in rainfall patterns are relatively unknown. A study by Fay et al. explored different possibilities of changes in rainfall patterns and their possible effects on grassland productivity. With a controlled plot that was based entirely on natural rainfall, a plot with reduced quantity in provided rainfall, a plot with altered timing in rainfall, and a plot with both reduced quantity and altered timing, possible changes were explored. Each plot was measured in terms of biomass, soil water content, and aboveground net primary productivity (ANPP). The results showed that with all three of the altered rainfall patterns, ANPP was decreased by as much as 20% (Fay et al., 2003).
Additionally, grasslands in cooler regions could suffer a loss of productivity due to the increased decomposition that is expected to occur with climate change (Hall et al., 1995). When organizations like the Greening Earth Society claim that increased Carbon will lead to a more bountiful Earth, they base their claims only on the increase of Carbon Dioxide. In doing so, they not only leave out other important greenhouse gases, they also refuse to acknowledge the countless other variables involved in the process of primary productivity. Ironically, a recent article featured on the GES’s website claims that “some scientists refuse to abandon their preconception that computer-based climate models reliably forecast future climate,” (GES, 2004) effectively accusing said scientists of replicating the mistakes of their accusers.
Fig. 1 (top/left). NPP of ecosystems across the globe. There is a significant difference between the more productive areas (wetlands-temperate forest) and the less productive areas (agriculture-desert).
Fig 2 (bottom/right) Areas covered by each ecosystem.
Climate Change Case Study:
There is no significant difference between the productivity levels of the boreal
forest and grasslands, except for the tropical savannas, yet both are still
only moderately productive when compared to the tropical rainforests and wetlands.
The purpose of this analysis is to show how all terrestrial ecosystems will
be affected by rising carbon-dioxide levels and temperature.
White et al. (1999) conducted a study by using a model called the Hybrid, which measures carbon fluxes, pre-industrial global distribution of vegetation types, NPP, and carbon. The model used previous data from carbon rate emissions for the simulation, and presented the best case scenario for the simulation. They ignored the effects of land use change and fire, and the hybrid does not represent dispersal processes; it assumes that all vegetation types are available to grow wherever and whenever the climate permits. While obviously this is not a realistic model, it does demonstrate how even under the best conditions possible, climate change can drastically change ecosystems across the globe. The table and image below are the results from this study.
Table 1 (left/top). The effects of temperature and CO2 emissions on ecosystems.
Figure 3 (right/bottom). Shows the change in ecosystems over a 100 year time span.
As you can see, the most productive areas are replaced by the less productive biomes. The NPP increased in response to warming, increased CO2, and in some areas, increased precipitation. However, NPP decreased in the areas where forests declined and were transformed to savanna, grassland, or desert (White et al., 1999). The model also measured net ecosystem productivity (NEP) during the simulations and found that after an initial increase in productivity, there was a drastic decline, below zero PgC/yr. Because there is less carbon being released into the atmosphere, one would assume that microbial activity is storing the carbon (acting as a carbon sink). However, the model shows the opposite, there is a collapse and reversal of carbon sinks (White et al., 1999). As previously mentioned, studies have found the boreal region to be a large sink for carbon, and the sink collapse and reversal would lead to an increase in CO2 in the atmosphere, causing even more global warming.
In all cases decreases in NEP and NPP occurred in areas where there was a decline and/or death of tropical and temperate forests. While there are some early benefits of climate change due to increased carbon dioxide, such as increased canopy photosynthesis, water use efficiency, increased NPP, forest biomass, litter-fall, and potential soil carbon, these benefits are not long term (White et al., 1999). Increasing carbon-dioxide diminishes photosynthesis when soils become saturated with CO2, which decreases NPP and NEP despite accelerated soil respiration (White et al., 1999). Saturation occurs when plants are unable to keep up with CO2 emissions. Just like humans, plants are only able to consume so much carbon before becoming satiated. Imagine eating Chipotle till you are ready to burst.
The predicted loss of forests has major socio-economic and environmental implications and the loss of terrestrial carbon sinks implies an accelerated increase in greenhouse gas concentrations (White et al., 1999). The World Bank and UN are currently conducting research on how detrimental climate change will be to poor and developing countries because of the potential affect this change will have on agricultural crops and ecotourism.