Effects of Climate Change on Decomposition

Now it's time to consider how global climate change will effect the process of leaf litter decomposition and what ecosystem changes might take place due to this alteration.

On the previous page I discussed the importance of leaf litter as a source of nutrients for trees. I also briefly mentioned the fact that CO2 is released through the process of decomposition. It's important to take note of this last point because as it relates to climate change, the sequestration of carbon within the global cycle can be influenced by changes in litter decomposition rates. As stated earlier, it's estimated that the decomposition of litter contributes approximately 70% to the global carbon flux (the global carbon flux is the transition of carbon from one pool to the other). As such, changes in the global rate of litter decomposition can result in potentially detrimental shifts withing the carbon cycle. (Hint: an analogous system to this is permafrost)

It's a well known scientific fact that the global average temperature has risen by as much as 3/4 degrees Celsius within the last century. In addition to this, there is evidence that the rising temperature rates have been, and will continue to be higher in the temperate regions as opposed to the tropics. As a consequence of this, recent work has focused on the effects of rising temperature on rates of litter decomposition within the northern temperate regions.

       Fig. 1        (CLICK ON GRAPHS TO ENLARGE)

The main way by which temperature has been shown to increase the rate of litter decomposition is based on the impact of micro-organisms. This process is dependent on the physiological capacity of bacteria and fungi to break down litter using enzymes. Since physiology plays such an important role in the process of decomposition, a rise in temperature is likely to cause increased metabolic activity which in turn would quicken the rate of decomposition (Aerts, 2006). This hypothesis was strengthened by Hobbie (1996) who conducted experiments which showed a rise in both litter decay rates (Fig. 1) as well as CO2 respiration in response to a temperature increase of 6 degrees Celsius (Fig. 2).


                                               Fig. 2           (CLICK TO ENLARGE)

However a few things must be noted when considering these results. The experiments were performed under
conditions where moisture was optimal for decomposition, this will not always be the case outside of experimental conditions. Precipitation, or more specifically soil moisture, appears to play a vital role in the effect of temperature on decay rates (Aerts, 2006). A few studies, including work done by Robinson et al. (1995), show the ability of precipitation to limit decay rates even under conditions of elevated temperature. This makes it difficult to predict the effects of climate change on decomposition because of our inability to accurately predict changes in precipitation over the next 100 years.

 In addition to this, the results in Fig. 1. show that two out of the 7 species did not respond to the treatment as expected. There are a few reasons for this discrepancy but the most likely cause is the difference in litter chemistry and composition between the experimental species. Mainly a compound named lignin is responsible for lessening the immediate effects of temperature on decomposition. Lignin is a complex compound found in secondary cell walls and is to wood what cellulose is to herbaceous tissue. Due to this quality, often the ratio between lignin and N or P in leaf litter is used as measure of potential decay rate (Aerts, 1997). In the study performed by Hobbie (1996) we can see that litter chemistry varies by species, but this can also be influenced by long term temperature trends. The implications are that temperature can cause direct change in the decomposition rate by altering the litter chemistry (higher air temperatures can lead to an increased growth rate greater than the rate of soil nutrient mineralization, resulting in decreased nutrition), or change can occur indirectly where the changing climate can cause a shift in plant community. Knowing that decay rates vary by species, a shift in plat communities will also cause a shift in regional decomposition rates (Aerts, 2006).

                                                                                   Fig. 3             (CLICK TO ENLARGE)

Here Fig. 3 (Quested et al., 2003) reinforces this point by displaying how different types of plants show varying degrees of decomposition. For example, grasses and sedge plants experience a significantly higher percentage of leaf litter decay than do forb plants (forb plants are defined to be all herbaceous flowering plants other than grasses).

So What are the Implications