Thus far many assessments of the effects of climate change on decomposition have predicted increases in loss rates as temperature increases. Nothing surprising there, but what happens if one considers the interactions of temperature with moisture? Our preliminary results indicate things get far more interesting!

Establishing decomposition study at PRF

A root decomposition simulation model (ROOTDK) was used to predict 30 years of decomposition of Douglas-fir woody roots (3 cm in diameter) under four climate change scenarios at Cascade Head (CAH), H. J. Andrews (HJA), and Pringle Falls (PRF) Experimental Forests. The four climate change scenarios consisted of increased temperature, but unchanged moisture (ITUM); increased temperature and decreased moisture (ITDM); unchanged temperature and increased moisture (UTIM); and both increased temperature and moisture (ITIM). In these simulations, soil temperature was increased 3 ⁰C more than the current seasonal temperatures and soil moisture was changed by 10% from the current seasonal averages of soil gravimetric moisture. The latter is equivalent to a 25, 24, and 21% change in the moisture of roots decomposing at CAH, HJA, and PRF, respectively. The time step for these simulations was 3 months.

Fresh root samples buried in upper soil for decomposition study

Root decomposition at CAH and PRF was more sensitive to climate change than HJA even though they are thought to represent the same region (Figure 1). The ITUM scenario enhanced root decomposition slightly among all three sites, a result in keeping with prior assessments. However, the other three scenarios (all of which involved changes in the moisture regime) had more profound impacts on root decomposition. For ITDM scenario, three patterns of root decomposition responses occurred among the three sites. Root decomposition increased at CAH due to the mitigation of excess moisture, was unchanged at HJA, and slowed down at PRF due excessive drying (Figure 2). In both the UTIM and ITIM scenarios, root decomposition was slower at CAH due to the limiting effects of excess moisture, but higher at PRF where more moisture became available for decomposers. These two climate change scenarios also slightly increased root decomposition of HJA.

What explains these results? High moisture content of woody roots at CAH and low soil temperature and low moisture content of woody roots at PRF are the dominant limiting abiotic factors of root decomposition at these sites. In contrast, soil temperature and moisture regimes at HJA are more optimal for root decomposition, so changed climates have less impact. This study suggests that the responses of root decomposition to altered climate within a region such as the Pacific Northwest can be highly divergent.