Aspen and pine leaf litter decomposition in laboratory microcosms. II. Interactions of temperature and moisture level

To explore the relative influences of substrate type, temperature, and moisture on litter decomposition rates, leaf litter of aspen (Populus tremuloides Michx.) and pine (Pinus contorta Loud, × P. banksiana Lamb.) was decomposed in laboratory microcosms for 16 weeks at 2, 10, 18, and 26 °C and 15, 30, or 60 mL∙week−1 watering rate. Multiple regressions on mass remaining indicated temperature was a more important influence than watering rate for both species, but the degree and nature of response were different for pine compared with aspen. Decay rates of aspen were strongly affected by temperature and less so by watering rate, but pine decomposition was quite insensitive to both. For aspen, watering rate was a more important influence on decay rates at low temperatures (2 and 10 °C), while for pine it was more important at high temperatures (18 and 26 °C). There was a very strong interaction of time with temperature in the determination of aspen decomposition rate, but none for pine. All these differences are attributable to the disparate chemical and physical natures of the two litter types. The time × watering rate interaction was weak for both species, and there was no temperature × watering rate interaction at all. As a consequence of these differences in response to climatic variables, aspen leaves decomposed faster than pine needles under most conditions, but under cold, dry conditions pine decomposed faster than aspen.