Barry R. Taylor

Air-drying depresses rates of leaf litter decomposition

Abstract Air-dried leaf litter is universally used in decomposition experiments, but limnological research indicates that air-drying considerably increases the susceptibility of litter to leaching. In an aspen forest in the Rocky Mountains of Alberta, Canada, I compared first-year decomposition of air-dried leaf litter (7–8% moisture) of trembling aspen (Populus tremuloides) and green alder (Alnus crispa) against decomposition of freshly fallen leaf litter that was still moist (57–62% moisture) when placed in the litterbags. Contrary to expectations, mass loss was substantially faster from fresh litter than from air-dried litter of both species, even in the first month in the field. Over the 334-d experiment, fresh alder leaves decayed nearly twice as fast as air-dried leaves; fresh aspen leaves decayed 62% faster than air-dried leaves. Air-drying was estimated to add at least another year to the time required to reach 50% mass loss. Moisture content of fresh leaves was higher than that of air-dried leaves at every sampling time. Fresh aspen litter rapidly accumulated external N during only the first month, while air-dried aspen litter accumulated external N throughout the experiment. N-rich alder leaves released N in the first week in the field, but the loss was greater for fresh leaves than for air-dried leaves and the content difference persisted for the remainder of the experiment. Both mass loss rates and rates of N uptake and release may be substantially underestimated by using air-dried leaves in decomposition experiments.

Does repeated freezing and thawing accelerate decay of leaf litter

Abstract Repeated cycles of freezing and thawing of leaf litter could accelerate decomposition if they caused physical damage to leaves or chemical changes which improve access or render leaf material more easily degradable by decomposers. Moistened leaf litter of aspen ( Populus tremuloides Michx.) and pine ( Pinus contorta Loud. × P. banksiana Lamb.), which had been frozen and thawed 14 times, absorbed water faster than litter frozen for the same total duration, before thawing once. Aspen, but not pine, began leaching sooner. Total solubles content of aspen leaves was reduced 8% by repeated freezing and thawing, but leaching loss from pine needles doubled from 4 to 8% of total mass. Previously frozen and thawed litter (14 cycles) decomposing at 10° C in laboratory microcosms lost more mass initially than litter frozen only once, but the difference was not maintained after 2 or 3 months decay. Litter exposed in microcosms to + 10°C (days) and −6°C (nights) for 3 months lost mass no faster or slower than litter maintained at the same mean temperature (+2° C) without variation, once differences in moisture content were allowed for. However, mass losses from frozen and thawed litter were substantially greater than expected if losses are prorated for the fact that these leaves were frozen 12 h each day. This difference reflects either the acceleration of decomposition due to freezing-thawing, possibly arising from weakening of the cuticle, or continuing decomposition at below-zero temperatures. For aspen litter, this effect was much smaller after 1 month of nightly freezing, followed by 2 months at 10° C. Frequency of freeze-thaw cycles in the field was estimated at 2–11 cycles month −1 during spring or fall at the aspen field site, and 0–8 cycles month −1 at the pine site. Evidence indicates that simple freezing (as opposed to freezing-thawing) and decomposer activity beneath snow may be more important factors in winter time litter decomposition than freeze-thaw cycles.

Does repeated wetting and drying accelerate decay of leaf litter

Abstract Needle litter of lodgepole-jack pine ( Pinus contorta Loud. × P. banksiana Lamb.) previously wetted and dried 14 times absorbed water faster than needles wetted only once for the same total duration (controls); trembling aspen ( Populus tremuloides Michx.) leaf litter showed the reverse response. Wetting-drying did not enhance leaching in either species. Aspen litter decaying in laboratory microcosms at 26°C decomposed faster than controls for 1 month if wetted and dried either previously (14 times) or while decomposition proceeded. However the difference was not maintained past 2 or 3 months decay. Accelerated water absorption by treatment leaves accounted for some, but not all, of the difference in decay rates. Similarly treated pine needles showed a slightly greater mass loss after 1 month, but no consistent difference after 2 or 3 months. Litter layers on the forest floor exhibited persistent trends of increasing (occasionally decreasing) moisture content with depth; only the top centimetre experienced wetting-drying cycles, and no more than 10–15 cycles yr − are likely. In these ecosystems, repeated wetting-drying is probably not a significant influence on litter decomposition rates.

Variable effects of air-drying on leaching losses from tree leaf litter

Leaching of soluble substances may be an important first step in leaf litter decomposition in small streams, but recent research has suggested that large leaching losses (up to 30% of initial mass in 48 h) may be an artifact created by using air-dried leaves in decomposition experiments. In laboratory experiments, we compared 3 d leaching losses from freshly fallen and air-dried senescent leaves of 27 tree species from different regions across Canada. Air-dried leaves from all species leached measurable amounts of original mass (3.6–32.8% dry mass), but leaching losses from fresh leaves (0–35%) were detectable in all but two species. Air-drying increased leaching losses in many species, but in others it reduced leaching losses or had no measurable effect. Results for leaves of the same species collected in different regions or in different years were generally similar, but species within the same genus often behaved very differently. Neither moisture content (fresh or air-dried), leaf thickness, nor cuticle thickness proved of any value as predictors of leaching losses or the effect of air-drying. The propensity of autumn-fallen leaves to leach, whether fresh or air-dried, appears to be a property of the individual tree species.