Influence of Dissolved Organic Matter and Invertebrates on the Function of Microbial Films in Groundwater

Microbial films play a central role in mediating energy flux in groundwater ecosystems. The activity of these microbes is likely to be influenced by the availability of resources, especially dissolved organic matter (DOM), and also by consumers, such as invertebrates that feed on microbial films. We used microcosm experiments to examine how bacterial production and extracellular enzyme activity on rocks and fine sediments from cave streams responded to amendments of DOM of varying form and to cave amphipods (Gammarus minus) that feed on microbial films. Glucose and mixtures of DOM extracted from soils and leaves stimulated bacterial production on rocks by 89–166% relative to unamended controls. In contrast, tannic acid amendment did not influence production. Microbial films on fine sediment were not consistently responsive to DOM amendment. Glucose amendment led to increased activity of enzymes associated with C acquisition, but other forms of DOM generally did not alter enzyme activity. DOM amendment led to removal of nitrate and this was correlated with bacterial production, suggesting microbes can link carbon and nitrogen cycling in groundwater as is the case in surface systems. Amphipods reduced bacterial production on rocks, but not fine sediments. The reduction caused by amphipods offset the stimulatory effect of glucose amendment, but there was no interactive effect of DOM and grazing on bacterial production or enzyme activity. Both resources and consumers play important roles in regulating microbial activity in groundwater with important implications for higher trophic levels that use microbes for food.     

Trophic plasticity among spring vs. cave populations of <Emphasis Type="Italic">Gammarus minus:</Emphasis> examining functional niches using stable isotopes and C/N ratios

In some environments, species may exhibit trophic plasticity, which allows them to extend beyond their assigned functional group. For Gammarus minus, a freshwater amphipod classified as a shredder or detritivore, cave populations have been observed consuming heterotrophs as well as shredding leaves, and therefore may be exhibiting trophic plasticity. To test this possibility, we examined the C and N stable isotope and C/N ratios for cave and spring populations of G. minus. A 15-day feeding experiment using leaves and G. minus from a spring population established that the diet-tissue discrimination factor was 3.2 ‰ for δ¹⁵N. Cave G. minus were 8 ‰ higher in δ¹⁵N relative to cave leaves, indicating they did not derive nitrogen from leaves, whereas field collected spring populations were 2–3 ‰ higher than spring leaves, indicating that they did. Cave G. minus were 2.6 ‰ higher in δ¹⁵N than the cave isopod, Caecidotea holsingeri. Relative to spring populations, Organ Cave G. minus were ¹⁵N enriched by 6 ‰, suggesting they occupied a different trophic level, or incorporated an isotopically distinct N source. While stable isotopes cannot tell what the cave G. minus are eating, the isotopes certainly show that G. minus are not eating leaves and are trophically distinct form the surface populations. Differences in C/N ratios were observed, but reflect the size of the G. minus examined and not feeding group or habitat. The isotope data strongly support the hypothesis that cave populations of G. minus have become generalist or omnivorous by including animal protein in their diet.     

Effect of predatory fish presence on habitat use and diel movement of the stream amphipod, Gammarus minus

SUMMARY.

• 1 We examined how the presence of predatory fish affected macro—and microhabitat use and movement patterns of the amphipod, Gammarus minus Say, in a second order stream.
• 2 Among macrohabitats, amphipods were consistently more abundant in runs than in pools. Densities in pools were low regardless of fish presence, whereas low densities in runs were correlated with fish presence. Siltation and the lack of coarse substrate particles probably accounted for the scarcity of G. minus in pools. Among microhabitats, G. minus was more abundant in leaf litter and gravel than in silt/sand substrates. Fish presence did not affect microhabitat use in the field. Laboratory manipulations showed, however, that predation rates by green sunfish (Lepomis cyanellus Rafinesque) were significantly lower on amphipods in leaf litter than in other microhabitats.
• 3 Most movement by G. minus in runs occurred at night and was directed downstream. In the laboratory, amphipods significantly reduced swimming activity at night in response to water conditioned with green sunfish. In the field, however, amphipods were only slightly less active in runs at night when exposed to chemical cues of confined green sunfish. The distribution of amphipods in streams is influenced by the presence of fish, but chemical cues alone may not be important in triggering predator avoidance behaviours in nature.

     

Interactions Between Hyphosphere-Associated Bacteria and the Fungus <Emphasis Type="Italic">Cladosporium herbarum</Emphasis> on Aquatic Leaf Litter

We investigated microbial interactions of aquatic bacteria associated with hyphae (the hyphosphere) of freshwater fungi on leaf litter. Bacteria were isolated directly from the hyphae of fungi from sedimented leaves of a small stream in the National Park “Lower Oder,” Germany. To investigate interactions, bacteria and fungi were pairwise co-cultivated on leaf-extract medium and in microcosms loaded with leaves. The performance of fungi and bacteria was monitored by measuring growth, enzyme production, and respiration of mono- and co-cultures. Growth inhibition of the fungus Cladosporium herbarum by Ralstonia pickettii was detected on leaf extract agar plates. In microcosms, the presence of Chryseobacterium sp. lowered the exocellulase, endocellulase, and cellobiase activity of the fungus. Additionally, the conversion of leaf material into microbial biomass was retarded in co-cultures. The respiration of the fungus was uninfluenced by the presence of the bacterium.     

Importance of fungi in the diet of Gammarus pulex and Asellus aquaticus

An important component of the interaction between macroinvertebrates and leaf litter in streams in the extent to which consumers can differentiate between undecomposed and decomposing leaves. The detritivores Gammarus pulex and Asellus aquaticus fed preferentially on conditioned rather on unconditioned leaf material. Growth in A. aquaticus was significantly reduced when unconditioned leaves were provided, but in G. pulex no significant effect of conditioning on growth was observed. The capacity of G. pulex to tolerate reductions in food quality seems to be a consequence of a compensatory system in which respiration rates change to compensate for reductions in food quality. In this way a constant growth rate is maintained. Increases in ingestion rates to compensate for low quality food were not observed.     

Protozoa,Nematoda and Lumbricidae in the rhizosphere of Hordelymus europeaus (Poaceae): faunal interactions,response of microorganisms and effects on plant growth

Interactions among protozoa (mixed cultures of ciliates, flagellates and naked amoebae), bacteria-feeding nematodes (Pellioditis pellio Schneider) and the endogeic earthworm species Aporrectodea caliginosa (Savigny) were investigated in experimental chambers with soil from a beechwood (Fagus sylvatica L.) on limestone. Experimental chambers were planted with the grass Hordelymus europeaus L. (Poaceae) and three compartments separated by 45-m mesh were established: rhizosphere, intermediate and non-rhizosphere. The experiment lasted for 16 weeks and the following parameters were measured at the end of the experiment: shoot and root mass of H. europaeus, carbon and nitrogen content in shoots and roots, density of ciliates, amoebae, flagellates and nematodes, microbial biomass (SIR), basal respiration, streptomycin sensitive respiration, ammonium and nitrate contents, phosphate content of soil compartments. In addition, leaching of nutrients (nitrogen and phosphorus) and leachate pH were measured at regular intervals in leachate obtained from suction cups in the experimental chambers. Protozoa stimulated the recovery of nitrifying bacteria following defaunation (by chloroform fumigation) and increased nitrogen losses as nitrate in leachate. In contrast, protozoa and nematodes reduced leaching of phosphate, an effect ascribed to stimulation of microbial growth early in the experiment. Earthworms strongly increased the amount of extractable mineral nitrogen whereas it was strongly reduced by protozoa and nematodes. Both protozoa and nematodes reduced the stimulatory effect of earthworms on nitrogen mineralization. Microbial biomass, basal respiration, and numbers of protozoa and nematodes increased in the vicinity of the root. Protozoa generally caused a decrease in microbial biomass whereas nematodes and earthworms reduced microbial biomass only in the absence of protozoa. None of the animals studied significantly affected basal respiration, but specific respiration of microorganisms (O₂ consumption per unit biomass) was generally higher in animal treatments. The stimulatory effect of nematodes and earthworms, however, occurred only in the absence of protozoa. The sensitivity of respiration to streptomycin suggested that protozoa selectively grazed on bacterial biomass but the bacterial/fungal ratio appeared to be unaffected by grazing of P. pellio. Earthworms reduced root biomass of H. europaeus, although shoot biomass remained unaffected, and concentrations of nitrogen in shoots and particularly in roots were strongly increased by earthworms. Both nematodes and protozoa increased plant biomass, particularly that of roots. This increase in plant biomass was accompanied by a marked decrease in nitrogen concentrations in roots and to a lesser extent in shoots. Generally, the effects of protozoa on plant growth considerably exceeded those of nematodes. It is concluded that nematodes and protozoa stimulated plant growth by non-nutritional effects, whereas the effects of earthworms were caused by an increase in nutrient supply to H. europaeus.     

Decomposition of Fire Exposed Eucalyptus Leaves in a Portuguese Lowland Stream

We compared fire exposed with normal abscised eucalyptus leaves incubated in a stream running through eucalyptus plantations in central Portugal, in terms of breakdown rates, microbial activity, diversity and macroinvertebrate abundance. Although leaves exposed to fire had lower nutritional value, mass loss was similar for both leaf types (k = 0.0089–0.0095 d^–1 for fire and k = 0.0084–0.00103 d^–1 for normal leaves). Fungal biomass was similar among treatments, whereas sporulation and microbial respiration were lower in fire exposed leaves. Both leaf types had similar aquatic hyphomycetes communities. Physical fragmentation was important in fire exposed leaves breakdown. Invertebrates colonized leaves in low numbers in both treatments. Alteration of leaf litter quality determined by fires in streams does not seem to determine changes in ecosystem functioning in a short term. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The quality of organic matter mediates the response of heterotrophic biofilms to phosphorus enrichment of the water column and substratum

1. Heterotrophic biofilms are important drivers of community respiration, nutrient cycling and decomposition of organic matter in stream ecosystems. Both organic matter quality and nutrient levels have been shown to affect biofilm biomass and activity individually, but both factors have rarely been manipulated simultaneously. 2. To experimentally manipulate the organic matter quality and phosphorus (P) levels of both the substratum and water column, we first used cellulose cloth as a low‐quality organic material and enhanced its quality and P‐content by amending the underlying agar with maltose and P, respectively (Experiment I). To manipulate water column P, artificial substrata were incubated in low‐ and high‐P sites of a whole‐stream P‐enrichment in lowland Costa Rica. 3. Results from Experiment I suggest that heterotrophic biofilm respiration on cellulose cloth is co‐limited by carbon (C) and P. Biofilm respiration responded in an additive manner to combined effects of maltose and P‐enrichment of water column and synergistically to maltose and high‐P in substrata. 4. As decomposing organic matter that supports heterotrophic biofilms varies naturally in its labile C content along with other physical and chemical properties, we conducted a second experiment (Experiment II) in which we amended leaf discs from two species (Trema integerrima, a labile C source and Zygia longifolia, a recalcitrant C source) with maltose. We incubated the substrata in low‐ and high‐P sites of the P‐enrichment stream. 5. Results from Experiment II indicate that biofilm respiration on a labile C source (Trema) was not C‐limited, while biofilm respiration on a recalcitrant C source (Zygia) was C‐limited. Phosphorus stimulated the biofilm respiration and breakdown rate on Trema, but not on Zygia, supporting the hypothesis that the stimulatory effect of P‐enrichment is dependent on the availability of labile C in decomposing leaves. 6. Our results suggest that the interactive effects of organic matter quality and nutrient loading of streams can significantly increase microbial biofilm activity, potentially altering the trophic base of stream food webs. Researchers should consider both the organic matter quality and the enrichment of both water column and substrata to better predict the effects of anthropogenic nutrient loading to stream the ecosystems.     

Palatability of Leaves Conditioned in Streams Affected by Mine Drainage: A Feeding Experiment with Gammarus Pulex (L.)

Both the absence of leaf shredding macroinvertebrates and low microbial activity are of major importance in determining slow and incomplete leaf decay in extremely acidic (pH<3.5) mining streams. These streams are affected by a heavy ochre deposition causing the formation of massive iron plaques on leaf surfaces that hinder microbial exploitation. An investigation was carried out to determine whether iron plaques and leaf conditioning status (acid conditioned with and without iron plaques, neutral conditioned, unconditioned) affect the feeding preference of the shredder Gammarus pulex (L.). Leaf respiration rates and fungal biomass (ergosterol contents) were measured to determine microbial colonization. Neutral conditioned leaves had significantly higher microbial colonization than acid conditioned leaves with iron plaques. Notwithstanding, leaves of both conditioning types were consumed at high rates by G. pulex. The microbial colonization had no influence on feeding preference in the experiment. It is presumed that iron adsorbed organic material caused the high palatability of leaves with iron plaques. The results indicate that the large deposits of leaves coated with iron plaques will be available to the stream food web when water quality will be restored to neutral as planed in scenarios for the future development of mining streams.     

The amphipod Gammarus minus has larger eyes in freshwater springs with numerous fish predators

Abstract. Intraspecific variation in eye size in relation to ecological factors has not been well studied. Here, for the first time, we show that larger eyes in a freshwater crustacean may be associated with the presence of predators. In central Pennsylvania (USA), individuals of the amphipod crustacean Gammarus minus have significantly larger eyes in two freshwater springs with numerous fish predators (Cottus cognatus) than in three springs with few or no fish predators. Although we do not know the precise causes of these differences, this study and previous work on cave populations of G. minus suggest that eye size is an evolutionarily malleable trait that may respond to multiple selection pressures, either directly or indirectly. Three plausible explanations for the eye‐size variation observed among our study populations include (1) larger eyes may enable amphipods to better detect and avoid fish predators, (2) fish predation favors nocturnal or shallow interstitial activity that is facilitated by larger, more light‐sensitive eyes, or (3) the presence of fishes is associated with other environmental factors that may favor relatively large eyes. Available evidence suggests that the first hypothesis is the most viable explanation, but further study is required.     

Floristic study of <Emphasis Type="Italic">Geastrum</Emphasis> in Japan: three new records for Japanese mycobiota and reexamination of the authentic specimen of <Emphasis Type="Italic">Geastrum minus</Emphasis> reported by Sanshi Imai

Geastrum berkeleyi, G. fornicatum, and G. minimum are newly recorded from Japan. A peristome of G. fornicatum has hitherto been described as indistinct, whereas the Japanese specimens have a well-delimited, fibrillose peristome. Geastrum minus, reported for the first time from Japan by Sanshi Imai, represents G. quadrifidum. Macroscopic and microscopic features of those four taxa are described and illustrated based on Japanese specimens.     

短期氮素添加和模拟放牧对青藏高原高寒草甸生态系统呼吸的影响

氮沉降和放牧是影响草地碳循环过程的重要环境因子,但很少有研究探讨这些因子交互作用对生态系统呼吸的影响。在西藏高原高寒草甸地区开展了外源氮素添加与刈割模拟放牧实验,测定了其对植物生物量分配、土壤微生物碳氮和生态系统呼吸的影响。结果表明:氮素添加显著促进生态系统呼吸,而模拟放牧对其无显著影响,且降低了氮素添加的刺激作用。氮素添加通过提高微生物氮含量和土壤微生物代谢活性,促进植物地上生产,从而增加生态系统的碳排放;而模拟放牧降低了微生物碳含量,且降低了氮素添加的作用,促进根系的补偿性生长,降低了氮素添加对生态系统碳排放的刺激作用。这表明,放牧压力的存在会抑制氮沉降对高寒草甸生态系统碳排放的促进作用,同时外源氮输入也会缓解放牧压力对高寒草甸生态系统生产的负面影响。     

Fungal importance extends beyond litter decomposition in experimental early‐successional streams

Fungi are important decomposers of leaf litter in streams and may have knock‐on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand‐bottom streams in an early‐successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sediment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early‐successional catchment circulate through the channels for 5 weeks. Whole‐stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflecting higher fungal activity on leaves. Bacterial communities on leaves were also significantly affected. Similarly, increases in net primary production, sediment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosystems beyond the well‐established direct involvement in leaf litter decomposition.     

Allelopathic interference of invasive <Emphasis Type="Italic">Acacia dealbata</Emphasis> Link on the physiological parameters of native understory species

The tree Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain, where its expansion is assumed to reduce populations of native species and threaten local plant biodiversity. In order to investigate the potential involvement of allelopathic mechanisms in this process, net photosynthetic and respiration rates of four test native understory species (Hedera hibernica (G. Kirchn.) Bean, Dicranum sp., Dactylis glomerata L. and Leucobryum sp.) were evaluated using a Clark-type electrode in the presence of canopy leachate collected under A. dealbata stands at four times of the year for 2 years and macerate from their apical branches at the same sites and times. The first two test species were present both inside and outside of A. dealbata stands, while the last two were only located outside the stands. We found that there were significant differences in respiration and net photosynthetic rates between the control and A. dealbata extracts in all test species. The respiration rate was increased by both canopy leachate and macerate extracts on certain collection dates, but net photosynthetic rate was stimulated by macerate and inhibited by canopy leachate on other dates. The main phenological stages of A. dealbata in which respiration and net photosynthetic rates were more affected were blossoming, inflorescence formation and in periods after severe drought, in this decreasing order. Our results also showed that Dicranum sp., Leucobryum sp. and D. glomerata were more affected by aqueous extracts than other species during a 2-year period. We suggest that the observed inhibitory or stimulatory effects on the physiological parameters studied could have an adverse effect on the understory species, and that allelopathic interference seems to participate in this process.     

Decomposition of <Emphasis Type="Italic">Typha angustifolia</Emphasis> and <Emphasis Type="Italic">Phragmites australis</Emphasis> in the littoral zone of a shallow lake

Decomposition of air-dried live Typha angustifolia (L) stems and leaves and Phragmites australis (Cav. Trin ex Steud.) leaves and culms were studied in a shallow freshwater lake (Lake Fehér, Fertő-Hanság National Park, Hungary) using the litter bag technique. Samples were analyzed for dry mass, fiber (cellulose, hemicelluloses, lignin) and nutrient (C, N, P, S) contents, litter-associated fungal biomass (ergosterol concentration), potential microbial respiration (electron transport activity: ETS) and cellulolytic bacteria. In terms of mesh size, there were no significant differences in the examined parameters of P. australis leaves and culms and T. angustifolia stems with leaves. P. australis leaves had the highest rate of decomposition and P. australis culms the lowest. Hemicellulose degraded more rapidly than the other fibers, while the lignin had the slowest rate of decomposition. The ETS activity of the examined plant litter types increased from day 91^st to 237^th while decomposition processes were most active, ergosterol contents were high, and there were few cellulolytic bacteria. The counts of cellulolytic bacteria fluctuated during the decomposition period, they were high at the beginning then they decreased. In each case bacteria were found to be the first colonizers of plant detritus, and were followed by fungal growth.     

Microbial Biomass,Respiration, and Decomposition of Hura crepitans L. (Euphorbiaceae) Leaves in a Tropical Stream1

The processing of leaves in temperate streams has been the subject of numerous studies but equivalent tropical ecosystems have received little attention. We investigated leaf breakdown of a tropical tree species (Hura crepitans, Euphorbiaceae), in a tropical stream using leaf bags (0.5 mm mesh) over a period of 24 days. We followed the loss of mass and the changes in adenosine triphosphate (ATP) concentrations and respiration rates associated with the decomposing leaves. The breakdown rate was fast (k=?0.0672/d, k_d=?0.0031/degree‐day), with 81 percent loss of the initial mass within 24 days. This high rate was probably related to the stable and high water temperature (22°C) favoring strong biological activity. Respiration rates increased until day 16 (1.1 mg O₂/h/g AFDM), but maximum ATP concentrations were attained at day 9 (725 nmol ATP/g AFDM) when leaf mass remaining was 52 percent. To determine the relative importance of fungi and bacteria during leaf decomposition, ATP concentrations, and respiration rates were determined in samples treated with antibiotics, after incubation in the stream. The results of the samples treated with the antifungal or the bacterial antibiotic suggest a higher contribution of the fungal community for total microbial biomass and a higher contribution of the bacterial community for microbial respiration rates, especially during the later stages of leaf decomposition. However, these results should be analyzed with caution since both antibacterial and antifungal agents did not totally eliminate microbial activity and biomass.     

The effects of irradiance on growth, respiration and nitrate reductase activity of Terminalia ivorensis and Terminalia superba

Controlled-environment experiments were conducted to determine the effect of three irradiance levels obtained by artificial shading (40%, 65% and 100% light) on the growth, distribution of photosynthate, relative growth rate, net assimilation rate, respiration and nitrate reductase activities in the leaves of seedlings of Terminalia ivorensis and Terminalis superba, two important tropical tree species. Total dry weights of both species increased with increasing irradiance level during growth. Shading affected the percentage dry matter in the roots and number of leaves of both species. Relative growth rate, net assimilation rate, respiration and nitrate reductase in the leaves of both species increased with increases in irradiance level during growth. Significant differences between the species were observed in most of the parameters studied.     

Acidic Cave-Wall Biofilms Located in the Frasassi Gorge,Italy

Acidic biofilms present on cave walls in the sulfidic region of the Frasassi Gorge, Italy, were investigated to determine their microbial composition and their potential role in cave formation and ecosystem functioning. All biofilm samples examined had pH values &lt; 1.0. Scanning electron microscopy of the biofilms revealed the presence of various filaments and rods associated in large clusters with mineral crystals. Qualitative energy-dispersive x-ray analysis was used to determine that the crystals present on the cave walls, associated with the microbial biofilm, were composed of calcium and barium sulfate. Ribosomal RNA-based methods to determine the microbial composition of these biofilms revealed the presence of at least two strains of potential acidophilic, sulfur-oxidizing bacteria, belonging to the genera Thiobacillus and Sulfobacillus. An acid-producing strain of Thiobacillus sp. also was obtained in pure culture. Stable isotope ratio analysis of carbon and nitrogen showed that the wall biofilms are isotopically light, suggesting that in situ chemoautotrophic activity plays an important role in this subsurface ecosystem.     

Respiration Rate, Adenosine Diphosphate and Triphosphate Concentrations of Leaves Showing Necrotic Local Lesions Following Infection by Tobacco Mosaic Virus

The respiration rate of TMV-infected leaves of Nicotiana tabacum L. variety Xanthi has been shown to be greater than the respiration rate of healthy leaves. 10^?5M DNP gave maximum stimulation of the respiration rate of the healthy leaf but had no effect on the respiration of the virus-infected leaf with dense lesion cover. The concentration of ATP increased with greater lesion density while the ADP concentration decreased. The increase in ATP concentration was greater than could be accounted for by the decrease in ADP concentration. Thus, increased respiration in the virus-infected leaf is accompanied by a decrease in the ADP/ATP ratio. Illumination of the virus infected leaf increased the ADP/ATP ratio.     

Decomposition of litter in sub-alpine forests of Eucalyptus delegatensis,E. pauciflora and E. dives

The rate of decomposition of summer leaf-fall (abscised leaves), winter leaf-fall (containing some green leaves) and mature green (picked) leaves was assessed in sub-alpine forests of E. delegatensis (R. T. Baker), E. pauciflora (Sieb. ex Spreng) and E. dives (Schau.) in the Brindabella Range, Australian Capital Territory, using litter bag and tethered leaf techniques. The relative contribution of leaching, microbial respiration and grazing by invertebrate macrofauna to loss of leaf weight was determined. The effect of leaching and microbial respiration was assessed in terms of weight loss per unit area of leaf (specific leaf weight), while losses due to macro-faunal grazing were assessed by measuring reductions in leaf area. Litter decomposition constants for litter components (leaf, bark, wood) and total litter were determined from long-term records of litterfall and accumulated litter. Weight losses of abscised leaves during the initial 12 months ranged from 25% for E. pauciflora to 39% for E. delegatensis and were almost entirely due to reduction in specific leaf weight. Losses in the weight of leaves falling in winter ranged from 38 to 49%, while green leaves lost 45 - 59%. Approximately 50% of the total weight loss of green leaves was due to a loss in leaf area caused by skeletonization by litter macrofauna. Thus abscised leaves rather than green leaves must be used for measuring litter decomposition rates since abscised leaves constitute most of the litterfall in eucalypt forests. Leaves placed in the field in autumn decomposed slowly during the first summer, while the rate increased during the second winter and summer. Low litter moisture content appears to limit decomposition in the initial summer period in all communities, after which litterfall provides a mulch which reduces the rate of desiccation of lower litter layers. A simple linear regression model relating decomposition rate to the number of days (D) when litter moisture content exceeded 60% ODW accounted for 63-83% of the variation in decomposition of leaves in the field. Inclusion of mean monthly air temperature (T) and the product of D and T (day degrees when litter was wet) in a multiple linear regression increased the variation in decomposition accounted for to 80 – 90%. The rate of weight loss showed a positive linear relationship with the initial concentration of nitrogen (N) or phosphorus (P) in the leaf. These concentrations are an index of the decomposability of leaf substrates (e.g. degree of sclerophylly or lignification). The rate of loss of specific weight was similar for tethered leaves and for leaves enclosed in mesh bags. Measured loss in specific leaf weight after 70 – 90 weeks was less than that predicted using decomposition constants (k).