Stuart Findlay

Effects of Damage to Living Plants on Leaf Litter Quality

The leaves of plants in nature are commonly subjected to damage from a wide variety of agents, including herbivory, air pollutants, and simple physical damage. Despite the attention paid to damage effects on living plants, the potential effects on the quality of litter derived from damaged leaves has not been considered. We used controlled laboratory assays of decomposition to show that both ozone (0.2 mL/m3, 4 h) and mite damage, but not ultraviolet radiation (UV-B) exposure, to living leaves of cottonwood plants resulted in a decrease in decomposition rate of litter derived from damaged leaves. De- composition rates were -50% slower for litter from damaged plants, and there was a twofold increase in the refractory fraction. Contrary to expectation, there was a negative relationship between rate of decomposition and litter nitrogen content. Our finding of slow decompo- sition of high-nitrogen litter is explained by a general mechanism whereby cellular damage causes increases in complex phenolic material. Such materials can lead to reductions in decomposition and binding of available nitrogen. We suggest that this mechanism can translate a common occurrence, damage by a diversity of processes, into long-term and possibly large-scale alterations in detritus processing.

Comparison of Detritus Dynamics in Two Tidal Freshwater Wetlands

We have examined the generation and persistence of detritus in two con- trasting tidal freshwater wetlands on the Hudson River. These wetlands offer a difference in vegetation, with Tivoli South Bay dominated by a floating-leafed macrophyte (water- chestnut, Trapa natans) and North Bay a typical Typha marsh. In South Bay, there was a large amount of water-chestnut dry biomass (400 g/m2) available to enter the detritus pool, but there was no increase in the standing stock of benthic organic matter following senescence of water-chestnut. Our estimates show that mineralization plus leaching of dissolved material are sufficient to remove much of this detritus. In the Typha marsh, there is a large amount of detritus generated (_25% of annual primary production) and this material persists as a thick litter layer. Decomposition of this litter is very slow (0.3/yr). A portion of the litter may be exported because decomposition alone cannot account for the observed rate of disappearance from the marsh surface. Microbial abundance was used to estimate the amount of heterotrophic biomass sup- ported by these different types of detritus. Bacterial growth on water-chestnut detritus is relatively slow (106 cells mg-I d-l), resulting in a turnover of bacterial biomass in 10-36 d. Bacterial and fungal biomass associated with Typha were low, and could not account for the observed increase in nitrogen content.

Decomposition of Hudson Estuary Macrophytes: Photosynthetic Pigment Transformations and Decay Constants

Plant pigment decay constants were determined for four macrophytes collected from the Hudson Estuary.Typha angustifolia andScirpus fluviatilis were used as representatives of emergent aquatic vegetation (EAV), andPotamogeton sp. andVallisneria americana were used to represent submerged aquatic vegetation (SAV). Litter bags were maintained in an environmental chamber in the dark for 104 d. The fastest rate of total mass loss was in the SAVV. americana and slowest in the EAVT. angustifolia. Changes in carotenoid and chloropigment concentration resulting from microbial and meiofaunal heterotrophy in each of the macrophytes were quantified using reverse-phase, high-performance liquid chromatography (RP-HPLC) techniques. Chlorophyllc and the carotenoid, fucoxanthin, provided useful biomarkers in determining the presence of epiphytic diatom growth, which only occurred on the SAV. The highest concentrations of phaeophorbidea, commonly used as an indication of metazoan grazing, were found in the SAVV. americana. Low concentrations of phaeophorbidea in the SAVPotamogeton sp. indicate inefficient use of this SAV by meiofaunal grazers. Lutein decayed slower than all other carotenoids in both EAV and SAV. Microcosm studies such as this are necessary to further understand the mechanisms and kinetics of photosynthetic pigment transformations in natural systems.

Extracellular enzyme kinetics scale with resource availability

Microbial community metabolism relies on external digestion, mediated by extracellular enzymes that break down complex organic matter into molecules small enough for cells to assimilate. We analyzed the kinetics of 40 extracellular enzymes that mediate the degradation and assimilation of carbon, nitrogen and phosphorus by diverse aquatic and terrestrial microbial communities (1160 cases). Regression analyses were conducted by habitat (aquatic and terrestrial), enzyme class (hydrolases and oxidoreductases) and assay methodology (low affinity and high affinity substrates) to relate potential reaction rates to substrate availability. Across enzyme classes and habitats, the scaling relationships between apparent Vmax and apparent Km followed similar power laws with exponents of 0.44 to 0.67. These exponents, called elasticities, were not statistically distinct from a central value of 0.50, which occurs when the Km of an enzyme equals substrate concentration, a condition optimal for maintenance of steady state. We also conducted an ecosystem scale analysis of ten extracellular hydrolase activities in relation to soil and sediment organic carbon (2,000–5,000 cases/enzyme) that yielded elasticities near 1.0 (0.9xa0±xa00.2, nxa0=xa036). At the metabolomic scale, the elasticity of extracellular enzymatic reactions is the proportionality constant that connects the C:N:P stoichiometries of organic matter and ecoenzymatic activities. At the ecosystem scale, the elasticity of extracellular enzymatic reactions shows that organic matter ultimately limits effective enzyme binding sites. Our findings suggest that one mechanism by which microbial communities maintain homeostasis is regulating extracellular enzyme expression to optimize the short-term responsiveness of substrate acquisition. The analyses also show that, like elemental stoichiometry, the fundamental attributes of enzymatic reactions can be extrapolated from biochemical to community and ecosystem scales.

Photosynthesis-irradiance relationships for three species of submersed macrophytes in the tidal freshwater Hudson River

Net photosynthesis under a range of natural light intensities was determined for three common macrophytes of the tidal freshwater Hudson River:Vallisneria americana Michx.,Potamogeton perfoliatus L., andMyriophyllum spicatum L. Light-saturated net photosynthetic rates did not differ among species, nor were there differences among species in the light intensity at which respiration balanced photosynthesis. The initial slope (α) of the photosynthesis-irradiance (P-I) curve was greater forV. americana than the other two species. As a result, maximum photosynthetic rates were reached inV. americana at significantly lower irradiances than those required to saturate photosynthesis inP. perfoliatus andM. spicatum. This characteristic would give this species a competitive advantage under conditions in which growth is limited by light availability. Macrophyte biomass distribution showed no clear correlation with depth.V. americana was the most abundant of the three species, both in frequency of occurrence and absolute biomass. The low irradiance required for the saturation of photosynthesis inV. americana may contribute to its predominance in the turbid Hudson River.

Exposure of cottonwood plants to ozone alters subsequent leaf decomposition

SummaryCottonwood saplings were exposed to ozone or charcoal-filtered air in a closed chamber. After leaf abscission, decomposition of individual leaf discs was measured in containers of stream water. Exposure of plants to 200 ppb ozone for 5 h caused early leaf abscission and changes in the chemical composition of leaves at time of abscission. Early-abscised leaves from O3-exposed plants had higher nitrogen, but decomposed more slowly than leaves from control plants. Leaves from O3-exposed plants that abscised at the normal time had lower nitrogen content and lower specific leaf mass than control leaves, but decomposed at the same rate as leaves from control plants. The results imply that O3 exposure can alter fundamental processes important to the functioning of detritus-based aquatic ecosystems.

Experimental degradation of plant materials in Hudson river sediments

We examined photopigment degradation and transformation in sediment microcosms that received different detrital source materials (planktonic, littoral, terrestrial) in the presence or absence of amphidpods (Gammarus sp.). Additions of realistic quantities of particulate organic matter resulted in detectable changes in pigment concentration and composition despite insignificant changes in total organic matter. The transformation of chlorophyll a to total phaeophorbide was significantly higher in all high quality (high nitrogen) detritus treatments containing amphipods. The highest production of phaeophorbide was in the higher quality detritus (blue-green algae, Anabaena cylindrica, and macrophyte, Vallisneria americana) when compared to red maple (Acer rubrum). Phaeophytin formation was not related to amphipod grazing and thus may be determined more by microbial heterotrophic processes. The degradation product of the carotenoid lutein, lutein 5,6 expoxide, was formed in all treatments. Phaeopigment composition can be used to infer differences in heterotrophic activity and will help in the interpretation of photopigment distribution in field samples.

Scaling microbial biomass, metabolism and resource supply

The microbiome concept has drawn attention to the complex signal and syntrophic networks that underlie microbial community organization. This self-organization may lead to patterns in the allometric scaling of microbial community metabolism that differ from those of macrobial communities. Using meta-analyses, we analyzed the power scaling relationships between community production, respiration, extracellular enzyme activity and biomass for bacteria and fungi across aquatic and terrestrial ecosystems. The scaling exponents for community production versus biomass for fungi and bacteria were 0.85xa0±xa00.06 (95xa0%xa0CI) and 0.72xa0±xa00.07, respectively. The scaling exponent for fungal respiration versus production was 0.61xa0±xa00.06. Previous studies reported exponents of 0.41, 0.44 and 0.58 for bacterial respiration versus production. Carbon use efficiency increased with biomass for both fungi and bacteria with an exponent of 0.27xa0±xa00.06. The potential activities of four widely measured extracellular enzymes were directly related to community production with power scaling exponents of 1.0–1.2. The frequency distribution of biomass turnover times (median 112xa0h for bacteria and 1,128xa0h for fungi) overlapped substantially with those for environmental substrate turnover, presented in a prior analysis of extracellular enzyme kinetics. These metabolic relationships, which have scaling exponents of 0.5, are linked by the ratio of assimilation to carbon use efficiency. This connection ties ecological stoichiometry and metabolic theory to microbial community homeostasis. At the ecosystem scale, allometry of microbial communities has similarities to that of eusocial insects but differs from that of plant communities, perhaps as a result of proto-cooperative processes that contribute to microbial community organization.

Effects of ozone on interactions between plants, consumers and decomposers

Ambient levels of ozone (O3) air pollution are sufficient to directly reduce plant growth and yield. Ozone decreases photosynthesis, or carbon (C) gain, and causes cellular injury, which increases C costs for repair and maintenance; less C is then available for allocation to growth and reproduction (Koziol & Whatley, 1984; Guderian et al., 1985; Reich & Amundson, 1985; Amthor, 1988). Nevertheless, predicting the net impact of O3 from direct reductions in growth and yield may be problematical for at least thee reasons. First, O3 may change plant resistance or tolerance to insect herbivores and plant pathogens (Hughes & Lawrence, 1984; Jones & Coleman, 1991; Coleman et al., 1992) potentially exacerbating or offsetting direct O3 effects on plant growth and yield. Second, O3 exposure of living plants could alter the timing of leaf litter fall and the quality of leaf litter, affecting decomposition and hence plant growth in terrestrial ecosystems; or nutrient flow though detritus-based food webs in aquatic ecosystems (Findlay & Jones, 1990). Third, O3 is not the only air pollutant and air pollution is not the only anthopic or natural cause of plant stress and damage that affects growth and yield (e.g. CO2, UV-B, temperature, precipitation, soil quality; Mooney et al., 1991). These factors could all interact with O3 effects.

How Can We Improve the Reception of Long-Term Studies in Ecology?

The initial discussion by the group pointed to several problems leading to a negative perception of LTS (long-term studies). The most common feeling was that LTS, due to the required simplicity of methodology and mundane parameters monitored (e.g., soil depth, rainfall, litter abundance), are perceived as inelegant, unexciting, and therefore not worthwhile science.