Whole-system Estimates of Nitrification and Nitrate Uptake in Streams of the Hubbard Brook Experimental Forest

Although they drain remarkably similar forest types, streams of the Hubbard Brook Experimental Forest (HBEF) vary widely in their NO₃ ⁻ concentrations during the growing season. This variation may be caused by differences in the terrestrial systems they drain (for example, varying forest age or composition, hydrology, soil organic matter content, and so on) and/or by differences between the streams themselves (for example, contrasting geomorphology, biotic nitrogen [N] demand, rates of instream nitrogen transformations). We examined interstream variation in N processing by measuring NH₄ ⁺ and NO₃ ⁻ uptake and estimating nitrification rates for 13 stream reaches in the HBEF during the summers of 1998 and 1999. We modeled nitrification rates using a best-fit model of the downstream change in NO₃ ⁻ concentrations following short-term NH₄ ⁺ enrichments. Among the surveyed streams, the fraction of NH₄ ⁺ uptake that was subsequently nitrified varied, and this variation was positively correlated with ambient streamwater NO₃ ⁻ concentrations. We examined whether this variation in instream nitrification rates contributed significantly to the observed variation in NO₃ ⁻ concentrations across streams. In some cases, instream nitrification provided a substantial portion of instream NO₃ ⁻ demand. However, because there was also substantial instream NO₃ ⁻ uptake, the net effect of instream processing was to reduce rather than supplement the total amount of NO₃ ⁻ exported from a watershed. Thus, instream rates of nitrification in conjunction with instream NO₃ ⁻ uptake were too low to account for the wide range of streamwater NO₃ ⁻. The relationship between streamwater NO₃ ⁻ concentration and rates of instream nitrification may instead be due to a shift in the competitive balance between heterotrophic N uptake and nitrification when external inputs of NO₃ ⁻ are relatively high. Received 11 October 2000; accepted 14 December 2001.     

Phenology of a northern hardwood forest canopy

While commonplace in other parts of the world, long‐term and ongoing observations of the phenology of native tree species are rare in North America. We use 14 years of field survey data from the Hubbard Brook Experimental Forest to fit simple models of canopy phenology for three northern hardwood species, sugar maple (Acer saccharum), American beech (Fagus grandifolia), and yellow birch (Betula alleghaniensis). These models are then run with historical meteorological data to investigate potential climate change effects on phenology. Development and senescence are quantified using an index that ranges from 0 (dormant, no leaves) to 4 (full, green canopy). Sugar maple is the first species to leaf out in the spring, whereas American beech is the last species to drop its leaves in the fall. Across an elevational range from 250 to 825 m ASL, the onset of spring is delayed by 2.7±0.4 days for every 100 m increase in elevation, which is in reasonable agreement with Hopkin's law. More than 90% of the variation in spring canopy development, and just slightly less than 90% of the variation in autumn canopy senescence, is accounted for by a logistic model based on accumulated degree‐days. However, degree‐day based models fit to Hubbard Brook data appear to overestimate the rate at which spring development occurs at the more southerly Harvard Forest. Autumn senescence at the Harvard Forest can be predicted with reasonable accuracy in sugar maple but not American beech. Retrospective modeling using five decades (1957–2004) of Hubbard Brook daily mean temperature data suggests significant trends (P≤0.05) towards an earlier spring (e.g. sugar maple, rate of change=0.18 days earlier/yr), consistent with other studies documenting measurable climate change effects on the onset of spring in both North America and Europe. Our results also suggest that green canopy duration has increased by about 10 days (e.g. sugar maple, rate of change=0.21 days longer/yr) over the period of study.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Stable isotopes identify dispersal patterns of stonefly populations living along stream corridors

1. Populations in different locations can exchange individuals depending on the distribution and connectivity of suitable habitat, and the dispersal capabilities and behaviour of the organisms. We used an isotopic tracer, ¹⁵N, to label stoneflies (Leuctra ferruginea) to determine the extent of adult flight along stream corridors and between streams where their larvae live. 2. In four mass, mark‐capture experiments we added ¹⁵NH₄Cl continuously for several weeks to label specific regions of streams within the Hubbard Brook Experimental Forest, NH, U.S.A. We collected adult stoneflies along the labelled streams (up to 1.5 km of stream length), on transects through the forest away from labelled sections (up to 500 m), and along an 800‐m reach of adjacent tributary that flows into a labelled stream. 3. Of 966 individual adult stoneflies collected and analysed for ¹⁵N, 20% were labelled. Most labelled stoneflies were captured along stream corridors and had flown upstream a mean distance of 211 m; the net movement of the population (upstream + downstream) estimated from the midpoint of the labelled sections was 126 m upstream. The furthest male and female travelled approximately 730 m and approximately 663 m upstream, respectively. We also captured labelled mature females along an unlabelled tributary and along a forest transect 500 m from the labelled stream, thus demonstrating cross‐watershed dispersal. 4. We conclude that the adjacent forest was not a barrier to dispersal between catchments, and adult dispersal linked stonefly populations among streams across a landscape within one generation. Our data on the extent of adult dispersal provide a basis for a conceptual model identifying the boundaries of these populations, whose larvae are restricted to stream channels, and whose females must return to streams to oviposit.     

The geochemistry of two hard water rivers,the Rhine and the Rhone: Part 3: The relations between calcium,bicarbonate, sulphate and pH

In the hard water rivers Rhine and Rhone the quotient Ca/HCO₃ is strongly related to the sulphate concentration and not to the pH. The relationship can be described (by least square analysis) for the Rhine: Ca/HCO₃) = 0.70 + 0.5 (SO₄), for the Rhone: (Ca/HCO₃) = 0.85 + 0.43 (SO₄). With a Teissier analysis (reduced major axis) a slope for both rivers of 0.58 has been found. These values equal the theoretically expected value of 0.5, when a solution of CaSO₄ is added to a saturated solution of CaCO₃. The source of the CaSO₄ (gypsum) is thought to be natural in the Rhone and anthropogenic in the Rhine. Acidification of both rivers is probably the result of decomposition of disposed organic matter.     

The use of body water, sodium, potassium and calcium content to investigate the nutritional status of first year Atlantic salmon parr in two Scottish Highland streams

Water content of Atlantic salmon parr fell from about 84% at emergence (late May) to just under 79% in September but rose again towards March. Na⁺ content consequently rose from 3·3 mg g⁻¹ dry wt at the beginning of June to 6·2 mg g⁻¹ in early July. It then fell to 4·4 mg g⁻¹ in September, rising again towards March. K⁺ content rose to a maximum in July to stabilize at 16·6 mg g⁻¹ dry wt in September. The resultant Na⁺/K⁺ ratio peaked at 0·43: 1 in mid-June, falling to a minimum in mid-August but rising again in March reflecting changes in the relative proportions of intra and extracellular water. The changes in whole-body chemistry suggest a period of nutritional stress immediately after emergence and during the winter. In streams at higher altitude and of lower nutrient status, nutritional stress during the winter appears to be more severe.     

Trace metal loss following whole-tree harvest of a northeastern deciduous forest, U.S.A.

We examined changes in the biogeochemistry of trace metals following a commercial whole-tree harvest (WTH) at the Hubbard Brook Experimental Forest in New Hampshire. Within 6 months after completion of the WTH, maximum streamwater concentrations of Ni, Cd, Ba, Sr, Mn, Zn and Fe increased two- to nine-fold. Streamwater concentration of Cu remained unchanged after harvest. Streamwater pH decreased from 5.2 to 4.5 after the harvest, and correlated strongly with trace metal concentrations except for Fe. The decrease in pH apparently resulted from increased nitrogen mineralization and nitrification following harvest. All streamwater metal concentrations (except Mn and Fe) in the disturbed watershed increased prior to the decrease in streamwater pH, suggesting that the loss of readily exchangeable metals, not increased mineral dissolution, was responsible for the initial increase in streamwater trace metal concentrations. In contrast, streamwater Mn concentrations did not increase until streamwater pH dropped to 4.5, due in part to increased mineral dissolution. Although pH related strongly to trace metal concentrations in the harvested watershed, it did not account for much of the variation in metal concentrations in the reference (W6) watershed. Annual flux of trace metals increased two- to eight-fold following WTH. Annual losses of Mn and Sr were 14% and 12%respectively of the forest floor pool for each element, and less than 10% of forest floor pools for all other elements. Except for Cd and Cu, annual trace metal losses in streamwater exceeded annual inputs in bulk precipitation.Deceased     

Response of secondary production by macroinvertebrates to large wood addition in three Michigan streams

1. We measured responses in macroinvertebrate secondary production after large wood additions to three forested headwater streams in the Upper Peninsula of Michigan. These streams had fine‐grained sediments and low retention capacity due to low amounts of in‐channel wood from a legacy of past logging. We predicted that wood addition would increase macroinvertebrate secondary production by increasing exposed coarse substrate and retention of organic matter. 2. Large wood (25 logs) was added haphazardly to a 100‐m reach in each stream, and a 100‐m upstream reach served as control; each reach was sampled monthly, 1 year before and 2 years after wood addition (i.e. BACI design). Macroinvertebrate secondary production was measured 1 year after wood addition in two habitat types: inorganic sediments of the main channel and debris accumulations of leaf litter and small wood. 3. Overall macroinvertebrate production did not change significantly because each stream responded differently to wood addition. Production increased by 22% in the main‐channel of one stream, and showed insignificant changes in the other two streams compared to values before wood addition. Changes in main‐channel macroinvertebrate production were related to small changes in substrate composition, which probably affected habitat and periphyton abundance. Macroinvertebrate production was much greater in debris accumulations than in the main‐channel, indicating the potential for increased retention of leaf litter to increase overall macroinvertebrate production, especially in autumn. 4. Surrounding land use, substrate composition, temperature and method of log placement are variables that interact to influence the response of stream biota to wood additions. In most studies, wood additions occur in altered catchments, are rarely monitored, and secondary production is not a common metric. Our results suggest that the time required for measurable changes in geomorphology, organic matter retention, or invertebrate production is likely to take years to achieve, so monitoring should span more than 5 years, and ecosystem metrics, such as macroinvertebrate secondary production, should be incorporated into restoration monitoring programs.     

Biogeochemistry of calcium in a broad-leaved forest ecosystem

This study of the biogeochemical cycle of Ca has been carried out in an oak-hornbeam forest ecosystem on a soil derived from the alteration of Famennian (upper Devonian) shales. The physical nature of the geologic substrate made it possible to use the watershed-ecosystem approach by monitoring the loss of Ca by streamflow. Input of hydrogen ions and Ca from rain were average as well as that of mineral nitrogen of which 50% was ammonium-N. Modelling the biogeochemical cycle of Ca in this ecosystem proved to be difficult owing to the heterogeneity of the geologic substrate.The cationic denudation occured mostly with bicarbonate ion as a carrier of the base cations towards the streamflow. Seasonal effects on streamwater cationic composition were in relation to maximal soil respiration occuring in the autumn. In addition to this effect, the electrolyte concentration of streamwater was determined by rain and by changes in concentration of the soil solution either by evapotranspiration or by freezing. In spite of widely fluctuating values of flow rate, the concentration of the streamflow remained constant within a factor of two or at the most three. The mechanism underlying this buffering action of the soil on the concentration of the solution flowing through it, was shown to be related to ion-exchange processes.     

Effects of calcium ions on responses of two jute species under water-deficit stress

Water-deficit stress (–1.0 MPa through polyethylene glycol 6000 for 2 h) decreased the relative water content (RWC) and leaf water potential (LWP) in the two jute species Corchorus capsularis L. (cv. JRC 212) and C. olitorius L. (cv. JRO 632), more so in the latter. Pretreatment of seeds with 5 m M CaCl₂ improved the water uptake capacity without altering stomatal movement, whereas foliar spraying with 0.01 m M ABA reduced transpiration through the reduction of stomatal aperture. A combination of both treatments, i.e. seed treatment followed by foliar spraying, additively improved the water status under water-deficit stress. Efflux of K⁺ from the guard cells followed by stomatal closure was more rapid in C. capsularis than in C. olitorius under water-deficit stress. Uptake of [³²P]-phosphate under water-deficit stress decreased more in C. olitorius than in C. capsularis and treatment of seeds with Ca²⁺ counteracted this decrease more markedly in the former species. These findings indicate that C. olitorius is more susceptible than C. capsularis to water-deficit stress.     

Secondary production,longevity and resource consumption rates of freshwater shrimps in two tropical streams with contrasting geomorphology and food web structure

1. Freshwater shrimps often dominate the biomass of tropical island streams and are known to have strong effects on stream ecosystem structure and function, but little effort has been dedicated toward quantifying basic energetic and life history attributes such as growth, production and longevity. Such information is critical for understanding both the role of shrimps in ecosystem dynamics and the gravity of threats to shrimp populations posed by human activities such as shrimp harvesting, dam construction and water withdrawal. 2. We quantified growth rates and secondary production of dominant freshwater shrimps for 3 years in two Puerto Rican headwater streams that differ in food web structure because of the presence or absence of predatory fishes that are excluded from reaches above waterfalls. Using growth data, we constructed a minimum longevity model to explore the likely minimum life spans of the two dominant taxa (Atya spp. and Xiphocaris elongata). Finally, we used a bioenergetics model to quantify annual consumption rates of major basal resources by the two taxa. 3. Daily growth rates ranged from ?0.001 to 0.011 day^?1, were inversely related to body size, and were higher for small individuals of X. elongata than Atya spp. Mean annual shrimp biomass and secondary production were an order of magnitude higher in the stream that lacked predatory fishes (biomass: 4.34 g AFDM m^?2; production: 0.89 g AFDM m^?2 year^?1) than in the stream with predatory fishes (biomass: 0.12 g AFDM m^?2; production: 0.02 g AFDM m^?2 year^?1). Production : biomass ratios ranged from 0.01 to 0.38. 4. Our longevity model predicted a minimum life span of 8 years for Atya spp. and 5 years for X. elongata in the stream lacking predatory fishes. In contrast, due to a larger average size of X. elongata in the stream with predatory fishes, our model predicted a minimum life span of 11 years. Actual life spans of these taxa are likely to be much longer based on long‐term observations of marked individuals. 5. Estimated consumption rates from the bioenergetics model indicated that Atya spp. and X. elongata are important processors of organic matter resources in streams where they occur at high densities. Atya spp. and X. elongata appeared capable of consuming a large proportion of algal and insect production and the proportion of direct leaf litter inputs consumed was also appreciable (c. 40–60%). However, the consumption of suspended fine particulate organic matter (SFPOM) by Atya spp. is probably only a minor proportion of total SFPOM flux in these streams. 6. Our study suggests that geomorphic features such as waterfalls may play an important role in controlling the distribution and production of freshwater shrimps through their effects on predatory fish movement. Spatial differences in shrimp densities result in landscape‐scale variation in the significance to ecosystem processes of these long‐lived organisms, particularly as processors of major organic matter resources.     

Life histories, production dynamics and resource utilisation of mayflies (Ephemeroptera) in two tropical Asian forest streams

SUMMARY 1. A 2‐year study of the life histories, production dynamics and resource utilisation of five mayfly species was undertaken in two forest streams in Hong Kong [Tai Po Kau Forest Stream (TPKFS) and Shing Mun River (SMR)]. Afronurus sp. and Cinygmina sp. (Heptageniidae), Procloeon sp. and Baetiella pseudofrequenta (Baetidae), and Choroterpes sp. (Leptophlebiidae) were abundant in both streams and contributed more than 50% of the total mayfly populations. 2. All species had asynchronous larval development with recruitment occurring throughout the year. Mean annual production (all mayflies combined) was 3.1 and 2.0 g dry weight m^?2 year^?1 in SMR and TPKFS, respectively – the higher value at SMR reflecting greater mayfly densities – with more than 70% of production occurring during the wet season. Mayfly production varied between years, decreasing by 5% in TPKFS and 43% in SMR during 1996–97, reflecting lower densities of heptageniids relative to 1995–96. Annual biomass turnover rates (P/B) were high in both sites ranging from 27.2 to 94.6 in TPKFS (Cinygmina sp. and Procloeon sp.) and from 31.8 to 109.8 in SMR (Cinygmina sp. and B. pseudofrequenta). 3. Patterns of daily production in both streams showed that Afronurus sp., Cinygmina sp. and Choroterpes sp. were most productive during the wet season, while Procloeon sp. maintained high production levels throughout the year. The highest daily production of B. pseudofrequenta occurred during the wet season in TPKFS, but in the dry season at SMR. Temporal overlap in production and hence resource utilisation in both streams, calculated using the proportional similarity index (PS), ranged from 0.39 to 0.81. It was highest (0.63–0.81) between pairs of species of Heptageniidae and Baetidae, and lowest between Choroterpes sp. and other mayflies (0.39–0.61). No clear temporal segregation was observed among any species. However, when using the fraction of production attributable to each food, lower PS values were obtained for all species in both sites. In SMR, trophic segregation may have occurred between the two species pairs Procloeon sp.–Cinygmina sp. and Procloeon sp.–Choroterpes sp. (PS=0.17 and 0.03, respectively). 4. A combination of production data and information on the stable isotope signature of mayflies revealed that, during both the wet and dry seasons, more than 50% of total mayfly production in TPKFS was derived from autochthonous foods. In SMR, 68% of production was supported by allochthonous foods during the wet season, and 72% by autochthonous sources in the dry season. Considering that more than 70% of the total production occurred in the wet season, the trophic basis of mayfly production in SMR is mostly allochthonous (58%) while in TPKFS it is mainly of autochthonous origin (66%). The year‐round importance of autochthonous foods in shaded streams such as TPKFS is surprising, but the wet season contribution of allochthonous foods (especially in SMR) may have resulted from depletion of algal biomass during spates.     

Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams

1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem‐level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole‐system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem‐level N and P excretion and uptake. Finally, we used a 3‐year record of discharge and NH₄‐N concentration in the high‐biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH₄‐N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16–3.80 μmol NH₄‐N shrimp^?1 h^?1, 0.23–5.76 μmol total dissolved nitrogen (TDN) shrimp^?1 h^?1 and 0.002–0.186 μmol total dissolved phosphorus (TDP) shrimp^?1 h^?1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH₄‐N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between‐stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH₄‐N m^?2 h^?1, 37.5 versus 0.26 μmol TDN m^?2 h^?1 and 1.1 versus 0.015 μmol TDP m^?2 h^?1), equivalent to 21% and 0.5% of NH₄‐N uptake and 5% and <0.1% of P uptake measured in the high‐ and low‐biomass stream, respectively. Distances required for excretion to turn over the ambient NH₄‐N pool varied more than a hundredfold over the 3‐year record in the high‐shrimp stream, driven by variability in discharge and NH₄‐N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community‐level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.     

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1⁻¹), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l⁻¹). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1⁻¹, and inorganic aluminium levels as low as l·0 μg 1⁻¹. The cpue was inversely correlated with lake altitude ( r ²=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue ( r ² = 0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue ( r ² = 0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.     

The geochemistry of two hard water rivers,the Rhine and the Rhone. Part 2: The apparent solubility of calcium carbonate

The ionic products of CaCO₃ have been calculated for 7 stations in the Rhine and for 7 stations in the Rhone over periods of 9 and 7 years respectively. The ionic products exceed the solubility product, indicating a supersaturation with CaCO₃. It is demonstrated that the ionic product is related to the pH according to IP_c = A.pH^B.The values obtained for A and B for the different stations vary per station and per year. The four regression lines, however, differ numerically very little between pH 7 and 8. Supersaturation with CaCO₃ of hard waters thus seems to depend on the pH.     

Changes in chironomid assemblage composition in two Mediterranean mountain streams over a period of extreme hydrological conditions

An opportunity to test Coffman's (1989) proposal that ecological heterogeneity is one of the main factors for chironomid species richness occurred when prolonged drought in southern Spain was broken by heavy rain. Chironomid assemblage composition was studied in two Mediterranean mountain first-order streams, one permanent and the other temporary. Samples were collected in the permanent stream over extreme hydrological conditions: 1991–95 with relatively low and stable flow, and 1996–97 with relatively high and unstable flow. The temporary stream flowed, and therefore was sampled, only during the second period. In the permanent stream, spate events resulted in a notable increase in species richness due to the arrival of 32 species and to the permanence of 81% of the species resident over the prespate period. This relatively high percentage suggests that chironomids, as a group, have important resilience properties. However, some species disappeared from the permanent stream after the spates and the considerable decrease in abundance of other species, shows that, at the species level, resilience may vary greatly within the Chironomidae. Most of the species that appeared in the temporary stream were the same as those of the permanent stream during the fast flow period despite the great ecological differences between the two streams under study, suggesting the key role of discharge in structuring chironomid assemblages. The outstanding exception was the fugitive species Orthocladius calvus, the most abundant species in the temporary stream during the initial successional stages but never collected in the permanent stream. This revised version was published online in July 2006 with corrections to the Cover Date.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Methane uptake rates in Japanese forest soils depend on the oxidation ability of topsoil, with a new estimate for global methane uptake in temperate forest

To clarify the reason for the higher CH₄ uptake rate in Japanese forest soils, twenty-seven sites were established for CH₄ flux measurement. The first order rate constant for CH₄ uptake was also determined using soil core incubation at 14 sites. The CH₄ uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH₄ uptake rates ranged from 2.7 to 24.8 kg CH₄ ha⁻¹ y⁻¹ (the average of these rates was 9.7 or 10.9 kg CH₄ ha⁻¹ y⁻¹, depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH₄ uptake rate correlated closely with the CH₄ oxidation rate of the topsoil (0–5 cm) in the study sites. The CH₄ oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio (p < 0.05, R ² = 0.684). This result and comparison with literature data suggest that the high CH₄ uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH₄ uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH₄ uptake rate in temperate forests was estimated to be 5.4 Tg y⁻¹ (1 SE is 1.1 Tg y⁻¹) on a continental basis.