Microtubules and 10 nm-filaments in the retinal pigment epithelium during the diurnal light-dark-cycle

Summary Microtubules and 10 nm-filaments appear to be involved in the functions of the retinal pigment epithelium (RPE). The presence of microtubules in the RPE of light-adapted eyes, but not in dark-adapted eyes, suggests that they may be involved primarily in organelle movement. On the other hand, the random and constant presence of 10 nm-filaments within the basal portion of the PE implies a cytoskeletal role for these filaments.The authors thank their colleagues Pierre Couillard and Michel Anctil for helpful advice and criticism during the course of this study. Financial support was provided by the C.R.S.N.G. du Canada and the Ministère de l'Education du Québec (F.C.A.C.)     

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO₃ ^–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO₃ ^– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO₃ ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO₃ ^-. At the regional scale, we observed high variability in the magnitude of NO₃ ^- losses: while six of the surveyed watersheds showed accelerated rates of NO₃ ^– loss, three did not. Moreover, in contrast to the strong linear relationship between NO₃ ^– loss and crown damage within HBEF watersheds [r ²: (W1 = 0.91, W6 = 0.85)], stream water NO₃ ^– concentrations were weakly related to crown damage (r ² = 0.17) across our regional sites. The efflux of NO₃ ^– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO₃ ^– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA.     

Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest

Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha⁻¹ as wollastonite (CaSiO₃) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO₂, N₂O, CH₄) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha⁻¹ decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N₂ and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient cations in soil.     

Buffering an Acidic Stream in New Hampshire with a Silicate Mineral

Ground and pelletized Wollastonite (Wo; CaSiO₃) was added to a 50‐m reach of an anthropogenically acidified stream within the Hubbard Brook Experimental Forest, New Hampshire, to evaluate its buffering and restoration potential. The Wo was highly effective in raising the pH, acid‐neutralizing capacity (ANC), dissolved inorganic carbon (DIC), and Ca²⁺ concentrations of the stream water, but during the short duration of the experiment had no discernable effect on the stream biota. After initial, spike‐like fluctuations in pH and concentrations of ANC, DIC, and Ca²⁺, the relatively slow dissolution rates of the Wo dampened extreme concentrations and contributed to relatively long‐lasting (4 months) amelioration of streamwater acidity. Changes in concentrations of Ca²⁺, dissolved Si, ANC, and DIC were inversely related to streamflow. After several high, stream‐discharge events, concentrations quickly and consistently returned to pre‐event conditions.     

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.     

湖南宏门冲溪鱼类多样性研究初报

2002年9～10月,我院生物系对湖南省通道县国家阔叶林采种基地宏门冲溪鱼类多样性进行了初步研究,共采集到标本鱼10种,分属于2目5科9属。通过对其多样性分析,结果表明：宏门冲鱼类多样性复杂,群落结构稳定。     

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.     

Morphometric and meristic characteristics of two groups ofLethenteron reissneri

Morphometric and meristic characteristics were compared between two genetically divergent groups ofLethenteron reissneri, collected from Hokkaido and Honshu islands, Japan. Whereas significant differences were found in some morphometric and meristic characters, the ranges overlapped somewhat in all of the characters examined between the two groups. Thus, it was difficult to distinguish exactly between the two groups using only the morphological characters. In sympatric populations of the two groups, no significant differences were found in almost all of the morphometric and meristic characters examined. The two groups ofL. reissneri, which are reproductively isolated from each other, should be regarded as sibling species. Assortative mating, based on morphological features, appears unlikely as the reproductive isolating mechanism between the two groups.