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The lower hybrid current drive (LHCD) system on Alcator C-Mod is capable of sustaining fully non-inductive discharges for multiple current relaxation times (τcr ∼ 200 ms) at line averaged densities in the range of 5x1019 m-3. Some of these non-inductive discharges develop unstable MHD modes that can greatly reduce current drive performance, particularly in discharges with plasma current of 0.5 MA or less [1,2]. Avoiding these unstable MHD modes motivated an experiment to test if the stable current profile shape of a higher current non-inductive discharge could be achieved in a lower current discharge. Starting from a discharge at 0.8 MA, the plasma current was ramped down to 0.5 MA over 200 ms. The surface voltage of the plasma swings negative during the ramp, with the loop voltage reversal impacting the edge fast electron measurements immediately. Little change can be seen during the Ip ramp in the core fast electron measurements, indicating that the loop voltage reversal does not penetrate fully to the magnetic axis on the timescale of the current ramp. The resulting discharge did not exhibit deleterious MHD instabilities, however the existence of this one discharge does not necessarily represent a robust solution to the problem.

(1) Modes of recruitment of herbs and shrubs, and the extent to which availability of propagules affected distribution of the various species after disturbance were examined in a field experiment of canopy removal at the Hubbard Brook Experimental Forest, New Hampshire, U.S.A. (2) The most common shrubs after canopy removal were Rubus idaeus and Viburnum alnifolium; the most common herbs were Aster acuminatus, Dennstaedtia punctilobula and Dryopteris spinulosa. All of the first-year recruitment of Rubus occurred through germination of buried seeds. In contrast, first-year recruitment of the other four species occurred through survival of pre-existing stems or expansion of pre-existing patches, or both. Recruitment of Viburnum, Dennstaedtia and Dryopteris during the second and third years of regrowth continued to be strictly vegetative, whilst recruitment of Aster occurred through newly dispersed seeds as well as rhizomatous growth. (3) The spatial distribution of herbs and shrubs after overstorey removal was strongly related to the location of predecessors before removal. During the first year of regrowth, the locations of almost all stems of all species except Rubus were associated with presence of pre-disturbance individuals on that site. (4) Collections from seed traps indicate that seed production in the site with overstorey removal was higher than in the surrounding, intact forest for most species. Some unoccupied sites were colonized by these seeds during the second and third years of regrowth. (5) The overall distribution of woodland species such as Viburnum alnifolium, Dryopteris spinulosa and Lycopodium lucidulum was unaffected by overstorey removal. After three years of regrowth, the spatial distribution of these species was entirely restricted to locations where predecessors grew in the pre-disturbance forest. (6) Life-history traits of herbs and shrubs at the Hubbard Brook Experimental Forest closely corresponded to Grime's c̀ompetitive', s̀tress-tolerant' and r̀uderal' strategies. It appears that ruderals obscured but did not affect spatial patterns of stress-tolerators and competitors that were present in the forest before overstorey removal. This suggests that, in the absence of physical destruction to existing herbs and shrubs in the forest, d̀isturbance' has, in the short term at least, little effect on spatial patterns of some understorey species.

The complexity of natural ecosystems makes it difficult to compare the relative importance of abiotic and biotic factors and to assess the effects of their interactions on ecosystem development. To improve our understanding of ecosystem complexity, we initiated an experiment designed to quantify the main effects and interactions of several factors that are thought to affect nutrient export from developing forest ecosystems. Using a replicated 2 × 2 × 4 factorial experiment, we quantified the main effects of these factors and the factor interactions on annual calcium, magnesium, and potassium export from field mesocosms over 4 years for two Vermont locations, two soils, and four different tree seedling communities. We found that the main effects explained 56%-97% of total variation in nutrient export. Abiotic factors (location and soil) accounted for a greater percentage of the total variation in nutrient export (47%-94%) than the biotic factor (plant community) (2%-15%). However, biotic control over nutrient export was significant, even when biomass was minimal. Factor interactions were often significant, but they explained less of the variation in nutrient export (1%-33%) than the main effects. Year-to-year fluctuations influenced the relative importance of the main effects in determining nutrient export and created factor interactions between most of the explanatory variables. Our study suggests that when research is focused on typically used main effects, such as location and soil, and interactions are aggregated into overall error terms, important information about the factors controlling ecosystem processes can be lost.

A survey of cubicles and indoor and outdoor walking surfaces on 37 farms served by four veterinary practices in Somerset, Cheshire, Wirral and west Wales was carried out in 1989 to 1991. A study of the space requirements of Friesian/Holstein cows at pasture showed that they required approximately 240 cm x 120 cm lying space and a further 60 cm lunging space for rising. By these standards, 87 per cent of the cubicles were too short and 50 per cent were too wide or too narrow. Over 1500 observations on cows lying down, rising and standing indicated that only 12 per cent of the cubicles permitted real freedom of movement; 91 per cent of top partition rails were judged to be too low and 70 per cent of bottom rails too low or too high. In addition, the kerb was very high in 76 per cent of the cubicles. As a result, 10 per cent of cows appeared moderately or severely restricted when lying down, 33 per cent when rising and 55 per cent when standing. Over 2000 cubicle beds were also studied; 75 per cent had a concrete base and of those, 63 per cent were judged to have too little bedding and 11 per cent next to none. Higher incidences and prevalences of lameness were associated with limited borrowing space (P<0.01) low bottom rails (P<0.05), high kerbs (P<0.05) and inadequate bedding (P<0.01). Of 3190 walking surfaces, only 25 per cent were classified as satisfactory in the first winter and 34 per cent in the second. In general, surfaces in silage bays were too rough and those in other sites were too smooth. The farms with the smoothest indoor walking surfaces had a significantly higher incidence of lameness (P<0.01). Of 3335 outdoor walking surfaces only 25 per cent were classified as satisfactory, and 70 per cent were too rough. The incidence of lameness was not significantly related to these findings.

Retention of riparian buffers is a common management practice used to protect streams from the effects of upslope forest harvest. We compared bird use of riparian buffers along main stem rivers, tributary streams, and reference riparian zones having intact, upslope forests. Community composition differed considerably between buffers and references, and also between main stems and tributaries. Density of the more common species (those detected >20 times) was significantly higher along main stems than along tributaries. Four species (bay-breasted warbler, black-throated green warbler, blue jay, Cape May warbler) were more abundant along main stems than along tributaries; no species was more abundant along tributaries. The overall density of less common species was significantly higher in buffer strips than in reference sites, but four of the more common species (bay-breasted, blackburnian, black-throated green, and Cape May warblers) were more abundant in reference sites than in buffer strips. We did not detect differences in species diversity or richness among the different site types, but edge-species were significantly more common in buffer strips than in reference sites. Interior-species, in contrast, were significantly more common in reference sites.

1. Patterns of fine root biomass, morphology, growth and longevity were examined in the northern hardwood zone of Hubbard Brook Experimental Forest to aid understanding of the role of roots in ecosystem function. 2. Fine root biomass in the mature hardwood forest was 471 g m-2 for < 2 mm roots in June 1987 and was concentrated in the surface soil, with 43% in the forest floor horizons. After clearcutting, fine root biomass accumulated rapidly in the regrowing forest, reaching 71% of that in the mature forest after only four years of recovery. 3. Fine root diameter distributions and specific root length (SRL; length/mass) differed among species. SRL was higher in the forest floor than mineral soil horizons. and decreased with increasing root diameter. 4. Fine root production in the mature forest, measured with in-growth cores, averaged 254 g m-2 year-1, but this method probably underestimated production. Rapid disappearance of fine roots was observed for roots growing through in situ screens, and these ephemeral roots are difficult to quantify. 5. The initiation of fine root growth in the forest floor was coincident with leaf expansion in the forest canopy; root growth in the mineral soil began 1-2 weeks later. Root growth was most rapid in early summer (mid-June to early July), and the lifespan of these early season roots averaged about 8-10 months across three years of study. This estimate of longevity was consistent with that obtained from the ratio of fine root biomass to production, after correcting the production value for the observed root disappearance from in situ screens (about 50% of fine roots disappeared from screens within an annual cycle). These longevity estimates also appeared to be consistent with an analysis of the soil C budget based upon soil and fine root respiration and total root allocation. Fine root production was apparently nearly twice as high as leaf production in this ecosystem. 6. These fine root production and turnover estimates are not consistent with results from previous studies of fine root decomposition, and we suggest that fine root decay has been underestimated because existing methods inhibit the saprotrophic activity of rhizosphere organisms.

Growth of spring ephemerals in northern forests is limited to early spring when competition for resources from other species is at a minimum. The abundance of resources during this vernal period suggests that spring ephemerals might grow continuously over a wide range of sites, but distributions tend to be patchy. I hypothesized that co-occurring plants that grow later in the season compete for resources to a limited extent, but that competition from these other species is sufficient to restrict the spread of spring ephemerals into unoccupied sites. Population dynamics of Erythronium americanum were compared on sites at the Hubbard Brook Experimental Forest in New Hampshire subjected to varying removals of co-occurring plants. During the 3-year period after removal of only overstory trees the density and frequency of occurrence of Erythronium increased by 225% and 180%, respectively, and removal of all co-occurring vegetation after 1 year's regrowth had an even more pronounced effect (400% and 195% increases, respectively). The abundance of Erythronium in the undisturbed forest did not change over the 3-year period. Elevational distribution of Erythronium also was directly related to the extent of removal of summer plants. Recruitment was mostly vegetative, but flower production on the tree removal site increased by a factor of six, and some distant sites apparently were colonized by seedlings. For several years following large-scale disturbances that eliminate co-occurring vegetation, the vernal growing season is lengthened and resources are more available, and spring ephemerals such as Erythronium americanum exploit these resource-rich opportunities to expand populations and colonize new sites. It appears that, as forest succession proceeds, the size and survival of newly established colonies are gradually constrained by competition from co-occurring species that grow later in the season

A method of program design is described which leads naturally to the expression of a program as a pipeline network of simple processes. Starting from the problem statement the valid inputs and outputs are specified by grammars, which can be combined to define the requisite translation. A notation for translation grammars is described informally which allows a translation to take into account semantic as well as syntactic information. Terminal symbols may be attributed by data types and may be qualified by Boolean expressions. The notation is capable of direct compilation but in this paper we show how it may be used to derive a program in a conventional high level language such as Pascal or Cobol. It is shown that more complex problems can be solved by simple pipeline structures of simple translations. Provided that nonbacktracking grammars are used to specify translations, the pipeline structure is well-suited to concurrent execution on a multiprocessor. The method is illustrated by examples from data processing.