Gaps in the canopy: the missing dimension in vegetation dynamics

Canopy gaps are important as entry points for new genotypes and new species into many types of vegetation, yet little is known about them in any type of vegetation but forests. Forest gaps are too large for manipulative experiments to be readily undertaken, and hitherto grassland gaps have been too small to be easily mapped. Preliminary results from mapping small (>1 cm) grassland gaps with a new fibre-optic device suggest that experiments need to be performed at a smaller physical scale than has hitherto been achieved.     

Seasonal carbon isotope discrimination in a grassland community

Summary Grassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture.     

Glucocorticoid effects on zinc transport into colostrum and milk of lactating cows

Colostrum Zn concentrations were measured in eight randomly selected Holstein dairy cows. Overall mean Zn concentrations were highest within 12 h postpartum (257 +/- 14 microM, mean +/- SEM), fell to 141 +/- 8 microM by 24 h, and then declined at a linear rate of 30 microM/d during the following 48 h. Zn concentrations at 3 d (82 +/- 5 microM) were not different from 150-d milk samples (72 +/- microM). In a second experiment, 32 early-gestation cows were blocked by stage of lactation into four groups in a randomized block design and injected with 0, 15, 30, or 45 mg of dexamethasone. Milk and blood samples were collected at 0, 12, and 24 h after injection and analyzed for Zn, and for fat, protein, and lactose in milk. Cows administered 0 and 15 mg of dexamethasone showed no difference in milk Zn concentrations compared to pretreatment measurements; however, milk Zn concentrations in cows administered 30- and 45-mg doses increased significantly. Plasma cortisol decreased in the dexamethasone-treated cows. Plasma Zn and milk fat, protein, and lactose did not change. These data indicate that glucocorticoids can mediate Zn uptake and transport by the mammary glands of lactating cows and suggest that the high Zn concentration in colostrum could be a result of the preparturient surge of cortisol.     

Binding protein dependent transport of C4-dicarboxylates in Rhodobacter capsulatus

The characteristics of malate transport into aerobically grown cells of the purple photosynthetic bacterium Rhodobacter capsulatus were determined. A single transport system was distinguished kinetically which displayed a K_t value of 2.9 ± 1.2 μM and V_max of 43 ± 6 nmol · min^-1 · mg^-1 protein. Competition experiments indicated that the metabolically related C4-dicarboxylates succinate and fumarate are also transported by this system. Malate uptake was sensitive to osmotic shock and evidence from the binding of radiolabelled malate and succinate to periplasmic protein fractions indicated that transport is mediated by a dicarboxylate binding protein. The activity of the transport system was studied as a function of external and internal pH and it was found that a marked activation of uptake occurred at intracellular pH values greater than 7. The use of a high affinity binding protein dependent system to transport a major carbon and energy source suggests that Rhodobacter capsulatus would be capable of obtaining growth sustaining quantities of C4-dicarboxylates even if these were present at very low concentrations in the environment.     

island biogeography of Day Geckos (Phelsuma) in the Indian Ocean

Summary Step-wise multiple regression was employed to probe the determinants of species diversity of day geckos (Phelsuma) in the Indian Ocean. Independent variables were area, elevation, and two measures of isolation. Distance from Madagascar and island height (an indicator of habitat diversity) were the two most important predictors of species richness. Similar studies on other taxa rarely find isolation to be a major factor. The relatively poor dispersal abilities of reptiles may explain why isolation, rather than attributes of the islands, are more important in this case. The regressions also indicate that habitat diversity (assumed to correlate with maximum island elevation) is more important than area per se in determining species diversity. These results agree with predictions of the equilibrium theory of island biogeography, but historical processes have also greatly influenced species richness.     

Spatial variation in larval concentrations as a cause of spatial variation in settlement for the barnacle,Balanus glandula

Summary Settlement rates of the high intertidal barnacle, Balanus glandula, were monitored at three sites in the rocky intertidal zone in Central California simultaneously with measurements of larval concentrations in the adjacent water column. In both 1983 and 1984, settlement rates onto vacant substrate differed among the sites by nearly two orders of magnitude. For all sampling dates, this spatial variation in settlement mirrored the spatial distribution of Balanus glandula cyprid concentration in the water column. A perfect rank correlation was found between cyprid concentrations near a site and subsequent settlement. A noteworthy observation was that the sites switched rank in their settlement rates from 1983 to 1984. This change in settlement rankings matched a switch in rankings for cyprid concentrations.Settlement itself appears to be an important cause of the spatial pattern of cyprid concentrations. Comparing the rates of settlement to estimates of the number of cyprids available at a site suggests that settlement causes a large drain on the cyprid population as a water mass passes over successive sites. No consistent spatial patterns were found in the distribution of other major plankton groups (calanoid copepods) that are similar in size to Balanus cyprids but do not settle.The large differences in settlement rates among these sites were previously shown to be a leading cause of large differences in the structure of benthic barnacle populations. The close correspondence shown here between these large differences in settlement and differences in larval concentrations suggests that nearshore oceanic processes affecting larval arrival contribute to the control of benthic community structure.     

Enhanced enzymatic hydrolysis of lignocellulosic biomass pretreated by low-pressure steam autohydrolysis

Summary The use of low-pressure steam autohydrolysis in the pretreatment of corn stover and hybrid poplar has been assessed. In terms of yield of prehydrolyzed solids, minimal by-product formation and extent of subsequent enzymatic saccharification, the results of low-pressure steam pretreatment were found to be as good as or better than those reported for more severe pretreatment processes. Almost complete saccharification of the cellulose in the prehydrolyzed biomass solids was obtained within 24h with a commercial cellulase preparation — Celluclast. The presence of grinding elements (glass beads) during the enzymatic hydrolysis was found to increase the extent of saccharification by 40% to 50% over controls without any grinding elements.     

Carbon content of the neritic scyphomedusa Chrysaora fuscescens

An analysis of the role of scyphomedusae in a planktonic ecosystemrequires that biomass or numerical abundance estimates be convertibleinto standard units for comparison with other components ofthe planktonic community. One species under investigation isthe brown sea nettle, Chrysaora fuscescens, which is very abundantin coastal waters of the west coast of North America (Shenker,1984). For this species, the carbon content of whole immatureanimals and the body components of sexually mature medusae weredetermined. Immature medusae contained a mean carbon content0.202% of the wet weight. The bell, oral arms and gonadal tissueof mature medusae had mean carbon levels 0.156, 0.554 and 0.576%of the wet weight, respectively. When the relative proportionsof these body tissues were calculated, the mean carbon contentof whole mature medusae was determined to be 0.280% of the wetweight.