Cent Cover Research Articles

Influence of vegetation cover on sand transport by wind: field studies at Owens Lake, California

Field studies conducted at Owens Lake, California, provide direct measurements of sand flux on sand sheets with zero to 20 per cent cover of salt grass. Results from 12 different sand transport events show that aerodynamic roughness length and threshold wind shear velocity increase with vegetation cover as measured by vertically projected cover and roughness density (λ). This results in a negative exponential decrease in sediment flux with increasing vegetation cover such that sand transport is effectively eliminated when the vertically projected cover of salt grass is greater than 15 per cent. A general empirical model for the relation between sand flux and vegetation cover has been derived and can be used to predict the amount of vegetation required to stabilize sand dune areas. © 1998 John Wiley & Sons, Ltd.

Stochastic estimation of plant-available soil water under fluctuating water table depths

Preservation of native valley-floor phreatophytes while pumping groundwater for export from Owens Valley, California, requires reliable predictions of plant water use. These predictions are compared with stored soil water within well field regions and serve as a basis for managing groundwater resources. Soil water measurement errors, variable recharge, unpredictable climatic conditions affecting plant water use, and modeling errors make soil water predictions uncertain and error-prone. We developed and tested a scheme based on soil water balance coupled with implementation of Kalman filtering (KF) for (1) providing physically based soil water storage predictions with prediction errors projected from the statistics of the various inputs, and (2) reducing the overall uncertainty in both estimates and predictions. The proposed KF-based scheme was tested using experimental data collected at a location on the Owens Valley floor where the water table was artificially lowered by groundwater pumping and later allowed to recover. Vegetation composition and per cent cover, climatic data, and soil water information were collected and used for developing a soil water balance. Predictions and updates of soil water storage under different types of vegetation were obtained for a period of 5 years. The main results show that: (1) the proposed predictive model provides reliable and resilient soil water estimates under a wide range of external conditions; (2) the predicted soil water storage and the error bounds provided by the model offer a realistic and rational basis for decisions such as when to curtail well field operation to ensure plant survival. The predictive model offers a practical means for accommodating simple aspects of spatial variability by considering the additional source of uncertainty as part of modeling or measurement uncertainty.

Percentage vegetation cover of a degrading rangeland from SPOT

Abstract Multispectral SPOT imagery is used to map percentage vegetation cover in the Mackenzie Basin of the South Island, New Zealand. Twenty ground measurements of quadrats 60 by 60 m2 relate percentage vegetation cover to normalized vegetation index. The non-linear calibration curve is C = 50 tanh (6·1(NV1-0·22))+ 50 (1) where C is percentage vegetation cover and NVI is normalized vegetation index. The vertical spread of quadrat points about the calibration curve gives the accuracy of a percentage cover prediction from NVI. In the linear part of the calibration curve the upper and lower prediction limits (80 per cent confidence interval) are plus or minus 15 per cent cover.

Ground and aircraft infrared observations over a partially-vegetated area

Abstract Thermometric observations over a row crop (cotton, Gossypium hirsutum L.) with 20 per cent cover using hand-held radiometers were made during several clear days near Maricopa, a town in south central Arizona. A ground sampling routine developed for estimating a composite temperature for the cotton field compared favourably with surface temperatures taken for one day from an aircraft flying at an altitude of approximately 150 m. Both ground and airborne radiometric measurements were from nadir angle traversing the row crop. It was found that by considering an equation for the radiative balance at the surface, a composite surface temperature could be calculated which was within ± 1.5°C of the value given by the labour-intensive ground sampling method or the aircraft. The data requirements were temperatures of sunlit and shaded soil and the sunlit canopy temperature and their respective fractional areas. The fractional areas of the sunlit and shaded soil as a function of time and the vegetative cover were estimated with extensive agronomic measurements and nadir photographs. A model similar to that of Kimes (1983) requiring only information on row orientation and the height and width of the vegetation produced estimates of the fractional areas close to the labour-intensive ground measurements. Therefore, it is possible to obtain estimates of the composite surface temperature comparable to low-flying aircraft observations over row crops with sparse cover without having to devise a labour-intensive ground-sampling routine and extensive agronomic measurements and photographs to determine fractional areas of the composite scene. However, there is some uncertainty as to the number of thermometric observations needed to obtain reliable estimates of sunlit and shaded soil and sunlit canopy temperatures.

Response of Three Mangroves to Salinity in Two Geoforms

1 The variation in the physiognomy of the mangroves Avicennia germinans L., Rhizophora mangle L. and Laguncularia racemosa (L.) Gaertn., as related to salinity in two different geomorphic habitats (mudflats and interdistributary basins) was investigated. 2 Due to the high intercorrelation between variables, principal component analysis was used to examine data on (a) water salinity on two sampling dates and in two geoforms and (b) diameter at breast height, height and per cent cover of each of the three species. The first component of each of these four data sets summarized 77, 64, 90 and 93% of the total variance in the intervening variables. 3 The projections of the intervening variables on their respective principal component was calculated, and response curves of species against environmental component scores were fitted by generalized linear models. The resulting curves were bell-shaped, except for A. germinans. 4 The highest diversity and the maximum height and diameter for the three species is at low salinities and in interdistributary basins but the cover of A. germinans is higher in the mudflat and under high salinities. The range of response to environmental change is wider in A. germinans. 5 There was a negative association between the cover of A. germinans and that of the other two species, and the slope of the regression line suggests a substitution of one unit of cover of A. germinans by one unit of cover of any of the other two species in the interdistributary basins as salinity decreases. 6 The results suggest that while abiotic environmental conditions may account for the absence of L. racemosa and R. mangle in the mudflats, biological interactions are more important in the formation of the mixed forests in the interdistribu-

Succession after Elimination of Buried Seeds from a Recently Plowed Field

MARKS, P. L. AND C. L. MOHLER (Ecol. & Syst., Cornell University, Ithaca, NY 14853). Succession after elimination of buried seeds from a recently plowed field. Bull. Torrey Bot. Club 112: 376-382. 1985.-The buried seed pool was eliminated from selected plots in a recently plowed goldenrod field by steam sterilization in order to determine the importance of stored seed in the early stages of old field succession. Recolonization by plants was slow despite the close proximity of abundant seed sources in the surrounding goldenrod field. Midway through the second growing season the mean per cent cover of plants in the sterilized plots was 12% (?6 SD) compared to 121% (?10) in the control plots. The results underscore the importance of both the buried seed pool and the population of perennial plants that may survive tillage and herbicide spraying in determining the dominant plants early in old field successions.

The vegetation of the Balsam Mountains of southwest Virginia: A phytosociological study

RHEINHARDT, R. D. AND S. A. WARE (Dept. Biol., College of William and Mary, Williamsburg, VA 23185). The Vegetation of the Balsam Mountains of southwest Virginia: a phytosociological study. Bull. Torrey Bot. Club 111: 287-300-1984. A quantitative phytosociological survey was made of the woody and herbaceous vegetation of theBalsam Mountains of southwest Virginia. Sixty-five forest stands were sampled between 1125 and 1547m in elevation. Density measurements for understory vegetation and per cent cover for herbaceous species were measured in 20 of the 65 stands. Stands were ordinated using detrended correspondence analysis (DCA), and divided into nine community-types: spruce, spruce-fir, northern hardwoods (beech-maple and dwarf beech subtypes), yellow birch (ravine and boulder field subtypes), mesophytic, mesophytic-oak, and northern red oak. A correlation analysis was performed to test for significant correlations between community composition and environmental parameters. Canopy species importance values tended to correlate predictably with most environmental parameters (particularly with elevation and moisture, and associated parameters such as pH). Herbaceous vegetation commonly associated with mesic conditions tended to show a strong correlation with magnesium and calcium. The first ordination axis of both the canopy and herbaceous vegetation correlate with an elevation-moisture gradient. Axis 2 of the canopy ordination, which correlates with calcium levels and herbaceous species diversity, separates mesophytic from mesophytic-oak communities. Axis 2 of the herbaceous ordination, which correlates with magnesium levels and organic matter percentage, separates mesophytic, mesophytic-oak, and northern red oak communities. One fairly extensive community-type, located on open north slopes, resembles the high elevation cove forests of the Great Smokies. In fact, the vegetation of all topographic types was found to be much more mesic than forests of similar topography elsewhere in the southern Appalachians. Slopes below 1500 m in elevation may have been dominated by spruce prior to the large scale logging operations of the early 1900's. Perhaps the disturbance of former boreal forests has enabled mesic hardwood forests to exist at these higher elevations where moisture is abundant.

The Vegetation of the Balsam Mountains of Southwest Virginia: A Phytosociological Study

RHEINHARDT, R. D. AND S. A. WARE (Dept. Biol., College of William and Mary, Williamsburg, VA 23185). The Vegetation of the Balsam Mountains of southwest Virginia: a phytosociological study. Bull. Torrey Bot. Club 111: 287-300-1984. A quantitative phytosociological survey was made of the woody and herbaceous vegetation of theBalsam Mountains of southwest Virginia. Sixty-five forest stands were sampled between 1125 and 1547m in elevation. Density measurements for understory vegetation and per cent cover for herbaceous species were measured in 20 of the 65 stands. Stands were ordinated using detrended correspondence analysis (DCA), and divided into nine community-types: spruce, spruce-fir, northern hardwoods (beech-maple and dwarf beech subtypes), yellow birch (ravine and boulder field subtypes), mesophytic, mesophytic-oak, and northern red oak. A correlation analysis was performed to test for significant correlations between community composition and environmental parameters. Canopy species importance values tended to correlate predictably with most environmental parameters (particularly with elevation and moisture, and associated parameters such as pH). Herbaceous vegetation commonly associated with mesic conditions tended to show a strong correlation with magnesium and calcium. The first ordination axis of both the canopy and herbaceous vegetation correlate with an elevation-moisture gradient. Axis 2 of the canopy ordination, which correlates with calcium levels and herbaceous species diversity, separates mesophytic from mesophytic-oak communities. Axis 2 of the herbaceous ordination, which correlates with magnesium levels and organic matter percentage, separates mesophytic, mesophytic-oak, and northern red oak communities. One fairly extensive community-type, located on open north slopes, resembles the high elevation cove forests of the Great Smokies. In fact, the vegetation of all topographic types was found to be much more mesic than forests of similar topography elsewhere in the southern Appalachians. Slopes below 1500 m in elevation may have been dominated by spruce prior to the large scale logging operations of the early 1900's. Perhaps the disturbance of former boreal forests has enabled mesic hardwood forests to exist at these higher elevations where moisture is abundant.

The effects of tidal height, wave-exposure, seasonality and rock-pools on grazing and the distribution of intertidal macroalgae in New South Wales

The effects of grazers (mostly gastropods), height on the shore, wave-exposure, season of the year and the presence of shallow rock-pools on the abundance of intertidal macroalgae were examined on shores at Cape Banks (Botany Bay). At the beginning of each of the four seasons, experimental plots with and without grazers were cleared at four heights on three shores, of increasing exposure to waves, The colonization and growth of algae in these plots were monitored (by measurement of per cent cover and dry wt) for approximately the next 3 months in each season. In grazed plots, foliose algae only grew at the lowest levels on the shores. They were more abundant where wave-action was greater, and during the cooler periods of the year, when growth of the plants was enhanced. Higher on the shore, there was a positive correlation between algal cover in grazed plots and the amount of rainfall during the previous 10 days. In all seasons, there was much greater colonization of foliose algae where grazers were excluded. There was greater algal growth at lower levels on the shore, and where wave-action was stronger. Less algae grew in sheltered areas during warmer times of the year. The major seasonal difference found was the more rapid growth and occupancy of the rock by algae during the cooler seasons of the year. Experimental rock-pools were colonized more rapidly at lower levels on the shore, and during the winter. There was no difference between pools and control (non-pool) areas during winter. During summer, however, there was a greater per cent cover and biomass of algae in pools from which grazers had been excluded than in similar control areas. The results can be interpreted as being due to the greater survival and more rapid growth of algae under conditions of increased moisture, decreased emersion and decreased temperatures and light regimes during low tide (i.e. when physical stresses were reduced). These physical factors were, however, less important to the distribution of the algae than were the effects of grazers. Although few algae were present in any experimental plot, the number of species of algae per plot was considerably reduced in grazed areas. The results confirm that the patterns of vertical distribution and abundance of algae on rock-platforms in New South Wales are primarily the result of the activities of grazers. All algae in the present study were capable of living higher on the shore than they were normally found. Much of the variation along a gradient of wave-exposure, from season to season, and small-scale variation from place to place at any time can be explained by the complex interactions between the activities of grazers, and the effects of variations in the physical environment that influence the recruitment, survival and growth of the algae.

Microhabitat factors affecting abundance and diversity of Conus on coral reefs.

Frequency and per cent cover of particular substratum types are shown to be important factors influencing abundance and diversity of mobile, predatory benthic invertebrates on spatially complex coral reef platforms. The most favorable portions of reefs for the gastropod Conus have <20% cover of algal-bound sand and <20% cover of living coral. The former microhabitat provides diurnal shelter for smaller Conus, harbors dense prey populations, and is typically interspersed with sand- and rubble-filled depressions in which many of the gastropods shelter during the day. In Micronesia and Australian Great Barrier Reef censuses, 39 1x10-m transects on favorable habitat supported an average of 7 Conus of 3 species. In striking contrast, portions of reef with >20% living coral are most unfavorable for Conus. Here density averaged 0.3 individual/10 m2, in 17 transects. Living coral harbors few suitable prey organisms, and contact with it elicits a strong avoidance response by Conus. Comparison of subtidal reef and intertidal bench habitats indicates that species diversity and population density are inversely related, but within both habitat types these attributes of Conus assemblages are positively associated.

The distribution of epifauna on Ecklonia radiata (C. Agardh) J. Agardh and the effect of disturbance

The epifaunal community of the kelp, Ecklonia radiata (C. Agardh) J. Agardh, was investigated at Portsea, Australia. The per cent cover of epifauna on the primary blade of Ecklonia was significantly greater on plants from the inner and outer pilings than on those from low on the pilings, under the pier and from an adjacent reef. Species diversity, but not the per cent cover of epifauna, was significantly greater on crinkled than on smooth plants. Various factors of disturbance were measured, including lashing by other plants, which increased with the density of plants. Other abrasive forces were found to increase closer to the bottom, as did the density of gastropods on the fronds. For each site, the number of disturbance factors present was summed giving a disturbance index (D), which was negatively correlated with per cent cover. Transplants and laboratory experiments indicated that three of these disturbance factors reduced the abundance of hydroids, but that increasing the amount of plant lashing had no effect on Tricellaria porteri MacGill. Seven species were chosen for further analysis. The abundance of most species correlated with either plant length or bushiness, although the variation accounted for had a range of 1–43%. For all species, the percentage of zones which were colonized on the Ecklonia blade increased distally, following the age gradient. While the abundance of some species also increased distally, that of hydroids decreased. Membranipora membranacea L. was found to smother the hydroids on 60% of their encounters but neither the distribution nor the abundance of hydroids was altered in the absence of Membranipora. It appears that disturbance rather than competition is the major modifying influence of this community.

Une methode simple et rapide pour decrire les habitats d'oiseaux: le stratiscope

The described permits a rapid quantification of avian habitats. The stratiscope is an optical device which divides the biological space into horizontal planes (layers of vegetation) and vertical planes for each layer by the appropriate position of a diaphragm. Several structural parameters can be measured, i.e. number of layers, per cent cover, and horizontal and vertical structural diversity. The use of the stratiscope is exemplified along a gradient of increasing vegetational complexity, where it allows drawing of foliage profiles. Simultaneously, the structure of the breeding bird communities were determined by census in each habitat, in order to measure the richness S of the communities, their overall density D and the information-theoretical diversity H' It is shown that these basic parameters of bird communities are significantly correlated with those of habitat structure: 5 out of 15 coefficients of correlation are significant at the 1 % level, and 8 at the 5% level. The main importance of this technique is that it is standardized and leads to a general classification of habitats regardless of, e.g., species composition.

Effects of chronic gamma radiation on the composition of the herb community of an oak-pine forest

A complete inventory of the herb populations of the inner 50 m of the Brookhaven irradiated forest was made in 1968 and 1973. The data are compared with those of earlier inventories, giving information on changes through a ten-year period in a community exposed to chronic radiation stress. Per cent cover of Carex pensyluanica, which dominates the zone 20–40 m from the source (125-25 R/day in 1973), increased rapidly at first and then more slowly through 1968, then declined somewhat by 1973. There was no shift in its distribution over ten years. Sharp increase in Rubus spp. cover in this zone contributed to the most recent Carex decline. A greater than eight-fold rise in total density of species other than Carex occurred since 1968, mostly due to a large increase in density of important species, while only those of infrequent occurrence declined or disappeared by 1973. Rate of change in herb populations was slowed by high exposures close to the source and by competition with Carex and Rubus in the sedge zone. The distribution of Rumex acetosella changed in a manner suggesting possible genetic alteration of radiosensitivity. There was a net increase of four species between 1968 and 1973. Species diversity increased with distance from the source, increased with time from 1968 to 1973 in the outer 40–50 m zone, and changed relatively slowly with time in the inner zones of higher exposure rates. The reasons for the comparatively slow rates of successional change are discussed. We predict that with future reduction in radiation stress due to source decay and with continuing expansion of Rubus spp. clones, Carex dominance will continue to decline as species diversity and herb population densities increase slowly.

Quantitative assessment of the rhizoplane microflora by direct microscopy

The bacterial populations on roots of several grassland species were estimated by combining the techniques of direct counting of stained bacteria in situ with standard sampling techniques of plant ecology. The eight plant species examined had bacterial cover of the root surface within the range 4–10 per cent. In a more detailed experiment with two plant species, the method was sufficiently precise to demonstrate that a 7.7 per cent cover of roots of Lolium perenne L. by bacteria was significantly different ( P < 0-05) from a 6.3 per cent cover on Plantago lanceolata L. The length of fungal mycelium per unit root surface area was estimated by a modification of the line intercept method, which was originally developed for measuring root lengths. When hyphal diameters were also measured, fungal cover per cent and volume could be estimated. Estimates of the numbers of rhizoplane bacteria and fungi made by the spread-plate counting method on a non-selective agar showed that the mean number of bacteria from Lolium was greater (but not significantly different) than the counts on Plantago: however. Lolium did have significantly more Gram-negative bacteria as estimated by crystal violet agar than did Plantago. The numbers of bacteria estimated by direct microscopy were about 10-fold greater than estimates by plate counts. For a given period of time, direct microscopy gave a more precise estimate of total bacterial and fungal abundance than did plate counts hut the technique does require continuous concentration by the observer over long periods.

Recolonization of Hermatypic Corals on Submerged Lava Flows in Hawaii

Recovery time for recolonization of coral communities on submerged lava flows in Hawaii may largely depend on exposure to sea and swell. In exposed areas, recovery time (in terms of number of species, per cent cover, and diversity) of areas decimated by lava and perhaps similar disturbances appears to be about 20 yr. At sheltered stations, more than 50 yr may be required for complete recovery. This difference in recovery time is apparently related to differences between undisturbed coral communities at exposed and sheltered stations: in exposed areas succession seems to be constantly interrupted, resulting in pioneer stages, whereas at sheltered stations reefs are more fully developed in terms of percent cover and thickness, and are thus closer to being climax coral communities; they therefore require more time for full recovery. The data, although meager, indicate that diversity increases during succession, but attains a peak value before climax is reached. The apparent decline in diversity as climax is approached may be due to interspecific competition for space, which leads to resource monopolization. Comparison of community structurewith more tropical coral reefs reveals that this pattern may apply only to physically controlled communities.

Dispersion Patterns of Mice in California Annual Grassland

Dispersion patterns of grassland mice were contagious. The frequency of occurrence of Microtus californicus tracks and pooled tracks of Pero-myscus maniculatus and Reithrodontomys megalotis showed positive correlations with the per cent cover of wild oats {Avena). No such correlations existed with other widespread species of grass nor with the total amount of grass cover. The distribution of cuttings suggested that Avena was the major food of Microtus at this time thus implicating food as a major factor in the control of dispersion. Trapping records revealed seasonal changes in the concentrations of mice on different parts of the study plot. The concentration of Microtus was inversely related to the concentration of the other species, which suggested that dispersion was influenced by interspecific interactions.

Precision in Poster Research

Another important move towards precision in poster research has been taken on the initiative of Mills &amp; Rockleys Ltd. In I949 they published a report of a research survey which dealt with the number of people who remembered seeing certain poster campaigns. They have now published the reslalts of a survey into the facts about people who have an "opportunity-to- see" an average poster campaign. A typical fact revealed by the Survey is that in a town of about I00,000 population, I6 locations will give a repetition value of about 20 opportunities-to-see per week, and at least 80 per cent cover. If the number of locations in the campaign is increased to 24, then repetition would rise to 29 and the cover to about 85 per cent.

Discussion of “Flood frequencies and sedimentation from forest watersheds”

The history of flood studies might be said to consist of seeking answers to: “How big can floods get?”, then “How often will floods come?”, and now “What causes floods?” ‐ or design flood studies, flood frequency studies, and flood cause studies. Hydrologists have lately devoted more and more time to the last. Peak discharges and sedimentation from watersheds as related to storm and watershed variables are one part of these flood cause studies.Schiff's discussion [Trans., v . 30, pp. 584–586, 1949] of our report on such a study brings out some important points. In the present primitive state of development of the relationship of peak discharges to meteorological and watershed variables such suggestions are sure to be helpful. He suggests and presents data to show that for two Ohio watersheds the effects on discharges of increasing forest cover varied considerably between two areas, one of .001 and one of .068 sq mi. In the smaller of these (his Watershed 134) he reports significant reductions in peak discharges occurred when forest cover was changed from a density of ten to 95 per cent. This was due to a reduction in surface runoff. Of the larger (his Watershed 172) he makes the statement that, “Had the actual peak discharges been used to establish an equation for Watershed 172, increases in cover would seem to have little influence. Better cover increased permeability rates and consequently infiltration rates so that most of the flow of Watershed 172 is now subsurface flow which reappears above the measuring flume. Peak discharges have not materially reduced and reflect the particular soil and geologic conditions of Watershed 172.” This is an interesting interpretation, and the factors which he talks of are probably operating; however, the results on peak discharges seem to be not at all what he hypothesizes. First, we would expect a reduction in peak discharges when surface runoff is changed to subsurface flow. And second, that such a reduction occurs and is of large magnitude may be shown when Schiff's data are actually “used to establish an equation.] Taking the data from his Table 1 and testing the relation of Q172, the peak discharges in inches depth per hour to the associated per cent cover density (C), with the maximum hourly precipitation (P1) in inches per hour as the meteorological control we find thatequation imageThe regression coefficient for cover density is found to be highly significant and the magnitude of the effect indicated is such that associated with the increase in cover from ten to 95 per cent there was a reduction of 61 per cent in the peak discharges. This is certainly not an insignificant decrease. With more storm data it would be possible to test other meteorological parameters and improve the equation but probably the cover density effect would remain about the same, since no obvious correlation appears between cover density and any of the meteorological characteristics listed in his Table 1.