Showing papers on "Growing season published in 1990"

Effects of soil resources on plant invasion and community structure in californian serpentine grassland

Abstract: Non-native annual grasses dominate most Californian mediterranean-cli- mate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine-derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non-native species in serpentine grassland. Factorial combinations of nitrogen, phos- phorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non-native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders sup- pressed some native forbs. Increased macronutrient availability can increase production on serpentine-derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy-metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant com- munities may be directly influenced by nutrient availability, independent of physical dis- turbance.

Countergradient variation in growth rate: compensation for length of the growing season among Atlantic silversides from different latitudes.

Abstract: How do organisms adapt to the differences in temperature and length of the growing season that occur with latitude? Among Atlantic silversides (Menidia menidia) along the east coast of North America, the length of the first growing season declines by a factor of about 2.5 with increasing latitude. Yet body size at the end of the first growing season does not decline. High-latitude fish must, therefore, grow faster within the growing season than do low-latitude fish. This geographical pattern has a genetic basis. Laboratory experiments on fish from six different locations revealed a latitudinal gradient in the capacity for growth (i.e., maximum growth potential). In two subsequent experiments using fish from Nova Scotia (NS), New York (NY) and South Carolina (SC) that had been separately reared in a common environment for several generations, differences in growth rate among populations were highly significant. The rank order was NS>NY>SC, but the difference among populations depended on temperature. High-latitude fish outperformed those from low latitudes primarily at the high temperatures that low-latitude fish would be expected to experience most often in nature. These results suggest that instead of being adapted for growth at low temperatures, fish from high latitudes are adapted for rapid elevation of growth rate during the brief interval of the year when high temperatures occur. Selection on growth rate results from sizedependent winter mortality: the importance to winter survival of being large increases with latitude but the length of the growing season simultaneously decreases. The end result is countergradient variation in growth rate, a phenomenon that may be much more widespread than currently recognized.

Effects of Different Resource Additions of Species Diversity in an Annual Plant Community

Abstract: A commonly observed phenomenon in plant communities is that the ad- dition of a limiting resource leads to an increase in productivity and a decrease in species diversity. We tested the hypothesis that the mechanism underlying this pattern is a dis- proportionate increase in mortality of smaller or shade-intolerant species in more produc- tive sites caused by reduction of light levels. We added water and/or one of three nutrients (nitrogen, phosphorus, and potassium) to a 1 st-yr old-field community dominated by weedy annuals and measured effects on productivity, species composition, diversity, and light levels after one growing season. Diversity was not clearly related to productivity in this experiment. Watering increased productivity, but, contrary to expectations, had no effect on density of surviving plants, species diversity, or abundance of low-growing species. Almost all the increase in biomass with watering was due to a positive response by Ambrosia artemisiifolia, an upright annual that was the most common species in the canopy in all treatments. The addition of nitrogen had only a small positive effect on productivity, but strongly decreased density of surviving plants, species diversity, and abundance of most low-growing species. Only Ambrosia increased in abundance with nitrogen addition. The phosphorus and potassium additions had little effect on the community. We suggest that the high mortality and low diversity in the nitrogen addition plots, but not in the more productive watered plots, was due to limitation by nitrogen earlier than limitation by water during the growing season. The consequence was earlier canopy closure and greater mortality due to light limitation.

The role of litter in an old-field community: impact of litter quantity in different seasons on plant species richness and abundance.

Abstract: We studied the effect of removing and adding plant litter in different seasons on biomass, density, and species richness in a Solidago dominated old-field community in New Jersey, USA. We removed all the naturally accumulated plant litter in November (658 g/m2) and in May (856 g/m2) and doubled the amount of litter in November and May in replicated plots (1 m2). An equal number of plots were left as controls. Litter removal and addition had little impact on total plant biomass or individual species biomass in the growing season following the manipulations. Litter removal, however, significantly increased plant densities but this varied depending upon the season of litter removal, species, and life history type. Specifically, the fall litter removal had a much greater impact than the spring litter removal suggesting that litter has its greatest impact after plant senescence in the fall and prior to major periods of early plant growth in spring. Annual species showed the greatest response, especially early in the growing season. Both spring and fall litter removal significantly increased species richness throughout the study. Litter additions in both spring and fall reduced both plant densities and species richness in June, but these differences disappeared near the end of the growing season in September. We concluded than in productive communities where litter accumulation may be substantial, litter may promote low species richness and plant density. This explanation does not invoke resource competition for the decline in species richness. Finally, we hypothesize that there may be broad thresholds of litter accumulation in different community types that may act to either increase or decrease plant yield and diversity.

Evolutionary and Ecophysiological Responses of Mountain Plants to the Growing Season Environment

Abstract: Publisher Summary This chapter highlights the evolutionary and ecophysiological responses of mountain plants to the growing season environment. The responses of mountain plants to their environment are due to a complex mixture of genetic and environmental influences. Plants growing on mountains experience reduced temperatures and vapor pressures with altitude, as well as a reduction in the partial pressure of air. There are many morphological, physiological, and biochemical features of plants that change with altitude, such as decrease in stature. Model simulations of canopy energy balance and CO2 fixation indicate that canopy structure and leaf area index (LAI) strongly influence both photosynthetic rate (A) and the ratio of 13C to 12C (δ13C) in leaves. δ13C measurements on expanded leaves provide a time integral of CO2 discrimination during the photosynthetic life of the leaf; they also include some unknown δ13C contribution from photosynthate exported or remobilized from other leaves and organs. The model simulations, for just the period of peak irradiance during the day, indicate that the energy balance and gas exchange of a leaf are dependent on its aerodynamic coupling with other leaves in the plant canopy, and with the air at some reference height above the canopy.

The Relation between Capacity for Growth and Length of Growing Season: Evidence for and Implications of Countergradient Variation

Abstract: Evidence suggests that the capacity for growth (i.e., maximum growth potential) within a species may vary inversely with the length of the growing season across a latitudinal gradient. I evaluated this hypothesis with data on three species—American shad Alosa sapidissima, striped bass Morone saxatilis, and mummichog Fundulus heteroclitus—having wide latitudinal ranges (≈29–46°N) along the east coast of North America. For each of these species, the length of the first growing season decreases by a factor of about 2.5 with increasing latitude within the species' range, yet body size at the end of the first growing season is independent of latitude. Northern fish must, therefore, grow substantially faster within the growing season than do southern fish. This “countergradient variation” in growth rate may be more widespread than has been recognized. A similar latitudinal pattern in growth rate has a genetic basis in the Atlantic silverside Menidia menidia, and data on Atlantic salmon Salmo salar, lar...

Growth and yield responses of rice to carbon dioxide concentration

Abstract: Rice plants (Oryza saliva L., cv. IR30) were grown in paddy culture in outdoor, naturally sunlit, controlled-environment, plant growth chambers at Gainesville, Florida, USA, in 1987. The rice plants were exposed throughout the season to subambient (160 and 250), ambient (330) or superambient (500, 660, 900 μmol CO2/mol air) CO2 concentrations. Total shoot biomass, root biomass, tillering, and final grain yield increased with increasing CO2 concentration, thegreatest increase occurring between the 160 and 500 μmol CO2/mol air treatments. Early in the growing season, root:shoot biomass ratio increased with increasing CO2 concentration; although the ratio decreased during the growing season, net assimilation rate increased with increasingCO2 concentration and decreased during the growing season. Differences in biomass and lamina area among CO2 treatments were largely due to corresponding differences in tillering response. The number of panicles/plant was almost entirely responsible for differences in final grain yield among CO2 treatments. Doubling the CO2 concentration from 330 to 660 μmol CO2/mol air resulted in a 32 % increase in grain yield. These results suggest that important changes in the growth and yield of rice may be expected in the future as the CO2 concentration of the earth's atmosphere continues to rise.

Fire affects ecophysiology and community dynamics of central Wisconsin oak forest regeneration

Abstract: In order to understand better the ecophysiological differences among competing species that might influence competitive interactions after, or in the absence of, fire, we examined the response to fire of four sympatric woody species found in intermediate—sized gaps in a 3—yr—old mixed oak forest in central Wisconsin. Selected blocks in the forest were burned in April 1987 by a low—intensity controlled surface fire. The fire had significant effects during the following growing season on community structure, foliar nutrient concentrations, and photosynthesis. Acer rubrum seedling density declined by 70% following the fire, while percent cover increased several—fold in Rubus allegheniensis. In general, leaf concentrations of N, P, and K were increased by the fire in all species, although the relative enhancement decreased as the growing season progressed. Daily maximum photosynthetic rates were 30—50% higher in burned than unburned sites for Prunus serotina, Quercus ellipsoidalis, and R. allegheniensis, but did not differ between treatments for A. rubrum. Mean sunlit photosynthetic rates and leaf conductances were stimulated by the burn for all species, with the greatest enhancement in photosynthesis measured in Q. ellipsoidalis. Leaf gas exchange in R. allegheniensis was most sensitive to declining leaf water potential and elevated vapor pressure gradient, with Q. ellipsoidalis the least sensitive. Fire had no discernable effect on water status of these plants during a year of relatively high rainfall. In comparison with other species, A. rubrum seedlings responded negatively after fire–both in terms of survival/reproduction (decline in the number of individuals) and relative leaf physiological performance. Fire enhanced the abundance of R. allegheniensis and the potential photosynthetic performance of R. allegheniensis, P. serotina, and particularly Q. ellipsoidalis. We conclude that post—fire stimulation of net photosynthesis and conductance was largely the result of enhanced leaf N concentrations in these species.

Alpine Seedling Establishment: The Influence of Disturbance Type

Abstract: The effects of disturbance type on seedling environment and establishment of alpine species with different physiological and life history traits were examined during a 2-yr study on the Beartooth Plateau in southwestern Montana, USA. We compared soil temperatures, water potentials, and nutrients on mineral soils of a gravel borrow area with those on highly organic soils of a Geum turf area. Seedling emergence, growth, and survival of six seeded species (Geum rossii, Artemisia scopulorum, Potentilla diversifolia, Sibbaldia procumbens, Deschampsia cespitosa, and Festuca idahoensis) and emergence and survival of five unseeded species (Draba crassifolia, Draba incerta, Cerastium arvense, Arenaria rubella, and Androsace septentrionalis) were evaluated on both areas. The effects of N and P nutrient addition and surface organic mulch on the soil environment and seedling es- tablishment were evaluated on the borrow area, while differences between uncleared turf and turf cleared of vegetation were compared on the Geum turf area. Plots cleared of vegetation on the Geum turf area had higher levels of soil N (NO3-) and P than uncleared turf and both higher levels of N (NO3- + NH4+) and P and higher soil temperatures (surface, 5, and 15 cm depths) than fertilized or not-fertilized borrow area treatments. Fertilization increased N and P on borrow area soils, but after 2 yr N had decreased significantly. Soil water potentials (5 and 15 cm depths) did not differ between cleared plots on the Geum turf area or any of the borrow area treatments and were never low enough to cause plant stress. Vegetated Geum turf had significantly lower water po- tentials than cleared plots, especially late in the growing season. Mulch had no effect on soil water potential or nutrients on the borrow area and increased soil temperatures only on clear days during the first growing season. Wind removed or redistributed the mulch over time, thus decreasing potential effects. Seedling emergence was highly dependent on soil surface stabilization and reflected species life history traits. Growth of seedlings was slow, and varied among species and treatments: 0.005-0.04 and 0.02-0.20 g total mass after the first and second growing seasons, respectively. Significantly higher total seedling mass was observed on cleared Geum turf plots than on any of the borrow area treatments, and on fertilized than on not-fertilized plots on the borrow area. Seedling mortality of most species was much lower than previously found for alpine ecosystems, rarely exceeding 50% even after 2 yr. On the borrow area mulch increased survival, probably through microenvironmental amelioration. The nu- trient pulse from fertilization increased mortality of several species, presumably by creating plant nutrient demands in excess of availability during year 2. Both disturbance characteristics and species life history and physiological traits affected seedling establishment. Pretreatment soil properties of the two disturbance types had the greatest effects on soil temperatures and nutrients and, consequently, on seedling growth and survival. Soil surface characteristics had the largest effects on seedling emergence; surface stabilization was essential for holding both soil and seed in place. Single species responses varied in magnitude but were similar on both disturbance types. In general, there were larger differences among species in emergence and growth than in survival. Thus, successful seedling establishment on different alpine disturbance types may depend more

Role of resources and disturbance in the organization of an old-field plant community.

Abstract: The roles of disturbance and resource availability in influencing species richness and plant abundance were examined by conducting a factorial experiment for 2 yr in an old field in New Jersey, USA, dominated by goldenrods (Solidago spp.). Replicate plots were treated by (1) adding macronutrients, (2) adding water, (3) tying back tall herbs to increase light to ground layer species, and (4) creating gaps in the vegetation by disturbing the soil with a hand trowel early in the growing season. Nutrient additions increased the cover of the herbaceous canopy and enhanced the dominance of Solidago spp. Water additions also increased canopy cover during the second growing season, a year with low seasonal rainfall. Light enrichment as a consequence of canopy manipulations resulted in the competitive release of subcanopy species, primarily Fragaria virginiana. Other subcanopy dominants were limited by different resources; Hier- acium pretense responded most strongly to water and Rumex acetosella to nutrients. These differences in limiting resources may be responsible for the continued coexistence of these three subcanopy species. Additionally, we observed marked seasonal and yearly fluctuations in the severity of resource limitation which might serve to favor different species in this community at different times. Species richness was decreased by light enrichment alone, whereas light with irrigation interacted to increase species richness. Nutrient additions had little direct effect on species richness. A single small disturbance at the beginning of the growing season had little impact on species richness or abundance. Relative to the influence of resources, small-scale dis- turbance plays only a minor role in this community. Our results suggest that nonequilibrium organization of old-field communities can occur via a shifting resource base in the absence of natural disturbances.

Competition, Fire, and Nutients in a Mixed-Grass Prairie

Abstract: Variation in the intensity of competition has been proposed as a mechanism that may control the species diversity and composition of many kinds of communities, including North American prairie. Competition intensity is predicted to increase with soil fertility and decrease with disturbance, causing variation in the abundances of competitively subordinate species. Variation in competition intensity was examined in an experiment in which fire and fertilizer were applied to native mixed—grass prairie. Treatments consisted of annual burns, a single burn, and untreated control, and fertilization (N ≥ 15 replicates) applied to 7 x 5 m plots for two growing seasons. The experiment was repeated at a second site 15 km away. Fire reduced standing crop and litter, and increased the frequency of bare ground. Nutrient addition increased standing crop and decreased bare ground. Species composition and diversity were largely unaffected by fire. Two common species, Bouteloua gracilis and Carex obusata, were significantly more abundant in fertilized vegetation than in controls at both sites; species diversity decreased in fertilized plots. Vegetation responses to the treatments were similar at the two sites. To tests for variation in competition intensity with fire and nutrient availability, removal experiments were performed in each treatment at one site. Two tussocks each of Festuca ovina and Stipa spartea were chosen in each plot. One plant of each pair had all neighbors within 30 cm clipped and removed during two growing seasons. Each species grew larger and faster in the absence of neighbors, but the magnitude of this effect did not vary with fire, fertilizer, or standing crop. Fire and nutrient addition produced significant changes in the community structure of mixed—grass prairie but did not alter competition intensity in the manner predicted.

The cost of stress: Dry matter partitioning changes with seasonal supply of water and nitrogen to dryland wheat

Abstract: Spring wheat cv. ‘Gutha’ was grown in continuous wheat (W/W) and narrow-leafed lupin (L. angustifolius L. cv. Yandee)-wheat (L/W) rotation on a yellow earth over mottled clay (Arenic Fragiudult) in a mediterranean climate for two years. The first year had a higher than average rainfall with adequate soil water until anthesis. The second year was very dry (only 232 mm total rainfall) and soil water contents were low throughout the growing season. Nitrogen fertilizer (+N) treatments were included in both years. In the first year an adjacent experiment compared the effects of loosening a pronounced traffic pan which existed on the site (LS)versus unloosened (US).

Long-term photosynthetic response in single leaves of A C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ

Abstract: Mono-specific communities of the C3 sedge, Scirpus olneyi and the C4 grass, Spartina patens, were exposed to normal ambient or elevated CO2, (ca. 680 μl l-1) throughout the 1987 and 1988 growing seasons in open-top field chambers located on a tidal marsh. Single stems of C3 plants grown in ambient or elevated CO2 showed an increased photosynthetic rate when tested at elevated CO2 for both seasons. This increase in photosynthetic response in the C3 species was maintained throughout the 1987 and 1988 growing season. The stimulation of photosynthesis with elevated CO2 appeared to increase as temperature increased and decreased as photosynthetic photon flux (PPF) increased. Analysis of the photosynthetic response of the C3 species during the 1988 season indicated that significant differences in light-saturated photosynthetic rate between ambient and elevated CO2 conditions continued until October. In contrast to the C3 sedge, the C4 grass showed no significant photosynthetic increase to elevated CO2 except at the beginning of the 1988 season.

Cotton Management Strategies for a Short Growing Season Environment: Water-Nitrogen Considerations

Abstract: (…) Field experiments were conducted during a 4-yr period at a sandy soil (fine, loamy, mixed, thermic family of Aridic Paleustalf) site. Water supply was varied through irrigation with treatments ranging from dryland to fully irrigated. Superimposed on the water supplies were N rate treatments applied preplant and sidedress in a factorial design. Lint yield (LY) was defined as a function of components including plant density, bolls per plant and average boll size.(...)

Forests and the temperature of upland streams in Wales: a modelling exploration of the biological effects

Abstract: SUMMARY. 1. Daily temperature data from six streams in upland Wales were used to explore the thermal effects of afforestation on stream ecology. The data were linked to published biological models to simulate fish and invertebrate development. 2. Mean daily temperatures in forest streams were lower than those of moorland streams in spring and summer, and higher in winter. These spatial comparisons were supported by the results of experimental bank-side clearance at a forest site, where there was evidence that stream temperatures fell in winter and rose in spring following treatment. 3. Simulations indicated that brown trout (Salmo trutta) could weigh over 30% more by the end of their second growing season in a moorland compared with a forest stream. Several species of insects showed slower simulated egg development at forest sites. For two ephemeropteran species simulated nymphal growth was also retarded, suggesting significant alterations to the life cycle. Two plecopteran species were affected only slightly by the different temperature regimes. 4. Overall, the simulations suggested that afforestation, by reducing summer temperatures, could lead to marked reductions in rates of development of some invertebrates and fish.

Effect of supplementary irrigation during reproductive growth on winter and spring chickpea (Cicer arietinum) in a Mediterranean environment.

Abstract: Inadequate soil moisture is one of the main constraints on the productivity of chickpea in the rainfed farming systems of the dry areas in West Asia and North Africa. The response to irrigation at flowering and pod filling of winter- and spring-sown kabuli chickpea was studied in 1983–86 at ICARDA's main research station at Tel Hadya in northern Syria. In 1983/84 when the cultivar ILC3279 was sown in winter, irrigation increased yield by 105% over a crop receiving 229 mm of precipitation. In 1984/85, ILC3279 was sown in winter and spring. Advancing the date of sowing to winter increased yield by 65% and irrigation increased seed yield by 73% in winter and 143% in spring sowings compared with crops grown receiving 373 mm rainfall.In 1985/86, six cultivars (ILC482, ILC3279, FLIP81–57W, FLIP81–293C, FLIP84–19C and FLIP84–80C) were compared, but differences in their response to irrigation were negligible. Advancing sowing from spring to winter increased seed yield by an average of 66%. Irrigation increased seed yield in winter and spring sowings by 56% and 72%, respectively, over those receiving 316 mm annual precipitation. Irrigation is, therefore, a way of increasing the productivity and yield stability of chickpea in northern Syria but the improvement in yield depends on the total rainfall and its distribution over the growing season.

spectral Inputs Improve Corn Crop Coefficients and Irrigation Scheduling

Abstract: An algorithm was developed to shift the basal crop coefficient (K^b) curve with respect to its time axis to obtain a K^b in accordance with the real time reflectance-based crop coefficient for corn. Adjustment of the emergence and effective cover dates during vegetative growth made these dates converge on their actual occurrence. Impact of adjustments to the K^b curve on irrigation dates was simulated using the USDA-ARS irrigation scheduling model for two seasons. Simulated results for the 1983 growing season showed one or two-day differences in the occurrence of irrigation dates. Prior to effective cover, irrigations occurred earlier for the simulation using feedback (adjusted K^b). After effective cover, irrigation dates lagged for the with-feedback simulation compared to the without-feedback simulation. An extra irrigation was required prior to effective cover in 1983 for the simulation using feedback. In 1986, one less irrigation was required for the simulation using feedback to the irrigation scheduling model. This was due to K^b curve adjustments that accounted for hail damage and ensuing cool weather that slowed plant growth during early vegetative growth. The first two irrigations were delayed by two and three days, respectively, due to these events for the with-feedback simulation when compared to simulated results obtained not using feedback. Results from the simulation indicated the following: 1) revised or adjusted K^b curves derived from spectral inputs are unique to the individual growing season, 2) conventional crop coefficients contribute to underestimation as well as overestimation of crop ET because they cannot account for variable crop growth rates, and 3) adjusting the K^b curve in response to actual crop growth allows proper timing of irrigations to ensure that soil moisture conditions are ideal throughout the growing season.

Fate of 15N-labelled fertilizer applied on snow at two forest sites in British Columbia

Abstract: Fertilizer was applied on snow in January 1981 at 100 kg N•ha−1 as [15N]urea, 15NH4NO3, and NH415NO3 to 11-year-old lodgepole pine (Pinuscontorta Dougl. var. latifolia Engelm.) at Spillimacheen in the British Columbia interior and as [15N]urea (200 kg N•ha−1) to 13-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) at Green Mountain, a coastal site in British Columbia. Recovery of labelled N after one growing season was determined in soil and biomass at both sites, and it was also monitored during the growing season in snow and soil at Spillimacheen. At Green Mountain, 5.5% of urea N was recovered in tree biomass, 10.8% in understory, and 33.4% in soil organic N (total recovery 49.7%). Leaching may have contributed to N losses at Green Mountain, but was probably not a direct consequence of the application on snow. At Spillimacheen, total recovery of [15N]urea was 93.3%, with 10.1% in tree biomass, 2.4% in understory, and 80.8% in soil. For 15NH4NO3, recoveries were 5.3% in tree biomass, 2.9% in un...

Whole-plant 14C-photosynthate allocation in Pinus taeda: seasonal patterns at ambient and elevated ozone levels.

Abstract: The seasonal patterns of carbon gain and allocation were examined in Pinustaeda L. seedlings grown under field conditions. To investigate how ozone stress may influence whole-plant carbon budgets over the growing season, the seedlings were grown in either ambient air or air enriched with ozone at twice-ambient levels. On five sampling dates during the 1987 growing season, seedlings were labeled with 14CO2, and whole-plant carbon budgets were constructed. Rate of assimilation of CO2 varied by a factor of 2 during the growing season, with a late spring maximum during the first growth flush. Respiratory losses were highest in the spring and then declined sharply during the summer when photosynthate allocation to the foliage increased rapidly. A second major shift in the carbon budget occurred in the autumn when allocation to the fine roots increased at the expense of the foliage. The proportion of photosynthate allocated to coarse roots and stems varied only slightly over the growing season. Allocation to an...

Patch structure in tallgrass prairies: dynamics of satellite species.

Abstract: Space in tallgrass prairie communities is dominated by a few core species. A large number of less abundant, satellite species occupy the remaining space. These satellite species define vegetation patches that vary within and between growing seasons. In order to determine if patch structure was random we established five permanent 100-m2 blocks in undisturbed tallgrass prairie in Oklahoma and Kansas. Presence of core and satellite species in each m2 was sampled over one or two growing seasons. Patch types were defined by cluster analysis. Characteristics of patch structure included number of patch types, patch type diversity, patch composition, number of spatial groups, group size, and fractal dimension of the spatial groups. We generated simulated data sets with random species associations, in which we quantified patch structure. Actual patch structure, defined mainly by satellite species, was not significantly different from simulated patch structure, except that simulated patches were more fragmented. Therefore, processes that affect species associations may not be important controls of patch structure of satellite species within the spatial and temporal scale of this analysis. Because there was some degree of spatial autocorrelation in patch structure, dispersal processes may have significant effects on patch structure at this scale. In order to understand grassland community dynamics, we propose that satellite species should be modelled using stochastic models constrained by core species dynamics. This approach may be applicable to any community with major components operating at different hierarchical levels.

Assessing and Modifying Ca and Al Levels in Acid Subsoils

Abstract: Subsoil acidity is a major problem in many parts of the world, particularly on highly weathered soils. In addition, subsoil acidity has been reported in some areas where the acidity is derived from the parent material rather than advanced weathering processes. The restricted rooting zone that results from excessive aluminum (Al) or insufficient calcium (Ca) in these soils becomes particularly acute during dry periods because the moisture in the subsoil is beyond the reach of roots. These soils are usually located in areas receiving adequate annual rainfall, but with a growing season during which drought conditions frequently occur. Much of the rainfall received during the growing season in these areas comes in the form of intense thunderstorms, resulting in a higher percentage of water being lost as runoff, which further exacerbates the problem.