Phenology and resource use in three co-occurring grassland annuals

Water resource partitioning among three co-occurring species of the California annual grassland was investigated. Plantago erecta, Clarkia rubicunda and Hemizonia luzulifolia differ in lifespan. The lifespan of Plantago is coincident with the October–May rainy season, but the other two species reproduce during summer when no precipitation occurs, and thus depend on stored water. Field studies indicated differential access to stored water commensurate with the phenology of each species. Studies of artificial stands under controlled conditions showed no difference in the species' ability to exploit stored water in the soil. However there was a striking difference in root behavior between Plantago and Hemizonia when plants were grown in a soil layer above a non-nutritive, waterstoring substrate. We concluded that Hemizonia, the longest lived species, survives on water stored in decomposed rock below the soil layer. Clarkia is restricted to cooler slope faces where a slightly longer growing season appears just suficient to complete reproduction. Productivity is enhanced by addition of later blooming species to the community, but there is no indication that the mixture is the most productive system.     

Biomass dynamics and water use efficiencies of five plant communities in the shortgrass steppe

Summary Standing crop biomass and water-use efficiency were estimated for five plant communities of the Central Plains Experimental Range in north central Colorado. Aboveground biomass by functional groups, surface litter amounts, and standing dead biomass were compared, as were vertical and size-class distributions of belowground biomass. Greater production and water-use efficiency values were found: (1) at coarse-textured sites, indicating the importance of the inverse texture effect, and (2) wherever site characteristics favored the establishment of lifeforms other than grasses, e.g., succulents, and shrubs. Seasonal aboveground biomass and water-use efficiencies for the grass component were similar among sites, even though the mixes of C₃ and C₄ grass species were different. Similar grass biomass values in very different communities suggested that high biomass and high water-use efficiencies were related less to grass types than to the abundance of non-grass life-forms.     

Growth and abundance of desert annuals in an arid woodland in Oman

Given the role of Scaevola plumieri as a major pioneer species in the dune environment, as well as the need forindigenous sand stabilisers for South Africa’s coastal dunes, the reproductive phenology was unravelled as a firststep to establish the potential usefulness of the species in dune stabilisation programmes. In the highly variablecoastal environment the plants must cope with many problems for successful reproduction to take place. Theplants of S. plumieri overcome these difficulties through vegetative reproduction, but sexual reproduction is byno means insignificant. Numerous peduncles, each carrying multiple buds, were produced per stem, but only fewdeveloped into ripe seeds. Several of the flowers were unfertilised and of the seeds produced many were infectedand/or aborted. There was a strong effect of dune position on the reproductive phenology of S. plumieri. Thestems situated at the landward face of the foredunes showed an overall higher reproductive performance numberof buds, flowers, unripe seeds, ripe seeds. The phenogram showed that the reproduction of stems at the landwardside of the foredunes started on average 34 days earlier and showed a tendency for a longer reproductionperiod. For the duration of the different stages in the reproduction sequence, the bud stage’ took the shortesttime and the ‘ripe seed stage’ longest time. Between the different sample years, the production of the number ofpeduncles, buds, flowers and seeds was the similar; only differences in reproduction time (duration) of the differentstages were observed for the bud, flower an unripe seeds stage between the sample years.     

Native and alien species diversity at the local and regional scales in a grazed California grassland

Serpentine meadows in Northern California supported higher species richness at the 1-m² scale than adjacent nonserpentine meadows, and had a considerably higher proportion of native species. Within each soil type, total species richness (natives plus aliens) was unrelated to biomass, cover, soil depth, or soil characteristics (N, P, Ca⁺⁺, Mg⁺⁺, water-holding capacity). However, the proportion of native species on serpentine was higher in meadows with lower levels of phosphorus and a lower calcium/magnesium ratio; the proportion of native species in nonserpentine meadows was higher on cool (north to northeast facing) slopes. At a regional scale, some of these effects were partly reversed; the rate at which new species accumulated with the addition of new sites, or beta diversity, was highest for native plant species in nonserpentine meadows. All of the above effects were independent of whether grazing by cattle was absent (removed 13 years ago) or present. The status of low-productivity serpentine soils as a refuge for native grassland species appears to be the result of their abiotic resistance to alien species, but not of a negative relationship between productivity and total species richness. Received: 10 April 1999 / Accepted: 4 June 1999     

Community and population dynamics of serpentine grassland annuals in relation to gopher disturbance

Summary This study examines the effects of soil disturbance by gophers on patterns of species abundance in an annual grassland community on serpentine soil. We assessed production, dispersal and storage of seed, germination, survivorship and growth of the most abundant species in undisturbed vegetation and on gopher mounds. Fewer seeds of the dominant species were dispersed onto gopher mounds due to the limited movement of seeds from within the closed vegetation. Species with taller flowering stalks were more likely to colonise gopher mounds. The timing of gopher disturbance in relation to the timing of seed fall determined which species could colonise mounds. Lower numbers of seeds falling onto gopher mounds resulted in lower seedling densities of several species compared with undisturbed areas. Survivorship of the commonest species differed between undisturbed areas and gopher mounds formed at different times of year. This resulted in characteristic spectra of species abundance on the different microhabitats, giving rise to distinct spatial patterning in the community. Plants growing on gopher mounds were generally larger and produced more seed than plants in undisturbed vegetation. We suggest that continued gopher disturbance is a factor allowing several species, including perennial grasses, to persist in this community.     

Time course of plant-plant interactions in experimental mixtures of annuals: density,frequency, and nutrient effects

Summary Stellaria media and Poa annua were grown in a greenhouse over a wide range of mixed densities, and at three nutrient levels. The relative performances and interactions between individuals of both species were assessed over four interharvest growth periods using a new approach based on response functions (Connolly 1987). Species relative performances and interactions were complex, varying through time, and with mixed density and the nutrient environment. Studying the dynamics of the individual, interharvest growth periods led to a different, more accurate mechanistic interpretation of the final yield results, than did studying only one overall period of growth. Starting from considerably smaller seeds, Stellaria averaged twice as much biomass as young Poa plants by harvest 2 (H2). Stellaria achieved this early dominance with little suppression of Poa. Interspecific interference became more significant, and species proportional growth (K) became more similar in the second interharvest period (H2-3). During the final period of growth (H3-4), species relative performances largely reversed, Poa showing greater proportional growth than Stellaria. This superior performance by Poa occured despite individual plants being, on average, one half the size of Stellaria. Lack of interference early on is attributed to complementary above ground resource use resulting from species different shoot architectures (prostrate vs. cespitose). This complementarity disappeared as plants aged and became larger. The decline in Stellaria's superior relative growth performance through time was mainly due to its earlier and more substantial diversion of resources to reproduction. Species perception of each others influence, relative to themselves, as quantified by substitution rates, varied with both mixed density and nutrient environments. Stellaria consistently perceived Poa as less influential at higher nutrient levels than at lower levels. High nutrients favored the production of biomass for Stellaria more than for Poa, particularly in H2-3. While accelerating the switch to reproduction for Stellaria, nutrients did not increase its final reproductive yield per unit biomass. After H2, proportional growth for both species was relatively independent of frequency and density, indicating that a species acted as an aggregate of similarly behaving units of biomass whether arranged on many small, or a few large plants. An example shows that the conclusions from the response function approach applied within can be qualitatively different from those derived from a substitutive approach such as replacement series.     

Effects of CO2 elevation on canopy development in the stands of two co-occurring annuals

Elevated CO₂ may increase dry mass production of canopies directly through increasing net assimilation rate of leaves and also indirectly through increasing leaf area index (LAI). We studied the effects of CO₂ elevation on canopy productivity and development in monospecific and mixed (1:1) stands of two co-occurring C₃ annual species, Abutilon theophrasti, and Ambrosia artemisiifolia. The stands were established in the glasshouse with two CO₂ levels (360 and 700 l/l) under natural light conditions. The planting density was 100 per m² and LAI increased up to 2.6 in 53 days of growth. Root competition was excluded by growing each plant in an individual pot. However, interference was apparent in the amount of photons absorbed by the plants and in photon absorption per unit leaf area. Greater photon absorption by Abutilon in the mixed stand was due to different canopy structures: Abutilon distributed leaves in the upper layers in the canopy while Ambrosia distributed leaves more to the lower layers. CO₂ elevation did not affect the relative performance and light interception of the two species in mixed stands. Total aboveground dry mass was significantly increased with CO₂ elevation, while no significant effects on leaf area development were observed. CO₂ elevation increased dry mass production by 30–50%, which was mediated by 35–38% increase in the net assimilation rate (NAR) and 37–60% increase in the nitrogen use efficiency (NUE, net assimilation rate per unit leaf nitrogen). Since there was a strong overall correlation between LAI and aboveground nitrogen and no significant difference was found in the regression of LAI against aboveground nitrogen between the two CO₂ levels, we hypothesized that leaf area development was controlled by the amount of nitrogen taken up from the soil. This hypothesis suggests that the increased LAI with CO₂ elevation observed by several authors might be due to increased uptake of nitrogen with increased root growth.     

Biomass accumulation and carbon utilization in layered sand filter biofilm systems receiving milk fat and detergent mixtures

This research evaluated utilization of organic compounds and biomass accumulation in three different layered sand filter biofilm systems renovating a mixture of detergent and milk fat. Organic compounds were measured as chemical oxygen demand (COD) and biomass as ATP. A two-layer sand filter with coarse sand on the top and fine sand at the bottom; a three-layer sand filter with pea gravel on the top, coarse sand in the middle, and fine sand at the bottom; and a two-filters in-series sand filter biofilm system each received mixtures of butterfat and detergent. While the three-layer sand filter system had greater COD utilization capacity due to a longer filter run, the two-filters in-series sand filter system had greater COD utilization efficiency. The COD utilization was highly associated with the period of filter run to clogging. The decrease of COD utilization rate could lead to the clogging of sand filters. To maintain long filter runs without clogging, a COD loading rate of 2100 mg O(2)/m(2)/h was recommended for three-layer sand filters and 1800 mg O(2)/m(2)/h for two-layer sand filters for high fat content wastewater. In the two-layer sand filters and the three-layer sand filters, biomass did not seem to accumulate in any particular layer on a unit volume of sand. However, based on unit surface area, the coarse sand layer accumulated more biomass than the fine sand layer and the pea gravel layer.     

Effects of fertiliser addition and subsequent gopher disturbance on a serpentine annual grassland community

Summary Application of slow release fertiliser to small (0.5x1 m) plots within a serpentine annual grassland community led to significant increases in above-ground biomass and a shift in species relative abundances. In fertilised plots the native forb species which usually dominate the grassland were almost totally replaced by grasses. In the years following initial fertiliser application, a heavy mulch formed from the previous year's grass growth allowed establishment of grass species such as Bromus mollis but significantly reduced forb establishment. Gopher disturbance of fertilised plots in the second and third years of the experiment effectively removed the grass mulch and allowed re-establishment of forb species.     

Warming and free-air CO2 enrichment alter demographics in four co-occurring grassland species

Species differ in their responses to global changes such as rising CO(2) and temperature, meaning that global changes are likely to change the structure of plant communities. Such alterations in community composition must be underlain by changes in the population dynamics of component species. Here, the impact of elevated CO(2) (550 micromol mol(-1)) and warming (+2 degrees C) on the population growth of four plant species important in Australian temperate grasslands is reported. Data collected from the Tasmanian free-air CO(2) enrichment (TasFACE) experiment between 2003 and 2006 were analysed using population matrix models. Population growth of Themeda triandra, a perennial C(4) grass, was largely unaffected by either factor but population growth of Austrodanthonia caespitosa, a perennial C(3) grass, was reduced substantially in elevated CO(2) plots. Warming and elevated CO(2) had antagonistic effects on population growth of two invasive weeds, Hypochaeris radicata and Leontodon taraxacoides, with warming causing population decline. Analysis of life cycle stages showed that seed production, seedling emergence and establishment were important factors in the responses of the species to global changes. These results show that the demographic approach is very useful in understanding the variable responses of plants to global changes and in elucidating the life cycle stages that are most responsive.     

Gopher mound soil reduces growth and affects ion uptake of two annual grassland species

Summary Portions of an annual serpentine grassland community in California are subject to frequent gopher mound formation. Consequently, studies were undertaken to characterize the effects of mound soils on plant growth and ion uptake. For two of the dominant annual species (Bromus mollis L. and Plantago erecta Morris), growth was reduced in gopher mound soil relative to that in inter-mound soil. A similar reduction in growth was found for plants grown in soils collected at a depth corresponding to the depth of gopher burrowing. This reduction in growth was associated with lower total P and N contents of the soil which were reflected in lower shoot contents of N and P. Additional experiments, however, showed that reduced N and P availabilities in mound soil were not entirely responsible for the growth reduction. Similarly, shoot Ca/Mg ratios were reduced in mound soil but additions of Ca improved the Ca/Mg ratio without improving growth. Growth reductions were associated with altered shoot concentrations of microelements, particularly elevated levels of Mn. A competition experiment between Plantago and Bromus showed that Bromus was more competitive than Plantago in mound and inter-mound soils and that soil type had only small affects on the nature of the interaction between the two species.     

Biomass accumulation and control strategies in gas biofiltration

Uneven distribution and excess accumulation of biomass within gas phase biofilters often result in operational problems such as clogging, channeling, and excessive head loss within biofilter beds, and consequently, the deterioration of performance. In this paper, the characteristics, mechanisms, and patterns of biomass accumulation in gas biofiltration were reviewed, and models for biomass accumulation were also summarized. Strategies for excess biomass control in gas biofiltration, categorized into either physical, chemical, or biological methods were also discussed, with improvements in design and operation of biofilters. Combinations of these approaches are usually necessary in order to maintain a reasonably even distribution and to minimize the accumulation of biomass in gas biofilters.     

Decomposition and nitrogen transformation rates in a temperate grassland vary among co-occurring plant species

Plant and Soil - Decomposition of organic matter varies depending upon interactions between the composition of the organic matter and the source of the microbial community, with differences in...     

Rainfall and the interaction of microclimate with larval resources in the population dynamics of checkerspot butterflies (Euphydryas editha) inhabiting serpentine grassland

Summary The interaction of host plant phenology and microclimatic heterogeneity was examined to determine its role in the population dynamics of checkerspot butterflies, Euphydryas editha, inhabiting serpentine grassland in California's outer Coast Range.Within the 2–3 hectares inhabited by a population of E. editha (Jasper Ridge Area H), microclimatic differences resulting from topographic heterogeneity largely determine the temporal and spatial pattern of senescence of the larval host plants, Plantago erecta and Orthocarpus densiflorus. Survival of larvae from hatching to diapause is extremely low as a result of unpredictable variation in the timing of larval development relative to the timing of host plant senescence, both of which are mediated by microclimatic patterns. During this study, population H declined to near extinction as a result of two consecutive years of record rainfall that apparently disrupted the tenuous temporal relationship between larval development and plant senescence. Retarded development of post-diapause larvae led to a late and extended flight season and delayed egg production; this in turn resulted in massive mortality of pre-diapause larvae due to starvation because host plant senescence occurred before larvae became large enough to enter diapause. Adult population size the following spring was the smallest in 25 years of study. This work emphasizes the importance of microclimatic heterogeneity for understanding population-level processes in small ectothermic animals and underlines the potential importance of such heterogeneity in the establishment of reserves designed to protect such animals     

Metal accumulation in cattle raised in a serpentine-soil area: Relationship between metal concentrations in soil, forage and animal tissues

Soils developed on serpentine rocks contain high levels of heavy metals such as copper (Cu), nickel (Ni) and chromium (Cr), and are deficient in some macronutrients. The crops and pasture grown on these soils may accumulate high levels of metals, which constitute a potential health hazard for cattle. The aim of this study was to evaluate Cr, Ni and Cu accumulation in cattle raised in a serpentine area in Southwest Europe (Galicia, NW Spain). Samples of liver, kidney and muscle of 41 animals aged 8–12 months were collected at slaughter. Representative samples of soil and forage were taken from 10 farms. Samples were acid-digested and metal concentrations determined by ICP-MS (Cr and Ni) and ICP-AES (Cu). The concentrations of the metals in soils and forage were in the range of those found in serpentine soils in other areas. Accumulation of Cr in animal tissues was generally low and within the normal range. However, 20% of the animals had toxic levels of Ni in kidney and 32% of the animals had liver Cu levels above the acceptable range. Serpentine soils had a significant effect on Ni and Cu accumulation in cattle, and a relatively high percentage of the animals showed tissue levels of Ni and Cu indicative of risk of toxicity.     

Biomass accumulation and energy conversion efficiency in aromatic rice genotypes

A field experiment was conducted to evaluate photosynthetic efficiency along with different growth parameters of aromatic rice genotypes. Forty genotypes including three non-aromatic checks exhibited enormous variations for leaf area index (LAI), crop growth rate (CGR), relative growth rate (RGR), net assimilation rate (NAR), grain yield, total dry matter, harvest index and photosynthetic efficiency or energy use efficiency (Eμ) at panicle initiation and heading stages. Minimum LAI-value was 0.52 in Khazar at PI stage and maximum was 4.91 in Sakkor khora at heading stage. The CGR-value was in the range of 4.80−24.11 g m⁻² per day. The best yielder BR39 produced grain of 4.21 t ha⁻¹ and the worst yielder Khazar gave 1.42 t ha⁻¹. Total dry matter (TDM) yield varied from 4.04 to 12.26 t ha⁻¹ where genotypes proved their energy use efficiency a range between 0.58 to 1.65%. Eμ showed a significant positive relation with TDM (r = 0.80^**), CGR (r = 0.72^**) and grain yield (r = 0.66^**). A negative correlation was established between TDM and harvest index and LAI and RGR. Path analysis result showed that NAR at heading stage exerted highest positive direct effect (0.70) on Eμ.     

Water resource partitioning,stem xylem hydraulic properties,and plant water use strategies in a seasonally dry riparian tropical rainforest

This study investigated seasonal variation in the origin of water used by plants in a riparian tropical rainforest community and explored linkages between plant water source, plant xylem hydraulic conductivity and response to the onset of dry conditions. The study focused on five co-dominant canopy species, comprising three tree species (Doryphora aromatica, Argyrodendron trifoliolatum, Castanospora alphandii) and two climbing palms (Calamus australis and Calamus caryotoides). Stable isotope ratios of oxygen in water (¹⁸O) from soil, groundwater, stream water and plant xylem measured in the wet season and the subsequent dry season revealed water resource partitioning between species in the dry season. Measurement of stem-area-specific hydraulic conductivity (K_S) in the wet season and subsequent dry season showed a significant dry-season loss of K_S in three of the five species (Castanospora alphandii, Calamus australis and C. caryotoides) and a decrease in mean K_S for all species. This loss of hydraulic conductivity was positively correlated with the difference between wet-season and dry-season midday leaf water potentials and with leaf carbon isotope discrimination, indicating that plants that were less susceptible to loss of conductivity had greater control over transpiration rate and were more water-use efficient.     

The effect of mean and variance in resource supply on survival of annuals from Mediterranean and desert environments

Resource availability is often characterized by mean annual amounts, while ignoring the spatial variation within habitats and the temporal variation within a year. Yet, temporal and spatial variation may be especially important for identifying the source of stress in low productivity environments such as deserts where resources are often pulsed and resource renewal events are separated by long periods of low resource availability. Therefore, the degree of stress will be determined in part by the length of time between recharge events. Here, we investigated the effect of timing and total amount of water application on two congeneric pairs, each with a population from a low (desert) and a high (Mediterranean) productivity habitat. As expected, highest survival and greatest growth were found at low or intermediate recharge intervals, and the magnitude of response to increases in total seasonal amounts was greater for Mediterranean species than desert species. The species that had greater survival switched in the hierarchy under high total water depending on interval length. These results demonstrate that temporal variation in resource availability can be as important as annual total amounts for plant performance and that response to temporal dynamics can vary between species. This has implications for community-level processes, as competitive hierarchies may switch based on resource dynamics rather than only total availability.     

Growth dynamics and population development in an alpine grassland under elevated CO2

Leaf expansion, population dynamics and reproduction under elevated CO₂ were studied for two dominant and four subdominant species in a high alpine grassland (2500 above sea level, Swiss Central Alps). Plots of alpine heath were exposed to 335 l l^-1 and 680 l l^-1 CO₂ in open-top chambers over three growing seasons. Treatments also included natural and moderately improved mineral nutrient supply (40 kg N ha^-1 year^-1 in an NPK fertilizer mix). Seasonal dynamics of leaf expansion, which was studied for the dominant graminoid Carex curvula only, were not affected by elevated CO₂ during two warm seasons or during a cool season. Improved nutrient supply increased both the expansion rate and the duration of leaf growth but elevated CO₂ did not cause any further stimulation. Plant and tiller density (studied in all species) increased under elevated CO₂ in the codominant Leontodon helveticus and the subdominant Trifolium alpinum, remained unchanged in two other minor species Poa alpina and Phyteuma globulariifolium, and decreased in Carex curvula. In Potentilla aurea elevated CO₂ compensated for a natural decline in shoot number. By year 3 the number of fertile shoots in Leontodon and individual seed weight in Carex were slightly increased under elevated CO₂, indicating CO₂ effects on sexual reproduction in these two dominant species. The results suggest that the effects of elevated CO₂ on the population dynamics of the species studied were not general, but species-specific and rather moderate effects. However, the reduction of tiller density in Carex curvula, in contrast to the increases observed in Leontodon helveticus and Trifolium alpinum, indicates that elevated CO₂ may negatively affect the abundance of the species most characteristic of this alpine plant community.     

Diurnal starch accumulation and utilization in phosphorus-deficient soybean plants

The effects of phosphorus deficiency on carbohydrate accumulation and utilization in 34-day-old soybean (Glycine max L. Merr.) plants were characterized over a diurnal cycle to evaluate the mechanisms by which phosphorus deficiency restricts plant growth. Phosphorus deficiency decreased the net CO₂ exchange rate throughout the light period. The decrease in the CO₂ exhange rate was associated with a decrease in stomatal conductance and an increase in the internal CO₂ concentration. These observations indicate that phosphorus deficiency increased mesophyll resistance. Assimilate export rate from the youngest fully expanded leaves was decreased by phosphorus deficiency, whereas starch concentrations in these leaves were increased. Higher starch concentrations in phosphorus-deficient youngest fully expanded leaves resulted from a longer period of net starch accumulation and a shorter period of net starch degradation relative to those for phosphorus-sufficient controls. Phosphorus deficiency decreased sucrose-P synthase activity by 27% (averaged over the diurnal cycle), and essentially eliminated diurnal variation in sucrose-P-synthase activity. Diurnal variations in nonstructural carbohydrate concentrations in leaves and stems were also less pronounced in phosphorus-deficient plants than in controls. In phosphorus-deficient plants, only 30% of the whole plant starch present at the end of a light phase was utilized during the subsequent 12-hour dark phase as compared with 68% for phosphorus-sufficient controls. Although phosphorus deficiency decreased the CO₂ exchange rate and whole plant leaf area, accumulation of high starch concentrations in leaves and stems and restricted starch utilization in the dark indicate that growth processes (i.e. sink activities) were restricted to a greater extent than photosynthetic capacity. Further experimentation is required to determine whether decreased starch utilization in phosphorus-deficient plants is the cause or the result of restricted growth.