Grazing-induced changes in plant composition affect litter quality and nutrient cycling in flooding Pampa grasslands

Changes in plant community composition induced by vertebrate grazers have been found to either accelerate or slow C and nutrient cycling in soil. This variation may reflect the differential effects of grazing-promoted (G+) plant species on overall litter quality and decomposition processes. Further, site conditions associated with prior grazing history are expected to influence litter decay and nutrient turnover. We studied how grazing-induced changes in plant life forms and species identity modified the quality of litter inputs to soil, decomposition rate and nutrient release in a flooding Pampa grassland, Argentina. Litter from G+ forbs and grasses (two species each) and grazing-reduced (G−) grasses (two species) was incubated in long-term grazed and ungrazed sites. G+ species, overall, showed higher rates of decomposition and N and P release from litter. However, this pattern was primarily driven by the low-growing, high litter-quality forbs included among G+ species. Forbs decomposed and released nutrients faster than either G+ or G− grasses. While no consistent differences between G+ and G− grasses were observed, patterns of grass litter decay and nutrient release corresponded with interspecific differences in phenology and photosynthetic pathway. Litter decomposition, N release and soil N availability were higher in the grazed site, irrespective of species litter type. Our results contradict the notion that grazing, by reducing more palatable species and promoting less palatable ones, should decrease nutrient cycling from litter. Plant tissue quality and palatability may not unequivocally link patterns of grazing resistance and litter decomposability within a community, especially where grazing causes major shifts in life form composition. Thus, plant functional groups defined by species’ “responses” to grazing may only partially overlap with functional groups based on species “effects” on C and nutrient cycling.     

Genotypic response to sodium chloride salinity of four major olive cultivars (Olea europea L.)

Anin situ resin bag technique was used to measure the relative availabilities of N and P along a chronosequence of soils in southern New Mexico, and was compared to two more common indices of nutrient availability. Accumulations of N and P during 10-week intervals over an 18 month period were separable into wet season (September–January) and dry season (February–August) groups, with wet season values significantly greater than dry season values. Only accumulations during the wet season showed significant differences among sites, thus stressing the role of field water regime in interpreting resin accumulation results. Total mineral N (NO₃+NH₄) sorbed by resins was significantly correlated to laboratory N mineralization rates. Although accumulation patterns of N and P were similar to patterns of %N and %P in shrub species growing along the chronosequence, these similarities were not statistically significant. A laboratory experiment demonstrated that bicarbonate-form anion resins are preferable to hydroxyl-form resins, as long as standards are made from solutions extracted by resins to account for variable ion recovery efficiencies.     

Dominance of legume trees alters nutrient relations in mixed species forest restoration plantings within seven years

Failures in reforestation are often attributed to nutrient limitation for tree growth. We compared tree performance and nitrogen and phosphorus relations in adjacent mixed-species plantings of contrasting composition, established for forest restoration on Ultisol soil, originally covered by tropical semi-deciduous Atlantic Forest in Southeast Brazil. Nutrient relations of four tree species occurring in both planting mixtures were compared between a legume-dominated, species-poor direct seeding mixture of early-successional species (“legume mixture”), and a species-diverse, legume-poor mixture of all successional groups (“diverse mixture”). After 7 years, the legume mixture had 6-fold higher abundance of N₂-fixing trees, 177% higher total tree basal area, 22% lower litter C/N, six-fold higher in situ soil resin-nitrate, and 40% lower in situ soil resin-P, compared to the diverse mixture. In the legume mixture, non-N₂-fixing legume Schizolobium parahyba (Fabaceae-Caesalpinioideae) had significantly lower proportional N resorption, and both naturally regenerating non-legume trees had significantly higher leaf N concentrations, and higher proportional P resorption, than in the diverse mixture. This demonstrate forms of plastic adjustment in all three non-N₂-fixing species to diverged nutrient relations between mixtures. By contrast, leaf nutrient relations in N₂-fixing Enterolobium contortisiliquum (Fabaceae-Mimosoideae) did not respond to planting mixtures. Rapid N accumulation in the legume mixture caused excess soil nitrification over nitrate immobilization and tighter P recycling compared with the diverse mixture. The legume mixture succeeded in accelerating tree growth and canopy closure, but may imply periods of N losses and possibly P limitation. Incorporation of species with efficient nitrate uptake and P mobilization from resistant soil pools offers potential to optimize these tradeoffs.     

Keystone Interactions: Salmon and Bear in Riparian Forests of Alaska

The term “keystone species” is used to describe organisms that exert a disproportionately important influence on the ecosystems in which they live. Analogous concepts such as “keystone mutualism” and “mobile links” illustrate how, in many cases, the interactions of two or more species produce an effect greater than that of any one species individually. Because of their role in transporting nutrients from the ocean to river and riparian ecosystems, Pacific salmon (Oncorhynchus spp.) and brown bear (Ursus arctos) have been described as keystone species and mobile links, although few data are available to quantify the importance of this interaction relative to other nutrient vectors. Application of a mass balance model to data from a southwestern Alaskan stream suggests that nitrogen (N) influx to the riparian forest is significantly increased in the presence of both salmon and bear, but not by either species individually. The interactions of salmon and bear may provide up to 24% of riparian N budgets, but this percentage varies in time and space according to variations in salmon escapement, channel morphology and watershed vegetation characteristics, suggesting interdependence and functional redundancy among N sources. These findings illustrate the complexity of interspecific interactions, the importance of linkages across ecosystem boundaries and the necessity of examining the processes and interactions that shape ecological communities, rather than their specific component parts.     

Temporal patterns of soil nutrients in a Panamanian moist forest revealed by ion-exchange resin and experimental irrigation

The effect of seasonal water availability on soil nutrients and soil N transformations was investigated by irrigating two large plots of mature tropical forest on Barro Colorado Island (BCI), Panama, during the dry season for five consecutive years. Methods included (i) nutrient accumulation by ion-exchange resins placed on the surface of the mineral soil for contiguous 21-day periods, (ii) monthly mineral soil (0–10 cm) extractions and incubations for inorganic N and P concentrations, and (iii) leaching loss of nutrients from leaf litter samples. Rates of nutrient accumulation by the resins showed a great deal of variation between sampling dates and among years in control plots; albeit, seasonal patterns were slight, except for the highest Ca values near the end of the wet season and inorganic P (P_i) and SO₄ values that peaked during the dry season. Irrigation had remarkably little effect on nutrient accumulation rates by resins, except for an increase in Mg and Na values, but did affect the timing in the temporal variation in K, Na, N_i and P_i values. In contrast, inorganic N (N_i) and P_i pools and N transformation rates in the mineral soil hardly varied among sampling dates and did not show any response to irrigation. We hypothesize that the timing of leaf litterfall and nutrient leaching from forest floor litter can set up temporal patterns in the levels of soil nutrient at the surface of the mineral soil, but the temporal patterns essentially disappear with depth in the mineral soil.     

Metal complexes increase uptake of Zn and Cu by plants: implications for uptake and deficiency studies in chelator-buffered solutions

The allocation of resources among roots and shoots represents the largest flux of resources within a plant and therefore should have been selected to maximize benefits to plants. Yet, it is unclear why some species like temperate grasses have such high root length density (RLD). Either the slow rate of diffusion of inorganic N in soils or interplant competition could explain the high RLD of temperate grasses. Using a fine-scale model of nutrient dynamics in the soil and plant growth, a cost–benefit approach was used to assess optimal allocation rates for plants that accounted for value of both carbon and nitrogen. In the absence of interplant competition, resource benefits are maximized with very little root length except in extremely dry soils for ammonium. In the presence of a competitor, optimal allocation of N to roots is much greater and increases as ability of competitors to produce root length increase. Competition for inorganic nitrogen generates a classic aspect of the tragedy of the commons, the “race for fish”, where plants must allocate more resources to acquisition of the limiting resource than is optimal for plants in the absence of competition. As such, nutrient competition needs to be directly addressed when understanding plant- and ecosystem-level resource fluxes as well as the evolution of root systems.     

Spatial and Temporal Distribution of Dissolved Inorganic Nutrients and Phytoplankton in Mida Creek,Kenya

The spatial and temporal distributions of dissolved inorganic nutrients were investigated between May 1996 and April 1997 in Mida Creek, a mangrove area along the north coast region of Kenya. The nutrient levels of pore water from boreholes/wells within the surrounding area of the creek were also investigated for comparison. In addition, phytoplankton distribution in Mida Creek was assessed in three stations within the creek in order to determine the structure and succession stages of the phytoplankton community and to provide an indication of the status of primary productivity of the creek. Measurements carried out within the creek revealed that the mean concentration ranges for NH₄⁺ –N, (NO ₂⁻ + NO₃⁻)−N, PO₄³⁻ −P and SiO₃²⁻ −Si were: 0.002–5.45; 0.12–5.63; 0.10–0.58 and 1.31–81.36 μM, respectively. For the case of boreholes/wells found in the surrounding area, their respective nutrient levels were found to lie in the ranges 0.4–907.0; 16.7–4897.0; 1.09–22.39 and 83.9–596.0 μM. A total of 295 species of phytoplankton belonging to 78 genera were identified with great temporal variability in abundance in all the stations sampled. The most dominant algal members in the Creek included Chaetoceros spp., Chroococcus limneticus and Oscillatoria spp. The diversity values recorded were indicative of mesotrophic conditions. The highest nutrient concentration levels within the creek were measured during the wet season as compared to dry season and this trend closely corresponded with that of the phytoplankton productivity. However, no significant variation ( p > 0.05) was found in all cases with respect to the tidal cycles. On the contrary, diurnal nutrient concentrations especially in areas with high flooding duration (>12 h) were found to be highest during the dry season as opposed to wet season for all nutrients except for SiO₃²⁻. The relatively high nutrient laden groundwater outflow into the creek water, coupled with surface runoff events during wet season, are the two main factors responsible for the elevated nutrients in the creek waters in the absence of river inflow into the creek.     

Nutrient Budget in the Seasonal Wetland of the Okavango Delta, Botswana

The nutrient (P and N species) and chloride budgets were investigated in a representative floodplain in the seasonal wetlands of the Okavango Delta, Botswana. A variety of sources of nutrients in the surface water were considered, namely ion species coming with the floodwater, those generated from dry floodplain soils and those from water-soluble dust deposition (both local and long-range sources). Concentrations of total-nitrogen and chloride in surface water were below 1 mg l⁻¹. Total-phosphorus concentrations were 0.05 mg l⁻¹, reflecting the oligotrophic character of the system. Dust deposition rates were highest for chloride at 2.44 g m⁻² year⁻¹ followed by 0.79 g m⁻² year⁻¹ for total-N, 0.40 g m⁻² year⁻¹ for ammonia and only 0.02 g m⁻² year⁻¹ for total-P, respectively. Chloride was derived primarily from long-range transport, while N and P species were of more local origin. Dissolution rates for these ions combined were calculated to be 3.9 g m⁻² for the flooded area in the 1999 season and thus all dry deposits must be re-dissolved. The accumulation of dust deposits on dry surfaces and their subsequent dissolution causes 2–5 times higher concentrations of nitrogen, phosphorus and chloride with the onset of the flood, thus boosting the nutrient stock in the crucial phase of the onset of flooding. Chloride dissolved from dry soil surfaces and dust contributed approximately 40% to the overall floodplain budget. Although contributions from the soil surface and dust to the nitrogen and phosphorus pools of the floodplain are less prominent (with 10% of total), they nonetheless represent a significant source of nutrients in the entire system. Extrapolation to annually flooded swamps (10,000 km²) indicates a maximum contribution of 40% for total-nitrogen and 60% for total-phosphorus from dust deposition on wet or dry surfaces to the nutrient pool of the water body.     

Wetland plant decomposition under different nutrient conditions: what is more important, litter quality or site quality?

Plants in nutrient poor environments are often characterized by high nutrient resorption resulting in poor litter quality and, consequently, slow decomposition. We used oligotrophic, P-limited herbaceous wetlands of northern Belize as a model system, on which to document and explain how changes in nutrient content along a salinity gradient affect decomposition rates of macrophytes. In 2001 we established a nutrient addition experiment (P, N, and N&P) in 15 marshes of a wide range of water conductivities (200–6000 μS), dominated by Eleocharis spp. To determine what is more important for decomposition, the initial litter quality, or site differences, we used reciprocal litter placement and cellulose decomposition assay in a combined “site quality” and “litter quality” experiment. Our prediction of the positive effects of P-enrichment on decomposition rate due to both the quality of litter and the site was confirmed. The site effect was stronger than the litter quality although both were highly significant. Strong site quality effect was apparently the result of more active decomposer community in P-enriched plots as supported by finding of higher microbial biomass in litter decomposing there. The strong effect of site quality on decomposition was further confirmed by the cellulose assay. The cellulose decomposition was significantly slower at high salinity sites indicating lower decomposer microbial activity. Litter nutrient N and P content and nutrient ratios were well correlated with decomposition with the best fit found for log C/P. At C/P mass ratio of >4000 decomposition processes were extremely slow. We hypothesize that in a long run, the increased decomposition will compensate the increase in primary production resulting from increased nutrient loading and there will be no differences in accumulation of organic material between the controls and nutrient enriched plots.     

Measuring Nitrogen and Phosphorus Uptake by Intact Roots of Mature Acer saccharum Marsh., Pinus resinosa Ait., and Picea abies (L.) Karst

A common method for measuring uptake by intact roots in situ is the depletion method, wherein intact fine roots are separated from soil and placed in nutrient solution. The difference between initial and final amounts of nutrient in solution is attributed to root uptake. Variations on this method include applying pretreatment solutions, training roots to grow into bags or trays, and varying concentrations of nutrient solution. We tested whether variations in methods affected measured net uptake rates of NH ₄ ⁺ , NO ₃ ⁻ , and PO ₄ ³⁻ . Intact roots of 60 year-old sugar maple (Acer saccharum Marsh.), red pine (Pinus resinosa Ait.), and Norway spruce (Picea abies (L.) Karst.) were given one of four treatments prior to measuring net uptake. “Trained” roots were grown in a sand-soil mixture. “Recovered” roots were excavated and allowed to recover in nutrient solution for two or four days (“two-day recovery” and “four-day recovery”, respectively). “No recovery” roots were excavated and used immediately in experiments. We exposed roots to three concentrations of nutrient solutions to observe the effects of initial nutrient solution concentration. Initial nutrient solution concentration was an important source of variation in measured uptake rates, and N uptake was stimulated by low antecedent concentrations. We found no significant differences in net uptake rates between pretreatments for any of the species studied, indicating that our pretreatments were not effective in improving measurement of uptake. Such pretreatments may not be necessary for measuring net uptake and may not hinder the comparison of rates measured using variations of the depletion method.     

Identification of Barley Varieties Tolerant to Cadmium Toxicity

Two successive hydroponic experiments were carried out to identify barley varieties tolerant to Cd toxicity via examining Soil–Plant Analyses Development (SPAD) value, plant height, leaves and tillers per plant, root number and volume, and biomass accumulation. The results showed that SPAD values (chlorophyll meter readings), plant height, leaf number, root number and volume, and biomass accumulation of shoot/root were significantly reduced in the plants grown in 20 μM Cd nutrient solution compared with control, and the uptake and translocation of Zn, Mn, and Cu was also strictly hindered. Furthermore, there was a highly significant difference in the reduction in these growth parameters among varieties, and varieties “Weisuobuzhi” and “Jipi 1” showed the least reduction both in the two experiments, suggesting their high tolerance to Cd toxicity, while “Dong 17” and “Suyinmai 2” with the greatest reduction and the toxicity symptoms appeared rapidly and severely, denoting as Cd-sensitive varieties. Significant variety difference in Cd concentration was also found, with Weisuobuzhi containing the highest and Jipi 1 the lowest Cd concentration in shoots.     

Nitrogen relations of natural and disturbed plant communities in tropical Australia

Nitrogen relations of natural and disturbed tropical plant communities in northern Australia (Kakadu National Park) were studied. Plant and soil N characteristics suggested that differences in N source utilisation occur at community and species level. Leaf and xylem sap N concentrations of plants in different communities were correlated with the availability of inorganic soil N (NH⁺ ₄ and NO⁻ ₃). In general, rates of leaf NO⁻ ₃ assimilation were low. Even in communities with a higher N status, including deciduous monsoon forest, disturbed wetland, and a revegetated mine waste rock dump, levels of leaf nitrate reductase, xylem and leaf NO⁻ ₃ levels were considerably lower than those that have been reported for eutrophic communities. Although NO⁻ ₃ assimilation in escarpment and eucalypt woodlands, and wetland, was generally low, within these communities there was a suite of species that exhibited a greater capacity for NO⁻ ₃ assimilation. These “high- NO⁻ ₃ species” were mainly annuals, resprouting herbs or deciduous trees that had leaves with high N contents. Ficus, a high-NO⁻ ₃ species, was associated with soil exhibiting higher rates of net mineralisation and net nitrification. “Low-NO⁻ ₃ species” were evergreen perennials with low leaf N concentrations. A third group of plants, which assimilated NO⁻ ₃ (albeit at lower rates than the high-NO⁻ ₃ species), and had high-N leaves, were leguminous species. Acacia species, common in woodlands, had the highest leaf N contents of all woody species. Acacia species appeared to have the greatest potential to utilise the entire spectrum of available N sources. This versatility in N source utilisation may be important in relation to their high tissue N status and comparatively short life cycle. Differences in N utilisation are discussed in the context of species life strategies and mycorrhizal associations. Received: 5 July 1997 / Accepted: 13 July 1998     

Responses of wetland graminoids to the relative supply of nitrogen and phosphorus

The biomass production of wetland vegetation can be limited by nitrogen or phosphorus. Some species are most abundant in N-limited vegetation, and others in P-limited vegetation, possibly because growth-related traits of these species respond differently to N versus P supply. Two growth experiments were carried out to examine how various morphological and physiological traits respond to the relative supply of N and P, and whether species from sites with contrasting nutrient availability respond differently. In experiment 1, four Carex species were grown in nutrient solutions at five N:P supply ratios (1.7, 5, 15, 45, 135) combined with two levels of supply (geometric means of N and P supply). In experiment 2, two Carex and two grass species were grown in sand at the same .ve N:P supply ratios combined with three levels of supply and two light intensities (45% or 5% daylight). After 12-13 weeks of growth, plant biomass, allocation, leaf area, tissue nutrient concentrations and rates and nutrient uptake depended signi.cantly on the N:P supply ratio, but the type and strength of the responses differed among these traits. The P concentration and the N:P ratio of shoots and roots as well as the rates of N and P uptake were mainly determined by the N:P supply ratio; they showed little or no dependence on the supply level and relatively small interspeci.c variation. By contrast, the N concentration, root mass ratio, leaf dry matter content and speci.c leaf area were only weakly related to the N:P supply ratio; they mainly depended on plant species and light, and partly on overall nutrient supply. Plant biomass was determined by all factors together. Within a level of light and nutrient supply, biomass was generally maximal (i.e. co-limited by N and P) at a N:P supply ratio of 15 or 45. All species responded in a similar way to the N:P supply ratio. In particular, the grass species Phalaris arundinacea and Molinia caerulea showed no differences in response that could clearly explain why P. arundinacea tends to invade P-rich (N-limited) sites, and M. caerulea P-limited sites. This may be due to the short duration of the experiments, which investigated growth and nutrient acquisition but not nutrient con­servation.     

Redistribution of the nitrogen reserves of15N-enriched stem cuttings and dinitrogen fixed by 90-day-old sugarcane plants

Sugarcane plants were grown in sandculture from¹⁵N stem cuttings approximately 3 cm long with one node (“nodes”) or 15 cm long, with two nodes (“seedpieces”), in nutrient solution without or with 8 mM of N. The decrease in the¹⁵N enrichment of plants growing in zero-N solution relative to the initial¹⁵N enrichment of the cuttings indicated N₂ fixation. The contribution from atmospheric dinitrogen to the total N of the plants amounted to 9.3% for those growing from nodes and 15.2% for those growing from seedpieces after 90 and 60-day growth periods, respectively. Most of the fixed nitrogen went to the root-N pool. From 9 to 39% of the N initially present in the stem cuttings was lost from the plants while about 45% of the remaining N was translocated to the shoots and roots during a 30-day growth period for seedpieces and a 15-day growth period for the nodes. More than 85% of the translocated N went to the shoots. At the end of a 60-day growth period (for nodes and seedpieces, respectively) about 35% of the initial N remained in the stem cuttings, irrespective of type of cutting or of the presence or absence of N in the nutrient solution.     

Application of a 15N tracer to simulate and track the fate of atmospherically deposited N in the coastal forests of the Waquoit Bay Watershed, Cape Cod, Massachusetts

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling. Received: 11 July 1996 / Accepted: 5 December 1996     

Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot‐dry period when soil water is limiting, while the subsequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during the dry summer period, however, is substantial in these species. We tested the hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a significant extent. We examined the leaf traits (with parameters such as leaf life span, leaf nutrient content and retranslocation of nutrients during senescence) in eight selected tree species in northern India. The concentrations of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a substantial amount of nutrient resorption before senescence and a ‘tight nutrient budgeting’. The resorbed N‐mass could potentially support 50 to 100% and 46 to 80% of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 100% and 20 to 91%, for K 29 to 100% and 20 to 57%, for Na 3 to 100% and 1 to 54%, and for Ca 0 to 32% and 0 to 30%. The species differed significantly with respect to their efficiency in nutrient resorption. Such interspecific differences in leaf nutrient economy enhance the conservative utilization of soil nutrients by the dry forest community. This reflects an adaptational strategy of the species growing on seasonally dry, nutrient‐poor soils as they tend to depend more or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new foliage biomass in summer when the availability of soil moisture and nutrients is severely limited.     

The plant vigour hypothesis revisited – how is browsing by ungulates and elephant related to woody species growth rate?

The way herbivores select what to eat is of considerable practical and theoretical interest, and has given rise to different theories and hypotheses. The plant vigour hypothesis predicts that herbivores feed preferentially on vigorous, i.e., large and/or fast-growing plants or plant parts. These predictions have previously primarily been tested on variation within plant species. Here we test whether differences in vigour among plant species in the same environment can explain differences in herbivore attack. We studied variation in browsing pressure by a guild of large herbivores on different woody species in an African savanna ecosystem. Shoot growth rate, annual shoot length, basal shoot diameter and annual shoot volume of 14 woody plant species were measured in the field. Plant species’ shoot vigour represented by the first PCA axis scores generated from the four shoot variables were then related to browsing pressure (% utilisation) on each of the species by native ungulates and elephant. Nutrient and fibre concentrations and tannin activity were also determined for the 14 woody plant species. We found ungulate browsing pressure to show a unimodal relationship with plant species’ shoot vigour. The heaviest browsing pressure was on plant species with shoots of intermediate vigour. We suggest that species with less vigorous shoots had low nutrient and high fibre concentrations and offered small bite sizes, whereas species with vigorous shoots had high nutrient concentrations but larger shoot diameters than the bite diameters of browsing ungulates. Elephant browsing pressure was not related to plant species’ shoot vigour.     

Ecological explanations for successful invasion of exotic plants

The intentional introduction of exotic species can increase the level of local biodiversity, enrich people’s material lives, and bring significant social and economic benefits that are also the symbols of human progress. However, along with the frequent intercourse among countries and regions, the frequency of uncontrolled crossregional migration of species is increased and there is a lack of scientific management strategy for the intentional introduction of exotic species. Exotic species invasion, which is behind habitat fragmentation, has become the second largest threatening factor to the maintenance of the global-scale level of biological diversity. Exotic species invasion can destroy the structure of an ecosystem, disturb the economic life of a society, and do harm to human health. In this paper, the authors review some of the ecological explanations for issues such as “what causes or mechanisms have led to the successful invasion of exotic species”, including the “ideal weeds characteristics”, “biodiversity resistance hypothesis”, “enemies release hypothesis”, “evolution of increased competitive ability hypothesis”, “niche opportunity hypothesis”, and “novel weapon hypothesis”. The authors also analyze and evaluate the background and theoretical basis of the hypotheses, providing explanations for some phenomena, as well as the deficiencies of these explanations.     

Regeneration niche differentiates functional strategies of desert woody plant species

Plant communities vary dramatically in the number and relative abundance of species that exhibit facilitative interactions, which contributes substantially to variation in community structure and dynamics. Predicting species’ responses to neighbors based on readily measurable functional traits would provide important insight into the factors that structure plant communities. We measured a suite of functional traits on seedlings of 20 species and mature plants of 54 species of shrubs from three arid biogeographic regions. We hypothesized that species with different regeneration niches—those that require nurse plants for establishment (beneficiaries) versus those that do not (colonizers)—are functionally different. Indeed, seedlings of beneficiary species had lower relative growth rates, larger seeds and final biomass, allocated biomass toward roots and height at a cost to leaf mass fraction, and constructed costly, dense leaf and root tissues relative to colonizers. Likewise at maturity, beneficiaries had larger overall size and denser leaves coupled with greater water use efficiency than colonizers. In contrast to current hypotheses that suggest beneficiaries are less “stress-tolerant” than colonizers, beneficiaries exhibited conservative functional strategies suited to persistently dry, low light conditions beneath canopies, whereas colonizers exhibited opportunistic strategies that may be advantageous in fluctuating, open microenvironments. In addition, the signature of the regeneration niche at maturity indicates that facilitation expands the range of functional diversity within plant communities at all ontogenetic stages. This study demonstrates the utility of specific functional traits for predicting species’ regeneration niches in hot deserts, and provides a framework for studying facilitation in other severe environments.     

Cultivation of Miscanthus under West European conditions: Seasonal changes in dry matter production,nutrient uptake and remobilization

There is increasing interest in cultivation of Miscanthus as a source of renewable energy in Europe, but there is little information on its nutrient requirements. Our aim was to determine the nutrient requirement of an established Miscanthus crop through a detailed study of nutrient uptake and nutrient remobilization between plant parts during growth and senescence. Therefore dry matter of rhizomes and shoots as well as N, P, K and Mg concentration under three N fertilizer rates (0, 90, and 180 kg N ha-1) were measured in field trials in 1992/93 and at one rate of 100 kg N h-1 in 1994/95.Maximum aboveground biomass in an established Miscanthus crop ranged between 25-30 t dry matter ha-1 in the September of both trial years. Due to senescence and leaf fall there was a 30% loss in dry matter between September and harvest in March. N fertilization had no effect on crop yield at harvest. Concentrations of N, P, K and Mg in shoots were at a maximum at the beginning of the growing period in May and decreased thereafter while concentrations in rhizomes stayed fairly constant throughout the year and were not affected by N fertilization.Nutrient mobilization from rhizomes to shoots - defined as the maximum change in nutrient content in rhizomes from the beginning of the growth period measured in 1992/93 was 55 kg N ha-1, 8 kg P ha-1, 39 kg K ha-1 and 11 kg Mg ha-1. This is equivalent to 21 N, 36 P, 14 K and 27 Mg of the maximum nutrient content of the shoots. Nutrient remobilization from shoots to rhizomes defined as the increase in nutrient content of rhizomes between September and March measured in 1994/95 was 101 kg N ha-1, 9 kg P ha-1, 81 kg K ha-1 and 8 kg Mg ha-1 equivalent to 46 N, 50 P, 30 K and 27 Mg of nutrient content of shoots in September. Results showed that nutrient remobilization within the plant needs to be considered when calculating nutrient balances and fertilizer recommendations.