Nutrient turnover studies in alpine ecosystems

Summary Four plots of alpine meadow communities (mats) of the Northern Calcareous Alps are compared with regard to dry substance (DS), N-, P-, and K-content of living and dead overground and total underground phytomass. The Caricetum firmae F (altitude 2,010 m) produces the least green mass but accumulates the highest amount of dead substance (litter). In all parts it has the lowest content of N, P, K. The opposite is the Caricetum ferrugineae C (1,900 m): Small dead mass, but high production of green mass and high content of N and K in the living overground and total underground phytomass. The Nardetum N (1,930 m) is marked by relatively high amounts of P in living and dead parts. The Seslerio-Semperviretum S (2,150 m) is intermediate but with rather low contents of K.In each plot the average total nitrogen found in the underground phytomass is about 200% of the amount found in the living overground phytomass. This suggests a similar relation of living mass in the two components of the total phytomass.The fluctuations in the phytomass throughout the growing season are represented for S. The green mass increase is about 200g DS with about 3 g N/m². This amount corresponds to the average mineralization supply of N during the growing season as obtained by the field incubation method (3.3 g/m², in 0–15 cm depth of the soil). In C the average mineralization is lower whereas the green mass production amounts to 300 g DS/m² with 5.5 g N/m². This discrepancy supports again our hypothesis of an internal nitrogen cycle in the Carex ferruginea community. In F and N we find a lower incorporation of N into the annual green mass production compared with the N mineralized in the upper soil layers (Table 1).Lactate-soluble P and K were as a rule not accumulated in our soil incubation tests. The actual values and their amplitude during the growing season scarcely coincide with the differences of P and K in the green mass.     

Vertical canopy structure of Dutch chalk grasslands in relation to their management

Analysis of leaf canopy overtopping relationships was carried out using a non-destructive point quadrat method followed by a destructive stratified harvest of the above-ground phytomass in Dutch chalk grasslands with different management: summer sheep grazing and annual autumn mowing. The two methods of analysis are compared: e.g. relative leaf overtopping can be recorded by the point quadrat method but it is obscured in vertical vegetation profiles based on stratified phytomass distribution. However the stratified harvest method describes the relationship between canopy phytomass and light microclimate, recorded by measuring Photosynthetically Active Radiation (PAR) at different heights in the vegetation. Plant growth form during peak standing crop is of greater significance than Raunkiaerian life form in determining structure of chalk grassland vegetation. In annually mown grasslands, the tall graminoid growth form, shown by Brachypodium pinnatum, reduces PAR in the lower canopy and suppresses most other growth forms except those which can reach the higher canopy levels, e.g. clumped herbs such as Origanum vulgare. In contrast, grazing can result in a reduction of dominance from the tall graminoids and reduction of the abundance of taller, grazing-sensitive herbs, e.g. clumped and climbing herbs such as Origanum and Lathyrus pratensis, and an increase in grazing-tolerant species, e.g. smaller rosette herb growth forms, e.g. Leontodon hispidus and shorter rhizomatous or stoloniferous graminoids, e.g. Carex flacca and Briza media. Changes in both the overtopping hierarchy at the peak of the growing season and the intensity of overtopping in the course of a growing season are of conclusive importance in determining the relative abundance of species in the vegetation.     

Fire regimes and phytomass growth dynamics in a Quercus coccifera garrigue

An experiment was set up in a Quercus coccifera garrigue in southern France to analyze the effect of burning frequency and season on phytomass production. Fire regimes consisted of late spring or early autumn burns, every 6 yr, every 3 yr, or every 2 yr. The experiment started in 1969 and lasted for 19 yr. In May 1981 and May 1987, 10 samples, each 1 m², were harvested per treatment. Fire frequency had an effect on the quantity of phytomass which was produced: above-ground phytomass decreased with increasing fire frequency. This was mainly due to the lower biomass of woody plants. In all burning treatments the phytomass of herbs was higher than in the unburned vegetation. Within each burning frequency, the total phytomass of the spring-burned vegetation was always higher than that of the autumn-burned community. Generally, the herb phytomass produced was higher in the autumn-burned plots. There were two fairly distinct phases in the period following fire, each with a different level of annual phytomass production. For the first six years it was about 300 g m^?2 yr^?1, falling thereafter to about 50 g m^?2 yr^?1. This and other studies on Q. coccifera garrigue indicate that this community is very resilient with respect to fire, but possesses a low productive capacity and does not show any sign of degeneration up to 30 yr old.     

Herbaceous phytomass and nutrient concentrations of four grass species in Sudanian savanna woodland subjected to recurrent early fire

Fire is an integral ecological factor in African savanna ecosystems, but its effects on savanna productivity are highly variable and less understood. We conducted a field experiment to quantify changes in herbaceous phytomass and nutrient composition in a Sudanian savanna woodland subjected to annual early fire from 1993 to 2004. Fire effects were also assessed on two perennial and two annual grass species during the following growing season. Early fire significantly reduced above‐ground phytomass of the studied species (P = 0.03), their crude protein (P = 0.022), neutral detergent insoluble crude protein (P = 0.016) and concentrations of Ca, Fe and Mn (P < 0.05). Perennial grasses had higher above‐ground phytomass but lower total crude protein and fat than annual grasses. Nonstructural carbohydrates tended to be higher for annuals, while fibre and lignin contents were high for perennials. Except Na and Fe, the concentration of mineral elements varied between species. Fire did not affect measures of digestibility and metabolizable energy, but its effect differed significantly among species. In conclusion, the results illustrate that long‐term frequent fire will counterbalance the short‐term increase in soil fertility and plant nutrient concentrations claimed to be accrued from single or less frequent fire.     

Productivity and mineral cycling in an oak coppice forest 1. Structure and phytomass of the forest

Ecological studies were made on the structure and phytomass of the secondary coppice forest near Tokyo, which was dominated by a deciduous oak,Quercus serrata Thunb. Average height of dominant trees was about 10 m. The shoot density at the beginning of the study was 4,600 ha⁻¹ of which 89.5% belonged toQ. serrata. During the subsequent two growing seasons 8.3% of shoots, mainly small ones, died. All the tree shoots in a 10 m×10 m quadrat were cut and most of the underground parts were dug out. The phytomass calculated by the allometric relations of the dry weight of each plant organ to the square of DBH (D ²) agreed fairly well with the values directly weighed. The average phytomass of the overstory trees was 906 kg a⁻¹. The leaf area index (LAI) of the canopy was 3.85. Phytomass of the undergrowth, mainly a dwarf bamboo,Pleioblastus chino Makino, was 91.8 kg a⁻¹ with LAI of 3.46. The dead phytomass in the stand was 177 kg a⁻¹, so the phytomass alive and dead amounted to 1,170 kg a⁻¹. Heavy self-thinning of the coppice forest is discussed in relation to the rapid development of the log-normal distribution of tree sizes and to the large allometric constants for leaf and branch weight toD ².     

Soil water status influences plant nitrogen use: a case study

We studied differences in nitrogen use efficiency (NUE) among six species [Calamagrostis epigejos (L.) Roth., Carex duriuscula C.A. Mey., Phragmites communis (L.) Trin., Salix gordejevii Y.L. Chang, Salix cheilophila Schneid., and Typha minima Funk.] growing in two contrasting habitat types, i.e., a riverine wetland with high water supply and a riparian zone with low water supply. The two sites were different in soil water supply, but not in nitrogen supply. Here, NUE was defined as the total net primary production per unit nitrogen absorbed. There was no significant difference in NUE between the species growing in the riverine wetland (Carex duriuscula, P. communis, S. cheilophila, T. minima) and the species growing in the river bank (Carex duriuscula, Calamagrostis epigejos, P. communis, S. gordejevii). We further analyzed NUE as the product of the nitrogen productivity (A, the rate of dry matter production per unit of nitrogen in the plant) and the mean residence time of nitrogen (MRT, the period of time a unit of nitrogen is present in the plant). The species growing in the riverine wetland had larger A but lower MRT than the species growing in the river bank. There was an inverse relationship between A and MRT. Consequently, NUE was similar among species and habitats. These results suggested that environmental factors, such as soil water supply, can influence N use by plants.     

Nutrient turnover studies in alpine ecosystems

Summary The nutrient relations of five treeless plant communities on acid soils above siliceous rock of the Central Alps are investigated. Three of these communities, situated on Mt. Patscherkofel, are dominated by dwarf shrubs of the Ericaceae family: Loiseleurietum (P 1, 2175 m NN), Loiseleuria heath (P 2, 2000 m NN), and Vaccinium heath (P 3, 1980 m NN). The other two are bound to higher elevations (2500 m NN, at Timmelsjoch): Caricetum curvulae (T 1), forming the mats, and Salicetum herbaceae (T 2), covering the snow-beds.Phytomass productivity decreases with increasing altitude in the sequence P 3-P 2, P 1-T 1-T 2.Compared with the turf communities of the Northern Calcareous Alps, nitrogen reserves and experimental net-mineralization of the soils (0–15 cm) are extremely low in P 1, P 2, and P 3 (<0.5 g N/m² mineralized per GS¹). The fluctuation of N in the living above-ground phytomass during the GS is also low (about 1.6 g/m² in P 1 and P 2; 2.2 g/m² in P 3, although it exceeds the values of net mineralization. Additional uptake through mycorrhizal fungi or activation of mineralizing microbes in the rhizosphere by exudation is assumed.The P- and K-reserves are extremely small in the humic soils of P 2 and P 3, but somewhat higher in the more mineral soil of P 1. Mean lactatesoluble P of the three sites is low (0.3 g/m² or less) whereas K_lact (2.7–3.3 g/m²) is higher than the lowest level found in some turf communities, e.g. Caricetum firmae. The amounts of P in the phytomass are in the range of those of the turf communities and agree with the gradation in the mean P_lact values (P 1 and P 3>P 2). There are, however, almost no discernible fluctuations of P in the phytomass, and the K-fluctuations are far below the mean K_lact level.The Timmelsjoch communities generally have higher N/C-, P/C-, and K/C-ratios in the soils compared with those of Mt. Patscherkofel, although the N-reserves (g/m²) and the K-reserves (of T 1 only) are lower. The P_lact values are higher than those from Mt. Patscherkofel and also exceed those of the calcareous turf communities. K_lact is low in T 1 whereas in T 2 it is in the range of P 1, P 2, and P 3. Compared with T 1, T 2 has distinctly higher amounts of the three nutrients in the soils and a higher net mineralization of N, as well as higher values in the phytomass components and in the fluctuation of the latter.In conclusion, a general view is given (Fig. 9) of the most important nutrient parameters of the communities represented in this series, including some others of lower altitudes.     

Temporal patterns of inorganic nitrogen uptake by mature sugar maple (Acer saccharum Marsh.) and red spruce (Picea rubens Sarg.) trees using two common approaches

Background: Plant uptake of nitrogen influences many ecosystem processes, yet uptake by trees in northern forests of the United States has not been quantified throughout the growing season.

Aims: To measure NH₄ ⁺ and NO₃ ^? uptake by mature sugar maple (Acer saccharum) and red spruce (Picea rubens) trees during the early, mid and late growing season.

Methods: At Hubbard Brook Experimental Forest, New Hampshire, we used two approaches to measure nitrogen uptake capacity by mature trees: an in situ depletion method using intact roots and an ex situ ¹⁵N tracer method using excised roots.

Results: NH₄ ⁺ uptake was greater than NO₃ ^? for both methods and tree species (P < 0.05). NH₄ ⁺ uptake was lowest during the early growing season, while NO₃ ^? uptake was lowest during the late growing season. Measured rates of NH₄ ⁺ uptake were 2–3 orders of magnitude greater using the in situ depletion method compared with the ex situ ¹⁵N tracer method.

Conclusions: These results demonstrate seasonal differences in nitrogen uptake by two dominant tree species in a northern forest and show that the method employed can significantly impact measured rates of uptake, which could have implications for understanding the magnitude of plant nitrogen uptake and for cross-study comparisons of this process.     

MARSH PLANTS AS VECTORS IN TRACE METAL TRANSPORT IN OREGON TIDAL MARSHES

The role of Pacific coast marsh plants as vectors in the flux of trace metals was studied in natural and perturbated situations. The flux of Cr, Cu, Fe, Mg, Sr, and Zn were studied in natural stands of Carex lyngbyei, Distichlis spicata, Potentilla pacifica, and Salicornia virginica. Seasonal fluxes of trace metals into the shoots of the living plants were measured as was their transfer to the dead plant community through mortality. Disappearance from the latter community (through fragmentation, leaching and excretion) was calculated. Seasonal data were summed to produce annual input-output budgets. The flux varied from 3 mg/m² for Cr in a stand of Potentilla pacifica to 15,000 mg/m² for Fe in Carex lyngbyei.     

Phyto- and necromass above and below ground in a fen

Few investigations deal with the subterranean phytomass in fens (minerotrophic peatlands), their results differing widely. This study was carried out in a "rich fen" in central Sweden, and involved excavation of peat monoliths, depth-sectioning and hand-sorting as to species, type of organ, and living or dead condition. The below-ground living phytomass was large, over 4 kg m⁻² in each of two sample plots, being no <97–96% in the dominants, Carex limosa and C. lasiocarpa , 89% on average in Menyanthes trifoliata , and c. 93% for all species. Carex limosa had its rhizome maximum slightly higher than C. lasiocarpa , but a greater part of its roots penetrated deeper. For Menyanthes , also net annual production above and below ground was estimated. The subterranean phytomass decreased with depth, but living roots were found even at 42 cm. The recognizable necromass did not decrease with depth. These recent constituents would render a very unequal age to the peat. The imbalance between the small green aerial phytomass and the much greater subterranean phytomass was emphasized. It could be related to a relatively very great translocation of photosynthates from above to below ground, and/or to longevity of below-ground organs, the latter at least in Menyanthes.     

Restoration of riparian meadows degraded by livestock grazing: above- and belowground responses

Riparian meadows in the western US are highly productive areas withinthe riparian corridor that have been extensively utilized for livestock grazingand that are often degraded. This study aimed to increase our understanding ofboth the functional responses and restoration potential of mesic meadow systemsin central Nevada, USA. Nitrogen addition and clipping treatments were used toexamine grazing effects, while aeration and revegetation were included toevaluate potential restoration treatments. Belowground response was determinedby examining gravimetric soil water content and rooting depth and activity.Aboveground response was determined for two key mesic meadow species,Carex nebrascensis and Poa pratensis,by quantifying gas exchange and water relations. Large yearly and seasonaldifferences among water tables largely determined the rooting activity anddepthin these mesic meadows and influenced treatment responses. Little rootingactivity occurred within or at the surface of the water table and rooting depthincreased as water table level lowered during the growing season. In general,nitrogen addition decreased rooting activity and depth relative to controls. Itresulted in less negative water potentials and photosynthetic rates that werehigher early in the growing season, but lower later in the growing season. Theeffects of nitrogen addition could be attributed to accelerated phenology andearlier senescence. Clipping resulted in less negative plant water potentialsinCarex and Poa, andPoa had higher photosynthetic rates immediately afterclipping but only for certain dates and comparisons. Aeration increased rootingactivity and depth where there was no confounding effect of water table. Also,predawn and midday water potentials were generally less negative forCarex and Poa in aerated plots.Establishment on the revegetation plots was dominated by annual and early seralspecies and, consequently, these plots exhibited reduced rooting depth andactivity early in the growing season and high root turnover. Results indicatethat water table depth and its effects on soil water are dominant factors indetermining the functional processes and recovery potentials of these riparianmeadows. Clipping had minimal effects on above- and belowground responses,perhaps because clipping was performed late in the growing season after plantshad begun to senescence. Nitrogen addition decreased rooting activity andaltered phenology, indicating deleterious effects. Aeration was effective atovercoming some of the negative affects of overgrazing and may be an effectiverestoration treatment. However, revegetation may have limited potential due toestablishment constraints of the dominant species.     

甘肃中东部地区9种薹草属植物分布区群落特征和土壤养分状况

为探究薹草属(Carex L.)植物在不同植被类型中的分布状况,该研究对甘肃中东部地区9种薹草属(Carex L.)植物分布区的群落特征进行调查,并对土壤养分状况进行比较分析,以揭示野生薹草群落物种多样性和分布特征与土壤环境因子间的关系。结果表明：(1) 9种薹草群落物种多样性差异性较大,Shannon-Wiener多样性指数(H)、Simpson优势度指数(D_si)均以青绿薹草群落最高,亚柄薹草最低;Patrick丰富度指数(R)以异穗薹草群落最高,细叶薹草群落最低;Pielou均匀度指数(J_sw)以凹脉薹草群落最高,亚柄薹草最低。(2) 9种野生薹草属植物适宜生长的土壤pH呈中性或弱碱性,且有机质、氮素、钾素含量较丰富,磷含量偏低;土壤有机质、pH、全氮、全磷、全钾、碱解氮、速效磷、速效钾含量的平均值分别为41.07 g·kg^-1、8.35、1.16 g·kg^-1、0.65 g·kg^-1、5.60 g·kg^-1、47.94 mg·kg^-1、5.82 mg·kg^-1和100.60 mg·kg^-1。(3) 9种薹草属植物群落物种多样性与全氮、全磷、全钾、碱解氮、有机质、降雨量和海拔呈正相关关系,而与土壤pH、速效磷、速效钾呈负相关关系,且降雨量、土壤pH、速效磷和有机质对9种野生薹草属植物群落物种多样性影响较大。     

Simulation of forest carbon dynamics based on a dry-matter production model

Effects of dry season upon a tropical rainforest ecosystem are analysed by employing the microcomputer model developed in a previous paper (Oikawa, 1985). Surplus production (P _s) illustrated in a three-dimensional figure linearly decreases with the length of dry season (λ). Simulation experiments demonstrate that all the three strata constituting the tropical rainforest ecosystem model are able to exist stably and perpetually unless λ exceeds four months. However, it is suggested that the dry season lasting five months brings about a transition from a tropical rainforest to a subtropical deciduous forest, because the upper story fails in a stable and perpetual occurrence at λ =5 months. Though the total living phytomass gradually decreases with prolonged dry season within four months, supplementary increase of living phytomass is observed in the middle story. A considerable amount of soil organic dead accumulated is also observed with increasing dry season, so that fairly constant ecosystem biomass including soil organic dead is predicted independent of the length of dry season so long as it is less than five months.     

Seasonal and annual variability in the quality of important forage plants on Banks Island,Canadian High Arctic

Abstract. In vitro acid‐pepsin digestibility (IVDMD), crude protein (CP), fibre, lignin, and energy content were measured for a variety of forage plants collected annually from Banks Island over five summers and three winters from 1993–1998. Summer samples were collected during mid‐June (start of growing season), mid‐July (peak of growing season), and mid‐late August (senescence). Winter samples were collected in early (November), mid‐ (February), and late‐ (April/May) winter. Samples, collected in areas of both high and low muskox density, included Carex aquatilis, unidentified Carex, Salixarctica, Dryasintegrifolia, Cassiope tetragona, Saxifraga spp., Astragalus spp., Oxytropis spp., lichen, and grass. Seasonal dynamics in forage quality during the growing season were similar to those reported elsewhere in the arctic and high arctic and were consistent across years. However, there were significant year effects in lignin, fibre, and energy content of forages and the crude protein (CP) content of C. aquatilis in winter, indicating annual differences in the quality of forage available to herbivores. The quality of forages on Banks Island was similar between areas subjected to different densities of muskox (ca. 1.6–1.9 versus 0.3 ‐ 0.4/km²) implying that quality was not affected by these grazing intensities. The Banks Island high arctic ecosystem supports an abundance of herbivores. It has been hypothesized that this is because forage quality and/or quantity are superior on Banks Island than elsewhere in the high arctic. Our results regarding forage quality are equivocal. Although the maximum CP content of forages from Banks Island was generally higher than reported elsewhere in the arctic and high arctic, CP content reported elsewhere fell within the interannual range reported from Banks Island. Fibre and energy content of forages from Banks Island were similar to slightly lower than elsewhere in the arctic and high arctic. Such comparisons must be considered in light of the interannual variability in quality we report.     

Phytomass utilization and deposition of feces by ungulates on steppe pastures of eastern Mongolia

The amount of phytomass removed by a complex of livestock (horses, cattle, sheep, and goats) and wild ungulates (Mongolian gazelle Procapra gutturosa Pall.) grazing in plain and mountain pastures of eastern Mongolia has been estimated by taking account of feces deposited by these animals. The results show that at an animal density of up to 30 head/km², the total annual amount of feces reaches 140 kg/ha (dry weight), with the greater part (up to 90 kg/ha) being deposited by horses. The contribution of Mongolian gazelles in some pastures reaches 20–40 kg/ha per year. Decomposition of feces proceeds very slowly, with the annual loss of their weight averaging only 9–12%. This is evidence for gradual accumulation of nondecomposed matter in the soil. The removal of phytomass by the complex of ungulates, calculated from the amount of feces with regard to their annual loss and forage digestibility, varies in different grazing areas from 240 to 400 kg/ha (25–60% of the total aboveground phytomass). The greatest amount of phytomass is utilized by horses, reaching 200 kg/ha (13%), and Mongolian gazelles utilize up to 86 kg/ha per year. In the growing season, ungulates remove no more than 11–16% of the total aboveground phytomass. It is concluded that the impact of total ungulate stock does not impair the productivity of vegetation in the pastures studied.     

Above- and below-ground phytomass and net primary production in boreal mire ecosystems of Western Siberia

We measured phytomass stock and production in Western Siberian mire ecosystems (palsa, ridge, oligotrophic and mesotrophic hollows, fen). To determine the contribution of different phytomass fractions into total production, we developed a method to estimate below-ground production (BNP). Standing crop of living above-ground phytomass on treeless plots varied from 300 to 660 g m⁻², reaching maximum on palsa, where 81% of phytomass consisted of Sphagnum mosses and lichens. In the hollows and the fen, Sphagnum percentage varied from 70 to 95%. Standing crop of living below-ground phytomass varied from 325 to 1,210 g m⁻². It consisted of woody stems, stem bases, rhizomes and roots, with the latter contributing from 30 to 60%. Total production of mire ecosystems in northern taiga of Western Siberia ranged from 350 to 960 g m⁻² year⁻¹ and depended on microtopography of the ecosystem (the presence of permafrost and water table depth). Production of treeless plant communities located on the elevated sites depended on the presence of permafrost: in comparison with the ridge, palsa production was lower. Production on the low sites increased with increase pH and reached maximum (960 g m⁻² year⁻¹) in poor fens. Bryophytes were the major producers above ground. Their production varied from 100 to 272 g m⁻² year⁻¹ and reached maximum on ridges. BNP contributed 37–66%, increasing due to increased contribution of sedges.     

Yield and species composition of a mesic grassland savanna in South Africa are influenced by long‐term nutrient addition

Species composition and productivity of natural grasslands are influenced by soil nutrient status. With high resource availability, productivity is expected to increase, and competition is assumed to gain prominence with predicted exclusion of species of lower competitive ability. During 2010 and 2011 we used the dry weight rank method to measure above‐ground phytomass production of herbage in 96 plots (9 m × 2.7 m) fertilized for 60 years with two forms of nitrogen (N as limestone ammonium nitrate or ammonium sulphate at four levels: 0, 7.1, 14.1, 21.2 g m^?2), phosphorus (P as superphosphate at two levels: 0, 33.6 g m^?2), and lime (two levels: 0, 225 g m^?2). Light attenuation was measured as the proportion of photosynthetically active radiation reaching the lower leaf layers of the grasses and the ground surface. Light conditions beneath the grass layer were reduced by nutrient addition to 30% of full sunlight but remained above 60% in non‐fertilized plots. Grass total above‐ground phytomass production increased with nutrient addition. The strongest yield responses were attained with N plus P addition. Species responses showed that Themeda triandra and Hyparrhenia hirta decreased in above‐ground phytomass production with nutrient addition while Panicum maximum, Eragrostis curvula and E. plana increased. These findings are discussed in terms of competitive interactions among species, their position in the grass canopy and their physiological tolerances to high nitrogen environments.     

Does nitrogen fertilization have an impact on the trade-off between willow growth and defensive secondary metabolism?

The effect of nitrogen fertilization on the phytomass production, shoot length and leaf secondary phenolics in nine Salix myrsinifolia clones was investigated. Cuttings taken from 1-year-old and 2-year-old shoot parts of field cultivated clones were grown at three concentrations of nitrogen (7, 150 and 300 ppm) in a greenhouse for one growing season. The willow clones differed significantly in phytomass yield and secondary phenolics content. Nitrogen fertilization affected significantly the growth and secondary metabolism of willow clones. In most clones, the addition of nitrogen from a sub-optimum concentration (7 ppm) to an optimum concentration (150 ppm) appeared to reduce the amounts of salicortin, chlorogenic acid and unknown salicylate and increased shoot phytomass, but a supraoptimum nitrogen concentration (300 ppm) resulted in highly variable growth and secondary phenolic responses. A significantly negative correlation between leaf phytomass and amount of total phenolics at sub-optimum and optimum N-treatments indicates trade-off between growth and secondary metabolism in willow clones at these treatments. However, the leaf phytomass:total amount of phenolics ratio varied significantly among clones, and in all clones it was not significantly lower at sub-optimum N-treatment than at optimum N-treatment.     

Nitrogen and phosphorus accumulation and cycling by Nymphoides peltata (Gmel.) O. Kuntze (Menyanthaceae)

In 1980, the seasonal changes in nitrogen and phosphorus concentration of various plant parts of Nymphoides peltata (Gmel.) O. Kuntze, together with aspects of nitrogen and phosphorus cycling by this species were studied in an oxbow lake of the river Waal (The Netherlands). The nitrogen and phosphorus stores of the water, seston, sediment and macrophyte compartments were assessed each month.The underground Nymphoides structures had high nitrogen and phosphorus concentrations before and after the main growing season, while during summer the aboveground plant parts had high nutrient contents. Nymphoides peltata accumulated maximum amounts of nitrogen (334 mmol m⁻²) and phosphorus (56.6 mmol m⁻²) in July. The upper layers of the bottom appeared to be an enormous nutrient reservoir (94–99% of total) of which the largest part was not directly available to Nymphoides. Nutrient uptake from the sediments by N. peltata is suggested by the fact that the bottom and/or interstitial water of the sample station devoid of rooted macrophytes, contained higher concentrations of nitrogen and phosphorus than that of the Nymphoides stands. The annual flux of nutrients from Nymphoides to the detritus compartment was estimated to be ca. 1200 mmol nitrogen and 164 mmol phosphorus per m² of littoral. During breakdown of the detritus there was a relatively fast net conversion of organically bound nitrogen and phosphorus to inorganic forms, especially at higher temperatures.Nymphoides has the potential to function as an important nitrogen and phosphorus pump, which regenerates sediment nutrients.     

Season of grazing and stocking rate interactively affect fuel loads in Baikiaea plurijuga Harms woodland in northwestern Zimbabwe

Wildfire is a major disturbance in Baikiaea plurijuga Harms woodland savannas. We tested the hypothesis that the timing and intensity of herbivory influence fuel loads. We used three stocking rates namely light (three cows and four goats ha^?1), medium (six cows and eight goats ha^?1) and heavy (eleven cows and sixteen goats ha^?1) and three times of grazing namely early‐, middle‐ and late‐growing seasons. Season of grazing and stocking rate influenced herbaceous phytomass. Phytomass was generally the highest (53.5 g DM m^?2) in paddocks grazed during the early growing season and the lowest (27.8 g DM m^?2) in those grazed during the late growing season. Phytomass was also generally the highest (40.4 g DM m^?2) in lightly stocked paddocks and the lowest (32.7 g DM m^?2) in heavily stocked ones. Litter mass was the lowest (160.8 g DM m^?2) in paddocks grazed during the early season whereas there were no differences in ungrazed paddocks and those grazed during either mid‐ or late growing seasons (205.4 g DM m^?2). There was a negative relationship between litter mass and stocking rate. Baikiaea Benth. woodlands should be grazed during either the mid‐ or late‐growing season at stocking rates greater than 0.1 LU ha^?1 to reduce grass fuel loads.