Resource allocation patterns in a forb and a sedge in two arctic environments—short-term response to herbivory

The present work investigates C and N allocation patterns in two forage plants: a forb, Oxyria digyna , and a sedge, Eriophorum angustifolium , in subarctic Sweden and high arctic Canada. Short-term changes in concentrations after simulated or natural herbivory (caused by Gynaephora groenlandica on Oxyria in the high arctic habitat) were also investigated. There were no clear differences in concentrations of C and N between the high arctic and subarctic sites in either species. In Oxyria of the subarctic habitat, the minimum N concentrations occurred at earlier phenological stages compared with plants in the high arctic habitat. Simulated herbivory increased the concentration of C in belowground tissues relative to those in control plants in Oxyria at the subarctic site, which may indicate increased allocation of non-C compounds to the growing shoots or daughter ramets. Herbivory by Gynaephora groenlandica caterpillars increased the N concentrations of Oxyria both in aboveground and belowground tissues, possible indicating increased uptake of N in the high arctic habitat. Eriophorum did not show clear trends in concentrations relating to habitat, phenology or simulated herbivory. The difference between Oxyria and Eriophorum in their response to herbivory apparently resulted from contrasting growth habits between the species. Tiller death after reproduction and long leaf life span may be the main reasons for the lack of clear patterns in concentrations in Eriophorum. Compensation after herbivory may be attained by the early production of daughter tillers in Eriophorum instead of the regrowth of the damaged ramets, as in Oxyria. Monitoring the responses for only one season is apparently a too short time period in these long-lived plants.     

香港芒萁群落养分的研究

本文研究香港芒萁群落养分的分配、季节动态和循环。研究结果表明：(1)N、P、K浓度在活物质大于死物质,地上部分大于地下部分。(2)植物的养分贮量是活物质大于死物质,地下部分大于地上部分。(3)当年生芒萁地上部养分贮量随生物量的增加而增加,但是,干物质生产率超过养分吸收率,使得N、P、K浓度由于生物量的增加而降低。(4)虽然芒萁群落的净第一性生产力大于其邻近的草地和灌木林,但其净第一性生产量中的N和P量却小于灌木林。(5)以土壤中的总N和总P来计算,生态系统中的N和P主要贮存于土壤库中,但以土壤有效K来计算,则有大约36％～50％的K贮存于植被中。(6)在研究期间,立枯体和死地被物中的N、P、K贮量逐渐增加,这表明在火灾后生态系统的立枯体和死地被物养分库有一个累积过程。(7)N、P、K通过枯枝落叶的归还量分别占它们在地上部净第一性生产量中的49.1％、30.8％和13.1％,而地上部净第一性生产量中的N和P的31.2％和46.6％来自于养分的内部循环。     

缙云山常绿阔叶林四川大头茶果实中矿质营养元素含量及季节动态

本文测定了缙云山常绿阔叶林中四川大头茶果实中Ｎ,Ｐ,Ｋ,Ｃａ,Ｍｇ,Ｃｕ,Ｚｎ,Ｆｅ,Ｍｎ９种矿质营养元素的含量,研究了这些元素的季节吸收特点与繁殖分配,并探讨了这些元素与某些土壤化学因素之间的关系,结果表明：四川大头茶果实在生长期内矿质营养元素含量随季节而变化,各元素平均含量顺序为：Ｎ＞Ｋ＞Ｃａ＞Ｍｇ＞Ｐ＞Ｍｎ＞Ｆｅ＞Ｚｎ＞Ｃｕ。土壤中化学因子与四川大头茶果实中常量元素含量之间的关系密切,果实中Ｎ含量受土壤中Ｎ,Ｐ,Ｋ含量的综合影响,土壤有机质最终会影响到果实中Ｎ,Ｐ,Ｋ,Ｃａ的吸收,矿质元素的果实繁殖分配量与果实净生物生产量分配之间不存在特定的比例关系。果实元素分配所占比例的大小顺序为Ｐ＞Ｎ＞Ｋ＞Ｃａ＞Ｆｅ＞Ｍｇ＞Ｍｎ＞Ｚｎ＞Ｃｕ。     

Temporal Asynchrony in Soil Nutrient Dynamics and Plant Production in a Semiarid Ecosystem

A central goal of ecosystem ecology is to understand how the cycling of nutrients and the growth of organisms are linked. Ecologists have repeatedly observed that nutrient mineralization and plant production are closely coupled in time in many terrestrial ecosystems. Typically, mineralization rates of limiting nutrients, particularly of nitrogen, during the growing season determine nutrient availability while pools of mineral nutrients remain low and relatively constant. Although several previous reports suggest nitrogen mineralization has the potential to vary seasonally and out of phase with plant production, such a phenomenon has been poorly documented. Here we report results from a semiarid savanna ecosystem characterized by distinct temporal asynchrony in rates of soil nitrogen cycling and plant production. Periods of positive plant growth following the onset of rains coincide with periods of low N turnover rates, whereas higher rates occur late in the wet season following plant senescence and throughout dry seasons. Plant uptake from the substantial mineral N pool present early in the growing season is sufficient to explain most of the N allocation to aboveground plant biomass during the growing season, even in the absence of any wet-season mineralization. The mineral N pool is subsequently recharged by late wet- and dry-season mineralization, plus urine inputs at sites with high levels of ungulate activity. These findings suggest fundamental changes in the quality of substrates available to decomposers over a seasonal cycle, with significant implications for the partitioning of limiting nutrients by plant species, the seasonal pattern of nutrient limitations of aboveground production, and the effective use of N fertilizers in semiarid ecosystems.     

Production and nutrient dynamics of plant communities on a sub-Antarctic island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in two mire-grasslands at Marion Island (46°54S, 37°45E). Mire-grasslands are an important component of the island's vegetation, occurring on very wet peats and dominated by graminoids and bryophytes. Peak aboveground standing stocks of N, P and K in the vascular plant species of the mire-grasslands mostly occurred earlier in the season than did peak aboveground biomass, implying that aboveground accumulation rates of these nutrients were greater than the rate of biomass accumulation. Maximum Ca standing stocks coincided in the season with peak shoot biomass. Depending on the plant species, peak Mg and Na standing stocks occurred either before, or later than, peak shoot biomass. Total (above-plus belowground) standing stocks of nutrients (N+P+K+Ca+Mg+Na) at the time of peak aboveground biomass were 51 g m^-2 at study mire 1 and 44 g m^-2 at study mire 2. The most abundant element in the vegetation was N, followed by K. The net quantities of most nutrients translocated into the aboveground growth were mostly greater than the seasonal mean standing stocks in the aerial biomass. Except for Ca, nutrient standing stocks in the vegetation of the mire-grasslands are in the upper part of the range reported for sub-Arctic and Arctic graminoid communities. They are more similar to standing stocks at oceanic moorlands, montane grasslands and heath communities. Low Ca concentrations occur in the plants so that Ca standing stocks are lower than in most comparable northern hemisphere communities. Pool sizes (i.e. total quantities contained in the plant/soil system to a depth of 25 cm) of N, P, K and Ca are in the lower part of the range reported for wet, graminoid-dominated tundra and tundra-like communities of the northern hemisphere.     

Nutrient content and dynamics in north Swedish shrub tundra areas

Forbs and leaves of deciduous shrubs had high concentrations of Ca, Mg, K, P, and N. Deciduous dwarf shrubs had intermediate concentrations but higher than evergreen dwarf shrubs. Monocots, cryptogams, woody and belowground tissues had low concentrations.
Plant nutrient concentrations and nutrient content in soil organic matter increased from dry towards moist tundra areas.
The residence time of nutrients was considerably less than ten years in surface litter, but several decades or centuries in total organic matter. The longest residence time was found in the moist part of the tundra.
N, K, and P in Betula nana leaves were translocated to a great extent prior to leaf fall, whereas Mg and particularly Ca were only slightly translocated. As on other tundra areas shortage of nutrients probably limits plant growth. In that case short supply of N and F seems most probable due to retention in litter and soil organic matter.     

VARIATION IN THE NUTRIENT CONTENT OF LEAVES OF QUERCUS ALBA,QUERCUS COCCINEA,AND PINUS RIGIDA IN THE BROOKHAVEN FOREST FROM BUD-BREAK TO ABSCISSION

Leaves of Quercus coccinea, Q. alba, and Pinus rigida were collected at six dates during the growing season and analyzed for N, P, K, Ca, Mg, Fe, S, and Na. Leaf weights per unit of leaf area (or length) were determined for the same period. Quercus coccinea and Q. alba leaves increased in weight per unit area by about 30 % and 50 %, respectively. First-year pine leaves increased in weight per unit length by about 65 %. During the second year the weight of pine leaves changed little. Two broad patterns in the nutrient content of leaves were apparent when nutrient content was expressed on the basis of leaf area rather than leaf weight. N, P, and K concentrations increased to a peak in mid- or late summer and declined abruptly just prior to abscission. Concentrations of other elements tended to rise slowly throughout the life of the leaves in all three species. The differences among nutrients and among species support the hypothesis that differential partitioning of the nutrient pool occurs as a result of evolutionary adaptation. The changes in weight of leaves per unit area and in nutrient content during the growing season are important for studies of net primary production and in appraisals of the cycling of nutrients. Least distortion of nutrient relationships occurs when area or length of leaf is used as the basis for expression of nutrient content.     

Hydraulic compensation in northern Rocky Mountain conifers: does successional position and life history matter?

Environmental changes are likely to alter the chemical composition of plant tissues, including content and concentrations of secondary compounds, and thereby affect the food sources of herbivores. After 10 years of experimental increase of temperature, nutrient levels and light attenuation in a sub-arctic, alpine ecosystem, we investigated the effects on carbon based secondary compounds (CBSC) and nitrogen in one dominant deciduous dwarf shrub, Salix herbacea × polaris and two dominant evergreen dwarf shrubs, Cassiope tetragona and Vaccinium vitis-idaea throughout one growing season. The main aims were to compare the seasonal course and treatment effects on CBSC among the species, life forms and leaf cohorts and to examine whether the responses in different CBSC were consistent across compounds. The changes in leaf chemistry both during the season and in response to the treatments were higher in S. herbacea × polaris than in the corresponding current year’s leaf cohort of the evergreen C. tetragona. The changes were also much higher than in the 1-year-old leaves of the two evergreens probably due to differences in dilution and turnover of CBSC in growing and mature leaves paired with different rates of allocation. Most low molecular weight phenolics in the current year’s leaves decreased in all treatments. Condensed tannins and the tannin-to-N ratio, however, either increased or decreased, and the strength and even direction of the responses varied among the species and leaf cohorts, supporting views of influential factors additional to resource-based or developmental controls, as e.g. species specific or genetic controls of CBSC. The results indicate that there is no common response to environmental changes across species and substances. However, the pronounced treatment responses imply that the quality of the herbivore forage is likely to be strongly affected in a changing arctic environment, although both the direction and strength of the responses will be different among plant species, tissue types and substances.     

Production and nutrient dynamics of plant communities on a sub-Antarctic Island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in a fjaeldmark and two fernbrake communities on Marion Island (46°54S, 37°45E). These communities are an important component of the island's vegetation on rocky plateaux and slopes. For most species the aboveground accumulations of N, P and K early in the season were more rapid than increases in the aerial biomass. Rates of Ca, Mg or Na accrual were either similar to, or lower than, rates of aboveground growth. Nutrient (N+P+K+Ca+Mg+Na) standing stocks at the three communities were high; 71 g m^-2 at fjaeldmark, 116 g m^-2 at open fernbrake and 154 g m^-2 at closed fernbrake. The aboveground component accounted for 47% to 65% of these values. N was the most abundant element in the vegetation, followed by K (closed fernbrake) or Ca (open fernbrake and fjaeldmark). Nutrient standing stocks at the two fernbrakes were mostly higher than for most sub-Arctic and alpine dwarf-shrub tundras. Nutrient pool sizes (i.e. the total quantities of nutrients contained in the soil/plant system to a depth of 25 cm) were lower than those reported for arctic tundra meadows but were similar to, and often greater than, those found at heath communities, sub-Arctic dry meadows and dwarf-shrub tundras and some boreal forests. Annual net primary productions of the fernbrake vegetations were high and substantial quantities of nutrients are aquired annually from the soils by the vegetations. Depending on plant species, either N or K was the element taken up in the largest quantity, whereas P was mostly taken up in the lowest amount. A large proportion (mostly all) of the Ca and Mg and a substantial proportion of the N taken up aboveground was lost in the litterfall but little of K taken up was lost in this way.     

Aboveground net production and dry matter allocation ofPinus pumila forests in the Kiso mountain range,central Japan

Aboveground net production rates of the subalpine stone pine (Pinus pumila) forests in central Japan were estimated by the summation method; net production was defined as the sum of annual biomass increment and annual loss due to death. In the two pine stands of different scrub heights, P1 (200 cm) and P2 (140 cm), aboveground biomass reached 177 and 126 ton ha⁻¹, respectively. Leaf biomass was about 14 ton ha⁻¹ in each stand. The estimates of aboveground net production during the 2 year period (1987–1989) averaged 4.1 and 3.7 ton ha⁻¹ y⁻¹ in P1 and P2, respectively, which were at the lowest among the pine forests in the world. Two indices of efficiency of energy fixation, that is, the ratio of net production to the total radiation during a growing season and the ratio of net production to total radiation per unit of leaf weight, were evaluated. Both efficiency indices for the twoP. pumila stands fell in the range obtained for other Japanese evergreen conifer forests. This suggested that the low annual net production of the stone pine stands were mainly due to a limitation in the length of the growing season. The pine forests were also characterized by a small allocation (about 17%) of aboveground net production into biomass increment, in comparison with other evergreen conifer forest types. Annual net carbon gain in theP. pumila forests was suggested to be largely invested in leaf production at the expense of the growth of woody parts.     

Characteristics of foliar chemistry in a commerical spruce-fir stand of northern New England,USA

Monthly foliage samples were evaluated for elemental composition from red spruce and balsam fir trees in a commercial, low elevation spruce-fir stand at Howland, Maine during the 1987 growing season. Balsam fir showed consistently higher concentrations in A1 and to a lesser extent N, Ca, Mg, and Fe when compared to red spruce. Red spruce exhibited consistently higher Mn and K concentrations. Both species showed a marked seasonal trend in the foliar concentrations of N, P, K, and Ca in current year foliage. For N, P, and K current year foliar concentrations declined rapidly during the early part of the growing season. Calcium concentrations steadily increased during the growing season for both species and age class of needles. Foliar concentrations of N and P in both species suggests that the availability of these nutrients is limited for this site.     

长白山林线主要木本植物叶片养分的季节动态及回收效率

植物叶片养分含量的季节动态和回收效率对植被生态系统的养分循环和植物生长策略具有重要意义。以长白山高山林线上分布的3种主要木本植物——岳桦(Betula ermanii),牛皮杜鹃(Rhododendron aureum)和笃斯越橘(Vaccinium uliginosum)为研究对象,通过测定叶片中N、P、K、Ca、Mg、Fe等6种养分元素含量,分析在林线处植物叶片养分含量的季节动态及其与土壤养分含量的关系。结果表明岳桦和笃斯越橘叶片中养分元素的季节动态基本一致,即:N、P、K含量在生长季内逐渐降低,而Ca的含量逐渐增加;Mg在生长季旺盛期最低,而Fe含量却最高。牛皮杜鹃作为常绿灌木,叶片养分的季节动态与其余两种植物明显不同,表现为在生长季初期6种养分元素含量最低。岳桦和笃斯越橘植物叶片中N、P、K、Fe都有一定程度的回收,但笃斯越橘叶片的养分回收率更高,反映了笃斯越橘更能适应相对贫瘠的环境。3种林线植物叶片中养分含量与土壤养分并不存在显著的相关性,说明长白山林线上土壤中养分的分布没有对林线上3种主要的木本植物的生长和分布产生直接的影响。     

Ecology of heath communities dominated by Cassiope tetragona at Alexandra Fiord, Ellesmere Island, Canada

At a high arctic lowland on Ellesmere Island, heath communities dominated by Cassiope tetragona were widespread, and occurred in a variety of habitats that differred in time of snowmelt, relative site moisture, soil thaw depth, and air and soil temperature. Cover, standing crop, and production were dominated by woody plants, notably Cassiope tetragona and Dryas integrifolia . The Cassiope -dominated heaths were similar in composition to those at other Canadian high arctic localities, but were less similar to localities in Greenland and Spitzbergen. Compositional relationships among Cassiope -dominated heaths in different habitats at the study site may be largely determined by two interrelated environmental factors, time of snowmelt and site moisture.
Aboveground vascular plant biomass was concentrated near the ground surface, resulting in simple vertical structure that takes advantage of relatively warm soil and air temperatures in summer, and a protective snow cover in winter. Aboveground vascular plant standing crop was largely comprised of attached dead tissue, and there were equal proportions of above- and belowground biomass. Low community production was due to dominance by long-lived, slow-growing species, and to short growing seasons and cold temperatures. Overall character of Cassiope -dominated heaths reflects the conservative, stress tolerant growth strategy of the dominant species, Cassiope tetragona .     

Nutrient sequestration potential of water primrose Ludwigia stolinefera (Guill. & Perr.) P.H. Raven: A strategy for restoring wetland eutrophication

The current work investigates the capacity of the water primrose (Ludwigia stolinefera) to sequester inorganic and organic nutrients in its biomass to restore eutrophic wetlands, besides its nutritive quality as fodder for animals. The nutrient elements and nutritive value of the water primrose were assessed seasonally in polluted and unpolluted watercourses. The water primrose plants’ highest biomass was attained during summer; then, it was significantly reduced till it reached its lowest value during winter. In the polluted canal, the plant root and shoot accumulated higher contents of all nutrient elements (except Na and Mg) rather than in the unpolluted Nile. They accumulated most investigated nutrients in the growing season during summer. The shoots accumulated higher contents of N, P, Ca, and Mg than the root, which accumulated higher concentrations of Na and K. Therefore, summer season is the ideal time to harvest water primrose for removing the maximum nutrients for restoring eutrophic watercourses. The aboveground tissues had the highest values of ether extract (EE) during spring and the highest crude fibers (CF) and total proteins (TP) during summer. In contrast, the belowground tissues had the lowest EE, CF, and TP during winter. In spring, autumn, and winter seasons, the protein content in the grazeable parts (shoots) of the water primrose was within the range, while in summer, it was higher than the minimum requirement for the maintenance of animals. There was a decrease in crude fibers and total proteins, while an increase in soluble carbohydrates content in the below- and above-ground tissues of water primrose under pollution stress. The total protein, lipids, and crude fibers of the aboveground parts of water primrose support this plant as a rough forage.     

The effect of crown position and date of sampling on biomass,nutrient concentrations and contents of needles and shoots in European larch

 The nutrient concentrations and contents of needles and shoots of 22-year-old European larch (Larix decidua Mill.) were evaluated with respect to crown position, age of tissues and sampling date during a complete growing season. Concentrations of N, P, K, Ca, Mg and Zn in the needles and of N, P and K in the shoots differed significantly among the dates of sampling. The concentrations of N and Mn in the needles and all nutrients in the shoots (except Mg) also differed significantly with crown position. Maximum needle biomass was observed in the middle crown position (55% of the total) and maximum shoot biomass, in the lower crown position (52% of the total). Maximum needle and shoot nutrient contents were observed in the middle position of the living crown for long shoot, short shoot-1, short shoot-2, short shoot-3 and, short shoot-4 age classes while highest contents for short shoot-5 and short shoot-6 age classes were observed in the lower crown position. Biases up to 42% for Mg in the needles and 200% for K in the shoots were obtained when only long shoot tissues are used for content evaluation. For needles and shoots, Mg and K are more difficult nutrients to evaluate. A sampling methodology is proposed for evaluating nutrient contents of the living crown. Accepted: 10 August 1995     

Mineralization and distribution of nutrients in plants and microbes in four arctic ecosystems: responses to warming

Mineralization and nutrient distribution in plants and microbes were studied in four arctic ecosystems at Abisko, Northern Sweden and Toolik Lake, Alaska, which have been subjected to long-term warming with plastic greenhouses. Net mineralization and microbial immobilization were studied by the buried bag method and ecosystem pool sizes of C, N and P were determined by harvest methods. The highest amounts of organic N and P were bound in the soil organic matter. Microbial N and P constituted the largest labile pools often equal to (N) or exceeding (P) the amounts stored in the vegetation. Despite large pools of N and P in the soil, net mineralization of N and P was generally low during the growing season, except in the wet sedge tundra, and in most cases lower than the plant uptake requirement. In contrast, the microorganisms immobilized high amounts of nutrients in the buried bags during incubation. The same high immobilization was not observed in the surrounding soil, where the microbial nutrient content in most cases remained constant or decreased over the growing season. This suggests that the low mineralization measured in many arctic ecosystems over the growing season is due to increased immobilization by soil microbes when competition from plant roots is prevented. Furthermore, it suggests that plants compete well with microbes for nutrients in these four ecosystems. Warming increased net mineralization in several cases, which led to increased assimilation of nutrients by plants but not by the microbes.     

Seasonal variation in nutrient concentration in leaves and branches of Quercus pyrenaica

Abstract. Seasonal variation in nutrient concentration in leaves and branches of Quercus pyrenaica was studied in natural Q. pyrenaica forest in the Sierra de Gata (Salamanca Province, Spain). Two permanent plots were established at the two extremes of a rainfall gradient in this area: annual mean precipitation from 720 mm at Fuenteguinaldo (granite bedrock) to 1580 mm at Navasfrias (schists and graywackes). Leaf and branch samples were collected every three weeks during the growing season from May to October, at three height levels of the tree canopy. Seasonal changes and internal nutrient dynamics were investigated for N, Ca, Mg, K, Na, Mn, Fe and P during a two-year period. The concentrations of all nutrients varied among the seasons; these variations were related to nutrient mobility and the annual physiological cycle. Nutrient concentrations decrease in the case of K and P, while the sparse mobile nutrients Ca, Mg, Mn and Fe gradually accumulated during each growing season. In Navasfrias a considerable resorption of P from senescing leaves was detected. Different patterns were found for the other nutrients studied (Na and N).     

Significance of sequential leaf development for nutrient balance of the cotton sedge,Eriophorum vaginatum L.

Summary The sedgeEriophorum vaginatum in an interior Alaskan muskeg site produced leaves sequentially at about 1.5-month intervals. Each leaf remained active for two growing seasons. Young leaves (even those initiated late in the season) always had high concentrations of N, P, K and Mg and were low in Ca. Stems had high concentrations of nutrients, sugar, amino acid N and soluble organic P in autumn and spring but low concentrations in summer. Growth of leaves in spring was strongly supported by translocation from storage. Leaves approached their maximum nutrient pool before nutrient uptake began in late spring, one month before maximum biomass. Retranslocation of nutrients from aging leaves could support nutrient input into new, actively growing leaves as a consequence of the sequential leaf development. For instance retranslocation from aging leaves accounted for more than 90 and 85% of P and N input to new leaves appearing in early summer and 100% to leaves that appeared later. Leaching losses were negligible. Half time for decay of standing dead litter was 10 years. We suggest that sequential leaf development paired with highly efficient remobilization of nutrients from senescing leaves enables plants to recycle nutrients within the shoot and minimize dependence upon soil nutrients. This may be an important mechanism enablingEriophorum vaginatum to dominate nutrient-poor sites. This may also explain why graminoids with sequential leaf production cooccur with evergreen shrubs and dominate over forbs and deciduous shrubs in nutrient-poor sites in the boreal forest (e.g., in bogs) and at the northern limit of the tundra zone.     

Nutrient retention by Pseudoscleropodium purum and its relation to growth

Abstract

A complete mineral nutrient fertilizer solution was added to carpets of Pseudoseleropodium purum growing in Windsor Forest, Berkshire, to study the efficiency of nutrient capture from dilute solutions and its effect on growth. In the experiment different frequencies of application were balanced by employing different concentrations of fertilizer so that all treatment plots received the same quantities of nutrients over a six-month period. At the end of the experiment the concentrations of macronutrients (N, P, K, Mg, Ca) within the moss tissues were determined and shoot growth was assessed by the tagged shoot method.

Significant luxury uptake occurred only in the case of P where shoot concentrations in fertilized plots were 50% above the control. Smaller net increases were noted for other elements, particularly Mg. Shoot N concentration was hardly affected by fertilizer application possibly due to efficient internal recycling of the element in P. purum. Levels of exchange ably held cations were not markedly increased by fertilizer addition in the field experiment but when shoots were incubated with fertilizers in the laboratory for 30 min the concentrations of exchangeable Ca and Mg rose appreciably and exchangeable K fell. These changes appear to be quickly reversed under field conditions without transfer of cations to the protoplasts. The importance of elevated cation exchange capacity in the initial sequestration of nutrient cations is, therefore, questioned. Growth stimulation due to fertilizer treatment was negligible. Maximum net uptake of P, Mg and Ca occurred when plots were watered at weekly intervals with a dilute fertilizer and least occurred where a concentrated solution was applied at less frequent intervals implying that contact time between P. purum and its potential nutrients is an important factor. The period elapsing between fertilizer additions may also have significance, however, due to leaching away of nutrients and the re-establishment of a natural ionic balance at the cell wall exchange sites. Levels of nutrient absorption and growth were greatest at a site where the moss remained moist longest. This result indicates that the ability of P. purum to absorb and utilize additional nutrients is governed by general metabolic performance which is itself restricted by water supply in Windsor Forest.     

Seasonal nutrient variation in trembling aspen

Summary Variation in concentrations of N, K, Mg, and Ca in vegetative-buds, leaves, twigs, and male catkins of trembling aspen (Populus tremuloides Michx.), determined throughout a growing season in central Minnesota, reflected the different physiological roles of the nutrients. Practical implications are discussed.