Fertile forests produce biomass more efficiently.

Trees with sufficient nutrition are known to allocate carbon preferentially to aboveground plant parts. Our global study of 49 forests revealed an even more fundamental carbon allocation response to nutrient availability: forests with high-nutrient availability use 58 ± 3% (mean ± SE; 17 forests) of their photosynthates for plant biomass production (BP), while forests with low-nutrient availability only convert 42 ± 2% (mean ± SE; 19 forests) of annual photosynthates to biomass. This nutrient effect largely overshadows previously observed differences in carbon allocation patterns among climate zones, forest types and age classes. If forests with low-nutrient availability use 16 ± 4% less of their photosynthates for plant growth, what are these used for? Current knowledge suggests that lower BP per unit photosynthesis in forests with low- versus forests with high-nutrient availability reflects not merely an increase in plant respiration, but likely results from reduced carbon allocation to unaccounted components of net primary production, particularly root symbionts.

Europe-wide reduction in primary productivity caused by the heat and drought in 2003.

Future climate warming is expected to enhance plant growth in temperate ecosystems and to increase carbon sequestration. But although severe regional heatwaves may become more frequent in a changing climate, their impact on terrestrial carbon cycling is unclear. Here we report measurements of ecosystem carbon dioxide fluxes, remotely sensed radiation absorbed by plants, and country-level crop yields taken during the European heatwave in 2003. We use a terrestrial biosphere simulation model to assess continental-scale changes in primary productivity during 2003, and their consequences for the net carbon balance. We estimate a 30 per cent reduction in gross primary productivity over Europe, which resulted in a strong anomalous net source of carbon dioxide (0.5 Pg C yr(-1)) to the atmosphere and reversed the effect of four years of net ecosystem carbon sequestration. Our results suggest that productivity reduction in eastern and western Europe can be explained by rainfall deficit and extreme summer heat, respectively. We also find that ecosystem respiration decreased together with gross primary productivity, rather than accelerating with the temperature rise. Model results, corroborated by historical records of crop yields, suggest that such a reduction in Europe's primary productivity is unprecedented during the last century. An increase in future drought events could turn temperate ecosystems into carbon sources, contributing to positive carbon-climate feedbacks already anticipated in the tropics and at high latitudes.

The influence of climate and fructification on the inter-annual variability of stem growth and net primary productivity in an old-growth, mixed beech forest.

The periodic production of large seed crops by trees (masting) and its interaction with stem growth has long been the objective of tree physiology research. However, very little is known about the effects of masting on stem growth and total net primary productivity (NPP) at the stand scale. This study was conducted in an old-growth, mixed deciduous forest dominated by Fagus sylvatica (L.) and covers the period from 2003 to 2007, which comprised wet, dry and regular years as well as two masts of Fagus and one mast of the co-dominant tree species Fraxinus excelsior (L.) and Acer pseudoplatanus (L.). We combined analyses of weather conditions and stem growth at the tree level (inter- and intra-annual) with fruit, stem and leaf production, and estimates of total NPP at the stand level. Finally, we compared the annual demand of carbon for biomass production with net canopy assimilation (NCA), derived from eddy covariance flux measurements, chamber measurements and modelling. Annual stem growth of Fagus was most favoured by warm periods in spring and that of Fraxinus by high precipitation in June. For stem growth of Acer and for fruit production, no significant relationships with mean weather conditions were found. Intra-annual stem growth of all species was strongly reduced when the relative plant-available water in soil dropped below a threshold of about 60% between May and July. The inter-annual variations of NCA, total NPP and leaf NPP at the stand level were low (mean values 1313, 662 and 168 g C m(-2) year(-1), respectively), while wood and fruit production varied more and contrarily (wood: 169-241 g C m(-2) year(-1); fruits: 21-142 g C m(-2) year(-1)). In all years, an annual surplus of newly assimilated carbon was calculated (on average 100 g C m(-2) year(-1)). The results suggest that stem growth is generally not limited by insufficient carbon resources; only in mast years a short-term carbon shortage may occur in spring. In contrast to common assumption, stem growth alone is not a sufficient proxy for total biomass production or the control of carbon sequestration by weather extremes.

Air Pollution and Forest Decline in a Spruce (Picea abies) Forest.

Symptoms of forest decline of spruce in Europe range from needle yellowing and loss to tree and stand mortality. In a study area in northeast Bavaria, West Germany, where forest decline was initially detected, exposure to high concentrations of gaseous pollutants, SO(2,) NO(x,) and ozone has had no long-lasting direct effect on needles, and pathogens have only been secondary agents. Deposition of sulfur, nitrate, and ammonium, however, have significantly modified plant nutrition and soil chemistry. Spruce roots apparently take up ammonium rather than nitrate with an antagonistic effect on uptake of Mg. Nitrate left in the soil solution is leached together with sulfate to ground water, accelerating soil acidification and decreasing Ca/Al and Mg/Al ratios in the soil solution. Soil solution chemistry affects root development, and water and nutrient uptake. Had all nutrients become equally deficient, spruce trees probably could have adjusted by retarding their growth. However, canopy uptake of atmospheric nitrogen in addition to root uptake stimulated growth and caused a nitrogen to cation imbalance to develop; this imbalance resulted in the decline symptoms.

Xylem-tapping mistletoes: water or nutrient parasites?

Most mistletoes parasitize higher plants by tapping the xylem (a conduction tissue) of their hosts. Field observations of diurnal gas exchange parameters and carbon isotope ratios in xylem-tapping mistletoes from three continents support the hypotheses that water use efficiency and carbon isotope composition are related and that mistletoes which are parasitic for water are also nutrient parasites, differing in their water use efficiency relative to that of their hosts on the basis of host nitrogen supply in the transpiration stream.

Adaptive survival mechanisms and growth limitations of small-stature herb species across a plant diversity gradient.

Several biodiversity experiments have shown positive effects of species richness on aboveground biomass production, but highly variable responses of individual species. The well-known fact that the competitive ability of plant species depends on size differences among species, raises the question of effects of community species richness on small-stature subordinate species. We used experimental grasslands differing in species richness (1-60 species) and functional group richness (one to four functional groups) to study biodiversity effects on biomass production and ecophysiological traits of five small-stature herbs (Bellis perennis, Plantago media, Glechoma hederacea, Ranunculus repens and Veronica chamaedrys). We found that ecophysiological adaptations, known as typical shade-tolerance strategies, played an important role with increasing species richness and in relation to a decrease in transmitted light. Specific leaf area and leaf area ratio increased, while area-based leaf nitrogen decreased with increasing community species richness. Community species richness did not affect daily leaf carbohydrate turnover of V. chamaedrys and P. media indicating that these species maintained efficiency of photosynthesis even in low-light environments. This suggests an important possible mechanism of complementarity in such grasslands, whereby smaller species contribute to a better overall efficiency of light use. Nevertheless, these species rarely contributed a large proportion to community biomass production or achieved higher yields in mixtures than expected from monocultures. It seems likely that the allocation to aboveground plant organs to optimise carbon assimilation limited the investment in belowground organs to acquire nutrients and thus hindered these species from increasing their performance in multi-species mixtures.

Cytokinins in the xylem sap of desert-grown almond (Prunus dulcis†) trees: Daily courses and their possible interactions with abscisic acid and leaf conductance.

Xylem sap samples were obtained from one- to four-year-old almond trees [Prunus dulcis (Miller) D. A. Webb] that had been grown in lysimeters of different volumes with different amounts of available water and subjected to an annual drying cycle. The samples were analyzed for cytokinin patterns by ELISA during the growing season. Free bases, ribosides and nucleotides of the N6 -(Δ2 -isopentenyl) adenine (iP) and the zeatin (Z) type could be identified and quantified. Z-type cytokinins were always predominant. In many cases the concentrations of the cytokinin fractions were rather constant during the day. In a limited number of days, however, Z-type cytokinins showed peak concentrations in the morning and a rapid decrease in the afternoon. A correlation between water status of the trees and the concentrations of cytokinins or their daily variations in the xylem sap could not be established. When the concentration of abscisic acid in the xylem sap was not limiting leaf conductance, daily-variation of a cytokinin fraction preceded daily variation in leaf conductance. We conclude that in almond trees, cytokinins may affect stomatal behaviour on a short-term basis. This is an outcome of changes in their xylem sap concentrations during the course of a day. Abscisic acid, on the other hand, acts as an opposing signal, the size of which reflects long-term water deficit.

A new type of climatized gas exchange chamber for net photosynthesis and transpiration measurements in the field.

A temperature- and humidity-controlled plant chamber for CO2 and H2O exchange measurements in the field is described in which the heat exchanger assembly and humidity controlling water vapour trap are separated from the plant cuvette. The shape and construction material of the plant cuvette can vary according to the demands of the experimental conditions and the size and growth form of the plant. The natural illumination field is only slightly altered in this plant cuvette. In the chamber, the temperature and humidity conditions can either be held constant throughout a wide range of conditions or can be programmed to track ambient condition. In this manner, not only temperature and absolute humidity are replicated, but it is also possible to reproduce the natural conditions of water vapour gradient between the evaporating surfaces in the mesophyll and the atmosphere, the relative humidity of the air, and the temperature difference between the leaf and the ambient air. Thus, the chamber appears to be an appropriate instrument to investigate with sufficient accuracy the reactions of individual plants in cultivation or in natural communities under field conditions.

[The effect of light and temperature of the CO2 exchange of different life forms in the ground vegetation of a montane beech forest]. / Die wirkung von licht und temperatur auf den CO2-gaswechsel verschiedener lebensformen aus der krautschicht eines montanen buchenwaldes.

In a montane beech (Fagus sylvatica) forest the influence of the climatic factors, light and temperature, on net photosynthesis and on the CO2 balance of the ground vegetation was investigated. The total turnover of carbon was calculated. Species studied included: Athyrium filix-femina, Oxalis acetosella, Luzula luzuloides, Deschampsia flexuosa and young plants of Fagus sylvatica. 1. The light compensation point in all spp. is between 300 and 500 lux except for D. flexuosa where it is 2 klx. Light saturation is attained at 2-3 klx for A. filix-femina, at 5-6 klx for O. acetosella, and at 6-7 klx for L. luzuloides and F. sylvatica. The net photosynthesis of D. flexuosa increases linearly upto 12 klx. This plant, therefore, is more closely related to plants with high light requirements than all the other species under experiment. 2. The maximum rates of net photosynthesis in O. acetosella and A. filix-femina are higher than in all the other plants, independent of the reference system. Per unit dry weight they even attain rates of CO2 uptake (22-27 mg CO2/gdw·h) known from herbs under the much better light conditions of an open habitat. F. sylvatica and L. luzuloides exhibit per unit dry weight only 30% of this rate and D. flexuosa 25%. On a leaf surface area and chlorophyll content basis differences are smaller: F. sylvatics attains 75%, L. luzuloides reaches 50% and D. flexuosa only 30% of the maximal rates of net photosynthesis of O. acetosella and A. filix-femina. The higher CO2 uptake of O. acetosella and A. filix-femina points to a better adaptation of their photosynthetic apparatus in comparison to all the other species of the same habitat. 3. At light saturation the temperature optimum of A. filix-femina and O. acetosella covers a smaller range at lower temperatures than was found in the other species. These attain almost maximal rates of net photosynthesis over the whole range of temperatures of their natural habitat. At decreasing light intensities the temperature optimum of O. acetosella changes from 13-18° C at 8-12 klx to a range of even lower temperatures (9-12° C at 1 klx). 4. The respiration of the rhizome and the roots of O. acetosella is per unit dry weight 40% of the dark respiration rate in the above ground material. 5. The daily gain of net photosynthesis per unit dry weight of O. acetosella and A. filix-femina is 4 times as high as in L. luzuloides and in F. sylvatica and 7 times as high as in D. flexuosa. Per unit of surface area and chlorophyll content differences are smaller. The sequence in all cases remains the same. During the night D. flexuosa has the highest relative respiratory loss. Its CO2 gain over 24 hours is very small. 6. The importance of sun flecks on the CO2 balance is small in all species except D. flexuosa. More important is the mean light intensity and the rate of net photosynthesis which is attained under these conditions. The amount of CO2 photosynthetically bound in sun flecks is 6% of the daily balance in A. filix-femina, 16-19% of the daily balance in O. acetosella, L. luzuloides and F. sylvatica, and 27% of the daily balance in D. flexuosa. The existence of D. flexuosa is dependent on the occurrence of sun flecks on the forest floor. The ecological significance of the relative light intensity in the mosaic-like distribution of plants on the forest floor is discussed. 7. The varying success in adaptation to the conditions of the habitat becomes even more evident when compared with the primary production of the beech crown. The daily gain of net photosynthesis of O. acetosella and A. filix-femina per unit dry weight is much larger than in either the sun or shade leaves in the canopy of the same stand. Per unit surface area of the leaves they attain 18-20%, per unit chlorophyll content 32% (L. luzuloides and the young plants of F. sylvatica 16-27%, D. flexuosa 4%) of the gain of net photosynthesis in the beech sun leaves. 8. A comparison with a model of primary production (maximal rates of net photosynthesis under experimentally optimal conditions over the whole day = 100%) shows what effect the different climatic factors of the natural habitat have in limiting the CO2 balance, and to what extend the actual CO2 gain reaches the physiological optinum. On the forest floor the rate of net photosynthesis is reduced primarily through the intense shade of the beech canopy and by dawn and dusk (reduction of the maximal CO2 gain in O. acetosella and in A. filix-femina ca. 50%, in L. luzuloides and F. sylvatica ca. 60% and in D. flexuosa ca. 86%). The effect of additional clouds is smaller (reduction of the maximal CO2 gain in all species 4-6%, maximal 19%). The effect of temperature is very small for L. luzuloides, F. sylvatica and D. flexuosa. Corresponding to the low temperature optimum the influence of the prevailing temperatures is much higher in O. acetosella and A. filix-femina (4-6% reduction of the maximal CO2 gain through temperatures above optimum). 9. For an assessment of the competition potential of O. acetosella compared to L. luzuloides and F. sylvatica for the vegetation period of April to August, the carbon balances of the whole plants were estimated. These are compared with the dry weight increase. Among the three life forms of the rhizome geophyte (Oxalis), the hemicryptophyte (Luzula) and the phanerophyte (Fagus) there are striking differences in the use and in the distribution of the CO2 gain. L. luzuloides invests 65% of the net photosynthetic gain as dry weight increment (O. acetosella only 44%, F. sylvatica 40%). Moreover, the growth of L. luzuloides takes place primarily above ground with the establishment of new leaves. The relative proportion of the growth above ground to the total dry weight increment is for L. luzuloides 63%, as against 57% in F. sylvatica and only 42% in O. acetosella. In respect to the total carbon balance the better use of the CO2 gain gives L. luzuloides in this habitat a higher competition potential than the photosynthetically more active O. acetosella. The constitutional differences in the photosynthetic activity are compensated for by the distribution of the assimilates to shoot and root.

Carbon and nitrogen isotope ratios in different compartments of a healthy and a declining Picea abies forest in the Fichtelgebirge, NE Bavaria.

Natural carbon and nitrogen isotope ratios were measured in different compartments (needles and twigs of different ages and crown positions, litter, understorey vegetation, roots and soils of different horizons) on 5 plots of a healthy and on 8 plots of a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), which has recently been described in detail (Oren et al. 1988a; Schulze et al. 1989). The δ13C values of needles did not differ between sites or change consistently with needle age, but did decrease from the sun-to the shade-crown. This result confirms earlier conclusions from gas exchange measurements that gaseous air pollutants did no long-lasting damage in an area where such damage was expected. Twigs (δ13C between-25.3 and-27.8‰) were significantly less depleted in 13C than needles (δ13C between-27.3 and-29.1‰), and δ13C in twigs increased consistently with age. The δ15N values of needles ranged between-2.5 and-4.1‰ and varied according to stand and age. In young needles δ15N decreased with needle age, but remained constant or increased in needles that were 2 or 3 years old. Needles from the healthy site were more depleted in 15N than those from the declining site. The difference between sites was greater in old needles than in young ones. This differentiation presumably reflects an earlier onset of nitrogen reallocation in needles of the declining stand. δ15N values in twigs were more negative than in needles (-3.5 to-5.2‰) and showed age- and stand-dependent trends that were similar to the needles. δ15N values of roots and soil samples increased at both stands with soil depth from-3.5 in the organic layer to +4‰ in the mineral soil. The δ15N values of roots from the mineral soil were different from those of twigs and needles. Roots from the shallower organic layer had values similar to twigs and needles. Thus, the bulk of the assimilated nitrogen was presumably taken up by the roots from the organic layer. The problem of separation of ammonium or nitrate use by roots from different soil horizons is discussed.

The effect of nitrogen nutrition on growth and biomass partitioning of annual plants originating from habitats of different nitrogen availability.

The hypothesis was tested that faster growth of nitrophilic plants at high nitrogen (N) nutrition is counterbalanced by faster growth of non-nitrophilic plants at low N-nutrition. Ten annual plant species were used which originated from habitats of different N-availability. The species' preference for N was quantified by the "N-number" of Ellenberg (1979), a relative measure of nitrophily. The plants were cultivated in a growth cabinet at five levels of ammonium-nitrate supply. At low N-supply, the relative growth rate (RGR) was independent of nitrophily. At high N-supply, RGR tended to be higher in nitrophilic than in non-nitrophilic species. However, the response of RGR to N-supply was strongly and positively correlated with the nitrophily of species. Increasing N-supply enhanced partitioning to leaf weight per total biomass (LWR) and increased plant leaf area per total biomass (LAR). Specific leaf weight (SLW) and LWR were both higher in non-nitrophilic than in nitrophilic species at all levels of N-nutrition. NAR (growth per leaf area or net assimilation rate) increased with nitrophily only under conditions of high N-supply. RGR correlated positively with LAR, irrespective of N-nutrition. Under conditions of high N-supply RGR correlated with SLW negatively and with NAR positively.

Xylem water flow in tropical vines as measured by a steady state heating method.

A method for determining the mass flow rate of xylem water in thin stems under natural field conditions is presented. Diurnal courses of xylem water flow and stomatal conductance of the vines Entadopsis polystachya, Cyclanthera multifoliolata, and Serjania brachycarpa were examined in a tropical deciduous forest on the west coast of Mexico. E. polystachya (leaf area 23.6 m2) had a maximum water flow rate of 6.50 kg h-1 or 1.44 kg cm-2stem basal area h-1; daily water use was 2.00 kg m-2leaf area day-1. S. brachycarpa (leaf area 4.5 m2) and C. multifoliolata (leaf area 3.6 m2) had a maximum water flow rate of 0.72 and 0.19 kg h-1 or 0.63 and 0.92 kg cm-2stem basal area h-1. Daily water use was 1.26 and 0.39 kg m-2leaf area day-1, respectively. The daily courses of xylem water flow were strongly influenced by the orientation of the leaf area to irradiance and its intensity. While leaves of E. polystachya had a constant high stomatal conductance during the day, S. brachycarpa had a maximum stomatal opening in the morning followed by continuous closure during the rest of the day. In contrast to the woody species, the herbaceous C. multifoliolata exhibited a strong midday depression of stomatal conductance and wilting of its leaves. The leaf biomass accounted for 8% (Entadopsis), 16% (Serjania), and 23% (Cyclanthera) of above-ground biomass. The relation of sapwood area to leaf area supplied (Huber value) was 0.19 (Entadopsis), 0.18 (Serjania), and 0.06 (Cyclanthera) cm2 m-2.

Insect capture and growth of the insectivorous Drosera rotundifolia L.

Rates of insect capture increased with leaf area in the insectivorous plant Drosera rotundifolia, and growth of new leaves was related to insect capture. However, increased leaf growth was counterbalanced by leaf abscission which was in turn related to insect capture and leaf growth. Leaf loss equaled leaf growth in plants having natural rate of insect capture. A large proportion of the nitrogen gain from prey was stored in the hypocotyl; it was estimated from feeding experiments that about 24% to 30% of the nitrogen stored in the hypocotyl after winter originated from insect capture in the previous season. The effect of insect capture is discussed in relation to the life cycle of Drosera.

15N-ammonium and 15N-nitrate uptake of a 15-year-old Picea abies plantation.

Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either 15N-ammonium or 15N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). 15N-labeling (62 mg 15N m-2 under conditions of 1.5 g N m-2 atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The 15N recovery within the entire stand (including soils) was 94%±6% of the applied 15N-ammonium tracer and 100%±6% of the applied 15N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the 15N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the 15N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both 15N-ammonium and 15N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees. 15N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered 15N in spruce; no enrichment could be detected in stem wood. Although 15N-ammonium and 15N-nitrate were applied in the same molar quantities (15NH 4+ : 15NO 3- =1:1), the tracers were diluted differently in the inorganic soil N pools (15NH 4+ /NH 4+ : 15NO 3- /NO 3- =1:9). Therefore the measured 15N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate 15N uptake from inorganic N pools resulted in a 2-4 times higher ammonium than nitrate uptake by P. abies.

Growth, photosynthesis and storage of carbohydrates and nitrogen in Phaseolus lunatus in relation to resource availability.

Growth, photosynthesis, and storage of nitrogen (N) and total non-structural carbohydrates (TNC) of a perennial wild type and an annual cultivar of lima bean (Phaseolus lunatus) were examined at different light intensities and N supplies. Relative growth rate and photosynthesis increased with light and N availability. N limitation enhanced biomass allocation into root rather than into shoot, while light limitation enhanced growth of leaf area. The TNC concentrations increased with light intensity and thus with photosynthesis, while the concentrations of organic N and nitrate decreased. Increasing N supply had the opposite effect. Therefore, TNC and organic N concentrations were negatively correlated (r=-0.90). Pool size of N or TNC increased with N and light availability when either resource was non-limiting, but increased little or remained constant when either resource was limiting. Storage reached a minimum when both resources were supplied at an equal rate.

Evaporation and canopy characteristics of coniferous forests and grasslands.

Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (gs and ga, respectively) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar maximum hourly evaporation rates (E max) and maximum surface conductances (gsmax) (medians = 0.46 mm h-1 and 22 mm s-1). However, on a daily basis, median E max of coniferous forest (4.0 mm d-1) is significantly lower than that of grassland (4.6 mm d-1). Additionally, a representative value of ga for coniferous forest (200 mm s-1) is an order of magnitude more than the corresponding value for grassland (25 mm s-1). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in gs is >0.7 for coniferous forest, but as low as 0.3 for grassland. The proportional sensitivity of E to changes in ga is generally ±0.15 or less.Boundary-line relationships between gs and light and air saturation deficit (D) vary considerably. Attainment of gsmax occurs at a much lower irradiance for coniferous forest than for grassland (15 versus about 45% of full sunlight). Relationships between gs and D measured above the canopy appear to be fairly uniform for coniferous forest, but are variable for grassland. More uniform relationships may be found for surfaces with relatively small ga, like grassland, by using D at the evaporating surface (D0) as the independent variable rather than D at a reference point above the surface. An analytical expression is given for determining D0 from measurable quantities. Evaporation rate also depends on the availability of water in the root zone.Below a critical value of soil water storage, the ratio of evaporation rate to the available energy tends to decrease sharply and linearly with decreasing soil water content. At the lowest value of soil water content, this ratio declines by up to a factor of 4 from the non-soil-water-limiting plateau. Knowledge about functional rooting depth of different plant species remains rather limited. Ignorance of this important variable makes it generally difficult to obtain accurate estimates of seasonal evaporation from terrestrial ecosystems.

Nitrogen and carbohydrate storage in biennials originating from habitats of different resource availability.

Four biennial species (Arctium tomentosum, Cirsium vulgare, Dipsacus sylvester and Daucus carota) which originate from habitats of different nutrient availability were investigated in a 2-year experiment in a twofactorial structured block design varying light (natural daylight versus shading) and fertilizer addition. The experiment was designed to study storage as reserve formation (competing with growth) or as accumulation (see Chapin et al. 1990). We show that (i) the previous definitions of storage excluded an important process, namely the formation of storage tissue. Depending on species, storage tissue and the filling process can be either a process of reserve formation, or a process of accumulation. (ii) In species representing low-resource habitats, the formation of a storage structure competes with other growth processes. Growth of storage tissue and filling with storage products is an accumulation process only in the high-resource plant Arctium tomentosum. We interpret the structural growth of low-resource plants in terms of the evolutionary history of these species, which have closely related woody species in the Mediterranean area. (iii) The use of storage products for early leaf growth determines the biomass development in the second season and the competitive ability of this species during growth with perennial species. (iv) The high-resource plant Arctium has higher biomass development under all conditions, i.e. plants of low-resource habitats are not superior under low-resource conditions. The main difference between high- and low-resource plants is that low-resource plants initiate flowering at a lower total plant internal pool size of available resources.

Carbon and nitrogen partitioning in the biennial monocarp Arctium tomentosum Mill.

Growth and nitrogen partitioning were investigated in the biennial monocarp Arctium tomentosum in the field, in plants growing at natural light conditions, in plants in which approximately half the leaf area was removed and in plants growing under 20% of incident irradiation. Growth quantities were derived from splined cubic polynomial exponential functions fitted to dry matter, leaf area and nitrogen data.Main emphasis was made to understanding of the significance of carbohydrate and nitrogen storage of a large tuber during a 2-years' life cycle, especially the effect of storage on biomass and seed yield in the second season. Biomass partitioning favours growth of leaves in the first year rosette stage. Roots store carbohydrates at a constant rate and increase storage of carbohydrates and nitrogen when the leaves decay at the end of the first season. In the second season the reallocation of carbohydrates from storage is relatively small, but reallocation of nitrogen is very large. Carbohydrate storage just primes the growth of the first leaves in the early growing season, nitrogen storage contributes 20% to the total nitrogen requirement during the 2nd season. The efficiency of carbohydrate storage for conversion into new biomass is about 40%. Nitrogen is reallocated 3 times in the second year, namely from the tuber to rosette leaves and further to flower stem leaves and eventually into seeds. The harvest index for nitrogen is 0.73, whereas for biomass it is only 0.19.

Nitrogen nutrition and isotope differences among life forms at the northern treeline of Alaska.

Natural abundances of nitrogen isotopes, δ15N, indicate that, in the same habitat, Alaskan Picea glauca and P. mariana use a different soil nitrogen compartment from the evergreen shrub Vaccinium vitis-idaea or the deciduous grass Calamagrostis canadensis. The very low δ15N values (-7.7 ‰) suggest that (1) Picea mainly uses inorganic nitrogen (probably mainly ammonium) or organic N in fresh litter, (2) Vaccinium (-4.3 ‰) with its ericoid mycorrhizae uses more stable organic matter, and (3) Calamagrostis (+0.9 ‰) exploits deeper soil horizons with higher δ15N values of soil N. We conclude that species limited by the same nutrient may coexist by drawing on different pools of soil N in a nutrient-deficient environment. The differences among life-forms decrease with increasing N availability. The different levels of δ15N are associated with different nitrogen concentrations in leaves, Picea having a lower N concentration (0.62 mmol g-1) than Vaccinium (0.98 mmol g-1) or Calamagrostis (1.33 mmol g-1). An extended vector analysis by Timmer and Armstrong (1987) suggests that N is the most limiting element for Picea in this habitat, causing needle yellowing at N concentrations below 0.5 mmol g-1 or N contents below 2 mmol needle-1. Increasing N supply had an exponential effect on twig and needle growth. Phosphorus, potassium and magnesium are at marginal supply, but no interaction between ammonium supply and needle Mg concentration could be detected. Calcium is in adequate supply on both calcareous and acidic soils. The results are compared with European conditions of excessive N supply from anthropogenic N depositions.