ALBERT

Notizen: Based on review and original data, this synthesis investigates carbon pools and fluxes of Siberian and European forests (600 and 300 million ha, respectively). We examine the productivity of ecosystems, expressed as positive rate when the amount of carbon in the ecosystem increases, while (following micrometeorological convention) downward fluxes from the atmosphere to the vegetation (NEE = Net Ecosystem Exchange) are expressed as negative numbers. Productivity parameters are Net Primary Productivity (NPP=whole plant growth), Net Ecosystem Productivity (NEP = CO2 assimilation minus ecosystem respiration), and Net Biome Productivity (NBP = NEP minus carbon losses through disturbances bypassing respiration, e.g. by fire and logging). Based on chronosequence studies and national forestry statistics we estimate a low average NPP for boreal forests in Siberia: 123 gC m–2 y–1. This contrasts with a similar calculation for Europe which suggests a much higher average NPP of 460 gC m–2 y–1 for the forests there. Despite a smaller area, European forests have a higher total NPP than Siberia (1.2–1.6 vs. 0.6–0.9 × 1015 gC region–1 y–1). This arises as a consequence of differences in growing season length, climate and nutrition. For a chronosequence of Pinus sylvestris stands studied in central Siberia during summer, NEE was most negative in a 67-y old stand regenerating after fire (– 192 mmol m–2 d–1) which is close to NEE in a cultivated forest of Germany (– 210 mmol m–2 d–1). Considerable net ecosystem CO2-uptake was also measured in Siberia in 200- and 215-y old stands (NEE:174 and – 63 mmol m–2 d–1) while NEP of 7- and 13-y old logging areas were close to the ecosystem compensation point. Two Siberian bogs and a bog in European Russia were also significant carbon sinks (– 102 to – 104 mmol m–2 d–1). Integrated over a growing season (June to September) we measured a total growing season NEE of – 14 mol m–2 summer–1 (– 168 gC m–2 summer–1) in a 200-y Siberian pine stand and – 5 mol m–2 summer–1 (– 60 gC m–2 summer–1) in Siberian and European Russian bogs. By contrast, over the same period, a spruce forest in European Russia was a carbon source to the atmosphere of (NEE: + 7 mol m–2 summer–1 = + 84 gC m–2 summer–1). Two years after a windthrow in European Russia, with all trees being uplifted and few successional species, lost 16 mol C m–2 to the atmosphere over a 3-month in summer, compared to the cumulative NEE over a growing season in a German forest of – 15.5 mol m–2 summer–1 (– 186 gC m–2 summer–1; European flux network annual averaged – 205 gC m–2 y–1). Differences in CO2-exchange rates coincided with differences in the Bowen ratio, with logging areas partitioning most incoming radiation into sensible heat whereas bogs partitioned most into evaporation (latent heat). Effects of these different surface energy exchanges on local climate (convective storms and fires) and comparisons with the Canadian BOREAS experiment are discussed. Following a classification of disturbances and their effects on ecosystem carbon balances, fire and logging are discussed as the main processes causing carbon losses that bypass heterotrophic respiration in Siberia. Following two approaches, NBP was estimated to be only about 13–16 mmol m–2 y–1 for Siberia. It may reach 67 mmol m–2 y–1 in North America, and about 140–400 mmol m–2 y–1 in Scandinavia. We conclude that fire speeds up the carbon cycle, but that it results also in long-term carbon sequestration by charcoal formation. For at least 14 years after logging, regrowth forests remain net sources of CO2 to the atmosphere. This has important implications regarding the effects of Siberian forest management on atmospheric concentrations. For many years after logging has taken place, regrowth forests remain weaker sinks for atmospheric CO2 than are nearby old-growth forests.

Notizen: A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO2 assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters.For canopies low in total N, daily assimilation rates are ∼10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, Gc should be calculated as Gc=Aσ(fslgsl+fshgsh), where Δ is leaf area index, fsi and fsh are the fractions of sunlit and shaded leaves at each level, and gsi and gsh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation.Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.

Notizen: Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.

Notizen: Abstract Experiments were carried out to determine how decreased expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) affects photosynthetic metabolism in ambient growth conditions. In a series of tobacco (Nicotiana tabacum L.) plants containing progressively smaller amounts of Rubisco the rate of photosynthesis was measured under conditions similar to those in which the plants had been grown (310 μmol photons · m−2 · s−1, 350 μbar CO2, 22° C). (i) There was only a marginal inhibition (6%) of photosynthesis when Rubisco was decreased to about 60% of the amount in the wildtype. The reduced amount of Rubisco was compensated for by an increase in Rubisco activation (rising from 60 to 100%), with minor contributions from an increase of its substrates (ribulose-1,5-bisphosphate and the internal CO2 concentration) and a decrease of its product (glycerate-3-phosphate). (ii) The decreased amount of Rubisco was accompanied by an increased ATP/ADP ratio that may be causally linked to the increased activation of Rubisco. An increase of highenergy-state chlorophyll fluorescence shows that thylakoid membrane energisation and high-energy-state-dependent energy dissipation at photosystem two had also increased. (iii) A further decrease of Rubisco (in the range of 50–20% of the wildtype level) resulted in a strong and proportional inhibition of CO2 assimilation. This was accompanied by a decrease of fructose-1,6-bisphosphatase activity, coupling-factor 1 (CF1)-ATP-synthase protein, NADP-malate dehydrogenase protein, and chlorophyll. The chlorophyll a/b ratio did not change, and enolase and sucrose-phosphate synthase activity did not decrease. It is argued that other photosynthetic enzymes are also decreased once Rubisco decreases to the point at which it becomes strongly limiting for photosynthesis. (iv) It is proposed that the amount of Rubisco in the wildtype represents a balance between the demands of light, water and nitrogen utilisation. The wildtype overinvests about 15% more protein in Rubisco than is needed to avoid a strict Rubisco limitation of photosynthesis. However, this “excess” Rubisco allows the wildtype to operate with lower thylakoid energisation, and decreased high-energy-state-dependent energy dissipation, hence increasing light-use efficiency by about 6%. It also allows the wildtype to operate with a lower internal CO2 concentration in the leaf and a lower stomatal conductance at a given rate of photosynthesis, so that instantaneous water-use efficiency is marginally (8%) increased.

Notizen: Abstract Transgenic tobacco (Nicotiana tabacum L.) plants transformed with ‘antisense’ rbcS to produce a series of plants with a progressive decrease in the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) have been used to investigate the contribution of Rubsico to the control of photosynthesis at different irradiance, CO2 concentrations and vapour-pressure deficits. Assimilation rates, transpiration, the internal CO2 concentration and chlorophyll fluorescence were measured in each plant. (i) The flux-control coefficient of Rubisco was estimated from the slope of the plot of Rubisco content versus assimilation rate. The flux-control coefficient had a value of 0.8 or more in high irradiance, (1050 μmol·m−2·s−1), low-vapour pressure deficit (4 mbar) and ambient CO2 (350 μbar). Control was marginal in enhanced CO2 (450 μbar) or low light (310 μmol·m−2·s−1) and was also decreased at high vapour-pressure deficit (17 mbar). No control was exerted in 5% CO2. (ii) The flux-control coefficients of Rubisco were compared with the fractional demand placed on the calculated available Rubisco capacity. Only a marginal control on photosynthetic flux is exerted by Rubisco until over 50% of the available capacity is being used. Control increases as utilisation rises to 80%, and approaches unity (i.e. strict limitation) when more than 80% of the available capacity is being used. (iii) In low light, plants with reduced Rubisco have very high energy-dependent quenching of chlorophyll fluorescence (qE) and a decreased apparent quantum yield. It is argued that Rubisco still exerts marginal control in these conditions because decreased Rubisco leads to increased thylakoid energisation and high-energy dependent dissipation of light energy, and lower light-harvesting efficiency. (iv) The flux-control coefficient of stomata for photosynthesis was calculated from the flux-control coefficient of Rubisco and the internal CO2 concentration, by applying the connectivity theorem. Control by the stomata varies between zero and about 0.25. It is increased by increased irradiance, decreased CO2 or decreased vapour-pressure deficit. (v) Photosynthetic oscillations in saturating irradiance and CO2 are suppressed in decreased-activity transformants before the steady-state rate of photosynthesis is affected. This provides direct evidence that these oscillations reveal the presence of “excess” Rubisco. (vi) Comparison of the flux-control coefficients of Rubisco with mechanistic models of photosynthesis provides direct support for the reliability of these models in conditions where Rubisco has a flux-control coefficient approach unity (i.e. “limits” photosynthesis), but also indicates that these models are less useful in conditions where control is shared between Rubisco and other components of the photosynthetic apparatus.

Notizen: Abstract 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.

Notizen: Abstract The study presents a data set of above-ground biomass (AGB), structure, spacing and fire regime, for 24 stands of pristine Siberian Scots pine (Pinus sylvestris) forests with lichens (n = 20) or Vaccinium/mosses (n = 4) as ground cover, along four chronosequences. The stands of the “lichen” site type (LT) were stratified into three chronosequences according to stand density and fire history. Allometric equations were established from 90 sample trees for stem, coarse branch, fine branch, twig and needle biomass. The LT stands exhibited a low but sustained biomass accumulation until a stand age of 383 years. AGB reached only 6–10 kgdw m−2 after 200 years depending on stand density and fire history compared to 20 kgdw m−2 in the “Vaccinium” type (VT) stands. Leaf area index (LAI) in the LT stands remained at 0.5–1.5 and crown cover was 30–60%, whereas LAI reached 2.5 and crown cover was 〉100% in the VT stands. Although nearest-neighbour analyses suggested the existence of density-dependent mortality, fire impact turned out to have a much stronger effect on density dynamics. Fire scar dating and calculation of mean and initial fire return intervals revealed that within the LT stands differences in structure and biomass were related to the severity of fire regimes, which in turn was related to the degree of landscape fragmentation by wetlands. Self-thinning analysis was used to define the local carrying capacity for biomass. A series of undisturbed LT stands was used to characterise the upper self-thinning boundary. Stands that had experienced a moderate fire regime were positioned well below the self-thinning boundary in a distinct fire-thinning band of reduced major axis regression slope −0.26. We discuss how this downward shift resulted from alternating phases of density reduction by fire and subsequent regrowth. We conclude that biomass in Siberian Scots pine forests is strongly influenced by fire and that climate change will affect ecosystem functions predominantly via changes in fire regimes.

Notizen: Summary Xylem-tapping mistletoe species growing on Mimosaccae, non-Mimosaceae and hosts performing Crassulacean acid metabolism (CAM) were studied along an aridity gradient in the Namib desert. °13C-values of mistletoes became more negative with decreasing nitrogen (N)-concentration in their leaves, while the host plants showed no such relationship. This might suggest that mistletoes regulate their water use efficiency according to the nitrogen supply from the host. However, further inspection of the data indicates that the relations of δ13C-values with leaf nitrogen in mistletoes may result from carbon input from the host. This is especially true for mistletoes growing on CAM plants which exhibit a very high δ13C-value, but show no evidence of CAM. It is calculated that about 60% of the carbon in mistletoes growing on C3 and on CAM hosts originated from the host. The hypothesis of Marshall and Ehleringer (1990) that xylem tapping mistletoes are also carbon parasites could explain the change in δ13C-values with N-supply and the difference in δ13C-values between mistletoes growing on C3 and CAM hosts.

Notizen: Summary Plants of Drosera species, neighbouring noncarnivorous plants, and arthropods on or near each Drosera sp. were collected at 11 contrasting habitat locations in SW Australia. At three of the sites clones of the rare glandless mutant form of D. erythrorhiza were collected alongside fully glandular counterparts. The δ 15N value (15N/14N natural isotope composition) of insect-free leaf and stem fractions was measured, and the data then used to estimate proportional dependence on insect N (%NdI) for the respective species and growth forms of Drosera. The data indicated lower %NdI values for rosette than for self-supporting erect or for climbing vine species. The latter two groups showed an average %NdI value close to 50%. The %NdI increased with length and biomass of climbing but not erect forms of Drosera. δ 15N values of stems were positively correlated with corresponding values for leaves of Drosera. Leaf material was on average significantly more 15N enriched than stems, possibly due to delayed transport of recent insect-derived N, or to discrimination against 15N in transfer from leaf to the rest of the plant. The comparison of δ 15N values of insects and arthropod prey, glandless and glandular plants of D. erythrorhiza indicated %NdI values of 14.3, 12.2 and 32.2 at the respective sites, while matching comparisons based on δ 15N of insect, reference plants and glandular plants proved less definitive, with only one site recording a positive %NdI (value of 10.4%) despite evidence at all sites of feeding on insects by the glandular plants. The use of the δ 15N technique for studying nutrition of carnivorous species and the ecological significance of insect feeding of different growth forms of Drosera growing in a large range of habitats is discussed.

Notizen: Summary 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.