Library

Notes: How might wild relatives of modern cereals have responded to past, and how might they respond to future, atmospheric CO2 enrichment under competitive situations in a dry, low-nutrient environment? In order to test this, Aegilops and Hordeum species, common in semiarid annual grasslands of the Middle East, were grown in nine model ecosystems (400 kg each) with a natural matrix of highly diverse Negev vegetation established on native soil shipped to Basel, Switzerland. In a simulated, seasonally variable climate of the northern Negev, communities experienced a full life-cycle in 280 (preindustrial), 440 (immediate future) and 600 ppm of CO2 (end of the next century). Neither Aegilops (A. kotschyi and A. peregrina), nor Hordeum spontaneum showed a significant biomass response to CO2 concentrations exceeding 280 ppm The reproductive output remained unaffected or even declined (A. peregrina) under elevated CO2. Non-structural carbohydrates in leaf tissues increased and N concentration decreased with increasing CO2 concentration. N concentration, germination success and seedling development of newly formed grains were either unchanged or reduced in response to high CO2 treatment of parent plants. In a separate fertilizer × CO2 trial with A. kotschyi nested in smaller model communities, we found no effect of P addition, but a 2–3-fold biomass increase by NPK addition compared to the unfertilized control. A significant stimulation of biomass by CO2 enrichment (+ 44% between 280 and 600 ppm) was obtained only in the NPK treatment. These data suggest that increased CO2 concentration had little direct effect on growth and reproduction in these ‘wild cereals’ in the recent past, and the same seems to hold for their future, except if N-rich fertilizer is added.

Notes: The relationship between plant species diversity and ecosystem CO2 and water vapour fluxes was investigated for planted calcareous grassland communities composed of 5, 12, or 32 species assembled from the native plant species pool. These diversity manipulations were done in factorial combination with a CO2 enrichment experiment in order to investigate the degree to which ecosystem responses to elevated CO2 are altered by a loss of plant diversity. Ecosystem CO2 and H2O fluxes were measured over several 24-h periods during the 1994 and 1995 growing seasons. Ecosystem CO2 assimilation on a ground area basis decreased with decreasing plant diversity in the first year and this was related to a decline in above-ground plant biomass. In the second year, however, CO2 assimilation was not affected by diversity, and this corresponded to the disappearance of a diversity effect on above-ground biomass. Irrespective of diversity treatment, CO2 assimilation on a ground area basis was linearly related to peak above-ground biomass in both years. Elevated CO2 significantly increased ecosystem CO2 assimilation in both years with no interaction between diversity and CO2 treatment, and no corresponding increase in above-ground biomass. There were no significant effects of diversity on water vapour flux, which was measured only in the second year. There were indications of a small CO2 effect on water vapour flux (3–9% lower at elevated CO2 depending on the light level). Our findings suggest that decreasing plant species diversity may substantially decrease ecosystem CO2 assimilation during the establishment of such planted calcareous grassland communities, but also suggest that this effect may not persist. In addition, we find no evidence that plant species diversity alters the response of ecosystem CO2 assimilation to elevated CO2.

Notes: The current carbon supply status of temperate forest trees was assessed by analysing the seasonal variation of non-structural carbohydrate (NSC) concentrations in leaves, branch wood and stem sapwood of 10 tree species (six deciduous broad-leafed, one deciduous conifer and three evergreen conifer trees) in a temperate forest that is approximately 100 years old. In addition, all woody tissue was analysed for lipids (acylglycerols). The major NSC fractions were starch, sucrose, glucose and fructose, with other carbohydrates (e.g. raffinose and stachyose) and sugar alcohols (cyclitols and sorbitol) playing only a minor quantitative role. The radial distribution of NSC within entire stem cores, assessed here for the first time in a direct interspecific comparison, revealed large differences in the size of the active sapwood fraction among the species, reflecting the specific wood anatomy (ring-porous versus diffuse-porous xylem). The mean minimum NSC concentrations in branch wood during the growing season was 55% of maximum, and even high NSC concentrations were maintained during times of extensive fruit production in masting Fagus sylvestris. The NSC in stem sapwood varied very little throughout the season (cross species mean never below 67% of maximum), and the small reductions observed were not significant for any of the investigated species. Although some species contained substantial quantities of lipids in woody tissues (‘fat trees’; Tilia, Pinus, Picea, Larix), the lipid pools did not vary significantly across the growing season in any species. On average, the carbon stores of deciduous trees would permit to replace the whole leave canopy four times. These data imply that there is not a lot of leeway for a further stimulation of growth by ongoing atmospheric CO2 enrichment. The classical view that deciduous trees rely more on C-reserves than evergreen trees, seems unwarranted or has lost its justification due to the greater than 30% increase in atmospheric CO2 concentrations over the last 150 years.

Notes: The hypothesis that plants grown under elevated CO2 allocate more carbon to the production of latex and C-rich secondary compounds whereas nutrient addition counteracts this effect was tested. Two similar experiments were conducted in two different experimental facilities. In both facilities seedlings of Euphorbia lathyris were exposed to factorial combinations of two CO2 concentrations and two levels of nutrient availability for 2 months. The CO2 treatments and growth conditions differed substantially between these two experiments but treatment responses to elevated CO2 and fertilizer addition were remarkably similar, underlining the robustness of our findings. Elevated CO2 increased biomass to a greater extent in fertilized than in unfertilized  plants  and  reduced  the  leaf  biomass  fraction by accelerating leaf senescence. Concentrations of non-structural carbohydrates (NSC) increased in elevated CO2. However, this apparent carbon surplus did not feed into the whole plant latex pool. The latex harvest per leaf (−25%) and the concentration of latex-related hydrocarbons (−20%) even decreased under elevated CO2 (both experiments P 〈 0.05). Fertilization reduced NSC concentrations (−25%) but neither affected latex yield per leaf nor the concentration of latex-related hydrocarbons. It is concluded that latex and related hydrocarbons in CO2-enriched plants are a negligible sink for excess carbon irrespective of nutrient status and thus, vigour of growth.

Notes: Four- to seven-year-old spruce trees (Picea abies) were exposed to three CO2 concentrations (280, 420 and 560 cm3 m−3) and three rates of wet N deposition (0, 30 and 90 kg ha−1 year−1) for 3 years in a simulated montane forest climate. Six trees from each of six clones were grown in competition in each of nine 100 × 70 × 36 cm model ecosystems with nutrient-poor natural forest soil. Stem dises were analysed using X-ray densitometry. The radial stem increment was not affected by [CO2] but increased with increasing rates of N deposition. Wood density was increased by [CO2], but decreased by N deposition. Wood-starch concentration increased, and wood nitrogen concentration decreased with increasing [CO2], but neither was affected by N deposition. The lignin concentration in wood was affected by neither [CO2] nor N deposition. Our results suggest that, under natural growth conditions, rising atmospheric [CO2] will not lead to enhanced radial stem growth of spruce, but atmospheric N deposition will, and in some regions is probably already doing so. Elevated [CO2], however, will lead to denser wood unless this effect is compensated by massive atmospheric N deposition. If can be speculated that greater wood density under elevated [CO2] may alter the mechanical properties of wood, and higher ratios of C/N and lignin/N in wood grown at elevated [CO2] may affect nutrient cycles of forest ecosystems.

Notes: Of the many responses of plants to elevated CO2, accumulation of total non-structural carbohydrates (TNC in % dry weight) in leaves is one of the most consistent. Insufficient sink activity or transport capacity may explain this obvious disparity between CO2 assimilation and carbohydrate dissipation and structural investment. If transport capacity contributes to the problem, phloem loading may be the crucial step. It has been hypothesized that symplastic phloem loading is less efficient than apoplastic phloem loading, and hence plant species using the symplastic pathway and growing under high light and good water supply should accumulate more TNC at any given CO2 level, but particularly under elevated CO2. We tested this hypothesis by carrying out CO2 enrichment experiments with 28 plant species known to belong to groups of contrasting phloem-loading type. Under current ambient CO2 symplastic loaders were found to accumulate 36% TNC compared with only 19% in apoplastic loaders (P=0.0016). CO2 enrichment to 600 μmol mol−1 increased TNC in both groups by the same absolute amount, bringing the mean TNC level to 41% in symplastic loaders (compared to 25% in apoplastic loaders), which may be close to TNC saturation (coupled with chlornplast malfunction). Eight tree species, ranked as symplastic loaders by their minor vein companion cell configuration, showed TNC responses more similar to those of apoplastic herbaceous loaders. Similar results are obtained when TNC is expressed on a unit leaf area basis, since mean specific leaf areas of groups were not significantly different. We conclude that phloem loading has a surprisingly strong effect on leaf tissue composition, and thus may translate into alterations of food webs and ecosystem functioning, particularly under high CO2.

Notes: Few of the most common assumptions used in models of responses of plants and ecosystems to elevated CO2 and climate warming have been tested under realistic life con-ditions. It is shown that some unexpected discrepancies between predictions and experimental findings exist, suggesting that a better empirical basis is required for predictions. The following ten suggestions may improve our potential to scale up from experimental scales to the real world.(1) Experiments should be timed to account for non-linearity in system responsiveness, asynchrony of responses and developmental differences. (2) By altering mineral nutrient supply, a wide range of CO2 responses can be ‘produced’, thus requiring realistic soil conditions. (3) Distinctions should be made between ‘doubling CO2 sup-ply’ and biologically effective degrees of CO2 enrichment. (4) Because of the non-linearity of plant responses to CO2, studies of at least three instead of two CO2 concentrations are necessary to describe future trends adequately. (5) Edge effects, in particular unscreened side light, may lead to allometric anomalies, strongly constraining up-scaling to stand-scale CO2 responses. (6) Variables such as growth, yield, net primary production and C turnover are often confused with carbon pools, carbon sequestration or net ecosystem production. (7) Mono- and interspecific interactions between individuals may lead to completely unpredictable CO2 responses. (8) Experiments with seedlings benefit from the absence of prehistory effects but are likely to be irrelevant for the responses of larger trees which, on the other hand, may be constrained by carry-over effects. Tree ring research indicates immediate sensitivity of large trees to environmental changes, supporting their usefulness in short-term CO2-enrichment experiments. (9) In predicting temperature responses, acclimation deserves more attention. (10) The significance of developmental responses is largely under-represented in experimental research, although these responses may overrule many of the other effects of atmospheric change. Results of more realistic experiments which account for these problems will provide a better basis for modelling the future of the biosphere.

Notes: Abstract The experimental data presented here relate to the question of whether terrestrial ecosystems will sequester more C in their soils, litter and biomass as atmospheric CO2 concentrations rise. Similar to our previous study with relatively fertile growth conditions (Körner and Arnone 1992), we constructed four rather nutrient-limited model communities of moist tropical plant species in greenhouses (approximately 7 m2 each). Plant communities were composed of seven species (77 individuals per community) representing major taxonomic groups and various life forms found in the moist tropics. Two ecosystems were exposed to 340 μl CO2 l−1 and two to 610 μl l−1 for 530 days of humid tropical growth conditions. In order to permit precise determination of C deposition in the soil, plant communities were initially established in C-free unwashed quartz sand. Soils were then amended with known amounts of organic matter (containing C and nutrients). Mineral nutrients were also supplied over the course of the experiment as timed-release full-balance fertilizer pellets. Soils represented by far the largest repositories for fixed C in all ecosystems. Almost 5 times more C (ca. 80% of net C fixation) was sequestered in the soil than in the biomass, but this did not differ between CO2 treatments. In addition, at the whole-ecosystem level we found a remarkably small and statistically non-significant increase in C sequestration (+4%; the sum of C accretion in the soil, biomass, litter and necromass). Total community biomass more than quadrupled during the experiment, but at harvest was, on average, only 8% greater (i.e. 6% per year; n.s.) under elevated CO2, mainly due to increased root biomass (+15%, P=0.12). Time courses of leaf area index of all ecosystems suggested that canopy expansion was approaching steady state by the time systems were harvested. Net primary productivity (NPP) of all ecosystems-i.e. annual accumulation of biomass, necromass, and leaf litter (but not plant-derived soil organic matter)-averaged 815 and 910 g m−2 year−1 at ambient and elevated CO2, respectively. These NPPs are remarkably similar to those of many natural moist tropical forested ecosystems. At the same time net productivity of soil organic matter reached 7000 g dry matter equivalent per m2 and year (i.e. 3500 g C m−2 year−1). Very slight yet statistically significant CO2-induced shifts in the abundance of groups of species occurred by the end of the experiment, with one group of species (Elettaria cardamomum, Ficus benjamina, F. pumila, Epipremnum pinnatum) gaining slightly, and another group (Ctenanthe lubbersiana, Heliconia humilis, Cecropia peltata) losing. Our results show that: (1) enormous amounts of C can be deposited in the ground which are normally not accounted for in estimates of NPP and net ecosystem productivity; (2) any enhancement of C sequestration under elevated atmospheric CO2 may be substantially smaller than is believed will occur (yet still very important), especially under growth conditions which permit close to natural NPP; and (3) species dominance in plant communities is likely to change under elevated CO2, but that changes may occur rather slowly.

Notes: Summary This study is part of a series of investigations on the influence of altitude on structure and function of plant leaves. Unlike most other mountain areas, the Southern Alps of New Zealand provide localities where physiologically effective moisture stress occurs neither at high nor at low elevation, but the changes in temperature and radiation with elevation are similar or even steeper than in most other regions. In comparison with results from other mountains, where moisture may impair plant functioning at low elevation, this study allows an estimation of the relative role of water for the expression of various leaf features typically associated with alpine plants. Maximum leaf diffusive conductance (g), leaf nitrogen content (LN), stomatal density (n) and distribution, as well as area (A), thickness (d) and specific area (SLA) of leaves were studied. Three different plant life forms were investigated over their full altitudinal range (m): trees, represented by Nothofagus menziesii (1,200 m), ericaceous dwarf shrubs (1,700 m), and herbaceous plants of the genus Ranunculus (2,500 m). In all three life forms g, LN, and n increased, while SLA and A decreased with elevation. Recent investigations have found similar trends in other mountains from the temperate zone, but the changes are larger in New Zealand than elsewhere. Herbs show the greatest differences, followed by shrubs and then trees. It is concluded that g is dependent upon light climate rather than water supply, whereas SLA and related structural features appear to be controlled by the temperature regime, as they show similar altitudinal changes under different light and moisture gradients. The higher leaf nitrogen content found at high elevations in all three life forms, suggests that metabolic activity of mature leaves is not restricted by low nitrogen supply at high altitude. In general, the leaves of herbaceous plants show more pronounced structural and functional changes with altitude than the leaves of shrubs and trees.

Notes: Summary The spatial and temporal variation of lead conductance (g) in Eucalyptus pauciflora was analysed with respect to photon flux area density (I), temperature (T), water vapour concentration deficit (Δw), and leaf water potential (Ψ) at four different sites between 940 m and 2,040 m altitude in the Snowy Mountains of south-eastern Australia. Along this altitudinal gradient the precipitation/evaporation ratio increases from 1 to 4. The results show that gas diffusion in this tree species is primarily controlled by I and Δw at all sites, independently of the specific soil moisture regime. Even under dry midsummer conditions with predawn leaf water potentials of-1 MPa at the lowest altitude, Ψ had no striking effect on g. The humidity threshold for the onset of stomatal closure does not vary greatly between the study sites (12.2±1.3 Pa kPa-1). The highest and lowest values observed for Ψ, the osmotic potential at water saturation (from pressure/volume curves), the mean and maximum g and stomatal dentity, all increase with elevation. The highest (least negative) osmotic potentials were obtained at all sites in midsummer. It therefore appears that there is no osmotic adjustment to drought in the seasonal course. The maximum difference between osmotic potentials obtained at the lowest and highest sites is 0.46 MPa. In general osmotic potential varies less than has been reported for other plant species exposed to varying water regimes. This may be the consequence of the pronounced feed-forward response of the stomata to evaporative demand, which led to only moderate tissue desiccation, never exceeding the turgor loss point. E. pauciflora is a tree species with a very conservative utilisation of soil water, which adjusts to drought via stomatal control of water loss, rather than via osmotic properties. These results explain previous reports of the comparatively high susceptibility of E. pauciflora to severe drought and its positive influence on the hydrological balance of mountain ecosystems in the Australian Alps.