Growth under elevated [CO 2 ] promoted spring frost damage in field grown seedlings of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.), one of the most frost tolerant of eucalypts. Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO 2 ] (-5·7°C versus -4·3°C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO 2 fertilization and may affect predictions of vegetation change based on increasing temperature. [CO 2 ]; freeze-induced damage; global climate change; ice nucleation; open-top chambers. Key-words: Eucalyptus pauciflora; elevated atmospheric INTRODUCTIONAtmospheric CO 2 concentrations have been increasing since the industrial revolution, and are predicted to reach twice present levels late next century (Boden et al. 1994). It is thought that temperature will increase as [CO 2 ] rises, and that warming may be greater in winter than in summer (Plummer, Lin & Torok 1996). This could lengthen the growing season if evergreen species were able to take advantage of warm conditions earlier in spring when below-ground resources are relatively abundant. However, predictions of plant responses to global climate and atmospheric change are complicated by weather variability (Katz & Brown 1992) and the extent to which elevated [CO 2 ] might affect plant responses to temperature (Long 1991), particularly low temperatures. Some studies have predicted that frost damage may increase for trees that break dormancy too early in spring (Cannell & Smith 1986;Repo, Hänninen & Kellomäki 1996) Here we report the results of a serendipitous experiment. The experiment naturally occurred during a larger fieldbased study aimed at understanding the effects of elevated [CO 2 ] on the interactions between grass and trees during spring. The results were unexpected and showed that one of the most frost-hardy of broadleaved, evergreen species suffered greater frost damage when grown under elevated than ambient [CO 2 ]. MATERIALS AND METHODS Plant material and growth conditionsSeeds of Eucalyptus pauciflora Sieb. ex Spreng. were collected from three trees growing along the floor of the Orroral Valley at an elevation of 850 m in New South Wales, Australia. The seeds were cold stratified under moist conditions at 3°C for 4 weeks before germinating on sand flats in a mist house. Seedlings of similar size were transferred to individual containers (5 cm diameter, 25 cm deep) and grown out of doors for 6 months before the start of the experiment.In a pasture near Bungendore in southeastern Australia (35°15' S, 149°27' E; elevation 700 m), 10 open-top chambers were installed as five replicate pairs, flushed with air containing either ambient or elevated CO 2 conce...
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A b s t r a c tA model of the interacting global carbon and nitrogen cycles (CQUESTN) is developed to explore the possible history of C-sequestration into the terrestrial biosphere in response to the global increases (past and possible future) in atmospheric CO2 concentration, temperature and N-deposition. The model is based on published estimates of pre-industrial C and N pools and fluxes into vegetation, litter and soil compartments. It was found necessary to assign low estimates of N pools and fluxes to be compatible with the more firmly established C-cycle data. Net primary production was made responsive to phytomass N level, and to CO2 and temperature deviation from preindustrial values with sensitivities covering the ranges in the literature. Biological N-fixation could be made either unresponsive to soil C:N ratio, or could act to tend to restore the preindustrial C:N of humus with different N-fixation intensities. As for all such simulation models, uncertainties in both data and functional relationships render it more useful for qualitative evaluation than for quantitative prediction.With the N-fixation response turned off, the historic CO2 increase led to standard-model sequestration into terrestrial ecosystems in 1995AD of 1.8 Gt C yr -l. With N-fixation restoring humus C:N strongly, C sequestration was 3 Gt yr -1 in 1995. In both cases C:N of phytomass and litter increased with time and these increases were plausible when compared with experimental data on CO2 effects. The temperature increase also caused net C sequestration in the model biosphere because decrease in soil organic matter was more than offset by the increase in phytomass deriving from the extra N mineralised. For temperature increase to reduce system C pool size, the biosphere "leakiness" to N would have to increase substantially with temperature. Assuming a constant N-loss coefficient, the historic temperature increase alone caused standard-model net C sequestration to be about 0.6 Gt C in 1995. Given the disparity of plant and microbial C:N, the modelled impact of anthropogenic N-deposition on C-sequestration depends substantially on whether the deposited N is initially taken up by plants or by soil microorganisms. Assuming the latter, standard-model net sequestration in 1995 was 0.2 Gt C in 1995 from the N-deposition effect alone. Combining the effects of the historic courses of CO2, temperature and N-deposition, the standard-model gave C-sequestration of 3.5 Gt in 1995. This involved an assumed weak response of biological N-fixation to the increased carbon status of the ecosystem. For N-fixation to track ecosystem C-fixation in the long term however, more phosphorus must enter the biological cycle. New experimental evidence shows that plants in elevated CO2 have the capacity to mobilize more phosphorus from so-called "unavailable" sources using mechanisms involving exudation of organic acids and phosphatases.
The relative importance of thermal interference and competition for below-ground resources in the inhibition of tree seedling growth by grass was determined under field conditions. Snow gum (Eucalyptus pauciflora) seedlings were grown in bare soil or soil covered with either live grass or straw. Covering soil with straw produced thermal conditions in soil and air that were indistinguishable from those associated with live grass. In contrast, seedlings grown in bare soil experienced more rapid increase in soil temperatures during late winter and spring, less frequent and less severe frosts, and temperature maxima that more closely followed those of the atmosphere than seedlings growing in live grass or straw. After 1 year, seedlings in bare soil had four times the biomass of those grown in grass or straw. Inhibition of seedling growth by grass was attributed to alteration of the thermal environment which caused (1) seedlings to have a short growing season largely restricted to summer, (2) temporal separation in competition for resources with consumption of below-ground resources by grass in spring reducing availability of resources to support tree seedling growth in early summer, and (3) seedlings to be more subject to stress from temperature extremes. These results show that thermal interference plays a major role in interactions between plants.
Microcosms of Danthonia richardsonii (Cashmore) accumulated more carbon when grown under CO2 enrichment (719 μL L–1 cf. 359 μL L–1) over a four‐year period, even when nitrogen availability severely restricted productivity (enhancement ratios for total microcosm C accumulation of 1.21, 1.14 and 1.29 for mineral N supplies of 2.2, 6.7 and 19.8 g N m–2 y–1, respectively). The effect of CO2 enrichment on total system carbon content did not diminish with time. Increased carbon accumulation occurred despite the development over time of a lower leaf area index and less carbon in the green leaf fraction at high CO2. The extra carbon accumulated at high CO2 in the soil, senesced leaf and leaf litter fractions at all N levels, and in root at high‐N, while at low‐and mid‐N less carbon accumulated in the root fraction at high CO2. The rate of leaf turnover was increased under CO2 enrichment, as indicated by increases in the carbon mass ratio of senesced to green leaf lamina. Microcosm evapotranspiration rates were lower at high CO2 when water was in abundant supply, resulting in higher average soil water contents. The higher soil water contents at high CO2 have important implications for microcosm function, and may have contributed significantly to the increased carbon accumulation at high CO2. These results indicate that CO2 enrichment can increase carbon accumulation by a simple soil–plant system, and that any increase in whole system carbon accumulation may not be evident from snapshot measurements of live plant carbon.
Dry weight (DW) and nitrogen (N) accumulation and allocation were measured in isolated plants of Danthonia richardsonii (Wallaby Grass) for 37 d following seed imbibition. Plants were grown at ≈ 365 or 735 µL L -1 CO 2 with N supply of 0·05, 0·2 or 0·5 mg N plant -1 d -1 . Elevated CO 2 increased DW accumulation by 28% (low-N) to 103% (high-N), following an initial stimulation of relative growth rate. Net assimilation rate and leaf nitrogen productivity were increased by elevated CO 2 , while N concentration was reduced. N uptake per unit root surface area was unaffected by CO 2 enrichment. The ratio of leaf area to root surface area was decreased by CO 2 enrichment. Allometric analysis revealed a decrease in the shoot-N to root-N ratio at elevated CO 2 , while the shoot-DW to root-DW ratio was unchanged. Allometric analysis showed leaf area was reduced, while root surface area was unchanged by elevated CO 2 , indicating a down-regulation of total plant capacity for carbon gain rather than a stimulation of mineral nutrient acquisition capacity. Overall, growth in elevated CO 2 resulted in changes in plant morphology and nitrogen use, other than those associated simply with changing plant size and non-structural carbohydrate content.Key-words: allometry; climate change; grass; growth analysis; resource use. INTRODUCTIONDanthonia spp. (Wallaby Grasses) are widespread over South-eastern Australia (Moore 1970). Under elevated CO 2 , swards of the Australian native wallaby grass Danthonia richardsonii Cashmore were shown to increase plant-soil system carbon accumulation at three productivity-limiting levels of nitrogen availability. That increase was attained without an increase in LAI at the higher rates of N availability, and without an increase in live above-ground carbon (Lutze & Gifford 1995, 1998. Little is known about the response of D. richardsonii to CO 2 or N when grown as an isolated plant, and many questions about isolated plant responses to CO 2 enrichment remain unanswered.Growth at high CO 2 is often accompanied by a downregulation of photosynthesis which is sometimes associated with a reduction in the nitrogen concentration of photosynthetic tissues (Stitt 1991;Arp & Berendse 1993;Bowes 1993). The decrease in nitrogen concentration in green leaf may relate to an increase in the proportion of plant nitrogen invested in root, as was shown for Xanthium occidentale Bertol. (Hocking & Meyer 1985) and Pinus sp. seedlings (Griffin, Winner & Strain 1995). Little is known about how such changes may be associated with changes in plant morphology and function. Increases in root to plant weight ratio in response to high CO 2 are often reported when plants are nitrogen-or water-stressed (e.g. Stulen & den Hertog 1993). This may result in relatively more root surface area for the same plant mass, allowing a more thorough exploration of the soil for nutrient and water acquisition (Allen et al. 1992). Another indicator of the functional balance between root and leaf is the ratio of root length to leaf surface...
Nitrogen-stressed microcosms of the C3 grass Danthonia richardsonii gained nitrogen from the environment when grown under ambient or enriched (359,`amb' or 719 mL L ±1 enr', respectively) atmospheric CO 2 concentrations over a 4-y period. This gain was apparent at all rates of supplied mineral N (2.2, 6.7 or 19.8 g N m ±2 y ±1 ± low-N, mid-N or high-N), although it was small at high-N. Small losses of N occurred from the microcosm as leachate, while gaseous losses of N were estimated to be between 10% and 25% of applied mineral N. Losses of applied mineral N were slightly lower under CO 2 enrichment only at the highest rate of mineral N supply. Levels of 15 N natural abundance in green leaf (d 15 N) of ±2½ (amb low-N) and of below ±4½ (enr low-& mid-N) suggest that absorption of atmospheric NH 3 may have been a source of some of the extra N in the low and mid-N treatments. Biological N 2 ®xation, of up to 2 g m ±2 y ±1 was hypothesized to form the remainder of the environmental N source. Microcosm C:N ratio was higher under CO 2 enrichment. Nitrogen productivity of microcosm carbon gain (g C accumulated g ±1 leaf N day ±1 ) was increased (up to 100%) by CO 2 enrichment at all rates of mineral N supply. Green leaf %N was reduced by CO 2 enrichment, and there was less nitrogen in the green leaf pool under CO 2 enrichment. Less, or the same amount of nitrogen was present in senesced leaf, surface litter and root under CO 2 enrichment while more nitrogen was present in the soil in organic forms, and as NH 4 + at the highest rate of mineral N supply.
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