Soil [N] modulates soil C cycling in CO2-fumigated tree stands: a meta-analysis.

Under elevated atmospheric CO(2) concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO(2) effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta-analysis to test the hypotheses that: (1) elevated atmospheric CO(2) stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO(2) induces a C allocation shift towards below-ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO(2). Soil N concentration strongly interacted with CO(2) fumigation: the effect of elevated CO(2) on fine root biomass and -production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO(2) are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.

Chemical fluxes in time through forest ecosystems in the UK - soil response to pollution recovery.

Long term trend analysis of bulk precipitation, throughfall and soil solution elemental fluxes from 12 years monitoring at 10 ICP Level II forest sites in the UK reveal coherent national chemical trends indicating recovery from sulphur deposition and acidification. Soil solution pH increased and sulphate and aluminium decreased at most sites. Trends in nitrogen were variable and dependant on its form. Dissolved organic nitrogen increased in bulk precipitation, throughfall and soil solution at most sites. Nitrate in soil solution declined at sites receiving high nitrogen deposition. Increase in soil dissolved organic carbon was detected - a response to pollution recovery, changes in soil temperature and/or increased microbial activity. An increase of sodium and chloride was evident - a possible result of more frequent storm events at exposed sites. The intensive and integrated nature of monitoring enables the relationships between climate/pollutant exposure and chemical/biological response in forestry to be explored.

Risk assessments for forest trees: the performance of the ozone flux versus the AOT concepts.

Published ozone exposure-response relationships from experimental studies with young trees performed at different sites across Europe were re-analysed in order to test the performance of ozone exposure indices based on AOTX (Accumulated exposure Over a Threshold of X nmol mol(-1)) and AF(st)Y (Accumulated Stomatal Flux above a threshold of Y nmol m(-2) s(-1)). AF(st)1.6 was superior, as compared to AOT40, for explaining biomass reductions, when ozone sensitive species with differing leaf morphology were included in the analysis, while this was not the case for less sensitive species. A re-analysis of data with young black cherry trees, subject to different irrigation regimes, indicated that leaf visible injuries were more strongly related to the estimated stomatal ozone uptake, as compared to the ozone concentration in the air. Experimental data with different clones of silver birch indicated that leaf thickness was also an important factor influencing the development of ozone induced leaf visible injury.

Stomatal conductance of forest species after long-term exposure to elevated CO2 concentration: a synthesis.

• Data from 13 long-term (> 1 yr), field-based studies of the effects of elevated CO2 concentration ([CO2 ]) on European forest tree species were analysed using meta-analysis and modelling. Meta-analysis was used to determine mean responses across the data sets, and data were fitted to two commonly used models of stomatal conductance in order to explore response to environmental conditions and the relationship with assimilation. • Meta-analysis indicated a significant decrease (21%) in stomatal conductance in response to growth in elevated [CO2 ] across all studies. The response to [CO2 ] was significantly stronger in young trees than old trees, in deciduous compared to coniferous trees, and in water stressed compared to nutrient stressed trees. No evidence of acclimation of stomatal conductance to elevated [CO2 ] was found. • Fits of data to the first model showed that growth in elevated [CO2 ] did not alter the response of stomatal conductance to vapour pressure deficit, soil water content or atmospheric [CO2 ]. Fits of data to the second model indicated that conductance and assimilation responded in parallel to elevated [CO2 ] except when water was limiting. • Data were compared to a previous meta-analysis and it was found that the response of gs to elevated [CO2 ] was much more consistent in long-term (> 1 yr) studies, emphasising the need for long-term elevated [CO2 ] studies. By interpreting data in terms of models, the synthesis will aid future modelling studies of responses of forest trees to elevated [CO2 ].

Short-term changes in carbon-isotope discrimination in the C3-CAM intermediate Clusia minor L. growing in Trinidad.

On-line instantaneous carbon isotope discrimination was measured in conjunction with net uptake of CO2 in leaves of exposed and shaded plants of the C3-CAM intermediate Clusia minor growing under natural conditions in Trinidad. At the end of the rainy season (late January-early February, 1992) C3 photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO2 at night for the synthesis of malic acid. Citric acid was the major organic acid accumulated by exposed leaves at this time with a citric: malic acid ratio of 11:1. Values of instantaneous discrimination (Δ) in exposed leaves during the wet season rose from 17.1‰ shortly after dawn to 22.7‰ around mid-day just before stomata closed, suggesting that most CO2 was fixed by Rubisco at this time. During the late afternoon, instantaneous Δ declined from 22.2‰ to 17‰, probably reflecting the limited contribution from PEPc activity and an increase in diffusional resistance to CO2 in exposed leaves. Shaded leaves showed no CAM activity and CO2 uptake proceeded throughout the day in the wet season. The decrease in instantaneous Δ from 27‰ in the morning to 19.2‰ in the late afternoon was therefore entirely due to diffusional limitation. Leaves sampled in the dry season (mid-March, 1992) had by now induced full CAM activity with both malic and citric acids accumulated overnight and stomata closed for 4-5 h over the middle of the day. Values of instantaneous Δ measured over the first 3 h after dawn (6.4-9.1‰) indicated that C4 carboxylation dominated CO2 uptake for most of the morning when rates of photosynthesis were maximal, implying that under natural conditions, the down regulation of PEPc in phase II occurs much more slowly than laboratory-based studies have suggested. The contribution from C3 carboxylation to CO2 uptake during phase II was most marked in leaves which accumulated lower quantities of organic acids overnight. In exposed leaves, measurements of instantaneous Δ during the late afternoon illustrated the transition from C3 to C4 carboxylation with stomata remaining open during the transition from dusk into the dark period. Uptake of CO2 by shaded leaves during the late afternoon however appeared to be predominantly limited by decreased stomatal conductance. The short-term measurements of instantaneous Δ were subsequently integrated over 24 h in order to predict the leaf carbon isotope ratios (δp) and to compare this with the δp measured for leaf organic material. Whilst there was close agreement between predicted and measured δp for plants sampled in the wet season, during the dry season the predicted carbon isotope ratios were 5-9‰ higher than the measured isotope ratios. During the annual cycle of leaf growth most carbon was fixed via the C3 pathway although CAM clearly plays an important role in maintaining photochemical integrity in the dry season.

The carbon isotope ratio of plant organic material reflects temporal and spatial variations in CO2 within tropical forest formations in Trinidad.

A method of monitoring and collecting CO2 samples in the field has been developed which has been used to study both temporal and spatial variations in canopy CO2 isotopic signatures in two contrasting tropical forest formations in Trinidad. These have been related to vertical gradients in the carbon isotope ratio (δ13C) of organic material in conjunction with measurements of other environmental parameters. The δ13C of leaf material from two canopies showed a gradient with respect to height, more negative values being found low in the understorey. The deciduous secondary forest, (Simla) showed a difference of 4.6‰ and the semi-evergreen seasonal canopy (Aripo), 2.8‰. The range of δ13C values at Simla was 4‰ less negative than those at Aripo. In order to relate these measurements to the interaction between diffusion or carboxylation limitation, and source CO2 effects, variations in environmental parameters through the canopy have been compared with changes in CO2 partial pressure (P a) and isotopic composition δ13C throughout the day during the dry season. Values of P a20 m above the ground at Aripo varied from 380 vpm at dawn to 340 vpm at midday, at which time the partial pressure 15 cm above the ground was 375 vpm. The CO2 partial pressure did not stabilise during the course of the day, and there was good correlation (r 2=0.82) between δa and P a, with more negative values of δa occuring in the understorey. Diuraal changes of 2‰ were evident at all canopy positions. In the more open canopy at Simla, these gradients were similar, but less marked. Leaf-air vapour pressure deficit (VPD) showed no relationship with height, possibly as a result of minimal water flux from both the soil and the canopy due to low soil water content; VPD was 1.5 kPa higher at midday than dawn. A 3° C temperature gradient between the understorey and upper canopy was observed at Aripo but not in the more open Simla canopy. CO2 partial pressure stabilised for only 4 h in the middle of the day, while other parameters showed no stable period. The proportion of floor respired CO2 reassimilated at Aripo has been calculated as 26%, 19%, and 8% for the periods 0600-1000, 1000-1400, and 1400-1800 hours. In order to quantify source CO2 effects, measurements of the environmental parameters and assimilation rate must be made at all canopy positions and throughout the day.

On the ecophysiology of the Clusiaceae in Trinidad: expression of CAM in Clusia minor L. during the transition from wet to dry season and characterization of three endemic species.

A study was made of photosynthesis and expression of crassulacean acid metabolism (CAM) in naturally exposed and shaded populations of Clusia minor L. during the transition from wet to dry season in Trinidad (mid-February to mid-April, 1990). At the start of the dry season, plants from exposed and shaded habitats showed a capacity for CAM either through the fixation of external or internal (respiratory) CO2 . Exposed plants showed continuous uptake of CO2 over 24 h although dark fixation accounted for only a small proportion of CO2 fixed over the day. The expression of CAM was considerably enhanced as the dry season progressed with substantial increases in the overnight accumulation of titratable acidity, particularly in leaves of exposed plants. This was accompanied by a reduction in day-time photosynthesis and an increase in dark fixation, with shaded plants showing only night-time fixation of CO2 . The magnitude of CAM in C. minor was substantial with a maximum ΔH+ of 1410 mol m-3 measured in leaves from exposed branches. Both malic and citric acids were accumulated overnight. The highest citric:malic acid ratios were found in young leaves from exposed plants with 250 mol m-3 malic and 125 mol m-3 citric acid accumulated near the time of maximum CAM activity. Photosynthetic efficiency, measured as light responses of O2 evolution, also varied on a daily basis dependent on the incident photosynthetic photon flux density (PPFD). Apparent quantum yield and photosynthetic capacity showed marked reductions depending on the degree of exposure, suggesting that photoinhibitory responses are important under natural conditions. An analysis of three members of the Clusiaceae endemic to Trinidad showed that each had the capacity to induce CAM activity, despite being found in a narrow range of habitats which have higher rainfall than those of C. minor. However, despite the variable expression of CAM activity, carbon isotope composition suggested that when integrated throughout the year, carbon accumulation is predominantly mediated via the C3 pathway in all the species studied.

Acquired resistance of sheep to larvae of Lucilia cuprina, assessed in vivo and in vitro.

The effect on subsequent larval survival of infesting sheep repeatedly with larvae of Lucilia cuprina was assayed in vivo and in vitro. One in vivo assay technique, in which implanted larvae were grown to third instar, indicated a significant reduction in larval survival; another in vivo technique, in which larvae were allowed to develop to second instar in small aluminium rings attached to the sheep, indicated no reduction in larval growth or survival. Larvae of Lucilia cuprina grown in vitro on media containing sera from previously infested sheep were significantly retarded in growth after 20 h compared with controls; no difference was detected when larvae were allowed to develop to pupation on two changes of the same media. No significant differences in survival of larvae either to 20 h or to pupation were obtained between the two treatments. ELISA antibody levels against crude soluble larval material were significantly higher for sera from infested sheep than for control sera, and the regression of antibody level on mean larval weight obtained after 20 h growth in vitro was significant. The immunoglobulin fraction isolated from sera of infested sheep significantly retarded larval growth when incorporated with normal serum in growth media. These results are consistent with an effect of specific anti-larval antibody produced by sheep in response to infestation.

Photosynthetic pathway, chilling tolerance and cell sap osmotic potential values of grasses along an altitudinal gradient in Papua New Guinea.

A total of 22 grass species were examined from 5 sites spanning the altitudinal range 1550-4350 m.a.s.l. The presence of the C3 or C4 photosynthetic pathway was determined from δ13C values and chilling tolerance was assessed on the basis of electrolyte leakage from leaf slices incubated on melting ice. Most of the grasses studied at the lower altitude sites of 1550 m.a.s.l. (annual mean of daily minimum temperature, 14.6° C) and 2600 m.a.s.l. (9.4° C) possessed C4 photosynthesis and were chill-sensitive. The single except ion was Agrostis avenacea, a montane chill-resistant C3 species which occurred at 2600 m.a.s.l. The three species apparently most sensitive to chilling were Ischaemum polystachyum, Paspalum conjugatum and Saccharum robustum, all occurring at 1550 m.a.s.l. At the higher altitude sites of 3280 (5.6° C), 3580 (4.0° C) and 4350 (-0.7°C) m.a.s.l., most of the grasses exhibited C3 photosynthesis and were chill-resistant. However, an Upland population of the C4 species, Miscanthus floridulus was found at 3280 m.a.s.l. which had acquired chill-resistance as confirmed by additional in vivo variable chlorophyll fluorescence measurements. Cell sap osmotic potential values of the upland grasses at altitudes of 3280-4350 m.a.s.l. were lower (-8.1 to -19.8 bars) than values in grasses from 1550 and 2600 m.a.s.l. (-3.9 to -7.5 bars) due mainly to the presence of non-electrolyte osmoticants, which may be involved in frost avoidance mechanism(s).

Short-term changes in carbon-isotope discrimination identify transitions between C3 and C 4 carboxylation during Crassulacean acid metabolism.

Short-term measurements of instantaneous carbon-isotope discrimination have been determined from mass-spectrometric analyses of CO2 collected online during gas exchange for the epiphytic bromeliad Tillandsia utriculata L. Using this technique, the isotopic signature of CO2 exchange for each phase of Crassulacean acid metabolism (CAM) has been characterised. During night-time fixation of CO2 (Phase I), discrimination (Δ) ranged from 4.4 to 6.6‰, equivalent to an effective carbon-isotope ratio (δ(13)C) of -12.3 to -14.5‰ versus Pee Dee Belemnite (PDB). These values reflected the gross photosynthetic balance between net CO2 uptake and refixation of respiratory CO2, characteristic of CAM in the Bromeliaceae. When Δ for the relative proportion of external (p a ) and internal (p i) CO2 is taken into account, calculated p i/p a decreased during the later part of the dark period from 0.68 to 0.48. Measurements of Δ during Phase II, early in the light period, showed the transition between C4 and C3 pathways, with carboxylation being increasingly dominated by ribulose bisphosphate carboxylase (Rubisco) as Δ increased from 10.5 to 21.2‰ During decarboxylation in the light period (Phase III), CO2 leaked out of the leaf and the inherent discrimination of Rubisco was expressed. The value of Δ calculated from on-line measurements (64.4‰) showed that the CO2 lost was considerably enriched in (13)C, and this was confirmed by direct analysis of the CO2 diffusing out into a CO2-free atmosphere (δ (13)C = + 51.6‰ versus PDB). Instantaneous discrimination was characteristic of the C3 pathway during Phase IV (late in the light period), but a reduction in Δ showed an increasing contribution from phosphoenolpyruvate carboxylase. The results from this non-invasive technique confirm the observations that "double carboxylation" involving both phosphoenolpyruvate carboxylase and Rubisco occurs during the transient phases of CAM (II and IV) in the light period.