It was hypothesized that high CO2 availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO2 concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO2 concentrations. Ponderosa pine grown at 700 μmol mol–1 CO2 exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol–1 CO2. Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol–1 CO2 exhibited increased photosynthetic rates as compared to growth at 350 μmol mol–1 CO2 in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO2 on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO2 plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO2 concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO2 may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO2 on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate.
Summary The alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their influence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO2 concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct influence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO2 concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution reflects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO2 concentration enhanced total modelled VOC emission by 81.8% (isoprene + 82.1%, monoterpenes + 81.6%, ORVOC + 81.1%), whereas a simulated doubled CO2 alone enhanced total modelled VOC emission by only + 11.8% (isoprene + 13.7%, monoterpenes + 4.1%, ORVOC + 11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene – 11.7%, monoterpenes – 18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf‐level adjustments to elevated CO2 (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem‐level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates).
Leaf 15N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO2 concentration. We tested the hypothesis that elevated CO2 significantly changes foliage δ15N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ15N of foliage of 27 field‐grown plant species from six free‐air CO2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ15N of foliage produced under elevated CO2 was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ15N by life form and growth habit revealed that the CO2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO2 concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO2 and leaf δ15N. In a select number of species, we measured bulk soil δ15N at a depth of 10 cm, and found that the observed depletion of foliage δ15N in response to elevated CO2 was unrelated to changes in the soil δ15N. While the data suggest a strong influence of elevated CO2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ15N responses to elevated CO2 reported here resulted from a general drop in δ15N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ15N under high CO2 may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.
Importance of this paper: Organic chemicals evaporating out of vegetation may aect various aspects of the earthÕs environment ± including the amount of haze in the air, which aects the earthÕs heat balance by re¯ecting sunlight back into space, and also the lifetime of methane gas in the atmosphere. Based on reconstructed vegetation zones for the coldest phase of the Last Glacial Period (about 20,000 years ago) and the warmer present interglacial (the last 11,000 years), we estimated the total amount of the substances evaporating from the world's vegetation. In the cooler, drier, less forested world of the Last Glacial, the¯uxes of these compounds were probably reduced by about one-third to one-half. The resulting reduction in haze might have tended to keep the earth slightly warmer during the Glacial than it would otherwise have been. In addition, Glacial methane levels may have been lowered because of the lack of volatile organics Ômopping upÕ reactive free radicals that would otherwise break down methane. AbstractThe¯ux of volatile organic chemicals from natural vegetation in¯uences various atmospheric properties including oxidation state of the troposphere via the hydroxyl radical (OH), photochemical haze production and the concentration of greenhouse gases (CH 4 , H 2 O, CO). Because the Volatile Organic Compound (VOC)¯ux in the present-day world varies markedly with both vegetation cover and with climate, changes in the emission of VOCs may have damped or ampli®ed past climate changes.Here we conduct a preliminary study on possible changes in VOC emission resulting from broad scale vegetation and climate change since the Last Glacial Maximum (LGM). During the general period of the LGM ($25±17,000 years before present {BP}), global forest cover was considerably less than in the present potential situation. The change in vegetation would have resulted in a $30% reduction in VOC emission at 643 Tg y À1 relative to the present potential vegetation (912.9 Tg y À1 ). Uncertainty in global vegetation cover during the LGM bounds the VOC estimate by AE15%. In contrast, during the warmer early-to-mid Holocene (8000 and 5000 BP), with greater forest extent and less desert than during the late Holocene (0 BP), emission rates of VOCs seem likely to have been higher than at present.Further modi®cations in VOC emission may have been mediated by a reduction in mean tropical lowland temperatures (by around 5±6°C) decreasing the LGM VOC emission estimate by 38% relative to the warmer LGM scenario. Increased VOC emissions due to forest expansion and increased tropical temperatures since the LGM may have served as a signi®cant driver of climate change over the last 15 ka y through the in¯uence of VOC oxidation; this can impact tropospheric radiative balance through reductions in the concentration of OH, increasing the concentration of CH 4 .The error limits on past VOC emission estimates are large, given the uncertainties of present-day VOC emission rates, paleoecosystem distribution, tropical paleoclimatic conditions, and ph...
We measured the relative control that resource availability (as a supply-side control) and wounding (as a demand-side control) exert on patterns of monoterpene synthesis and concentration in Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] needles. While supply-side controls should alter monoterpene production due to changes in the availability of substrate (carbohydrates), demand-side controls alter the need for a defensive product. We examined these relationships by measuring constitutive (preformed) and wound-induced rates of monoterpene synthesis and pool sizes in trees grown under ambient and elevated (ambient +200 µmol mol) CO, ambient and elevated (ambient +4°C) temperature, and in trees grown under four levels of nitrogen fertilization (0, 50, 100 and 200 µg g N by weight). Monoterpene pool size decreased at elevated CO, increased at elevated temperature and did not change in response to nitrogen fertilization. Overall, we did not find that foliar nitrogen, carbon balance, or rate of monoterpene synthesis alone were consistent predictors of monoterpene concentration in current-year Douglas fir needles. In addition, despite a wound-induced decrease in monoterpene pool size, we found no evidence for induction of monoterpene synthesis in response to wounding. The influence of either resource availability or wounding on rates of monoterpene synthesis or accumulation cannot be explained by traditional supply-side or demand-side controls. We conclude that monoterpene synthesis in first-year Douglas fir needles is controlled by fairly conservative genetic mechanisms and is influenced more by past selection than by current resource state.
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