In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

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High levels of ozone have been measured in rural and remote locations, far from significant anthropogenic sources of oxidant precursors. Elevated oxidant concentrations in these areas could be the result of transport into these areas and/or photooxidation of locally produced biogenic hydrocarbons. Volatile organics, including monoterpenes, have been detected in the atmosphere (I 1, 15, 18, 25), and the reports suggested that the hydrocarbons had a biogenic origin. Robinson (17) proposed that hydrocarbon concentrations were governed by both long distance transport and local production. However, there are only limited data available concerning the biogenic emission rates of potential photochemically reactive hydrocarbons such as the monoterpenes.The data used to estimate emission rates for monoterpenes were collected using a variety of experimental techniques ranging from static encapsulation chambers to profile measurements in the field.The degree of environmental control during the measurement period was highly variable making it difficult to establish a clear relationship between monoterpene emission rates and environmental conditions. The data base for monoterpene emission rates is limited almost exclusively to a-pinene. Only limited data are available conceming the influence of the environment, particularly light and temperature, on monoterpene emission rates. Apparently light does not directly influence monoterpene emissions even though photosynthate is required for biosynthesis (4,13).The emissions of camphor (23) and a-pinene increase with temperature (1, 10, 13).The objectives of this study were to (a) determine the monoterpene emission rates from intact plants under controlled environmental conditions using a dynamic mass balance gas exchange chamber; (b) determine the independent influence of light and temperature on monoterpene emission rates; (c) determine the emission rates of monoterpenes from dead needles; and (d) approximately 15 months old when sampled and appeared healthy. The plants had both mature and recently elongated needles, none of which appeared to be defective, and there were no significant bark lacerations or gum exudations.Gas Exchange Chamber. A dynamic mass balance gas exchange chamber was used to determine monoterpene emission and photosynthetic rates (22). The gas exchange chamber was housed in a controlled environment chamber which regulated light intensity and cooling. Ambient air was pumped through an Aadco pure air generator to remove hydrocarbons and CO2 and to reduce the dewpoint. CO2 and water vapor were added back to the air stream to obtain the desired concentrations within the gas exchange chamber. C02, water vapor and hydrocarbons were measured at the gas exchange chamber's inlet and outlet ports, and air flow was measured at the chamber inlet. Photosynthetic and monoterpene emission rates were determined using the equations for calculating gas fluxes in an open gas exchange chamber (19).During experiments, the CO2 concentration in the gas exchange chamber wa...

There is a growing awareness of the role of vegetation as a source of reactive hydrocarbons that may serve as photochemical oxidant precursors. A study was designed to assess independently the influence of variable light and temperature on isoprene emissions from live oak (Quercus virginiana Mill.). Plants were conditioned in a growth chamber and then transferred to an environmentally controlled gas‐exchange chamber. Samples of the chamber atmosphere were collected; isoprene was concentrated cryogenically and measured by gas chromatography. A logistic function was used to model isoprene emission rates. Under regimes of low temperature (20°C) or darkness, isoprene emissions were lowest. With increasing temperature or light intensity, the rate of isoprene emission increased, reaching maxima at 800 μE m‐2 s‐1 and 40–44°C, respectively. Higher temperatures caused a large decrease in emissions. Since the emissions of isoprene were light‐saturated at moderate intensities, temperature appeared to be the main factor controlling emissions during most of the day. Carbon lost through isoprene emissions accounted for 0.1 to 2% of the carbon fixed during photosynthesis depending on light intensity and temperature.

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 Elevated CO# increases root growth and fine (diam. 2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO # changes the proportion of C allocated to root biomass, measured as either the root : shoot ratio or the fine root : needle ratio. Elevated CO # tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO # on conifer fine-root life span, and there is no clear trend. Elevated CO # increases the absolute fine-root turnover rates ; however, the standing crop root biomass is also greater, and the effect of elevated CO # on relative turnover rates (turnover : biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO # can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO # concentration initially increases biomass, but this stimulation declines with the response to elevated CO # because increases in assimilation are not matched by increases in nutrient supply.

Although numerous studies indicate that increasing atmospheric CO2 or temperature stimulate soil CO2 efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO2 and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO2 and elevated temperature on these components of soil CO2 efflux in Douglas‐fir terracosms. We measured both soil CO2 efflux rates and the 13C and 18O isotopic compositions of soil CO2 efflux in 12 sun‐lit and environmentally controlled terracosms with 4‐year‐old Douglas fir seedlings and reconstructed forest soils under two CO2 concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO2 efflux. In most cases, litter decomposition was the dominant component of soil CO2 efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO2 concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO2, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual‐isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.

A new environmental‐tracking, sun‐lit controlled‐environment facility (terracosm) that can control and manipulate climatic and edaphic factors while maintaining natural environmental variability was developed to study the effects of environmental stresses on a model ecosystem (i.e., plant and soil processes). An analysis of terracosm performance data indicates that the terracosms simulated natural seasonal and diurnal changes in atmospheric CO2, air and soil temperatures, vapor pressure deficit (VPD), and soil moisture. The terracosm performance data indicate that between 92 and 100% of the hourly CO2 concentrations are within ±50 µmol mol−1 of the target concentrations for both ambient and elevated treatments (1 Nov. 1993 through 30 Nov. 1994). Air temperatures are within 2°C of the target temperature between 85 and 100% of the hours for both ambient and elevated temperature treatments. The VPD was approximately the same (0.09 kPa difference between treatments) in the ambient and elevated temperature treatments. Distributed process control was implemented to minimize downtime. Terracosm downtime, periods when terracosm environmental conditions could not be reliably controlled, varied between 2.4 and 2.8% of all hours, and was equally distributed between biological sampling and equipment problems.