The effect of elevated CO2 on photosynthesis is modulated by nitrogen supply and reduced water availability in Picea abies.

It is assumed that the stimulatory effects of elevated CO2 concentration ([CO2]) on photosynthesis and growth may be substantially reduced by co-occurring environmental factors and the length of CO2 treatment. Here, we present the study exploring the interactive effects of three manipulated factors ([CO2], nitrogen supply and water availability) on physiological (gas-exchange and chlorophyll fluorescence), morphological and stoichiometric traits of Norway spruce (Picea abies) saplings after 2 and 3 years of the treatment under natural field conditions. Such multifactorial studies, going beyond two-way interactions, have received only limited attention until now. Our findings imply a significant reduction of [CO2]-enhanced rate of CO2 assimilation under reduced water availability which deepens with the severity of water depletion. Similarly, insufficient nitrogen availability leads to a down-regulation of photosynthesis under elevated [CO2] being particularly associated with reduced carboxylation efficiency of the Rubisco enzyme. Such adjustments in the photosynthesis machinery result in the stimulation of water-use efficiency under elevated [CO2] only when it is combined with a high nitrogen supply and reduced water availability. These findings indicate limited effects of elevated [CO2] on carbon uptake in temperate coniferous forests when combined with naturally low nitrogen availability and intensifying droughts during the summer periods. Such interactions have to be incorporated into the mechanistic models predicting changes in terrestrial carbon sequestration and forest growth in the future.

Methane emission from stems of European beech (Fagus sylvatica) offsets as much as half of methane oxidation in soil.

Trees are known to be atmospheric methane (CH4 ) emitters. Little is known about seasonal dynamics of tree CH4 fluxes and relationships to environmental conditions. That prevents the correct estimation of net annual tree and forest CH4 exchange. We aimed to explore the contribution of stem emissions to forest CH4 exchange. We determined seasonal CH4 fluxes of mature European beech (Fagus sylvatica) stems and adjacent soil in a typical temperate beech forest of the White Carpathians with high spatial heterogeneity in soil moisture. The beech stems were net annual CH4 sources, whereas the soil was a net CH4 sink. High CH4 emitters showed clear seasonality in their stem CH4 emissions that followed stem CO2 efflux. Elevated CH4 fluxes were detected during the vegetation season. Observed high spatial variability in stem CH4 emissions was neither explicably by soil CH4 exchange nor by CH4 concentrations, water content, or temperature studied in soil profiles near each measured tree. The stem CH4 emissions offset the soil CH4 uptake by up to 46.5% and on average by 13% on stand level. In Central Europe, widely grown beech contributes markedly to seasonal dynamics of ecosystem CH4 exchange. Its contribution should be included into forest greenhouse gas flux inventories.

Trees as net sinks for methane (CH4 ) and nitrous oxide (N2 O) in the lowland tropical rain forest on volcanic Réunion Island.

Trees are known to emit methane (CH4 ) and nitrous oxide (N2 O), with tropical wetland trees being considerable CH4 sources. Little is known about CH4 and especially N2 O exchange of trees growing in tropical rain forests under nonflooded conditions. We determined CH4 and N2 O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400-yr-old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows. The stems studied were net sinks for atmospheric CH4 and N2 O, as were cryptogams, which seemed to be co-responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH4 . Their N2 O fluxes were negligible. Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH4 and N2 O. The volcanic tropical lowland rain forest appears to be an important CH4 sink, as well as a possible N2 O sink.

The phytotoxic air-pollutant O3 enhances the emission of herbivore-induced volatile organic compounds (VOCs) and affects the susceptibility of black mustard plants to pest attack.

Stress-induced changes to plant biochemistry and physiology can influence plant nutritional quality and subsequent interactions with herbivorous pests. However, the effects of stress combinations are unpredictable and differ to the effects of individual stressors. Here we studied the effects of exposure to the phytotoxic air-pollutant ozone (O3), feeding by larvae of the large cabbage white butterfly (Pieris brassicae), and a combination of the two stresses, on the emission of volatile organic compounds (VOCs) by black mustard plants (Brassica nigra) under field and laboratory conditions. Field-grown B. nigra plants were also measured for carbon-nitrogen (C-N) content, net photosynthetic activity (Pn), stomatal conductance (gs) and biomass. The effects of O3 on interactions between plants and a herbivorous pest were addressed by monitoring the abundance of wild diamondback moth larvae (Plutella xylostella) and feeding-damage to B. nigra plants in an O3-free air concentration enrichment (O3-FACE) field site. Herbivore-feeding induced the emission of VOCs that were not emitted by undamaged plants, both under field and laboratory conditions. The combination of O3 and herbivore-feeding stresses resulted in enhanced emission rates of several VOCs from field-grown plants. Short-term O3 exposure (of 10 days) and P. brassicae-feeding did not affect C-N content, but chronic O3 exposure (of 34 and 47 days) and P. brassicae-feeding exacerbated suppression of Pn. Ozone exposure also caused visible injury and decreased the plant biomass. Field-grown B. nigra under elevated O3 were infested with fewer P. xylostella larvae and received significantly less feeding damage. Our results suggest that plants growing in a moderately polluted environment may be of reduced quality and less attractive to foraging herbivores.