James Heath

Forests have a key role as carbon sinks, which could potentially mitigate the continuing increase in atmospheric carbon dioxide concentration and associated climate change. We show that carbon dioxide enrichment, although causing short-term growth stimulation in a range of European tree species, also leads to an increase in soil microbial respiration and a marked decline in sequestration of root-derived carbon in the soil. These findings indicate that, should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration.

Above‐ground herbivory is ubiquitous in terrestrial ecosystems, yet its impacts on below‐ground processes and consequences for plants remain ambiguous. To examine whether physiological responses of individual trees may potentially modify soil nutrient availability, we subjectedFagus sylvaticaL. (European beech) andAbies albaMill. (silver fir) to simulated foliar herbivory over two growing seasons. Above‐ground herbivory enhanced N mineralization and inorganic N availability in the soil. The total input of C from the plant roots to the soil is not known; however, carbon sequestration in the soil, measured using stable isotopic techniques, was unaffected by herbivory.Fagusresponded to herbivory by producing larger leaves, with increased photosynthetic capacity and N content, which largely compensated for the loss of biomass;Abiesexhibited no such response. We conclude that despite large interspecific differences in the growth response, tree physiological responses to foliar herbivory are capable of directly modifying soil biological processes.

Stomatal conductance (g_s) and photosynthetic rate (A) were measured in young beech (Fagus sylvatica), chestnut (Castanea sativa) and oak (Quercus robur) growing in ambient or CO₂‐enriched air. In oak, g_s was consistently reduced in elevated CO₂. However, in beech and chestnut, the stomata of trees growing in elevated CO₂ failed to close normally in response to increased leaf‐to‐air vapour pressure deficit (LAVPD). Consequently, while g_s was reduced in elevated CO₂ on days with low LAVPD, on warm sunny days (with correspondingly high LAVPD) g_s was unchanged or even slightly higher in elevated CO₂. Furthermore, during drought, g_s of beech and chestnut was unresponsive to [CO₂], over a wide range of ambient LAVPD, whereas in oak g_s was reduced by an average of 50% in elevated CO₂. Stimulation of A by elevated CO₂ in beech and chestnut was restricted to days with high irradiance, and was greatest in beech during drought. Hence, most of the additional carbon gain in elevated CO₂ was made at the expense of water economy, at precisely those times (drought, high evaporative demand) when water conservation was most important. Such effects could have serious consequences for drought tolerance, growth and, ultimately, survival as atmospheric [CO₂] increases.