Summary• Data from 13 long-term (> 1 yr), field-based studies of the effects of elevated CO 2 concentration ([CO 2 ]) 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 [CO 2 ] across all studies. The response to [CO 2 ] 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 [CO 2 ] was found.• Fits of data to the first model showed that growth in elevated [CO 2 ] did not alter the response of stomatal conductance to vapour pressure deficit, soil water content or atmospheric [CO 2 ]. Fits of data to the second model indicated that conductance and assimilation responded in parallel to elevated [CO 2 ] except when water was limiting.• Data were compared to a previous meta-analysis and it was found that the response of g s to elevated [CO 2 ] was much more consistent in long-term (> 1 yr) studies, emphasising the need for long-term elevated [CO 2 ] studies. By interpreting data in terms of models, the synthesis will aid future modelling studies of responses of forest trees to elevated [CO 2 ].

Seedlings of Quercus petraea (oak), Fraxinus excelsior (ash) and Pinus sylvestris (Scots pine) were grown at two CO2 concentrations with two O3 and two water supply treatments for 3 yr in a factorial experiment. Oak was the most responsive species to all three treatments; elevated CO2 and irrigation increased biomass by an average of 79% and 41%, respectively, whereas the ozone treatment resulted in a 30% reduction in growth. Significant treatment interactions in this species demonstrated that CO2 ameliorated and irrigation exacerbated the effects of ozone. For Scots pine and ash, irrigation and elevated CO2 increased growth by approx. 60% and 20%, respectively, whereas ozone had no detectable effect on ash and resulted in a 15% reduction in growth of Scots pine. Carbon partitioning to the shoot was enhanced by both the CO2 and H2O treatments in oak, while branching was also increased in this species in response to elevated O3, resulting in changes to the allometric relationships. CO2 enhanced leaf production in oak and Scots pine, and together with the promotion of shoot allocation, this indicates an increased susceptibility to windthrow. Biomass accumulation expressed as relative growth rate, suggested three different time‐dependent growth responses to elevated CO2; the CO2 fertilization effect was maintained through the third year of growth in oak, had disappeared in Scots pine and a negative effect was evident in ash. Foliar nitrogen and chlorophyll concentrations indicated a CO2‐induced nitrogen deficiency in oak and ash, but not in Scots pine. Chlorophyll degradation in response to ozone was observed in oak, an effect that was enhanced by irrigation and reduced by CO2, presumably through stomatal mediated changes in effective ozone dose. These results therefore suggest that elevated atmospheric CO2 concentrations will enhance growth of some UK forest tree species, although this might only be apparent during the juvenile phase. However, nitrogen deficiencies might limit this enhancement on some sites while changes in allocation and leaf area might promote susceptibility to windthrow. Elevated CO2 also provides some protection against ozone pollution, especially in combination with limited soil moisture availability. These interactions between CO2, ozone and water supply should be taken into account when predicting the effects of environmental change on tree growth and forest productivity.

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Microstimulation can modulate the activity of individual neurons in order to affect behavior, but the effects of stimulation on neuronal spiking are complex and remain poorly understood. This is especially challenging in the human brain where the response properties of individual neurons are sparse and heterogenous. Here we use micro- electrode array (MEAs) in the human anterior temporal lobe in six participants (3 female) to examine the spiking responses of individual neurons to microstimulation delivered through multiple distinct stimulation sites. We demonstrate that individual neurons can be driven with excitation or inhibition using different stimulation sites, which suggests an approach for providing direct control of spiking activity at the single neuron level. Spiking responses are inhibitory in neurons that are close to the site of stimulation, while excitatory responses are more spatially distributed. Together, our data demonstrate that spiking responses of individual neurons can be reliably identified and manipulated in the human cortex.SIGNIFICANCE STATEMENT:One of the major limitations in our ability to interface directly with the human brain is that the effects of stimulation on the activity of individual neurons remains poorly understood. Here, Youssef et al. examine the spiking responses of neurons in the human temporal cortex in response to pulses of microstimulation. They find that individual neurons can either be excited or inhibited depending on the site of stimulation. These data suggest an approach for modulating the spiking activity of individual neurons in the human brain.