Aim: An elevated CO 2 concentration (eCO 2 ) can increase photosynthesis (A sat ) and reduce stomatal conductance (g s ); hence, it can benefit tree growth directly by increasing carbon availability and/or indirectly by saving water. However, uncertainties remain regarding the direction and magnitude of these effects, particularly in combination with changes in temperature and water availability.

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1. The elevated CO 2 concentration (eCO 2 ) is expected to improve plant water relations and carbon (C) uptakes, with a potential to mitigate drought stress.However, the interactive effects of eCO 2 and drought on plant physiology and growth are not clear.2. We performed a meta-analysis on the interactive effects of eCO 2 and drought on plant water relations, photosynthesis, biomass production and allocation.3. We found that eCO 2 did not lead to the conservation of soil water, but improved leaf water status under drought conditions as evidenced by a higher leaf relative water content (LRWC) and a less negative midday leaf water potential, resulting from reduced stomatal conductance (g s ) and increased root to shoot ratio.Elevated CO 2 retarded the response of g s to drought, which may be mediated by the decrease in leaf abscisic acid concentration under eCO 2 and drought.Drought imposed stomatal limitations on photosynthesis (A), which was alleviated by eCO 2 via increasing intercellular CO 2 concentration (C i ). This led to a stronger A response to eCO 2 under drought, supporting the 'low C i effect'.However, no interaction of eCO 2 and drought was detected on plant biomass production. Intrinsic water use efficiency (iWUE) increased proportionally with eCO 2 , while plant-scale WUE was less responsive to eCO 2 . C 3 plants had advantages over C 4 plants in terms of A and biomass production under eCO 2 and wellwatered conditions rather than under eCO 2 and drought conditions. Drought caused a greater reduction in biomass for woody plants than for herbs. Plants growing in pots showed greater decreases in the physiology and biomass under drought than those growing in field. 4. Synthesis. These findings suggest that eCO 2 can alleviate the adverse impacts of drought on plant water relations and C sequestration, and are of significance in the prediction of plant growth and ecosystem productivity under global changes. K E Y W O R D S biomass, drought, elevated CO 2 , gas exchange, global change ecology, water potential, water use efficiency | 2837

Aim: While drought and shade are critical factors that determine seedling survival and growth, whether the responses of woody seedlings to drought are facilitated, aggravated or unaffected by shade remains uncertain.

Elevated temperature (Te) and drought often co‐occur and interactively affect plant carbon (C) metabolism and thus the ecosystem C cycling; however, the magnitude of their interaction is unclear, making the projection of global change impacts challenging. Here, we compiled 107 journal articles in which temperature and water availability were jointly manipulated, and we performed a meta‐analysis of interactive effects of Te and drought on leaf photosynthesis (Agrowth) and respiration (Rgrowth) at growth temperature, nonstructural carbohydrates and biomass of plants, and their dependencies on experimental and biological moderators (e.g., treatment intensity, plant functional type). Our results showed that, overall, there was no significant interaction of Te and drought on Agrowth. Te accelerated Rgrowth under well‐watered conditions rather than under drought conditions. The Te × drought interaction on leaf soluble sugar and starch concentrations were neutral and negative, respectively. The effect of Te and drought on plant biomass displayed a negative interaction, with Te deteriorating the drought impacts. Drought induced an increase in root to shoot ratio at ambient temperature but not at Te. The magnitudes of Te and drought negatively modulated the Te × drought interactions on Agrowth. Root biomass of woody plants was more vulnerable to drought than that of herbaceous plants at ambient temperature, but this difference diminished at Te. Perennial herbs exhibited a stronger amplifying effect of Te on plant biomass in response to drought than did annual herbs. Te exacerbated the responses of Agrowth and stomatal conductance to drought for evergreen broadleaf trees rather than for deciduous broadleaf and evergreen coniferous trees. A negative Te × drought interaction on plant biomass was observed on species‐level rather than on community‐level. Collectively, our findings provide a mechanistic understanding of the interactive effects of Te and drought on plant C metabolism, which would improve the prediction of climate change impacts.