Summary: Understanding the drivers of geological‐scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function. Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (g_max) and leaf vein density (D_v) and physiological measurements including operational stomatal conductance (g_op), saturated (A_sat) and maximum (A_max) assimilation rates were investigated for 18 extant taxa in order to improve understanding of angiosperm diversification in the Cretaceous. Our study demonstrated significant relationships between g_op, g_max and D_v that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high g_max in angiosperms conferred a competitive advantage over gymnosperms by increasing the dynamic range (plasticity) of their gas exchange and expanding their ecophysiological niche space. We suggest that species with a high g_max (> 1400 mmol m⁻² s⁻¹) would have been capable of maintaining a high A_max as the atmospheric CO₂ declined through the Cretaceous, whereas gymnosperms with a low g_max would experience severe photosynthetic penalty. Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high g_max, D_v and increased plasticity in g_op, adds further functional insights into the mechanisms driving angiosperm speciation.

A rapid rise in CO ₂ has made evergreen trees more waterwise than deciduous trees, especially in cooler parts of the world.

Studies have indicated that plant stomatal conductance (g_s) decreases in response to elevated atmospheric CO₂, a phenomenon of significance for the global hydrological cycle. However, g_s increases across certain CO₂ ranges have been predicted by optimization models. The aim of this work was to demonstrate that under certain environmental conditions, g_s can increase in response to elevated CO₂. Using (1) an extensive, up-to-date synthesis of g_s responses in free air CO₂ enrichment (FACE)experiments, (2) in situ measurements across four biomes showing dynamic g_s responses to a CO₂ rise of ~50 ppm (characterizing the change in this greenhouse gas over the past three decades) and (3) a photosynthesis–stomatal conductance model, it is demonstrated that g_s can in some cases increase in response to increasing atmospheric CO₂. Field observations are corroborated by an extensive synthesis of g_s responses in FACE experiments showing that 11.8 % of g_s responses under experimentally elevated CO₂ are positive. They are further supported by a strong data-model fit (r² = 0.607) using a stomatal optimization model applied to the field g_s dataset. A parameter space identified in the Farquhar–Ball–Berry photosynthesis–stomatal conductance model confirms field observations of increasing g_s under elevated CO₂ in hot dry conditions. Contrary to the general assumption, positive g_s responses to elevated CO₂, although relatively rare, are a feature of woody taxa adapted to warm, low-humidity conditions, and this response is also demonstrated in global simulations using the Community Land Model (CLM4). The results contradict the over-simplistic notion that global vegetation always responds with decreasing g_s to elevated CO₂, a finding that has important implications for predicting future vegetation feedbacks on the hydrological cycle at the regional level.

Plant growth chambers provide a controlled environment to analyse the effects of environmental parameters (light, temperature, atmospheric gas composition etc.) on plant function. However, it has been shown that a ‘chamber effect’ may exist whereby results observed are not due to an experimental treatment but to inconspicuous differences in supposedly identical chambers. In this study, Vicia faba L. ‘Aquadulce Claudia’ (broad bean) plants were grown in eight walk-in chambers to establish if a chamber effect existed, and if so, what plant traits are best for detecting such an effect. A range of techniques were used to measure differences between chamber plants, including chlorophyll fluorescence measurements, gas exchange analysis, biomass, reproductive yield, anatomical traits and leaf stable carbon isotopes. Four of the eight chambers exhibited a chamber effect. In particular, we identified two types of chamber effect which we term ‘resolvable’ or ‘unresolved’; a resolvable chamber effect is caused by malfunctioning components of a chamber and an unresolved chamber effect is caused by unknown factors that can only be mitigated by appropriate experimental design and sufficient replication. Not all measured plant traits were able to detect a chamber effect and no single trait was capable of detecting all chamber effects. Fresh weight and flower count detected a chamber effect in three chambers, stable carbon isotopes (δ13C) and net rate CO2 assimilation (An) identified a chamber effect in two chambers, stomatal conductance (gs) and total performance index detected an effect only in one chamber. (1) Chamber effects can be adequately detected by fresh weight measurements and flower counts on Vicia faba plants. These methods were the most effective in terms of detection and most efficient in terms of time. (2) δ13C, gs and An measurements help distinguish between resolvable and unresolved chamber effects. (3) Unresolved chamber effects require experimental unit replication while resolvable chamber effects require investigation, repair and retesting in advance of initiating further experiments.