It’s highly cited because it’s one of the only reviews to focus on how stomatal conductance is affected by the increasing level of atmospheric carbon dioxide (CO₂). It’s well known that high CO₂ affects photosynthesis and therefore growth of plants, and those responses have been reviewed many times.

“The paper is a synthesis of data on effects of CO2 on stomatal conductance in forests.”

However, CO₂ also has an important effect on stomatal conductance, which is one of the major determinants of plant water use. With water availability becoming an important issue worldwide, particularly with climate change predicted to decrease rainfall in many regions of the globe, there’s increasing awareness that the impact of CO₂ on plant water use needs to be taken into account.

The paper is a synthesis of data on the effects of CO₂ on stomatal conductance in forests. I think another reason why the paper is highly cited is because we analyzed the data in particularly useful ways. Meta-analysis was used to estimate the overall response of stomatal conductance to CO₂, and how it differs among different categories of vegetation.

We also carried out several model-based analyses, which are useful because they interpret the data in terms that people can use directly in larger-scale models, and they provide insights into how the responses vary with environmental conditions. Another nice thing we did was to combine our data with a previous database, improving the power of both datasets.

Atmospheric carbon dioxide is increasing in concentration because of fossil fuel emissions. In addition to causing changes in climate, the increase in CO₂ has direct effects on plants. One of these effects is to cause plants to close their stomata, which are pores in the leaves through which water escapes.

By closing the stomata a little, the amount of water that plants use is reduced. This is a positive response for plants, since it helps them to conserve their soil water. We showed that this response was quite consistent for broad-leafed deciduous trees, but that coniferous trees tended not to close their stomata.

The research was part of a collaborative EU project that involved 12 institutional partners from nine countries. I was employed as a postdoc with responsibility for co-coordinating the modelling activities by this group. This particular review could only have been done as part of such collaboration, because we needed to reanalyze the original datasets to get all the information in a comparable form.

One of the main implications is that plant water use may be decreased in the future, which would tend to increase soil water storage and river runoff. There is also the potential for plants to better withstand droughts—although one of the major findings was that the response of water-stressed trees differed significantly from those of well-watered trees.

We concluded that more research was needed to understand how stomatal conductance would respond to elevated CO₂ in water-limited conditions. I’m now part of a team from four different institutions in Sydney, Australia, that is setting up a major field experiment to study exactly this problem (see photo below).