• Energy Research

Predicting how increases in surface temperature will modulate the response of plants to rising atmospheric CO2 concentrations is an increasingly urgent aspect of climate change research. Plant responses to elevated CO2 have been well documented over the last 40 years, and the mechanisms underlying these responses are well understood. Elevated CO2 affects plants mainly by increasing photosynthesis and decreasing stomatal conductance (Ainsworth and Rogers 2007). However, the scaling up of these primary, leaf-level CO2 responses to the whole plant and canopy levels is moderated by the plant’s growth characteristics (e.g., sink strength, biomass partitioning), and the availability of soil water and nutrients (Long et al. 2004). Unlike elevated CO2, temperature has effects that extend beyond direct leaf-level responses. Temperature affects plant growth through a number of processes at varying scales, including photosynthesis, respiration, meristem initiation, cell division, water transport and phenology (Berry and Bjorkman 1980, Atkin and Tjoelker 2003, Thomas et al. 2007, Way 2011). Importantly, the response of biological activity to temperature is not linear, and it has long been known that photosynthetic thermal responses depend on the plant’s ecological adaptation (Pearcy and Harrison 1974). Consequently, the response of plant growth to increasing temperature alone (not to mention its interaction with elevated CO2) is complicated by ecological adaptation of the species or genotype examined. This, in turn, will determine the thermal optima of temperaturesensitive processes and their potential for acclimation. Nevertheless, a survey of the literature indicates that growth in most tree species responds positively to warmer growth temperature, with deciduous species showing a greater positive response than evergreen trees (Way and Oren 2010). However, this generalization does not always hold; for example, Ghannoum et al. (2010) found that evergreen eucalypts (Eucalyptus saligna and Eucalyptus sideroxylon) had a strong positive growth response to higher temperatures. In this issue, Wertin et al. (2011) demonstrate that the deciduous, temperate species Quercus rubra (northern red oak) showed a negative growth response to elevated temperature, which was strong enough to negate growth enhancements of high CO2. While elevated CO2 alone increased total biomass by 38% relative to the ambient CO2 and ambient temperature (control) treatment, plants grown at elevated CO2 and a 3°C warming had similar biomass to their control counterparts, and plants exposed to elevated CO2 and a 6°C warming had 12% less biomass than control plants (Wertin et al. 2011). Declines in growth were associated with reduced net photosynthetic rates and photosynthetic capacity at higher temperatures, as well as higher dark respiration rates (Wertin et al. 2011). Since respiration usually acclimates to temperature more strongly and quickly than photosynthesis (Gunderson et al. 2000, Campbell et al. 2007, Ow et al. 2008, Way and Sage 2008a, 2008b, Way and Oren 2010), this result is surprising. These recent warming studies highlight the gaps in our understanding and the need to synthesize alternative hypotheses that can form the basis of future experiments in this field. One potential explanation for the diversity in results from warming experiments is presented by Wertin et al. (2011): populations from the equatorial distribution limit of a species may be more prone to warming-related growth declines than populations from the poleward distribution limit. While poleward range limits have received more research attention, equatorial range limits are where negative impacts of climate Commentary

AbstractPhotosynthetic acclimation to both warming and elevated CO2 of boreal trees remains a key uncertainty in modelling the response of photosynthesis to future climates. We investigated the impact of increased growth temperature and elevated CO2 on photosynthetic capacity (Vcmax and Jmax) in mature trees of two North American boreal conifers, tamarack and black spruce. We show that Vcmax and Jmax at a standard temperature of 25°C did not change with warming, while Vcmax and Jmax at their thermal optima (Topt) and growth temperature (Tg) increased. Moreover, Vcmax and Jmax at either 25°C, Topt or Tg decreased with elevated CO2. The Jmax/Vcmax ratio decreased with warming when assessed at both Topt and Tg but did not significantly vary at 25°C. The Jmax/Vcmax increased with elevated CO2 at either reference temperature. We found no significant interaction between warming and elevated CO2 on all traits. If this lack of interaction between warming and elevated CO2 on the Vcmax, Jmax and Jmax/Vcmax ratio is a general trend, it would have significant implications for improving photosynthesis representation in vegetation models. However, future research is required to investigate the widespread nature of this response in a larger number of species and biomes.

The UK Government is hosting COP26 in Glasgow between 31st October and 12th November 2021. It plans to make progress in four key areas which summarize as ‘coal, cars, cash and trees’ (Carbon Brief, 2021). The first two of these aims—to get agreement for the rapid phase out of coal, the most polluting of fossil fuels, and to ensure a rapid transition away for cars fuelled by fossil fuels—are very important, but are not directly related to the remit of Global Change Biology. The latter two aims—ensuring that the financial support of $100 billion per year promised in 2010 by wealthy countries to developing countries finally gets delivered and ensuring that climate solutions adopted also co-deliver to nature—are squarely within the remit of Global Change Biology. With respect to the ‘cash’ aim, this flow of finance is essential to allow poorer countries to adapt to, and to mitigate, climate change. We know that a vast proportion of the potential for natural climate solutions is located in the developing world (Griscom et al., 2020), so if we are to realize that global potential, developing countries must have the financial backing to ensure that this happens in an equitable and just way. Not all of this cash will be used for nature-based solutions, of course, but a proportion of it will be, and nature-based solutions would almost certainly not happen at the scale and speed required to help us meet net zero greenhouse gas emissions targets without this cash. With respect to the ‘trees’ aim, the first thing to note is that nature-based solutions are about so much more than just planting trees (Seddon et al., 2021)! ‘Trees’ is just shorthand for nature-based solutions, but the broad variety of nature-based solutions available, beyond just tree planting, must be encouraged at COP26. The recent joint workshop report by IPBES and IPCC (Pörtner et al., 2021) demonstrated that we cannot successfully resolve either of the existential threats of climate change or biodiversity loss unless we tackle ...

The IPCC (Intergovernmental Panel on Climate Change) "Special Report on Global Warming of 1.5°C" presented the ambitious target of needing to achieve zero net emissions by 2050 in order to meet the goals of the Paris Agreement (IPCC, 2018). This report led some governments and jurisdictions to declare a climate emergency (Climate Emergency Declaration, 2019) and prompted the rise of movements of activism and civil disobedience such as the School Strike for the Climate and Extinction Rebellion. The reach of these civil actions extends beyond those directly involved, potentially increasing wider public awareness of climate change. Here, we examine trends in indicators of this wider public awareness and engagement and compare these with major global movements of civil disobedience focussed on climate, the release of substantive climate reports, and global governmental gatherings on climate change. We show that these global movements may be increasing public awareness of, and stimulating public engagement with, issues of climate change. .

Abstract Warming nights are correlated with declining wheat growth and yield. As a key determinant of plant biomass, respiration consumes O2 as it produces ATP and releases CO2 and is typically reduced under warming to maintain metabolic efficiency. We compared the response of respiratory O2 and CO2 flux to multiple night and day warming treatments in wheat leaves and roots, using one commercial (Mace) and one breeding cultivar grown in controlled environments. We also examined the effect of night warming and a day heatwave on the capacity of the ATP-uncoupled alternative oxidase (AOX) pathway. Under warm nights, plant biomass fell, respiratory CO2 release measured at a common temperature was unchanged (indicating higher rates of CO2 release at prevailing growth temperature), respiratory O2 consumption at a common temperature declined, and AOX pathway capacity increased. The uncoupling of CO2 and O2 exchange and enhanced AOX pathway capacity suggest a reduction in plant energy demand under warm nights (lower O2 consumption), alongside higher rates of CO2 release under prevailing growth temperature (due to a lack of down-regulation of respiratory CO2 release). Less efficient ATP synthesis, teamed with sustained CO2 flux, could thus be driving observed biomass declines under warm nights.

This article is a Commentary on Walker et al. (2021), 229: 2413–2445.