By Sebastian Pfautsch
ASPS Representative ‘Environment and Ecophysiology: Global Change’
Hawkesbury Institute for the Environment, Western Sydney University
Every year plants respire about 64 Gt CO2 back into the atmosphere. That is six times as much as released from burning fossil fuels. Obviously, small proportional changes in the respiratory flux can result in dramatic changes in atmospheric CO2 concentrations and associated warming effects.
Plant respiration (R) is positively correlated to temperature (T). Plant scientists have believed that R will accelerate under global warming, generating a feed-forward loop where plant-respired CO2 leads to even faster rates of warming, that lead to even greater fluxes of R and so on. Clearly, this scenario, where plants are responsible for faster rates of global warming is a scary one. Should plants be put into the limelight of being irresponsible climate sinners? Have plants worked out an evil plan to overthrow Homo sapiens? Are we all working for the enemy?
Rest assured, the short answer is no. The slightly longer answer, according to plant ecologist Peter Reich is “that we have no silver bullet to stop global warming, but recent research on the temperature-R relationship provides a silver lining in an otherwise dark sky”.
Recently Peter published new research findings on plant R in Nature (http://www.nature.com/nature/journal/vaop/ncurrent/full/nature17142.html ). He and his team grew 10 North American tree species for five years in the field and measured an insane amount of R-to-T response curves of leaves. Trees were grown under ambient and warmed (+3.4 °C) conditions with and without canopy cover of dominant trees. When leaves of ambient trees were exposed to +3.4 °C warming for a short time, their rate of R increased by 23% compared to unwarmed leaves.
However, when R was measured in leaves that were constantly warmed, they respired 80% less than those plants that experienced only short-term warming. This indication of long-term acclimation of R to higher T has large implications for global simulation models that – until now – have falsely assumed that R increases exponentially when T is rising. Consequently, these models have grossly overestimated atmospheric concentrations of CO2 and associated rates of global warming.
Independent of the research in North America, two other studies have emerged that point in the same direction. A global team, spearheaded by colleagues from ANU (Heskel et al., 2016, published in PNAS) found that R did not increase exponentially as T rose along an environmental gradient reaching from the artic to equatorial biomes and covering 231 plant species. Most importantly, the shape of the response curve of R-to-T was similar for all species, pointing to a universally applicable sensitivity of leaf metabolism to T. Earth system modellers must have a field day as these results make life so much easier…
The second study I’d like to point out is the work by John Drake and colleagues, accepted for publication in New Phytologist. This work brings R-to-T responses home to Eucalyptus trees. 12 Forest Redgum trees were grown in 10 m tall whole-tree chambers in Richmond, of which half tracked ambient temperatures, the other half ambient +3 °C warming. Instrumentation of the chambers allowed John to track gas exchange of the trees on a day-to-day basis at high temporal resolution.
Results from these Redgums agreed with Peter’s work on North American trees. R did not increase with warming. Furthermore, the Redgums responded to warming by decreased photosynthetic carbon assimilation. This means that – although indirectly – warming may increase atmospheric CO2 concentrations, but not due to increased R rather than decreased assimilation of C. In the experiment this led ultimately to a small increase in the ratio of R-to-photosynthesis, but only during heatwaves.
John says “these gum trees happily adjusted their physiology to cope with warming during most conditions, but heat waves were different. The gums shut their stomata and waited out the heat, but in the process they lost a lot of C to respiration without gaining any via photosynthesis”. In a global warming perspective this means that higher atmospheric CO2 concentrations could be the result of decreased uptake of CO2 by trees during the more frequent and intense heat waves predicted for our future in Australia.
The capacity of plants to acclimate to higher temperatures without exponentially increasing R is great news for the modelling of the earth system. As Peter puts it, acclimation of R to temperature only represents “a delay in the race to the climate change cliff”. However, we must consider that ecosystem R is much more than leaf R. At least John’s paper reports R of the woody proportion of trees (R is slightly increasing with warming). Leaf R represents roughly 50% plant R, with fine roots being responsible for the majority of the other half. We know little about acclimation of R in fine roots.
We know even less of acclimation of R in soil microbial biomass. And on a ecosystem scale consider that: while under hot and dry conditions plants would close stomata, leading to reduced uptake of CO2 while R remains relatively constant, soil microbial activity would slow down, autotrophic R would decrease and soils would retain more C. How does that affect atmospheric concentrations of CO2?
It gets complicated really quickly if we leave experimental units and enter the real world where plants have to deal with competition for resources (e.g. light, water, nutrients), grow fast or slow, tolerate shade more or less, age, etc. And on top of all that, even one of the world’s most distinguished plant ecologist admits that we don’t even know how acclimation exactly modifies enzymatic and biochemical processes. Nevertheless, pushing the boundaries of our understanding of R-to-T responses remains important work.
The seminal work of Owen Atkin and Mark Tjoelker 13 years ago (published in TiPS) defined the current framework of leaf acclimation. The three most recent studies about R-to-T responses and their implications to the earth system are tribute to an ever-progressing understanding of the effects of global warming on the natural world.
And besides all the great results, the work by Reich, Heskel and Drake and their colleagues is proof of the importance of well-funded, long-term and field-based research programs – a type of plant science that is pushed towards extinction. So, are plants winning after all? Take a deep breath and … respire.