CO2 enrichment at treeline: help or hindrance for trees on the edge?
One of the most perplexing questions relating to climatechange effects is what underlies the negative feedbacks that gradually inhibit the initial CO2 response (Hyvönen et al. 2007; Norby et al. 2010). Hundreds of studies have shown that when CO2 concentrations are first increased above ambient, plants increase photosynthetic rates and growth. However, over longer periods at high CO2, the photosynthetic and growth responses are often, but not always, damped out (Fig. 1). How this happens and under what conditions has been the focus of considerable thought and effort (Hyvönen et al. 2007). In part, the issues related to CO2 acclimation are difficult to study because the feedbacks are slow. This makes research on the topic of acclimation to high CO2 expensive and underlines the value of the few long-term studies available. This month’s issue of Plant, Cell & Environment includes a paper by Streit et al. (2013) that describes 9 years of CO2 enhancement superimposed on a 4 year soil-warming experiment. The work was conducted at the upper treeline in the Swiss Alps, where feedbacks were expected to be exceptionally strong. But instead, and even after 9 years, the feedbacks were surprisingly small. Instead, the CO2-induced increase in photosynthesis has been maintained and, in one of the two species, the initial increase in growth rate has also been sustained (Dawes et al. 2013). This paper follows closely on two recent publications for the same site (Dawes et al. 2011, 2013), but the authors provide several new elements. Firstly, they show information on the CO2 response of photosynthesis, which provides a more direct measure of the lack of feedback effects than in situ gas exchange. Secondly, Streit et al. (2013) present stable isotope data, which support inferred gas exchange traits. For the purposes of a long-term study such as this one, the integrated inferences of leaf physiology derived from isotopes better match the treatment duration and vary less than measurements of instantaneous (ca. 60 s) gas exchange rates. The third new element is that the authors describe the results of a soil-warming treatment, which was expected to weaken the feedbacks on leaf gas exchange and maintain the initial stimulation of photosynthesis and growth by the high CO2 treatment. Leaf nitrogen status is perhaps the key feedback limiting the response to CO2 enrichment in high latitude and altitude conifer forests. Leaf nitrogen concentration is often low in the coldest forests, both boreal (Hyvönen et al. 2007) and near the treeline (Loomis et al. 2006; Birmann and Körner, 2009). Trees are able to access only the nitrogen that has been mineralized into soluble forms by soil microbes, but the microbes are intolerant of cold temperatures. In fact, soil microbial activity is often more sensitive than the physiological responses of the trees, as demonstrated by the pronounced growth response that occurs when inorganic fertilizers are added to boreal forests (Susiluoto et al. 2010; Sigurdsson et al. 2013; McNown & Sullivan 2013). Under CO2 enrichment, photosynthetic rates initially increase and, over time, leaf nitrogen concentrations begin to decline (Marshall & Linder 2013). As nitrogen status declines, growth rates also decline and the CO2 response is muted (Norby et al. 2010). This is the most common kind of feedback in CO2 effects research, having been found in all of 39 studies reviewed (Luo et al. 1994). At treeline, there is considerable evidence that low temperatures inhibit not only microbial nitrogen mineralization, but also plant growth processes. In fact, temperature limitation of growth is currently the most widely accepted explanation for treeline formation worldwide (Körner 2012). The notion is that growth processes are much more sensitive to low temperature than is photosynthesis. Because trees stand above the soil surface, their leaves and buds emerge from the layer of warm air that accumulates during the day near the soil surface. This exposure means that tree growth is preferentially inhibited by low temperatures in these sites. Upright trees therefore disappear and ground-hugging herbs continue to be found further upslope. The expected strength of the nitrogen and growth limitations at treeline make this a particularly interesting place to study CO2 enhancement. At the simplest level, a sufficiency of CO2 would seem to be the least of the challenges facing such trees. As a consequence of the other limitations, the feedbacks on any short-term CO2 effects would be expected to be rapid and strong. But the current study was full of surprises. Firstly, over 9 years of CO2 enrichment, leaf-level measurements of photosynthetic gas exchange showed clear evidence of CO2 enhancement, but the enhancement was as strong at the end of the experiment as it was at the beginning – that is, no feedback. Gas exchange results have been reported before (Dawes et al. 2013), but not in terms of photosynthetic capacity. The authors speculate that the excess photosynthate may be exuded to the soil microflora (Phillips et al. 2011), supporting the high microbial respiration rates seen in the CO2 enrichment treatment, especially when combined with soil warming (Hagedorn et al. 2010). The second surprise is that, in spite of the low temperatures, Larix (larch) trees grew faster under CO2 enrichment than in the control plots (Dawes et al. 2013), and did so until the end of the experiment – again, no feedback.The Pinus (pine) trees behaved as Correspondence: John D. Marshall. E-mail: jdm@uidaho.edu Plant, Cell and Environment (2014) 37, 312–314 doi: 10.1111/pce.12235 bs_bs_banner