Central European hardwood trees in a high‐CO₂ future: synthesis of an 8‐year forest canopy CO₂ enrichment project

Abstract

Summary1. Rapidly increasing atmospheric CO 2 is not only changing the climate system but may also affect the biosphere directly through stimulation of plant growth and ecosystem carbon and nutrient cycling. Although forest ecosystems play a critical role in the global carbon cycle, experimental information on forest responses to rising CO 2 is scarce, due to the sheer size of trees. 2. Here, we present a synthesis of the only study world-wide where a diverse set of mature broadleaved trees growing in a natural forest has been exposed to future atmospheric CO 2 levels (c. 550 ppm) by free-air CO 2 enrichment (FACE). We show that litter production, leaf traits and radial growth across the studied hardwood species remained unaffected by elevated CO 2 over 8 years. 3. CO 2 enrichment reduced tree water consumption resulting in detectable soil moisture savings. Soil air CO 2 and dissolved inorganic carbon both increased suggesting enhanced below-ground activity. Carbon release to the rhizosphere and/or higher soil moisture primed nitrification and nitrate leaching under elevated CO 2 ; however, the export of dissolved organic carbon remained unaltered. 4. Synthesis. Our findings provide no evidence for carbon-limitation in five central European hardwood trees at current ambient CO 2 concentrations. The results of this long-term study challenge the idea of a universal CO 2 fertilization effect on forests, as commonly assumed in climate-carbon cycle models.

Carbon release from rice roots under paddy rice and maize–paddy rice cropping

“…The absence of labeled DOC in 60 cm depths at the lysimeter bottom corresponds to findings of Lu et al (2000), who showed that organic compounds that are released from rice roots hardly leave the rhizosphere, likely because they are mineralized too quickly. This view is also supported by results from FACE experiments, which revealed only a small fraction of less than 20% of labeled DOC after up to 9 years of fumigation with isotopically distinct CO 2 (Bader et al, 2013;Dawes et al, 2013;Hagedorn et al, 2008;Siemens et al, 2012).…”

Water availability drives gas exchange and growth of trees in northeastern US, not elevated CO2 and reduced acid deposition

“…These findings match experiments in mature forests where elevated atm. CO 2 concentration and increases in i WUE do not necessarily translate into enhanced radial tree growth27. In those settings, heat and drought stress162829 or limited nutrient availability30 override CO 2 effect.…”

Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO2

“…S4). Whether or not eCO 2 stimulates LAI is a matter of debate, because responses differ among experiments (53,54), and there is contention as to whether nutrient limitation or direct environmental controls limit growth regardless of carbon assimilation rates (12,44,(55)(56)(57)(58). Hence, we examined the impact of eCO 2 on NPP while holding LAI constant.…”