Higher sensitivity of gross primary productivity than ecosystem respiration to experimental drought and warming across six European shrubland ecosystems.

Shrubland ecosystems across Europe face a range of threats including the potential impacts of climate change. Within the INCREASE project, six shrubland ecosystems along a European climatic gradient were exposed to ecosystem-level year-round experimental nighttime warming and long-term, repeated growing season droughts. We quantified the ecosystem level CO2 fluxes, i.e. gross primary productivity (GPP), ecosystem respiration (Reco) and net ecosystem exchange (NEE), in control and treatment plots and compared the treatment effects along the Gaussen aridity index. In general, GPP exhibited higher sensitivity to drought and warming than Reco and was found to be the dominant contributor to changes in overall NEE. Across the climate gradient, northern sites were more likely to have neutral to positive responses of NEE, i.e. increased CO2 uptake, to drought and warming partly due to seasonal rewetting. While an earlier investigation across the same sites showed a good cross-site relationship between soil respiration responses to climate over the Gaussen aridity index, the responses of GPP, Reco and NEE showed a more complex response pattern suggesting that site-specific ecosystem traits, such as different growing season periods and plant species composition, affected the overall response pattern of the ecosystem-level CO2 fluxes. We found that the observed response patterns of GPP and Reco rates at the six sites could be explained well by the hypothesized position of each site on site-specific soil moisture response curves of GPP/Reco fluxes. Such relatively simple, site-specific analyses could help improve our ability to explain observed CO2 flux patterns in larger meta-analyses as well as in larger-scale model upscaling exercises and thereby help improve our ability to project changes in ecosystem CO2 fluxes in response to future climate change.

Long-term temporal patterns in ecosystem carbon flux components and overall balance in a heathland ecosystem.

Terrestrial ecosystems have strong feedback to atmospheric CO2 concentration and climate change. However, the long-term whole life cycle dynamics of ecosystem carbon (C) fluxes and overall balance in some ecosystem types, such as heathland ecosystems, have not been thoroughly explored. We studied the changes in ecosystem CO2 flux components and overall C balance over a full ecosystem lifecycle in stands of Calluna vulgaris (L.) Hull by using a chronosequence of 0, 12, 19 and 28 years after vegetation cutting. Overall, the ecosystem C balance was highly nonlinear over time and exhibited a sinusoidal-like curvature of C sink/source change over the three-decade timescale. After cutting, plant-related C flux components of gross photosynthesis (PG), aboveground autotrophic respiration (Raa) and belowground autotrophic respiration (Rba) were higher at the young age (12 years) than at middle (19 years) and old (28 years) ages. The young ecosystem was a C sink (12 years: -0.374 kg C m-2 year-1) while it became a C source with aging (19 years: 0.218 kg C m-2 year-1) and when dying (28 years: 0.089 kg C m-2 year-1). The post-cutting C compensation point was observed after four years, while the cumulative C loss in the period after cutting had been compensated by an equal amount of C uptake after seven years. Annual ecosystem C payback from the ecosystem to the atmosphere started after 16 years. This information may be used directly for optimizing vegetation management practices for maximal ecosystem C uptake capacity. Our study highlights that whole life cycle observational data of changes in C fluxes and balance in ecosystems are important and the ecosystem model needs to take the successional stage and vegetation age into account when projecting component C fluxes, ecosystem C balance, and overall feedback to climate change.