Separating the effects of temperature and carbon allocation on the diel pattern of soil respiration in the different phenological stages in dry grasslands

Detecting drought stress occurrence using synergies between Sun induced fluorescence and vegetation surface temperature spatial records

Drought stress occurrence and recovery from drought can be detected using a single spatial set of simultaneous observations of SIF and canopy temperature records. Temporal and spatial responses to drought and heat stresses by plant stands of a drought-adapted diverse grassland ecosystem were studied using sun induced fluorescence (SIF,O2A and O2B bands) and further ecophysiological (canopy temperature (Tsurf), spatially modeled evapotranspiration, vegetation reflectance spectra) variables collected along spatial sampling grids while also utilizing eddy covariance measured carbon dioxide (net ecosystem exchange: NEE, gross primary production: GPP) and water flux (evapotranspiration: ET) data. The grids were of 0.5 and 5 ha spatial extents and contained 78 sampling points. Data were collected in four spatial sampling campaigns, two under drought (early summer) and another two during and after recovery (midsummer) at both spatial resolutions. Small values of spatial SIF_A averages (around 0.5 mW m-2 nm-1 sr-1) under strong early summer drought increased (to around 2 mW m-2 nm-1 sr-1) due recovery upon rain arrivals, showing high (R2: 0.8-0.88) positive temporal correlations to eddy covariance measured carbon (GPP, NEE) and water (ET) fluxes. Spatial averages of LAI, vegetation indices (NDVI, NIRv) and modeled ET followed similar temporal patterns. While SIF was depressed by drought, it showed higher values in high canopy temperature vegetation patches than in vegetation patches with lower Tsurf. The spatial pattern of higher SIF in higher Tsurf patches was persistent (2 weeks) under drought. The positive SIF_A-Tsurf spatial correlation turned into negative/not significant after recovery of the grassland from the drought, while hot summer weather persisted. It is proposed that, by using a single set of simultaneously measured spatial SIF and Tsurf data it is possible to infer whether the studied vegetation is under drought (and heat) stress while it could not be decided on the base of SIF data alone. Evaluation of the slope of the above relationship seems therefore beneficial before e.g. starting the (stress) classification procedure based on SIF.

Enhanced spring temperature sensitivity of carbon emission links to earlier phenology

Phenology has a great effect on the carbon cycle. Significant relationships have been well demonstrated between phenology and photosynthesis. However, few studies have been undertaken to characterize relationships between phenology and ecosystem respiration (Re). We conducted a reciprocal transplant experiment among three elevations for two-years to measure Re over six phenological sequences throughout the growing seasons. Our results showed that changes in phenological duration were mainly determined by the onset of phenology, as one day advance of phenological onset could lengthen 0.13 days of phenological duration. Advances in early spring phenophases (i.e., first leaf-out, first bud/boot-set and first flowering) under warming strengthened the temperature sensitivity of Re. However, the late phenophases (i.e., first seeding-set, first post-fruiting vegetation and first leaf-coloring) had non-significant relationships with Re. In total, after pooling all the data, one day advance of phenophases would increase Re by 2.23% under warming. In particular, Re would increase by 29.12% with an advance of phenophases by 8.46 days of under a 1.5 °C warming scenario. Our analysis of the coupling between temperature/moisture-phenology-Re may further supplement evidence that warmer spring temperature increases carbon emission by advancing early phenophases. This points to a faster and easier way to investigate how aboveground functional traits (phenology) affect unseen functional traits (Re) on the Tibetan Plateau.