Semi-arid tree-grass systems are considered one of the major contributors to the interannual variability of the global carbon cycle (Poulter et al., 2014). The SMANIE project is a small-scale manipulation experiment (SMANIE) running parallel to the large scale manipulation experiment with focusing on the grass layer. The site is located in Mediterranean savannah in Spain (39°56'24.68"N, 5°45'50.27"W; Majada del Tietar, Caceres). The site is characterized by a mean annual temperature of 16°C, mean annual precipitation of ca. 572 mm (~700 mm the man average precipitation collected in the last 10 years), falling mostly from November until May, and by a very dry summer. The focus of SMANIE was the effects of N and P fertilization on ecosystem level C and water fluxes, plant traits, hyperspectral vegetation indices (VIs) and solar induced chlorophyll fluorescence (SIF) of the grassland layer at the MaNiP experimental site.
The plot was broken up into four randomized plots 20 m x 20 m (Figure 1), and within each block another four separate 9 m x 9 m plots with 2 m in between to avoid boundary effects. In each of the blocks, 4 different treatments were applied one of the following four treatments was realized per plot:
(a) Control treatment (C) with no fertilization.
(b) Nitrogen addition (N treatment) fertilized by applying 100 kg N ha-1 as potassium nitrate (KNO3) and ammonium nitrate (NH4NO3).
(c) Phosphorous addition (P treatment) fertilized with 50 kg P ha-1 as monopotassium phosphate (KH2PO4).
(d) NP addition (NP treatment) were fertilized with both 100 kg N ha-1 and 50 kg P ha-1 as ammonium nitrate (NH4NO3) and monopotassium phosphate (KH2PO4).
Fertilizer was dissolved in water and sprayed on the foliage of the plants (~ 2 L m-2) in the early growing season, particularly in the tillering phenological period that same amount of water was applied to the controls.
Within each plot, two permanent collars (32 in total, see black squares in Figure 1) were dedicated to monitor CO2 fluxes (i.e., net ecosystem CO2 exchange NEE, and daytime ecosystem respiration Reco). The gross primary productivity (GPP) was then computed. Fluxes were observed with a manual static chambers built in-house (Perez-Priego et al.).
Spectral measurements were conducted immediately before to flux measurements at 16 collars. Hyperspectral data were measured with a built-in-house manual high resolution spectrometric system, based on Rossini et al., (2011) , and capable to collect canopy spectral reflectances with the spectral resolution adequate to compute the majority of VIs foreseen in the novel satellite missions, as well as SIF. The equipment developed for the field activity is shown in Figure 2.
Figure 1: SMANIE experimental facility (Perez-Priego et al.)
Figure 2: SMANIE equipment
From the hyperspectral canopy signature the following vegetation indices (VIs) have been selected: the normalized difference vegetation indices (NDVI); the Meris Terrestrial Chlorophyl Index (MTCI) (Dash and Curran, 2004), and the Photochemical Reflectance Index (PRI) (Gamon et al., 1992). Moreover the solar-induced fluorescence (SIF) and the apparent fluorescence yield (Fy760) were also computed.
To quantify the impact of the fertilization we evaluated statistical differences between treatments in the GPP, in its response to light availability (through LRCF), VIs, and Fy760. The direct correlation between the GPP and the VIs was also tested.
Finally, we implemented new formulations light use efficiency (LUE) models based only on remote sensing data related to plant structure and physiology (e.g. combining NDVI and PRI and Fy760).
Using the soil-canopy observation of photosynthesis and energy (SCOPE) model, we investigated how nutrient-induced changes in canopy structure (i.e. changes in plant forms abundance that influence leaf inclination distribution function, LIDF) and functional traits (e.g. N content in dry mass of leaves, N%, Chlorophyll a+b concentration (Cab) and maximum carboxylation capacity (V cmax )) affected the observed linear relationship between SIF measured at 760 nm (F760) and GPP. We conclude that the addition of nutrients imposed a change in the abundance of different plant forms and biochemistry of the canopy that controls F 760 . Changes in canopy structure mainly control the GPP-F 760 relationship, with a secondary effect of Cab and V cmax . In order to exploit F 760 data to model GPP at the global/regional scale, canopy structural variability, biodiversity and functional traits are important factors that have to be considered. [Migliavacca et al., 2017 | http://onlinelibrary.wiley.com/doi/10.1111/nph.14437/abstract]
Perez-Priego, O., Guan, J., Rossini, M., Fava, F., Wutzler, T., Moreno, G., Carvalhais, N., Carrara, A., Kolle, O., Julitta, T., Schrumpf, M., Reichstein, M., Migliavacca, M. (2015). Sun-induced Chlorophyll fluorescence and PRI improve remote sensing GPP estimates under varying nutrient availability in a typical Mediterranean savanna ecosystem. Biogeosciences, 12(14), 11891-11934. doi:10.5194/bgd-12-11891-2015
Migliavacca, M. , Perez-Priego, O., Rossini, M., El-Madany T.S., Moreno, G., van der Tol, C., Rascher, U., Berninger, A., Bessenbacher, V., Burkart, A. .. , Reichstein, M., '(2017). Plant functional traits and canopy structure control the relationship between photosynthetic CO2 uptake and far-red sunâ€induced fluorescence in a Mediterranean grassland under different nutrient availability, New Phytologist, 214, 3, 1078-1091.