Lai 2000 plant canopy analyzer

LAI was measured with a LAI-2000 plant canopy analyzer (LI-COR Inc., Lincoln, Nebraska, USA). In each plot, five representative positions were selected for LAI measurement, and two repeated measurements were performed at each position. Chl_leaf was obtained with a portable chlorophyll meter (SPAD-502, New Jersey, USA). The Chl_leaf measurement procedures and Chl_canopy calculation were described in detail by Huang et al. [18 ]. AGB was measured by randomly harvesting the aboveground fresh winter wheat plants in three subplots (0.2 m×0.2 m) and three maize plants in each plot. The fresh samples were oven dried at 65°C until the mass of the sample became constant. AGB measurement damaged the samples. Thus, we conducted this measurement when all the other measurements were finished. To reduce spatial sampling and measurement errors, we averaged the LAI, Chl_leaf, and AGB derived from each plot for both winter wheat and summer maize for further analysis.

To validate the model simulation in our study area, field experiments were performed during the 2017 winter wheat growing season in each county of Hengshui city. In the field experiments, we obtained the important biological parameters of the winter wheat, including the LAI, SM, chlorophyll content of wheat leaves, and above ground dry biomass. To validate the remotely sensed data and wheat yield assimilation results at the regional scale, 22 sample plots of homogenous wheat areas were selected from March to June 2017. Each county has two sample plots (Figure 1), and their positions in the field were located using a Trimble GeoXT3000 GPS from Trimble Navigation, Ltd. The wheat LAI of different phenological stages (green-up, jointing, heading, and milking) were measured by the LAI-2000 plant canopy analyzer from LI-COR Inc., and the surface SM was measured at 10 cm depth by FieldScout TDR300 from Spectrum Technologies Inc. The winter wheat yields were measured at 22 field plots during the wheat maturity stage. The winter wheat field measurements in 2014 from Li et al. [21 (link)] were also collected to calibrate the WOFOST model.

Corn and soybean intercrops were harvested concurrently. Corn was harvested when the kernel milk line was between 50 and 75%, and soybean was harvested at the seed-filling stage. The fresh weights of the harvested were obtained to determine fresh forage yield. The sampled area was 5 m² in the center of each plot for the monocropped corn and intercropping treatments, and the fresh biomass weight was determined in grams per DM per square meter; aboveground plant parts were harvested by cutting the plants 2 cm above the soil surface by hand. Samples were oven dried at 70 °C for at least 72 h. Forage DM yields were calculated from the fresh and dry weights of the respective components listed above. Prior to harvest, measurements of parameters such as the leaf area index (LAI), photosynthesis and leaf chlorophyll content were taken; the equipment used included a LICOR LAI-2000 Plant Canopy Analyzer, LICOR LI-6400 Portable Photosynthesis System (Lincoln Nebraska USA), and a Minolta SPAD-502 chlorophyll meter, respectively.

Stand attributes at the micrometeorological recorded sites were measured in transects at least 6 m long × 10 m wide parallel to isohypse and calibrated to account for the slope on both treelines (Table 1). Diameter at breast height, stem form index (i.e., stem length:stem height ratio in the first 2 m from the ground), total and live crown base heights were measured in all trees of the transect. Plant area index (PAI) was measured in summer during the fully leafed period by using the LAI 2000 Plant Canopy Analyzer (Li-Cor, Lincoln, NE, USA).

Vegetation parameters of experimental plots were assessed from April to September covering the growing season of the grassland ecosystem. Data on vegetation parameters, that is, species richness, species abundance, and LAI, were collected once per month, in May and July twice, which means in total 8 measurement campaigns. For vegetation measurements, a random subplot (2 × 1.5 m) was designated of each of the 16 plots (n = 4 per treatment). This subplot was used for measurements for the whole growing season. The species present on this subplot were assessed and their absolute canopy cover of the subplot estimated. We pooled all sweet-grass species as Poaceae. Species richness was defined as species present per subplot. Relative species abundance/cover was estimated as the percentage of species on total canopy cover on each subplot. For further analysis, species were pooled into the functional groups grasses and forbs and the absolute/relative cover of functional groups derived. The LAI was measured with the LAI-2000 Plant Canopy Analyzer (Li-Cor Biosciences, Lincoln, NE, USA) under constant atmospheric conditions. One atmospheric capture and three below canopy captures were taken three times on each subplot. If the standard error was >0.7, the measurement was repeated. The LAI was calibrated against the leaf area of biomass harvests.

We established 1 m² plots in the study area, each containing an individual tree in the center, in which light and LAI were measured. All measurements were done under a uniform overcast sky. Photosynthetic Photon Flux Density (PPFD, 400–700 nm) was measured in the center of each quadrant of each plot, summing up to four light profiles per study tree. These were averaged per plot. Light was measured at ground level, 0.25, 0.5, 0.75 and 1 m using spherical light quantum sensors and meters (LI-250, LiCor). Field testing revealed that light levels higher up in the canopy could be accurately calculated from these values. Simultaneously light measurements were done above the vegetation canopy. The Leaf Area Index (LAI) was measured four times in each plot at ground level from every corner of the (sub)plot facing the center (LAI-2000 Plant Canopy Analyzer, LiCor, NE, USA). Vertical leaf area distribution was determined by counting the number and recording the height of leaves touched by a telescopic rod when moved up through the vegetation. This was done in the center of each quadrant of a plot.

Leaf area index (LAI) was measured using the LAI-2000 Plant Canopy Analyzer (LI-COR Biosciences, Lincoln, NE, USA) as described by Villalobos et al. [51 (link)] on the same days as water potential and stomatal conductance measurements [52 (link)]. LAI_max was measured at 50–80 cm from the trunk and LAI_min at the center of the canopy since all plants had an open-shape canopy through pruning. LAI_avg was calculated by integrating plant ground cover (GC) using the following equation [53 (link)]: $${\mathrm{LAI}}_{\mathrm{avg}}={\mathrm{LAI}}_{\max }\ast \mathrm{GC}+{\mathrm{LAI}}_{\min }\ast \left(1-\mathrm{GC}\right)$$