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Forage monitoring in grassland is an important task to support management decisions. Spatial data on (i) yield,(ii) quality, and (iii) floristic composition are of interest. The spatio-temporal variability in grasslands is significant and requires fast and low-cost methods for data delivery. Therefore, the overarching aim of this contribution is the investigation of low-cost and non-calibrated UAV-derived RGB imagery for forage monitoring. Study area is the Rengen Grassland Experiment (RGE) in Germany which is a long-term field experiment since 1941. Due to the experiment layout, destructive biomass sampling during the growing period was not possible. Hence, non-destructive Rising Plate Meter (RPM) measurements, which are a common method to estimate biomass in grasslands, were carried out. UAV campaigns with a Canon Powershot 110 mounted on a DJI Phantom 2 were conducted in the first growing season in 2014. From the RGB imagery, the RGB vegetation index (RGBVI) and the Grassland Index (GrassI) introduced by Bendig et al. (2015) and Bareth et al. (2015), respectively, were computed. The RGBVI and the GrassI perform very well against the RPM measurements resulting in R2 of 0.84 and 0.9, respectively. These results indicate the potential of low-cost UAV methods for grassland monitoring and correspond well to the studies of Viljanen et al. (2018) and Näsi et al. (2018).

Non-destructive methods to derive spatial information on the development of forage mass are of key importance in managed grasslands. Established methods are rising plate meter (RPM) and rapid pasture meter, which both require in-field work, are rather time consuming, and do not provide spatially continuous data. Therefore, the overall objective of this study is to investigate low-cost unmanned aerial vehicle (UAV)-based RGB image acquisition for grassland monitoring. The idea of this paper is to transfer the successfully introduced approach of crop surface models (CSMs) for ultrahigh resolution analysis of plant height to managed grasslands. The study area is the Rengen Long-term Grassland Experiment, Germany, which is a two-cut experiment and was established in 1941. We conducted RPM and UAV-based data acquisition over six growth periods in 2014, 2015, and 2016. In 3 years, 26 RPM and 46 UAV campaigns were conducted under varying weather conditions (cloudy/sunny). The UAV-based RGB imagery was photogrammetrically processed with Structure from Motion and Multi-view Stereopsis techniques, producing multi-temporal CSMs for grassland sward height analysis. The regression analysis of UAV-derived sward height (CSM-SH) against RPM-measured sward height (RPM-SH) resulted in R2 of 0.91, 0.87, and 0.83 for 2014, 2015, and 2016, respectively. The pooling of the data for all 3 years resulted in an R2 of 0.86. These findings prove the successful transfer of the CSM approach for grassland monitoring and the potential of UAV-based monitoring to replace manual or in-field measurements with RPM or rapid pasture meter.

The aim of this study was to determine the effects of different long-term management options on re-establishment of traditional species-rich grassland in a Lolio-Cynosuretum grassland. The experiment was set up in 1987 in a low-fertilized mown pasture in the Eifel Mountains (Germany). Two and four cuts per year each with three levels of nitrogen fertilizer application were compared to no application of P and K fertilizer. Biomass production increased as a result of N application, whereas crude protein content and digestibility of organic matter improved in the four-cut treatments. During a 20-year period, we found a significant shift in dominant grass species without any significant effect on species richness. Lolium perenne and Elytrigia repens were replaced by Holcus lanatus and Alopecurus pratensis, especially in the two-cut treatments with N application. In the unfertilized plots, several individuals of Platanthera bifolia appeared when swards were cut only twice per year, probably as a result of nutrient depletion together with low cutting frequency. An increase in number of cuts mainly supported plant species well adapted to intensive management, even under low N application and relatively low plant-available P content in the soil. Therefore, we conclude that cutting frequency was a key driver on plant species composition in this experiment. The re-establishment of traditional species-rich grassland from intensively managed Lolio-Cynosuretum was not achieved within 20 years of cutting management combined with absence of P and K application.

Rhinanthus minor (yellow-rattle) can be used for restoration of species-rich grasslands but is vulnerable to competitive exclusion from high total aboveground biomass production of vascular plants. We asked (1) whether there is a threshold limit for total annual aboveground biomass production of vascular plants above which R. minor cannot establish viable population in grasslands and (2) how is cover of R. minor in grassland related to standing biomass of bryophytes. Data were collected in the Rengen Grassland Experiment (RGE) established in Germany in 1941 with following fertilizer treatments: unfertilized control, application of Ca, CaN, CaNP, CaNPKCl and CaNPK 2 SO 4 . Cover of R. minor and total annual aboveground biomass production of vascular plants were determined from 2005 to 2009. Further relationship between standing biomass of bryophytes and cover of R. minor was analyzed in 2006. Mean cover of R. minor over five years ranged from 0.7% to 12.3% in CaNPK 2 SO 4 and control treatments, respectively. Cover of R. minor was significantly negatively related to total annual aboveground biomass production of vascular plants and cover of R. minor was below 3% in all plots with total annual aboveground dry matter biomass production of vascular plants higher than 5 t/ha. Although cover of R. minor was markedly reduced in highly productive plots in the RGE, high standing biomass of bryophytes (1.8 t/ha) in low productive control was not an obstacle for establishment of its viable population. We concluded, that viable population of R. minor can be established in grasslands only if total annual aboveground dry matter biomass production of vascular plants is below 5 t/ha regardless on standing biomass of bryophytes.