Responses of the reflectance indices PRI and NDVI to experimental warming and drought in European shrublands along a north-south climatic gradient

The aim of this study was to evaluate the use of ground-based canopy reflectance measurements to detect 40 changes in physiology and structure of vegetation in response to experimental warming and drought 41 treatment at six European shrublands located along a North–South climatic gradient. We measured canopy 42 reflectance, effective green leaf area index (green LAIe) and chlorophyll fluorescence of dominant species. The 43 treatment effects on green LAIe varied among sites. We calculated three reflectance indices: photochemical 44 reflectance index PRI [531 nm; 570 nm], normalized difference vegetation index NDVI680 [780 nm; 680 nm] 45 using red spectral region, and NDVI570 [780 nm; 570 nm] using the same green spectral region as PRI. All three 46 reflectance indices were significantly related to green LAIe and were able to detect changes in shrubland 47 vegetation among treatments. In general warming treatment increased PRI and drought treatment reduced 48 NDVI values. The significant treatment effect on photochemical efficiency of plants detected with PRI could not 49 be detected by fluorescence measurements. However, we found canopy level measured PRI to be very sensitive 50 to soil reflectance properties especially in vegetation areas with low green LAIe. As both soil reflectance and LAI 51 varied between northern and southern sites it is problematic to draw universal conclusions of climate-derived 52 changes in all vegetation types based merely on PRI measurements. We propose that canopy level PRI 53 measurements can be more useful in areas of dense vegetation and dark soils.