Transpiration of grapevines and co-habitating cover crop and weed species in a vineyard. A “snapshot” at diurnal trends

The objective of this study was to quantify transpiration rates of two cover crops, Festuca rubra subsp. rubra (red fescue) and Medicago lupulina (black medick) and 4 weeds, Chenopodium album (fat hen), Cirsium arvense (creeping thistle), Malva neglecta (common mallow) and Taraxacum officinale (dandelion) occurring in a mixed stand in a commercial steep-slope, North-South oriented vineyard as compared to vines, cv. Riesling near Johannisberg (Rheingau), Germany. Leaf transpiration (E) was measured directly on the cover crop and weed species with a portable gas exchange measurement system. Grapevine transpiration was measured concomitantly using custommade Granier-type xylem sap flow gauges. Measurements were conducted on two days in August (15th and 22nd) in 2001 under hot and sunny conditions. All herbaceous species presented a similar diurnal pattern of E, with low values in the morning and afternoon and peak values between 12 and 15 h. In contrast E of grapevines peaked mid-morning (between 8 and 10 h) remained relatively stable until mid-afternoon (16 h) before decreasing continuously until darkness. Significant differences in E between the herbaceous species were observed throughout the day. In general transpiration rates were highest for M. neglecta and lowest for C. arvense, T. officinale and F. rubra subsp. rubra. We estimated the projected leaf area indices (leaf area per surface area covered) for each species and calculated possible transpiration rates for pure stands assuming that all leaves were well exposed. Potential transpiration rates ranged from about 1 mm d-1 (one l m-2 of soil surface) for F. rubra subsp. rubra to = 5 mm d-1 for M. neglecta as compared to only 0.9 mm d-1 for grapevine. These results underline the importance of appropriate cover crop species and the control of some weed species with respect to water use.