Effects of late mowing on plant species richness and seed rain in road verges and adjacent arable fields.

Delaying the first date of mowing of road verges is frequently practised as a way of preserving or enhancing biodiversity. Although the effects of this management practice on plant species richness remains controversial, it has been demonstrated that it allows more species to set seed. While an integrated management of the weed flora is currently promoted in arable fields adjacent to road verges, delaying the mowing of these field boundaries can be questioned, especially because seed dispersal from road verges to arable fields through field margins can be increased. In this context, the study aimed to assess the potential influence of delaying the first date of mowing of berms on (1) the species richness of berm standing vegetation (BSV), (2) the number of seeds and (3) the composition of seed rains in road verges and adjacent arable fields. To this end, we surveyed the standing vegetation and seed rain in four areas, i.e. in the berm, embankment, crop edge and field margin from a total of 252 sampling points. First, we analysed the effect of the mowing period (i.e. early or late summer) on species richness of both BSV and berm seed rain. Then, at each sample point in the four areas, we studied the variation in (i) seed abundance (i.e. number of seeds), (ii) contribution of BSV to each seed rain (i.e. Sorensen⿿s similarity and Bray-Curtis⿿ dissimilarity indices between the BSV and each sample point) and (iii) seed abundance of the most frequent species recorded in BSV. Our findings showed that delaying the first mowing to late summer for four years had no impact on the species richness of BSV, while it increased species number and abundance of seeds in the berm seed rain. We also demonstrated that BSV of the late mown berms contributed more to the berm seed rain, than that of early mown berms. Moreover, our results suggest that compared to early mowing, only Dactylis glomerata dispersed more seeds from the late mown berms to field margins. Regarding the most frequent weed species observed in the BSV, for example Elytrigia repens , Arrhenatherum elatius and Convolvulus arvensis , the number of seeds trapped in field margins was low except for A. elatius . Finally, under our conditions, late mowing remained inefficient in promoting plant species richness on berms, but did not represent a particular factor of weed risk in field margins. [ABSTRACT FROM AUTHOR]