Compensatory growth of fast-growing willow (Salix) coppice in response to simulated large herbivore browsing

We tested the effect of timing and intensity of deer browsing on compensatory response by willow grown for bioenergy. Browsing was simulated several times during summer and once during winter. The clipping was done at different intensities during the first year after establishment in a new willow plantation, and during the first year after harvest in an older willow coppice. Total aboveground biomass and biomass available for deer browsing were recorded both at the end of the first and the second growing periods after clipping. Both in the new plantation and in the older coppice, the willows fully compensated for biomass losses after winter clipping, irrespective of clipping intensity. On the other hand, total biomass production usually decreased after high-intensity summer clipping. Such a seasonal difference may be explained by the nutrition of remaining buds after clipping. In the new willow plantation, total produced biomass after summer clipping still bottomed at about 50% of the level of control stools at the end of the experiment. In the older coppice, usual browsing levels by deer in summer, roughly corresponding to the moderate clipping intensities used in this experiment, did not influence total produced biomass. The newly established willow stools reacted to summer clipping by undercompensating in terms of biomass available for deer browsing, while older stools could overcompensate. Compared to stools clipped in late summer, willow stools clipped during early summer were able to compensate earlier and stronger. The higher the proportion of twig biomass removed by summer clipping, the lower the compensatory growth by willow. Depending on the circumstances, the fast-growing willows responded within the whole continuum from under- to overcompensation, and some implications for the management of willow plantations and large herbivores are discussed.