The impact of ungulate browsing on black locust competitive abilities in a temperate oak forest

Invasive species such as black locust (Robinia pseudoacacia) cause several problems in conserving semi-natural forests in Europe. The selective browsing of ungulates can be a prominent regulating factor in the spread of black locusts. We set out a 4 year-long experiment in a fenced area of a Turkey oak (Quercus cerris) – sessile oak (Quercus petraea) forest in Hungary to 1) evaluate short-term responses of black locust saplings to simulated browsing; 2) identify the effects of browsing and soil conditions on the competition between black locust and oak saplings. We hypothesized that clipping treatments would reduce the growth and vitality of black locusts and indirectly enhance oak sapling development. Vegetation patches were designated to separate oak saplings with treated and intact black locust saplings, and without black locust. After the first round of treatments in September 2014, clipped black locusts compensated for height and shoot loss. The summer drought in 2015 reduced the survival of black locusts independently of the clipping and many saplings disappeared from previously occupied plots. In addition, the effects of the second clipping treatment in September 2015 have significantly slowed down, even hindering further growth of the clipped saplings. Soil nitrogen level was not higher in black locust plots, but low pH and significant soil compaction could be limiting factors for all saplings. Our results did not confirm any significant inhibitory effect of black locust on oak sapling development, but sessile oak had increased height growth compared to Turkey oak in black locust plots. It seems that browsing is only a secondary factor that influences the growth and competitiveness of black locusts since only repeated browsing after a heavy drought period could prevent black locust saplings from compensating for their losses.