Uncovering the little known impact of a millennia-old traditional use of temperate oak forests: free-ranging domestic pigs markedly change the herb layer, but barely affect the shrub layer

Free-range domestic pig keeping in forests is a millennia-old practice in Eurasia, and remains common in many silvopastoral systems worldwide. Despite the long history of its potential impact on forests, the influence of this practice on the structure, composition and species richness of the understorey is hardly known. We studied the impact of free-ranging domestic pigs on the herb and shrub layers, and the consequences of abandoning pig keeping in a hardwood floodplain forest in Serbia, SE Europe. We recorded the cover and composition of the herb layer (incl. woody species) using 30 subsamples at each of the 56 sampling plots (1680 subsamples) in seven disturbance intensity and abandonment categories. We measured the browsing, rooting and rubbing impact on the shrub layer by sampling more than 3100 woody specimens. The cover of the herb layer decreased as disturbance intensity increased. We observed only a small compositional shift from currently used, slightly to highly disturbed plots in the herb layer, while the most severely disturbed currently used resting places and the abandoned sites were markedly different. The number and cover of hardwood forest species were the highest at abandoned sites, but only their cover showed a decreasing trend along the disturbance intensity gradient, while the cover of forest generalist species showed an increasing trend in currently used plots. The number and cover of forest ruderals were the highest in resting places in both abandonment categories, while the cover of non-forest ruderal and neophyte species was significantly higher only in currently used resting places. Unexpectedly, neither disturbance intensity nor abandonment had a substantial effect on the composition of the shrub layer. The cover of both the upper and lower shrub layers was the highest in plots abandoned 40 years ago, decreasing slightly as disturbance intensity increased in currently used plots. The impact of rooting and rubbing by pigs on shrub morphology (increase of resprouting specimens and specimens with distorted stems) was significant, while that of browsing was negligible. Our study shows that this previously widespread, but now almost completely disappeared land-use practice may have had a marked impact on the understorey vegetation of Eurasian temperate oak forests.