Establishment patterns of woody species in low intensity-grazed pastures after the cessation of intensive agricultural use

Low intensity grazing with large herbivores is increasingly used to steer the development of mosaic landscapes (including wood-pastures) and woodland expansion on former agricultural land. We studied the establishment and early growth pattern of woody species in grasslands (formerly used for high intensity summer cattle grazing) in which a low intensity-grazing regime was set up between 4 and 11 years ago. We tested whether the developing vegetation patches, consisting of tall herbs and scrub, influenced this pattern. Further, we investigated if increased susceptibility to trampling disturbance by large herbivores on wet soils in floodplains influenced sapling establishment.
Formerly intensively used grasslands were studied at six sites, four in floodplains and two in interfluvial zones on nutrient rich, heavy soils. In total, we sampled 141 plots with grassland, tall herb or scrub vegetation and recorded individuals of established tree and shrub species, their height, diameter, browsing damage and distance to potential seed sources. In the floodplains, we recorded soil disturbance by large herbivores (hoofprints) in wet and moist soils.
Sapling frequencies, with Fraxinus excelsior as the most common species (>50% of all individuals), were two to three times higher in tall herb and scrub patches compared to grassland. Only the spiny Crataegus monogyna established in higher frequencies in grassland plots. In floodplains, we found two to three times more established woody saplings in wet tall herb plots, compared to moist and wet grassland plots and moist tall herb plots.
Browsing reduced sapling height in grassland and tall herb patches. Only in scrub patches did average height (±2.5 m) reach above the browse line. We found 30–70% less browsing damage to saplings in scrub compared to damage in grassland and tall herb plots. In particular, damage to the apical shoot was strongly reduced.
Our findings show that tree establishment occurs in spatial association with non-grassland patches that offer protection against browsing. Fast growing spiny shrubs like Rubus sp. offer protection for palatable saplings, allowing establishment and growing out beyond the browse line. Alternatively, increased establishment occurs in wet tall herb patches, where soil disturbance by moderate trampling favours germination and, subsequently, saplings are protected at least temporarily against browsing by unpalatable tall herbs.