Assessment of recent to medium-term impacts of bench terrace construction on forest ground cover and selected soil physical and fertility properties along a chrono-sequence in north-central Portugal.

Mankind has built terraces on sloping terrain since pre-historic times, especially for expanding croplands. Widespread terracing for forestry purposes is comparatively recent, presumably linked to the advent of fast-growing tree plantations together with technological advances in heavy forestry machinery. Perhaps not surprisingly therefore, the impacts of terracing on soil properties are poorly studied for forest stands. The present investigation addressed this knowledge gap using eucalypt plantations in north-central Portugal as study case and using a space-for-time approach. In total, 12 pairs of neighbouring, terraced (T) versus non-terraced (NT) plantations were studied, with the terraced plantations equally divided over four time-since-terracing periods: <1 year (recently terraced); 5 years; 10 years; 17 years. As such, the studied chrono sequence covered roughly half of a full eucalypt rotation cycle in the region. All field data and all litter and topsoil (0-5 cm depth) samples were gathered during an 2-weekly campaign, at 5 equidistant transect points per plantation. Terracing changed all four cover categories most strongly and markedly during the initial time-since-terracing period, while its effects on the averages of vegetation cover (T/NT: 19/37 %) and litter cover (T/NT: 47/62 %) contrasted with those of stone cover (T/NT: 26/1 %) and bare soil cover (T/NT: 8/1 %). Terracing increased dry bulk density across the entire chrono sequence (overall averages T/NT: 1.17/0.84 g cm), while it greatly decreased resistance to shear stress but only for the initial post-terracing period (T/NT: 0.8/ 3.2 kg cm) and not towards the middle and end of the first rotation cycle (five and ten years after terracing). Total carbon content in soils was significantly affected by terracing, while total nitrogen content not. Both properties revealed similar temporal patterns, with a marked initial decrease (TC: -32 mg C g soil; TN: -0.73 mg N g soil) and a gradual recovery during the first rotation cycle.