Twelve-year dynamics of alien and native understorey plants following variable retention harvesting in Nothofagus pumilio forests in Southern Patagonia

Disturbances have frequently been shown to increase the invasibility of plant communities. Harvesting, the most common and severe type of anthropogenic disturbance in forests, creates pulses of enhanced resource availability for alien plants and for native colonizers. However, it is unclear for how long initial changes in species composition of the understorey persist after harvesting. Using annually surveyed permanent plots (n = 72 plots across six stands), we analysed temporal changes of plant understorey composition of Patagonian Nothofagus pumilio forests during 12 years under three different conditions created by variable retention harvesting: retained forest patches (aggregates) of 30 m radius in a density of one patch ha(-1) (AR), dispersed retention within the influence of the aggregate (DRI), retained dispersed single trees (DR); and primary unharvested forests as control (PF). Our results show that: (i) cover of native forest species in the understorey declined only very slowly in AR, but alien species cover increased strongly and became dominant nine years after harvesting (YAH); (ii) DR and DRI supported higher cover of alien species in the understorey than AR and PF, and alien species became dominant two to three YAH. Yet, they started to decline following a peak at eight YAH; (iii the cover of native forest species in the understorey was high in the beginning but alien species became dominant in DR and DRI after four and seven YAH, respectively. Tree regeneration significantly influenced understorey dynamics, i.e. (iv) in DR, a negative relationship with alien species cover, and a positive relationship with native colonizers from other habitats; (v) in DR and in DRI, this relationship was negative with alien species richness; (vi) in DRI, it was positive with native colonizers from associated environments (cover and richness). We conclude that alien plants invade Patagonian forests after harvesting, but decline with time when tree regeneration is established. Our results highlight the role of harvesting in facilitating plant invasions in forests, and how the spatio-temporal trajectories of such invasions are influenced by different levels of disturbance created by harvesting according to the position within the resultant matrix.