Effects of artificial shading and weed mowing in reforestation of Mediterranean abandoned cropland with contrasting Quercus species

Large areas of abandoned cropland in the world can be reforested with native shrubs and trees to gain a number of environmental benefits. In abandoned Mediterranean croplands, establishment and growth of woody plants are limited by high radiation and low water availability during summer, and weeds are strong competitors for resources, particularly water. We conducted a 3-year experiment in central Spain to study the response of three Quercus species (Q. coccifera, Q. ilex and Q. faginea) that differ in their habitat requirements under four treatment field conditions resulting from the combination of full-light versus artificial shading and weed presence versus weed mowing. We measured seedling survival, resprouting capability and growth, weed production, microclimate (incident photosynthetic active radiation (PAR), air temperature, soil water evaporation and effective precipitation) and soil moisture. Shading and weeds reduced PAR reaching the seedlings and soil water evaporation, and shading also reduced effective precipitation. Shading and mowing increased soil moisture. We found a clear positive synergic effect of shading and mowing on seedling performance. Weed competition limited seedling survival in all species more than high radiation, whereas the relative importance of these factors in limiting growth depended on the growth measure and species. As hypothesised, the effects of stress release on plot cover, an integrated performance index that combines survival and growth, were most noticeable in Q. faginea, the most mesic species, and least in Q. coccifera, the most xerophytic species. The release of weed competition allowed Q. ilex seedlings to invest resources in above-ground and, apparently, in below-ground growth. Shading increased simultaneous growth in diameter and volume only for Q. faginea. It is important that planted Quercus seedlings in abandoned Mediterranean cropland take advantage of a low competitive environment from weeds during the period before the first dry season. Once seedlings have established, an artificially shaded environment would provide benefits in terms of plot cover by the oaks, particularly for mesic species. Studies like this will be of great interest to optimize resource investment in active restoration of other ecosystems of the world.