Shrub canopy matrix decreases reproductive output of a sheltered plant via pollinator exclusion

Shrubs establish microenvironments under their canopies that can favor the growth of other plants. However, the shrub canopy could impede pollination by reducing the number of pollinator visits to sheltered plants, resulting in pollen limitation and decreased reproductive output. We assessed whether the presence of a nurse shrub species (Flourensia thurifera) alters the reproductive output of a sheltered cactus (Eriosyce coimasensis) via the restriction of access by the giant hummingbird (Patagona gigas) to E. coimasensis flowers. During two consecutive years (2018 – 2019), we excluded hummingbirds from individual cacti (using cages) and studied fruit set and seed production in two microhabitats: underneath shrubs and in open sites. In addition, we compared the reproductive mode of E. coimasensis in the two microhabitats. We observed that shrubs limit the reproduction of E. coimasensis, which strongly depends on P. gigas for seed production. Plants in open sites produced 80% more fruit and 76% more seeds than those growing underneath shrubs. The reproduction of caged individuals was low and similar to those growing beneath shrubs. In addition, plants underneath shrubs, but not in open sites, may suffer pollen limitation. Our results offer novel insights into plant-plant interactions and suggest potential trade-offs for sheltered cacti between the mild microclimatic conditions under the canopy, that could lead to larger plants and pollinator preclusion, which decreases the reproductive performance of sheltered plants.