Reciprocal interactions between a non-native shrub and the dominant native trees of a high mountain woodland: who benefits?

Abstract

Facilitation by nurse plants is a common interaction, whereby climatic harshness and/or other limiting factors are reduced under the canopy of the “nurse” plant, favoring the regeneration of other plant species. In mountain systems, nurse plant interactions among coexisting native and non-native woody species might reciprocally affect the regeneration patterns of the involved species. Elucidating these relationships might contribute to the understanding of interactions that facilitate the success of non-native woody species at high elevations. We assessed whether the non-native shrub Cotoneaster franchetii and the dominant native trees Polylepis australis and Maytenus boaria, reciprocally affect each other’s regeneration in highland woodlands of central Argentina. We recorded woody recruits (seedlings and saplings) and micro-environmental conditions under the three nurse plants and in gaps without woody cover. The frequency of occurrence of Cotoneaster and Maytenus recruits differed among microhabitats. Polylepis regeneration in the area was almost null. The highest Cotoneaster recruit frequency was under the canopy of conspecifics and Polylepis. Maytenus recruit frequencies did not differ among microhabitats, but saplings had higher frequency under Polylepis than under conspecific nurses. The apparently higher quality of Polylepis as nurse plant might be related to intermediate conditions of photosynthetically active radiation and milder winter temperatures under its canopy. Non-native shrub regeneration seems to be facilitated by one of the native species, whereas neither of the natives seems to be favored by the invader. These results support the importance of biotic interactions as drivers of woody invasion success in mountain ecosystems.