Context-dependent effects of fire and browsing on woody alien invasion in mountain ecosystems
Anthropogenic activities have increased disturbances and alien woody invasion in mountain ecosystems worldwide. Whether disturbances promote or counteract upward movement of woody aliens is poorly understood. We assessed if the most successful woody invader of low mountains of central Argentina (Gleditsia triacanthos) might expand its elevational distribution in response to the principal disturbances of these ecosystems (fire and livestock browsing) across increasing climatic severity. We assessed seedling emergence, growth and mycorrhizal colonization on sown plots distributed in burned and unburned sites, with and without browsing at the lower and upper elevation belts (i.e. 1000 and 2400 m a.s.l.). Additionally, several abiotic variables were measured to relate their influence on the seedling establishment. Disturbances reduced seedling emergence at both elevations. Burned conditions increased seedling growth and arbuscular colonization only in the lower belt. Seedling success (total seedling biomass per plot) was not modified by disturbances at the upper elevation, but was reduced by browsing and enhanced by fire in the lower elevation. The overall reduction in seedling emergence and growth in the upper elevation despite the higher soil nutrient content places climate as the strongest regulator of G. triacanthos seedling establishment. Accordingly, climate rather than disturbances would be the main limiting factor of upward expansion of this woody alien. Our findings differ from general patterns described for mountain invasion by herbaceous species, highlighting that mountain invasibility is highly growth-form dependent, and that upper range expansion by woody aliens interacting with multiple disturbances should be assessed worldwide.
KeywordsGleditsia triacanthos Altitudinal expansion Disturbance Tree invasion Seedling establishment
Authors acknowledge the assistance of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), and Secretaría de Ciencia y Técnica (SECyT) of Universidad Nacional de Córdoba, both of which supported this investigation. We also thank M. Cabido for promoting this project, E. Kowaljow for field assistance, M. Pereyra for identification of ant species and J. Brasca for English edition of this article. We especially thank land owners (J. Astrada, Atum Pampa, Olguín), who provided access to the study site. The authors specially acknowledge the suggestions of the editor and two anonymous reviewers that improved the article.
Author contribution statement
MP, FA and TP conceived the ideas and designed methodology; MP, FA, TP, FG and ZS collected the data; UC and LS measured mycorrhizal colonization; MP, FA, TP and ZS analyzed the data; all authors were involved in the writing of the manuscript, contributed critically to the drafts and gave final approval for publication.
Compliance with ethical standards
Disclosure of potential conflicts of interest
The authors declare that they have no conflict of interest.
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