Reduction of understory cover by exotic deer promotes early establishment of introducedconifers in Patagonia

CACCIA, F; RELVA, A; NUÑEZ, MA; MARTÍN, V; AMICO, GC

Congreso; 2nd World Conference on Biological Invasions and Ecosystem Functioning; 2011

In Austrocedrus-Nothofagus forests at Isla Victoria, Nahuel Huapi National Park, southwestern Argentina, exotic deer reduce understory cover and facilitate conifer invasion by selectively browsing on native saplings. To examine how exotic deer alters recruitment opportunities for exotic and native trees at early stages decreasing understory cover we performed a transplant experiment with newly germinated seedlings of the dominant native trees (Austrocedrus chilensis and Nothofagus dombeyi) and two exotic potentially invaders (Pinus ponderosa and Pseudotsuga menziesii). Treatments were a factorial combination of deer access areas (exclosure vs control), understory cover microsites (high vs low) inside each deer access area, rodent access levels (exclosure vs control) and tree seedling species (as above). After one growing season mean survival of exotic tree seedlings was nearly twice than natives in the low cover understory microsites (74 vs 38%) irrespective of deer and rodent exclosures. High understory cover strongly decreased survival of both exotic and native tree seedlings (8 vs 16% respectively). These patterns resulted mostly from differences in mortality by invertebrate predation and abiotic stress. Invertebrate predation on native and exotic seedlings decreased in low compared to high understory cover microsites (14 vs 63%). Mortality by abiotc stress (desiccation) in the low understory cover microsites was higher for native compared with exotic seedlings (33 vs 18%). Rodent exclosures slightly increased survival of native and exotics irrespective of microsite cover and deer exclosures (38 vs 29%). By reducing understory cover exotic deer enhanced early establishment of the fastgrowing exotic conifers supporting the invasional meltdown hypothesis.