Cage closed: the effects of introduced herbivores in forest regeneration in Isla de los Estados, Argentina

SALOM, AMIRA; PÜTZ, KLEMENS; RAYA REY, ANDREA; BIGANZOLI, FERNANDO

Chistchurch

Congreso; Fourth International Congress on Biological Invasions; 2023

The presence of introduced herbivorous mammals on oceanic islands usually involves the exposure of native forest, evolved in isolation, to functional groups previously absent in these ecosystems. Such is the case of Isla de los Estados mixed-evergreen forest of Drimys winteri and Nothofagus betuloides in Tierra del Fuego, Argentina, where red deer (Cervus elaphus) and goats (Capra hircus) were introduced over 50 and 100 years ago, respectively. The early stages of tree life cycles are usually the most vulnerable to grazing and trampling, so the successful establishment and development of seedlings and saplings is a key indicator of the potential of forests to regenerate over time. We evaluated the effect of these introduced herbivores on the survival and establishment of the two dominant tree and two shrub species (Berberis ilicifolia and Chiliotrichum diffusum) which are widely distributed at the study site and are main components of the understory of the forest and in the diet of these species. We monitored the survival and growth of 577 seedlings over three years (2019-2021) within two treatments, exclosure and exposure to herbivores. Analysis were performed applying generalized linear mixed models (GLMMs). We found that seedlings of all four species showed a significant increase in survival and height inside the exclosure. Overall, 79% of seedlings survived inside the exclosure, while only 26% did so in the open plots. From the surviving seedlings, 40% passed to an advanced regeneration state, exceeding a height of 10 cm within the exclosure, while only a 4% of the seedlings exposed to herbivores did. In turn, the number of leaves of all species showed a general pattern of increase within the exclosure, but only D. winteri and B. ilicifolia exhibited significant changes. Our study suggest that the introduced herbivores could be affecting the establishment of seedlings and saplings of the species studied by increasing their mortality either by trampling or browsing and at the same time strongly diminishing growth of surviving seedlings. Red deer and goats clearly impact negatively on the development of seedlings into saplings and small trees, decreasing the recruitment of these species into larger size-classes. This results in forest degradation and possibly impeding its regeneration, which in turn could be affecting the structure and heterogeneity at landscape scale in the long-term.