María Andrea Relva

Gringos en el bosque: introduced tree invasion in a native Nothofagus/Austrocedrus forest

We studied invasion into native Nothofagus/Austrocedrus forest by many introduced tree species planted between 1910 and 1940 in plantations near the center of Isla Victoria, in northern Patagonia. We located virtually all individuals of these species in 30 ha of forest in two series of transects at increasing distances from the plantations. Although these species included many reported as highly invasive elsewhere, we found little evidence for invasion on Isla Victoria, with many ‘invasive’ species utterly failing to invade native forest. There was a notable decline with distance in number of introduced individuals, but wind direction appeared to be unimportant. Pseudotsuga menziesii and Juniperus communis were the only 2 species represented by many individuals, while 4 pine species plus Araucaria araucana were far less numerous and 6 other species were found fewer than 10 times each. Even those species found repeatedly were represented overwhelmingly by small individuals, and the great majority of introduced individuals were found not in native forest proper, but in somewhat open areas such as road verges, small remnant pastures, and deer trails. Invasion may be occurring, but too slowly to be clearly evidenced yet because of the longevity of the dominant native trees. Factors that may be stopping or slowing invasion include competition in gaps with native species, browsing by introduced deer, unfavorable soil, allelopathy, and natural enemies or other idiosyncratic factors for particular species. The absence of substantial invasion so far is no guarantee against future invasion, particularly if some major natural or anthropogenous disturbance were to occur.

Ungulates can control tree invasions: experimental evidence from nonnative conifers and sheep herbivory

Invasive conifer species are increasingly recognized as a serious problem in many parts of the world, where they are having large ecological and economic impacts. Understanding factors that trigger and can control invasion is key to management. Grazing and browsing by large herbivores have been suggested as a mechanism that may halt conifer invasions, although conflicting results have been reported (i.e. positive, negative or no effect of grazing on invasion). We believe that some of these opposing responses arise due to the absence of well-planned and replicated experiments, since current evidence is mostly observational, and for example, differences in animal densities can produce different results. Thus, in this study, we tested whether large herbivores can control invasion by nonnative conifers and whether the severity of the invasion process would be lessened by increased herbivory intensity. We evaluate experimentally herbivore damage on Pinus contorta, a highly invasive species in many countries of the Southern Hemisphere, under different sheep stocking rates in Patagonia, Argentina. We used four stocking rates, corresponding to 1, 2, 4 and 8 times the recommended sheep herbivory intensity for the study area. The response was not linear but rather presented a threshold. The greater stocking rate, the greater the browsing, higher reduction in seedling height, and decrease of survival of P. contorta. Also, the highest stocking rate damaged and killed 99% of them. This study provides evidence that large domestic herbivores can play a key role in the invasion process and, if managed properly, would provide a tool to help limit conifer invasion.

Not one but three: undetected invasive Alnus species in northwestern Patagonia confirmed with cpDNA and ITS sequences

Species of Alnus (alders) have become invaders in several parts of the world. Here we report the presence of three naturalized alien species: A. glutinosa, A. incana and A. rubra from several populations in nature reserves of northwestern Patagonia, an area of remarkably high biodiversity. Alnus glutinosa had been cited previously for Chile and southern Argentina, but A. incana and A. rubra are here reported for the first time. As we found morphological variation within and among the populations of these introduced species that makes their discrimination difficult, we used chloroplast (trnH-psbA) and nuclear ribosomal (ITS) DNA sequences to confirm their identifications from morphological characteristics. Results from nuclear and chloroplast sequence data confirm the morphological tentative identification of the three species and remark the utility of molecular information together with morphology for the detection of introduced species of taxonomically difficult groups. The invasive characteristics of these alien tree species are discussed in relation to the conservation of the nature reserves where they are found.