Earlier this century, humans introduced exotic conifers such as ponderosa pine and Douglas fir to Nahuel Huapi National Park (NHNP). Land managers in the Argentine national park service recently recognized that these trees have the potential to invade native plant communities, threaten biodiversity, and change ecosystem structure and function. To address the problem, the park service began clearcutting NHNP’s exotic conifer plantations in the 1980s, with the intention of reestablishing native trees on these sites. So far, native trees have largely failed in those clearcut sites, while many are actually supporting reinvasions of exotic trees. This project sought to examine the barriers to native tree success in those sites and investigate the possibility that a native vine (Mutisia spinosa) limits reinvasion of the clearcuts by exotic trees.
Parque Nacional Nahuel Huapi, Río Negro, Argentina, -41.0000256, -71.50000160000002
Country or Territory:
Temperate Forest - Coniferous
Primary Causes of DegradationDeforestation, Invasive Species (native or non-native pests, pathogens or plants)
Exotic conifers were introduced in NHNP beginning in the early 20th century, with attention to several species of pine, Douglas fir, maples and several other non-native varieties. The inexpensiveness of land in Patagonia and the high rate of growth of pines and other trees made such species attractive economically, encouraging the spread of plantations as the original growth forest was cut down. The exotic trees are wind-dispersed but have different reproductive needs than the native trees, further altering ecosystem function. The addition of exotic wildlife, including two species of deer that remain to this day, are notable for how they affect the recruitment of native trees. The exotic trees are not as frequently browsed as the native trees, with has serious consequences for the persistence of those native plant communities. In many areas, the exotic conifers have spread beyond the boundaries and into native vegetation because of their efficient wind-dispersal. The colonization and spread of these species is unknown in its entirety because they have not been tracked.
Reference Ecosystem Description
These forests in Argentina are are closed canopy temperate mesic forests with generally three dominant trees in the forest: Nothofagus dombeyi, Austrocedrus chilensis, and Lomatia hirsuta. The ecosystem historically knew little disturbance, although catastrophic fires or landslides are noted as a means for the regeneration of Nothofagus and Austrocedrus species. Nothofagus trees are broadleaved, evergreen species that flowers in austral spring and produces seeds in summer that are dispersed by gravity and wind, while Austrocedrus is in Cupressaceae, producing berrylike fruit. Lomatia is a large subcanopy species with thick tough leaves and wind-dispersed seeds.
Restoration at NHNP worked with two purposes in mind: first, to prevent reinvasion and second, to reestablish native vegetation. The difficulties of the Argentine national park service in having success with their clearcutting projects necessitated an evaluation of the successional patterns that were occurring in the park. Additionally, the lack of knowledge about both abiotic and biotic conditions as well as the community-level interactions between the native and exotic vegetation provided for an improvement in the state of knowledge about both the park and the forests. Finally, the project sought to develop what were the environmentally-limiting factors to native tree survival.
The project does not have a monitoring plan.
The project was undertaken by the Argentine national park service as part of a larger effort to eliminate exotic conifers from parts of NHNP and restore native vegetation under guidelines adopted in the late 1980s and early 1990s. The poor to nonexistent regeneration of native species in the park was cited as a motivation for taking a more active role in restoring the ecosystems. The project investigator was a graduate student from the University of Montana.
Description of Project Activities:
The first steps in the project were taken a number of years previous to the inception of this project with the clearcutting of the forest, some sites saw cutting in stages between 1986 and 1994, and another site was cut in 1997. A portion of one site, the Puerto Madera site (which was a P. ponderosa/P. menziesii plantation) was replanted with native seedlings in 1998. The project evaluated this planting for experiments beginning in summer of 1999. The Pampa Pseudotsuga site was clearcut in the austral autumn of 1997 and was replanted the following spring with N. dombeyi, A. chilensis and L. hirsuta, however, few seedlings survived. In May 1998, a second section of the site was chosen to be replanted again to evaluated under experimental conditions. To evaluate the effects of shrub proximity and herbivory on growth and survival of the native species, a 48 x 90m experimental grid was established within a ponderosa clearcut at Puerto Madera. Each 4x15m section of the grid contained a shrub and an open site. Shrub sites were considered those within 50 cm of a shrub minimally 50 cm tall and wide, with the plant on the shrubs southeast side (sheltered from afternoon sun and from the prevailing wind). Each shrub and open site was assigned a seedling species and herbivory treatment (with or without a cage). At Pampa Pseudotsuga trees were planted on a 54x24m grid with eighteen columns and eight rows spaced 3m apart. The trees were planted in a regular order throughout the grid, however the eight-row structure created an alternating pattern. Half the seedlings were randomly selected for shade treatments, which was a simple metal frame covered with shade cloth.
Ecological Outcomes Achieved
Eliminate existing threats to the ecosystem:
Succession in clearcut conifer plantations proceeds very differently that in nearby native forests. Data from Puerto Madera indicated that very few native tree species were regenerating in the clearcut, and those six native shrubs (two exotics shrubs were found to be recolonizing also) which were found to be recolonizing the area tended to be tolerant of xeric conditions and typically regenerate in steppe-forest ecotone. All three species fared better at Puerto Madera when they are near shrubs than in the open, which was thought to be a consequence of the microclimatic conditions present in both light and temperature. At the Pampa Pseudotsuga site the trees fared poorly with the shade treatment, actually declining in height. This decline was thought to have occurred because of the severe limiting of light through the shade cloth, which limited light to 15% of the normal. This could have resulted in a diminuation of the plant's photosynthetic capacity. Cages, on the other hand, were shown to be successful in preventing herbivory. Native trees suffered considerable herbivory overall on those uncaged plants, however, those located near shrubs were found to be unbrowsed. The data indicated that the effects of shrub proximity may differ and be more complex than simple shade treatments. The final block of data suggested that restoration treatments will differ from site to site and year to year, suggesting that an effective way of compensating for the vagaries of natural climatic variation may necessitate planting in successive years. The other element that the project looked at was the effect of the native Mutisia spinosa vine on reinvasion. This native vine was hypothesized to heavily colonize conifer seedlings based on early observations of the researchers. Mutisia spinosa is a perennial rhizomatous vine that produces pink daisy-like flowers and small plumed seeds, the leaves are tough, and are resistant to both insect and mammalian herbivory. At the Puerto Madera site, the vine covered large areas, growing in clumps that appeared to spread both outward and upward. However, in the course of the two year study there was no conifer mortality. The vine slowed the growth of the Douglas fir but did not significantly influence the growth of the Ponderosa pine. The conclusion of the researchers was that the vine had only a mild competitive effect on the conifers.
Factors limiting recovery of the ecosystem:
The experimental results indicated that the clearcuts were stressful environments for native tree seedlings. The data suggested a continual decline even in the face of shading and caging, although the data also suggested that nurse shrubs and protection from herbivory may both affect regeneration. The differences among all three tree species indicates that any restoration program must take into account the unique ecology of the species being planted. The dryness of the year in which the experiment took place meant that all the seedlings were exposed to major stress, in greater magnitude than usual. That indicated that for many tree species the recruitment is episodic and depends on years with particular climatic conditions. The absence of post-planting care may also account for a change in the effectiveness of the present project. The final condition that might alter future results is that the experiment was conducted in close proximity to one another, which because of dramatic changes in the Patagonian landscape over very short geographic distances indicates that further experimentation will be necessary to clarify who limiting factors vary from site to site.
Socio-Economic & Community Outcomes Achieved
Economic vitality and local livelihoods:
The biology of invasive species is well-known in the Northern Hemisphere where pines are native to, but in the southern Hemisphere, there are considerably fewer invasive species. The potential exists based on the ecology of pine and fir for the conifers introduced at the turn of the twentieth century to become troublesome species. Because this invasion is just beginning, the problem is small and localized, presenting a special opportunity to take action and prevent further spread of exotic species. While there are distinct social and economic incentives for planting pine and fir, there is a complex consideration of the social costs and benefits to the exotic conifers as they have ecological effects far beyond their rapid growth. The direct success and dispersal of these species may spell trouble for many native species in the Southern hemisphere. This project sought to expand the available ecological knowledge about the native species that were having difficulty regenerating themselves in those clearcuts left by Argentine national park service activities and in so doing managed to develop community-level understandings that can now guide future restoration efforts.
Because of the early nature of the invasion of pine and fir into these ecosystems, future research is critical to refine the understanding of what factors limit native tree regeneration and to design restoration strategies and practices to promote native species.
Sources and Amounts of Funding
National Science Foundation
Hourdequin, Marian. 1999. Ecological restoration after removal of exotic conifer plantations in Argentine Patagonia’s Nahuel Huapi National Park. Masters thesis, University of Montana.