Date Published: May 24, 2019
Publisher: Public Library of Science
Author(s): Persy Gómez, Maureen Murúa, José San Martín, Estefany Goncalves, Ramiro O. Bustamante, Dusan Gomory.
Pine invasion is a global threat that is occurring in native forests of diverse regions of the world. This process is arising in a scenario of rapid forest deforestation and degradation. Therefore, elucidate which forests attributes explain invasibility is a central issue in forest ecology. The Coastal Maulino forest is an endemic forest of central Chile, which has suffered a large history of disturbance, being replaced by large extensions of Pinus radiata plantations. This land transformation conveys high rates of pines invasion into native remnants. In this study we examined to what extent structural features of forest patches explains invasibility of this forest-type. Within eight forest fragments, we sampled 162 plots (10 x 10 m2 each). We quantified seedling pine density and related these estimates with tree cover, litter depth, PAR radiation, and diversity of the resident community. Our results indicate that canopy cover was the most important variable to determine seedling pine density within forest fragments. Our investigation highlights the importance to conserve the forests cover to reduce significantly their invasibility. This action can be effective even if we cannot avoid pine plantations in the region as a source of a massive seed dispersal to forests with well conserved canopy.
Annually, millions of hectares are deforested and fragmented worldwide, transforming native forests into disturbed habitats often used for agriculture and forestry practices . This land transformation produces a cascade of ecological effects, being one of them high rates of species invasion into the native remnants [2–5].
The Coastal Maulino forest is located in the coastal range between 35° and 37° latitude S . It is an endemic ecosystem which has declined significantly the last decades . For instance, ca. 67% of the original forest was replaced by Pinus radiata D. Don between 1975 and 2000 . Currently, forest patches are embedded in a matrix of pine plantations . Pine invasion into forest fragments occurred at least from 1945 and currently, there exists approx. 10% of reproductive individuals inside forest . During the last decades the intensity and extent of human-induced fires have increased significantly, reducing even more the remnant native forest . The study area was located at Cauquenes, Maule Region (-72.35°; -35.97°). Topography is heterogeneous with plains, gently slopes and creeks. The climate is Mediterranean-type with mean annual temperature of 18°C and mean annual precipitation of 709 mm, concentrated mainly in the winter season [36,37]. The landscape is highly anthropogenic presenting a mosaic of native forest fragments surrounded by Pinus radiata plantations. Dominant native tree are Nothofagus glauca associated with Persea lingue and Gevuina avellana in more humid habitats and with Nothofagus obliqua and Nothofagus alessandrii in drier habitats . Evidence indicates that P. radiata is invading actively these forest [11,38]. Our research focused on eight forest fragments dominated by N. glauca, which ranged from 3 to 152 ha, summing a total of 332 ha; the areal extent of the sampled zone was approx. 60 km2 (Fig 1). This study was conducted in private lands of
Mantel test indicated that there is no spatial dependence among pairs of plots (r = 0.015; P = 0.31), i.e. the geographical distance between pairs of plots did not affect differences in seedling abundance. From PCA, the PC1 accounted 38% of the variance (eigenvalue = 1.90), and correlated negatively with tree cover (r = -0.53), and litter depth (r = – 0.49) and positively with PAR radiation (r = 0.48) (Fig 2). PC1 is an axe that represents light availability, being negative values with low light availability and positive values an increase of light availability. On the other hand, PC2 accounted 33% of the variance (eigenvalue = 1.64), and was correlated positively with Shannon-Wiener index (r = 0.65) and species richness (r = 0.59) (Fig 2). Thus, PC2 is an axe that represents native species diversity, being negative values with low diversity while positive values with high diversity.
In this study, we have tested the hypothesis that forest invasibility can be predicted by the interplay between canopy cover (a proxy of disturbance) and resident species diversity (a proxy of competition intensity) . Our results indicate that canopy cover is sufficient to determine forest invasibility. As this variable is a proxy of light availability, it is the ultimate abiotic driver influencing pine regeneration. We did not detect direct effects of the native plant community on pine regeneration, suggesting no biotic resistance (competition) of the resident community. This result is of major importance because indicates that there are no biotic filters for the colonization and establishment of this invasive tree.
Our results, besides to corroborate the hypothesis that canopy cover is the best predictor to arrest pine invasion, as it has documented in other studies, provide basic information to conduct simple conservation actions in the Coastal Maulino forest. To preserve this valuable ecosystem and the containing biodiversity, it is mandatory to conserve the canopy cover beyond the detected threshold of 63%. These solely actions will be highly effective even if there exists a massive seed rain that arrives each year from the surrounding pine plantations, as they are unable to recruit under well conserved native forests . These criteria can be properly used in other regions of the world where native forests are in permanent risk to be invaded by pines.