Restoration of native forests from Pinus radiata plantations in southern Chile

Hochschulschrift

Abstract: Background: In view of their high productivity and rapid growth, plantation forests are expected to play an important role in reducing the pressure for wood production from natural forests, and in in achieving restoration goals for climate change mitigation. Nevertheless, given their lower capacity to deliver ecosystem services other than the production of raw materials and energy resources, clearing natural forests for establishment of plantations is socially and institutionally unaccepted. In south-central Chile, after a long process of deforestation and land degradation, a rapid expansion of commercial plantations took place, especially since 1974 in response to public policies promoting afforestation and economic development. This expansion, mostly with plantations of Pinus radiata D. Don and eucalypt species, occurred to some extent at the expense of clearing natural forests and shrublands (over 270,000 ha since 1976), in a region of high biological importance. In response to the requirements of certification schemes, some of the main forestry companies in the country are now committed to restoring at least 30,000 ha of native forests and shrublands that were replaced by plantations since 1994. However, despite the need for technical expertise, there is no documented experience for this form of restoration in Chile.

Aims and objectives: The purpose of this thesis was to provide a scientific basis for the initial steps of the restoration of native forests of south-central Chile that have been cleared for the establishment of plantations of P. radiata (i.e., the species that was most frequently used in this process). Due to the diverse nature of studies on this subject at the global scale on the one hand, and the lack of local documented experiences on the other hand, I aimed at synthesizing worldwide available knowledge, while providing evidence for the local context through experimental approaches. Specifically, I addressed the following research questions:

1.What are the main drivers of native species regeneration in the process of restoring native forests from planted forests?
2.What is the effect of harvesting intensity of P. radiata plantations on the performance of seedlings of native tree species from the natural forests of south-central Chile?
3.What is the potential of P. radiata plantations established through clearing of natural forest for a passive restoration approach based on natural regeneration of native woody species, and what are the drivers of this regeneration?

Methodology: For addressing the first research question, I conducted a review of 68 systematically-selected peer-reviewed studies reporting effects of interventions or of environmental conditions on the early survival, early growth, abundance, and/or diversity of native woody species regeneration, when restoring native forests from plantations. To identify the main factors determining regeneration success, I assessed the frequency upon which a specific environmental factor or intervention was reported to significantly affect regeneration of native woody species in plantations.
Meanwhile, the experimental part of this research took place along the Coastal Range of south-central Chile (~36º00’S - ~40º00’S), covering most of the area in which natural forests have been replaced through P. radiata plantations since 1974. Specifically, for addressing the second research question, a landscape-scale replicated silvicultural experiment was established in 20-year-old P. radiata plantations across three different sites, which were located at different altitudes and slopes on the eastern and western slopes of the Coastal Range. Different harvesting treatments were applied (clearcut, strip-cutting and unharvested control), and group plantings of shade-intolerant, semi-tolerant and shade–tolerant species were established across the treatments. By using mixed-effects models, I assessed the effect of the harvesting treatments on seedling performance and water status during the first two growing seasons.
For addressing the third research question, I conducted a survey of natural regeneration of native tree species in first-rotation P. radiata plantations approaching harvesting age. Across different landscapes and regions, 256 plots were established to assess regeneration and environmental conditions along 26 transects running from plantations into adjacent natural forests. I compared the density, diversity and composition of regeneration found in plantations with that of adjacent remnants of natural forest to determine the potential of plantations for passive restoration. Additionally, I conducted a canonical correspondence analysis to study the relationships between regeneration and the environmental conditions in plantations.

Results and discussion: According to the general evidence collected, spatial landscape configuration, overstory structure, ground vegetation structure, climate and geomorphology, and overstory composition are the most frequent drivers of native species regeneration in plantations, with significant effects in more than 47% of cases. Specifically, regeneration diversity and abundance increase with proximity to natural vegetation remnants and seed sources. Lower canopy and understory stocking levels positively influence regeneration, as well as interventions to reduce them. Canopy cover reduction proves especially effective in warmer regions, in stands of broadleaved species, younger ages (< 30 years), higher densities (> 1,000 trees ha-1), and taller canopies (> 20 m). According to this, restoration of native forests can be optimized by adopting interventions that prove most effective, and prioritizing more responsive stand types.
The silvicultural experiment showed that partial harvesting is a promising option for facilitating artificial regeneration of native tree species when restoring native forests from P. radiata plantations. Higher canopy cover in the control treatment (no harvesting) promoted high seedling survival rates, while limiting height and diameter growth. In contrast, the open conditions of the clearcut treatment (all trees removed) promoted high growth rates, while reducing survival. Under the intermediate canopy cover of the partial harvesting treatment (strip-cuttings), both high survival and high growth rates were found. Seedlings in this treatment also experienced lower water stress, suggesting that their higher performance is a result of higher irradiances combined with higher water availability.
Density and diversity of natural regeneration of native tree species were significantly higher in natural forests than in P. radiata plantations. Nevertheless, significant differences among these forest types were only observed for seedlings, and not for saplings. Additionally, the composition of natural forests and plantations differed significantly only in a few cases (two of the eight study sites). The similarity of the regenerating communities from both forest types suggests that under harvesting approaches that maintain advance regeneration, the transition from plantations to native forests could be largely facilitated through passive restoration. Compared to climatic and soil chemical variables, which varied mostly at regional scales, local environmental conditions showed little influence on regeneration, possibly due to the structural homogeneity of plantations. Yet, significantly higher basal area, litter thickness, and gap fraction of plantations compared to natural forests, suggest that these factors may explain the differences seen at the seedling stage. Thus, the use of advance natural regeneration for passive restoration should be further investigated by experimentally evaluating the effect of these factors on natural regeneration.

Conclusions: The findings described in this thesis represent the first scientific contribution to the restoration of native forests from of P. radiata plantations in Chile, as well as an important background for restoration in other regions with similar challenges. Based on the availability of advance regeneration of a variety of species with different traits, and on the positive response of artificial regeneration to intermediate canopy removal, partial harvesting approaches that promote site variability while protecting advance regeneration should be prioritized for restoration from P. radiata plantations in Chile. The consistency of these results across different climatic and vegetation regions of the study area, suggests their applicability under a wide range of environmental conditions. Yet, the feasibility of partial harvesting approaches and the specific design to apply them should be determined on the basis of economic, operational and social considerations. Additionally, further research focused on later stages of stand development is required for more comprehensive management recommendations

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2021

Classification and Topics

• Keyword: Plantations ; Pinus radiata ; Chile

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2022
• Creator: Kremer Ramírez, Klaus
• Contributor: Bauhus, Jürgen ; Bauhus, Jürgen ; Promis Baeza, Alvaro Andrés ; Kohnle, Ulrich ; Albert-Ludwigs-Universität Freiburg. Professur für Waldbau ; Albert-Ludwigs-Universität Freiburg. Fakultät für Umwelt und Natürliche Ressourcen

Further information

• DOI: 10.6094/UNIFR/224660
• URN: urn:nbn:de:bsz:25-freidok-2246604
• Rights: Kein Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:51 PM CET