Silvicultura en bosques nativos: experiencias en silvicultura y restauración en Chile, Argentina y el oeste de Estados Unidos

8 — Silvicultura en bosques nativos Abstract Advances in ecological research have led to better under- standing of the socio-ecological systems. However, un- certainty resulting from the overuse of natural resources in addition to global changes necessitate increased scien- tific efforts through the integration of ecological theories in order to provide effective solutions to environmental issues. An important threat to the integrity of forest ecosystems is forest degradation, which is recognized as an overwhelming planetary issue that is complicated to evaluate due to different socio-ecological constraints and values. Among the forest stages most impacted by degra- dation are primary forests due to the quality and yield of their timber; the harvest of this timber greatly heightens the impact on system productivity and biodiversity in different forest biomes. It is recognized that high-grading (i.e., selective harvesting of the best trees based on tim- ber goals) is mainly responsible for this issue, especially in developing countries. Typically, after high grading, little attention is paid to tree regeneration, and undesir- able understory vegetation often dominates for a long period, stagnating forest recovery and succession. After high-grading, managing understory vegetation through ground disturbance has been proposed as a restoration tool to encourage the regeneration of desirable tree spe- cies. In this work, we investigated the effectiveness of ground disturbance in high-graded Nothofagus (southern beeches) old-growth forests in the Chilean Andes. In these forests, after high grading the understory is typ- ically dominated by dense tickets of colihue ( Chusquea culeou ), which has been shown to delay succession by preventing the regeneration of pioneer tree species for extended periods. We utilized the Hutchinson’s niche concept, as related to regenerating tree species and colihue, to better understand the variables shaping the different plant groups and thus the mechanisms behind the impacts of ground disturbance on overcoming stag- nation in succession by encouraging tree regeneration. Multivariate ordination (CCA) and niche modeling (NPMR) were used to investigate the shape, direction, and magnitude of realized niche shifts, and the environ- mental variables responsible for these shifts. The results showed that ground disturbance expanded the realized niche for early-seral tree species and constrained it for bamboo and late-seral tree species. These patterns were supported by the impact of ground disturbance on litter cover (-) and soil water content (+), which affected bamboo cover (-) and favored to early-seral trees (+). Surprisingly, light had less influence on shifting the niche for tree regeneration during early stages of succession (8 years following ground disturbance). Utilizing the niche con- cept allowed us to understand the relative importance of factors that lead to delayed or arrested succession in these forests. Linking ecological concept to restoration treatments provided a better understanding of under- lying processes and mechanisms, and will be helpful to determine in which situations such restoration practices can be applied successfully. Keywords : arrested succession, colihue, high-grading, old- growth forests, Nothofagus , regeneration. Introducción En muchas partes del mundo gran parte del bosque nativo ha sido transformado en plantaciones u otro uso de suelo o ha sido degradado (FAO 2010). En los bosques andinos de Chile, la gran cantidad de bos- ques degradados que existen se debe en su mayoría a cortas selectivas (i.e., extracción de árboles de alta calidad y rendimiento volumétrico, dejando sólo ár- boles de menor calidad o en malas condiciones sani- tarias y de forma; Donoso y Lara 1996, Bahamondez et al. 2009) y al poco interés en la regeneración de bosques (Soto 2017, Soto y Puettmann 2017). Por ejemplo, en Chile, se ha estimado que la regene- ración en bosques sólo ha tenido un escaso éxito luego de intervenidos (Donoso y Lara 1996, Lara et al . 2016), lo que sugiere una falta de resiliencia de estos (i.e. la capacidad del ecosistema para absorber el disturbio y mantener sus funciones). El fracaso de la regeneración en bosques intervenidos pareciera deberse, al menos parcialmente, a la expansión del bambú ( Chusquea culeou ) en los sotobosques. En es- tas condiciones, las especies de Chusquea a menudo crecen en densos matorrales que impiden la rege- neración natural y de otras especies de árborea por largos períodos de tiempo (Veblen et al . 1980, Veblen 1982, González et al . 2002). Bosques con árboles dañados o con poca vigorosidad y un amplio do- minio de bambú en el sotobosque son considerados indeseables ecológicamente, en términos de condi- ciones estructurales y composicionales (González et al . 2002, Reyes et al . 2013, Soto et al . 2015, Soto y