Tree regeneration after fire and logging in sub-alpine forest on the southeastern Tibetan Plateau

Abstract

Forest ecosystems provide environmental services that benefit human communities. Despite their importance, forests have experienced deforestation and land-use changes that are particularly severe in the tropics and subtropics. Increasing concern about forest sustainability management and conservation supports the need for studies on forest recovery after deforestation. Many studies have estimated that recovery of forest structure, species composition and richness takes more than 50 years to reach a state similar to that of adjacent undisturbed forests. To date, most of these studies were carried out in the tropics and at high latitudes. However, less is known about the recovery process of sub-alpine forest after deforestation. The natural sub-alpine forest on the southeastern Tibetan Plateau is an important ecosystem, which ranges from 3000 to 4400 m above the sea level (a.s.l.), and these forests were disturbed by human before 1998. In the forests near the town of Lulang, Smith fir ( Abies georgei var. smithii ) is the dominant tree species above 3500 m a.s. l. The climate is mainly influenced by the Indian monsoon and westerlies. In summer, the Indian monsoon brings abundant precipitation to our study area. The winter climate is controlled by the westerlies producing cold and dry conditions. According to the instrumental record from the Nyingchi meteorological station, mean annual temperature and total annual precipitation are 8.9°C and 672.7 mm, respectively. This area has experienced a significant warming trend during the past five decades. Our study focused on logged and burnt sites of Smith fir forests (3460–3865 m a.s. l.) near Lulang in Nyingchi. Three and five rectangular plots (30 m ´ 30 m) were established in the logged (azimuth angle: 326°–328°) and fire-affected (azimuth angle: 243°–260°) gaps, respectively. Global Positioning System (GPS) was used to record the locations of the 8 plots. For each plot, the upper and lower sides were perpendicular to the direction of slope aspect. The lower-left side of each plot was regarded as the origin for each sampling grid. To assess the regeneration in each plot, we tagged all of the trees and measured their position coordinates ( x and y ). The tree species were also recorded. The ages of the gaps were estimated by taking increment cores from nearby adult trees surrounding the gap and examining the synchronous and abrupt growth release events. These cores were then air dried, carefully sanded, and crossdated in the lab. The ring-width series were measured under the Lintab 6 system (Rinntech, Heidelberg, Germany). We used several methods to estimate the age of each tree. For the seedlings and saplings, we estimated their age by counting the number of bud scars along the main stem. For the adult trees, we used the increment borer to get a core (close to the pith) at the base of the trunk. The ages of these samples were obtained by counting the number of rings under the stereomicroscope. The date of the burnt gap formation was estimated as 1942 and the logging gap formed in 1991, both based on abrupt growth release of nearby adult trees surrounding the gaps after fire and logging. According to the surveyed data and tree age, we obtained the decadal recruitment series for each tree species. In the burnt gaps (southwestern facing slope), we found that the regenerated tree species mainly consist of spruce ( Picea likiangensis var . linzhiensis ), juniper ( Sabina squamata ) and oak ( Quercus aquifolides ). In the logged site (northwestern facing slope), the regenerated tree species is mainly Smith fir. Compared with the natural Smith fir forest, the tree density is lower in the burnt and logged sites. In summary, once the forest on the southeastern Tibetan Plateau is logged or burnt, it will take longer time to recover to undisturbed Smith fir forest.