Above- and belowground carbon stocks and effects of enrichment planting in a tropical secondary lowland dipterocarp rainforest

Sammanfattning: The intact tropical rainforests are rapidly being degraded and subsequently converted to other land uses, with associated greenhouse gas emissions and loss of biodiversity. It is imperative that the effects of such conversions and large-scale restoration efforts on forest structure and ecosystem services are understood to effectively be able to counteract the negative consequences of deforestation and forest degradation. Assisted regeneration by line planting is one such restoration method that have been used in degraded forests. Here I studied a chronosequence of 0-19 years since planting in a secondary lowland dipterocarp forest in Sabah, Malaysian Borneo which was selectively logged in the 1970s and subsequently burned at varying intensity in the El Niño fires 1983-1984 resulting in forests that are in arrested in early stages of succession. The primary focus of this study was the assessment of above- and belowground carbon in total, in different carbon pools and by functional species group (dipterocarps, fruit trees, pioneers and other commercial) in a secondary rainforest, as well as assessing the potential influence of assisted regeneration through enrichment line planting on these carbon pools as well as on tree diversity. I found no significant relationship in total carbon, carbon in different pools or carbon in different functional species groups and time since planting. Also, there was no significant difference in tree diversity or species diversity between treated and untreated control plots. Combining all 12 (60 x 60 m) plots, the mean total carbon stock (± SE) was estimated to 231.4 ± 11.2 Mg C ha-1. This includes aboveground carbon pools: tree aboveground carbon (TAGC: 44.0%, 101.7 ± 8.5 Mg C ha-1), woody debris (3.4%, 7.9 ± 1.5 Mg C ha-1), standing dead wood (2.0%, 4.5 ± 1.0 Mg C ha-1), fine ground litter (FGL: 0.8%, 2.0 ± 0.1 Mg C ha-1), lianas (0.6%, 1.4 ± 0.4 Mg C ha-1) and belowground: soil organic carbon (SOC: 36.2%, 83.8 ± 8.2 Mg C ha-1), tree belowground carbon (TBGC: 9.3%, 21.6 ± 2.1 Mg C ha-1), fine & coarse roots (3.6%, 8.4 ± 2.1 Mg C ha-1). When testing for correlations of effects over time since treatment by linear regression analyses, the applied treatment was not found to significantly improve carbon storage in total, by carbon pools or by functional species groups (p > 0.05), nor was it found to improve overall tree diversity or species richness (p > 0.05). However, between the treated and untreated control plots, there was a 10% (~20 Mg C ha-1) increase in total carbon storage, which indicates that the treatment might still have a positive effect on carbon sequestration. Therefore, I performed a power analyses, which indicated that to significantly detect a such an effect (with a power of 0.8), I would have needed 5.5 times the number of plots. Additionally, soil edaphic factors (e.g. nutrients and texture) appeared to influence aboveground forest structure, both in terms of carbon storage and stem density, and may be contributing factors to why no clear positive effect of restoration was detected. For the twin goals of climate change mitigation and biodiversity retention, further study should be devoted to understanding the effects of restoration methods on secondary tropical rainforests and to what extent edaphic factors may influence aboveground forest structure.