Regeneration and recovery dynamics of logged forests in the Solomon Islands

Logging is among the major drivers of deforestation and forest degradation in tropical landscapes globally. Its enduring persistence over the past century was driven by the high demand for tropical wood. Tropical forests in the Solomon Islands have been heavily logged over the last century, but very little is known about the recovery dynamics and ecology of logged forests in this region. Furthermore, it is still unknown whether these forests can return to a pre-cut state via natural regeneration processes. In this thesis, the effects of industrial selective logging on forest floristics, functions and structure are assessed in the Solomon Islands. Furthermore, factors influencing the recovery of these attributes and their implications in forest management and conservation are considered. This thesis initially assesses the rich plant biodiversity in Oceania, in Chapter 2. Results from this study suggest that highly varied landscapes in Papua New Guinea contain high plant endemism and beta diversity via intense segregation and establishment of locally varied vegetation communities. Therefore, to effectively conserve biodiversity in such forests, protection must include landscapes that best represent the full range of topographic and edaphic variability throughout the island to account for locally endemic species restricted to specific ecological niches. Chapter 3 reviews the differences between forest-gap characteristics and gap-phase regeneration in natural and anthropogenically created forest gaps in tropical forests. It also evaluates whether or not forest degradation can provide an opportunity for generating floral compositions in rainforests that might be better adapted to continually changing environmental conditions. A review of the dynamics of logging in the Solomon Islands in chapter 4 highlights how the socioeconomic dynamics in this small developing nation promoted excessive logging, resulting in highly degraded forests. The key elements discussed here are the roles of (i) economic interests, (ii) corruption, (iii) poor employment conditions in the logging sector, (iv) high forest accessibility, (v) resource limitations for forest monitoring, (vi) disputes over logging benefits, and (vii) a paucity of information for policy development. The review also suggests that some logged forests in the Solomon Islands may require active restoration—especially those that have been most heavily damaged. Chapter 5 compares the taxonomic diversity and composition of trees between unlogged forest and sites that were logged 10, 30 and 50 years previously to evaluate the floristic dynamics of these forests following timber harvesting. The results reveal that distance to logging roads and to unlogged forest influenced postlogging recovery, emphasising the importance of edge effects in influencing forest composition. At least in the first 50 years after logging, tree-community composition did not appear to converge over time toward that in unlogged forests. Although species assemblages in previously logged forests generally tend to shift over time from light demanding-pioneers to old-growth species, a long-lived pioneer Campnosperma brevipetiolata dominated the forest even 50 years after logging. It was suggested that the persistence of C. brevipetiolata may have hindered the recovery of tree communities in logged forests. The removal and harvesting of such persistent, long-lived pioneer trees via careful silviculture techniques could potentially help to accelerate recovery of heavily logged forests. Chapter 6 explores the influence of light on the occurrence of understory plant functional traits and diversity in logged forests. Plant functional types–growth form, chlorotype and leaf area were used to predict the development of post-logging understory plant communities as such functional types show strong links with disturbance history and capture interspecific variation in strategies that drive plant performance. The response of the plant-trait elements of each plant functional types to changing light levels was determined across post-logging recovery times of 10, 30 and 50 years. The highly varied associations between the influence of light and the occurrence of each plant-trait element across recovery times revealed in the results suggest that plant-trait elements are closely linked to a range of environmental response strategies. Functional diversity can fully recover to pre-cut levels if logged forests are given ample time to regenerate, suggesting that the functional diversity of forests in logged Oceanic forests can be highly resilient under effective management regimes. Factors influencing the recovery of tree biomass, wood density and understory tree-specific leaf area (SLA) in logged forests were investigated in Chapter 7. Harvest intensity, tree abundance and liana abundance influenced tree biomass and wood density, whereas elevation and ground litter affected understory tree SLA during post-logging recovery. Tree biomass and wood density did not recover to pre-cut levels after 50 years of post-logging regeneration whereas SLA values were similar among logged-forest time classes and unlogged forests. The full recovery of tree biomass and wood density within 50 years of logging is unlikely in the Solomon Islands. Improving the recovery of wood functional traits in logged forests in this region requires the development and implementation of policies that regulate initial harvest intensity and re-entry harvesting in post-logged forests. Chapter 8 examines factors that influenced the recovery of several forest 'keystone structures' during 50 years of post-logging regeneration in the rainforests of Kolombangara Island. The results reveal that half a century of post-logging regeneration has been insufficient for full structural recovery of logged forests on Kolombangara. The results also suggest that proxies of logging, soil and tree attributes were more important than topographical factors in influencing the recovery of keystone structures over time. Based on these results, it was concluded that logging practices in the Solomon Islands must be regulated by more rigorous and effective management policies in order to facilitate full structural recovery of forests. Finally, Chapter 9 presents a 'one–stop shop' for forest-restoration practitioners. This chapter discusses forest-restoration approaches that have been successfully implemented in tropical forests, and recommends an integrative approach whereby forest-restoration techniques are amalgamated for potentially better outcomes in heavily logged forests. Because of the high intensity of logging on Kolombangara, natural regeneration alone is inadequate to permit full recovery of forests to pre-cut levels within 50 years since logging. The findings presented in this thesis can be used for both precautionary and remedial approaches to manage logged forests in the Solomon Islands. Nevertheless, much more information is required in terms of research and policy development to ensure that these and other logged forests in Oceania are appropriately managed to balance important economic and conservation outcomes.