Patterns of secondary forest recovery in two soil types

Tropical forests are biologically diverse and vital for their contribution to global biogeochemical cycles. However, agricultural expansion has led to the devastation of more than half of forests worldwide. In order to accelerate forest regeneration to provide habitat and ecosystem services, we need to observe what limits natural recovery in order to understand the regeneration process. In that context, secondary forests provide a means to increase forested area. Here, I refer to secondary forests as the spontaneous regrowth of vegetation after major disturbances without human intervention. By studying the regeneration of degraded lands, ecologists should be able to observe patterns of changes, make predictions and determine the potential for recovery and, if necessary, propose interventions to accelerate the recovery process. Soil condition is one major factor expected to limit forest recovery. However, the resilience of different types of soil to disturbance is still a matter of uncertainty. Chemical, physical and biological alteration could result in negative impacts on plant growth, as well the structure and composition of plant communities, with potential consequences for pathways of secondary forest recovery. In this context, my doctoral thesis investigates the patterns of secondary forest recovery in two contrasting soil types nearby the Australian Wet Tropics bioregion. Along a total 45 sites (33 secondary forest, 6 pastures and 6 mature forest sites), I documented changes in soil properties, woody plant vegetation, soil organic carbon stocks and arbuscular mycorrhizal communities and asked whether the different substrates affect the pathways of secondary forest recovery. To achieve my goals, I prepared a literature review where I observed that numbers of species in long chronosequences (>40 years) tended to plateau following a rapid increase in the first 20 years after land abandonment, but found only nine chronosequences that were older than 45 years that fitted the selection criteria. Even though the number of species tended to increase with forest age, more than half of the studies (56%) reported low floristic similarity between the oldest secondary forest and mature phase forests. Soil fertility and parent material were only directly investigated in four studies, indicating the scarcity of evidence on the relationship between soil properties and secondary forest regeneration. To answer my research questions, I collected extensive field data to address questions on the above- and below-ground changes in the biophysical environment after pasture abandonment. Firstly, I used as response variables, nine measures of woody vegetation recovery, representing structural, functional and compositional parameters to demonstrate that species diversity and species dominance showed distinct patterns of recovery in the two soil types. The community composition was also significantly different between basalt and granite soils. I conclude that time since abandonment is crucial for secondary forest recovery, however, soil types may influence the composition of the community that is able to reestablish. Spatial distribution of surrounding mature forest may also affect the rate of recovery. Contrary to my expectation, the poorer and sandier granite-based soil showed faster rates of species recovery compared with the more nutrient-rich basalt soils. Increasing woody species diversity was associated with increasing dominance, which may indicate a positive effect of the early establishment of dominant species on later successional species. The effect of soil type suggested that soil condition was important for the rate of forest recovery, yet in needs to be considered in context with the greater landscape as in a parallel study my collaborators and I found that distance to mature forests was also important for the forest recovery rates. Secondly, I was interested in the below-ground recovery of secondary forests, with a special focus on the most common plant mutualist: the arbuscular mycorrhizal fungi. In this chapter, I investigated changes in the arbuscular mycorrhizal fungi (AMF) community of secondary forests and their association with edaphic and vegetation properties. Despite the importance of AMF for plant nutrition and soil structure, little is known about their diversity in successional forests and the environmental factors that shape AMF communities during succession. I found that mature forest on the poorer soil had significantly higher AMF richness than mature forests on the more fertile soil. However, secondary forests on the more fertile soil tended to present higher AMF richness compared to the secondary forests on the poorer soils. The relatively greater species richness in mature forest on granite derived soils agrees with predictions that plant communities in less fertile soils are more dependent on mycorrhizal associations, however in secondary forests the trend did not hold. Soil pH and total phosphorous were the best environmental predictors of AMF richness and community composition. Thirdly, I investigated another below ground component, which represents an important ecosystem function played by forest soils: the soil organic carbon stocks. For this study, I estimated soil carbon stocks in the two soil types to allow comparisons before and after pasture abandonment. Secondary forests around the world have been found to recover aboveground biomass stocks relatively fast compared with species diversity and composition. However, soil organic carbon stocks do not always replicate stand age or above-ground biomass accrual. I asked whether soil and vegetation parameters are better predictors of soil carbon stocks than secondary forest age and if soil type influenced these relationships. I found that, in comparison with basalt soils, granite soils tend to accumulate SOC stocks and recover their isotopic composition at faster rates during forest succession. However, basalt soils tend to maintain higher SOC stocks after pasture abandonment. SOC variation was best explained by a model that included soil pH, woody species diversity and soil type, with significant changes mostly driven by sites on granitic soils. These results support the idea that predictions of SOC stocks can be improved with the inclusion of basic information on vegetation cover and soil type. To summarise, I found that nutritional and textural differences between soil types remain during forest succession, indicating that the major edaphic properties of some parent materials remain little or unaffected after long-term agricultural use. As expected, woody plant communities, mycorrhizal fungi, and soil carbon stocks seem to respond to such soil type differences at local scales, even though the variability within sites and successional categories suggest a strong idiosyncratic nature to secondary forest regeneration processes. Continued investigation of the interactions of plants, fungi and the mechanisms that stabilise C in the soil will likely produce further theoretical and practical ecological evidence to improve the understanding of secondary forest regeneration. Although primary forests may be irreplaceable, abandoned lands have a great capacity to become biologically and functionally rich systems again.