Linking tree community functional change with soil carbon dynamics during secondary succession in a naturally regenerating tropical forest in Panama

Naturally regenerating tropical forests are increasingly important for their role in the global carbon (C) balance, particularly due to their ability to rapidly sequester large amounts of C in aboveground biomass during forest regrowth. Over half of all C in tropical forests is stored belowground, yet in contrast to the predictable pattern aboveground, there is no clear pattern of soil C accumulation with time during forest regrowth, and we are therefore currently unable to predict and increase soil C sequestration during tropical forest regrowth. Soil C turnover and storage depends on the input of plant-derived organic matter which is likely to be affected by shifts in tree community resource-use strategy (functional group) during secondary succession from light-demanding to shade-tolerant species, and the corresponding reduction in litter quality. As tree community composition can differ between forest stands of similar ages, I hypothesised that tree community functional composition would better predict soil C dynamics during secondary tropical forest succession than stand age and specifically, that differences in litter quality between shade-tolerant and light-demanding tree species would influence rates of soil C turnover via litter decay rates and changes to the soil microbial community. The body of work presented in this thesis provides compelling evidence in support of my overarching hypothesis that soil C accumulation is more closely related to tree functional composition than forest age. My studies highlight some of the potential pathways by which tree community composition can influence soil C storage via plant-derived organic matter inputs representing substrate for the soil microbial community. Overall, the research presented in this thesis demonstrates that tree functional composition could be one of the main factors determining belowground C storage and therefore, my work represents an important first step towards using tree functional groups to predict soil C accumulation in secondary tropical forests

Litter inputs, but not litter diversity, maintain soil processes in degraded tropical forests — a cross-continental comparison

Land-use change in tropical forests can reduce biodiversity and ecosystem carbon (C) storage, but although changes in aboveground biomass C in human-modified tropical forests are well-documented, patterns in the dynamics and storage of C belowground are less well characterised. To address this, we used a reciprocal litter transplant experiment to assess litter decomposition and soil respiration under distinct litter types in forested or converted habitats in Panama, Central America, and in Sabah, Malaysian Borneo. The converted habitats comprised a large clearing on the Panama Canal and oil palm plantation in Borneo; forested habitats comprised a 60-year old secondary forest in Panama and a disturbed forest in Borneo that was selectively logged until 2008. In each habitat, we installed mesocosms and litterbags with litter collected from old-growth forest, secondary forest or an introduced species: Elaeis guineensis in Borneo and Saccharum spontaneum in Panama. We measured litter mass loss, soil respiration, and soil microbial biomass during nine months at each site. Decomposition differed markedly between habitat types and between forest vs. introduced litter, but the decay rates and properties of old-growth and secondary forest litters in the forest habitats were remarkably similar, even across continents. Slower decomposition of all litter types in the converted habitats was largely explained by microclimate, but the faster decay of introduced litter was linked to lower lignin content compared to the forest litter. Despite marked differences in litter properties and decomposition, there was no effect of litter type on soil respiration or microbial biomass. However, regardless of location, litter type, and differences in soil characteristics, we measured a similar decline in microbial activity and biomass in the absence of litter inputs. Interestingly, whereas microbial biomass and soil respiration increased substantially in response to litter inputs in the forested habitats and the converted habitat in Panama, there was little or no corresponding increase in the converted habitat in Borneo, indicating that soil recovery capacity had declined substantially in oil palm plantations. Overall, our results suggest that litter inputs are essential to preserve key soil processes, but litter diversity may be less important, especially in highly disturbed habitats

Litter Traits of Native and Non-Native Tropical Trees Influence Soil Carbon Dynamics in Timber Plantations in Panama

Tropical reforestation initiatives are widely recognized as a key strategy for mitigating rising atmospheric CO2 concentrations. Although rapid tree growth in young secondary forests and plantations sequesters large amounts of carbon (C) in biomass, the choice of tree species for reforestation projects is crucial, as species identity and diversity affect microbial activity and soil C cycling via plant litter inputs. The decay rate of litter is largely determined by its chemical and physical properties, and trait complementarity of diverse litter mixtures can produce non-additive effects, which facilitate or delay decomposition. Furthermore, microbial communities may preferentially decompose litter from native tree species (homefield advantage). Hence, information on how different tree species influence soil carbon dynamics could inform reforestation efforts to maximize soil C storage. We established a decomposition experiment in Panama, Central America, using mesocosms and litterbags in monoculture plantations of native species (Dalbergia retusa Hemsl. and Terminalia amazonia J.F.Gmel., Exell) or teak (Tectona grandis L.f.) to assess the influence of different litter types and litter mixtures on soil C dynamics. We used reciprocal litter transplant experiments to assess the homefield advantage and litter mixtures to determine facilitative or antagonistic effects on decomposition rates and soil respiration in all plantation types. Although litter properties explained some of the variation in decomposition, the microclimate and soil properties in the plantations also played an important role. Microbial biomass C and litter decomposition were lower in Tectona than in the native plantations. We observed non-additive effects of mixtures with Tectona and Dalbergia litter on both decomposition and soil respiration, but the effect depended on plantation type. Further, there was a homefield disadvantage for soil respiration in Tectona and Terminalia plantations. Our results suggest that tree species diversity plays an important role in the resilience of tropical soils and that plantations with native tree species could help maintain key processes involved in soil carbon sequestration

Soil carbon storage is related to tree functional composition in naturally regenerating tropical forests

1. Regenerating tropical forests are increasingly important for their role in the global carbon cycle. Carbon stocks in aboveground biomass can recover to old-growth forest levels within 60-100 years. However more than half of all carbon in tropical forests is stored belowground, and our understanding of carbon storage in soils during tropical forest recovery is limited. 2. Importantly, soil carbon accumulation does not necessarily reflect patterns in aboveground biomass carbon accrual during secondary forest succession and factors related to past land-use, species composition, and soil characteristics may influence soil carbon storage during forest regrowth. 3. Using tree census data and a measure of tree community shade tolerance (species-specific light response values), we assessed the relationship between soil organic carbon stocks and tree functional groups during secondary succession along a chronosequence of 40–120-year-old naturally regenerating secondary forest and old-growth tropical forest stands in Panama. 4. Whereas previous studies found no evidence for increasing soil C storage with secondary forest age, we found a strong relationship between tree functional composition and soil carbon stocks at 0-10 cm depth, whereby carbon stocks increased with the relative influence of light-demanding tree species. Light demanding trees had higher leaf nitrogen but lower leaf density than shade-tolerant trees, suggesting that rapid decomposition of nutrient-rich plant material in forests with a higher proportion of light-demanding species results in greater accumulation of carbon in the surface layer of soils. 5. Synthesis. We propose that soil carbon storage in secondary tropical forests is more strongly linked to tree functional composition than forest age, and that the persistence of long-lived pioneer trees could enhance soil carbon storage as forests age. Considering shifts in tree functional groups could improve estimates of carbon sequestration potential for climate change mitigation by tropical forest regrowth

Soil carbon dynamics are linked to tree species growth strategy in a naturally regenerating tropical forest

Secondary tropical forests are increasingly important for their role in the global carbon (C) balance as they can rapidly accumulate aboveground biomass C during regrowth. Substantial amounts of plant-derived carbon are also incorporated into the soil through decomposition processes, but our understanding of soil C dynamics during forest regrowth is limited. Secondary succession is characterised by a shift in tree functional groups from light-demanding to shade-tolerant species over time, which can influence rates of C turnover via differences in litter quality and by modifying the decomposition environment. Changes in decomposition processes in turn affect the amount of organic C stored in the soil or released to the atmosphere as CO2. Consequently, understanding how tree functional composition influences C turnover during decomposition could help us predict soil C storage during tropical forest regrowth. We experimentally explored the relationship between tree functional groups and soil C dynamics (decomposition and respiration) by conducting a litter decomposition experiment across a successional gradient of naturally regenerating tropical forest. We created litter mixtures representing tree communities differing in their shade tolerance, as well as a functionally diverse litter mixture, and observed litter mass loss and soil respiration as measures of C turnover over a 6 month period. Litter from light-demanding species decomposed faster than litter from shade-tolerant species, which was reflected in the pattern of soil respiration. There were no clear patterns of increasing or decreasing rates of litter decay or soil respiration with increasing forest age, but there was an interaction between stand age and litter type which influenced both decomposition and soil respiration rates. Interestingly, soil respiration from the functionally diverse litter mixture was significantly higher in the younger than older forest stands, and the functionally diverse litter mixture decayed more rapidly than expected in one of the younger stands. Our findings highlight the potential importance of functionally diverse plant inputs, as well as the interaction between local environmental attributes and litter type, for soil C dynamics in tropical forests