This study examines the way nutrients are used and recycled in natural rainforests and in two mature plantation systems established on former rainforest sites. Studies were carried out at adjacent mature (ca. 60 years old) forest plantations of hoop pine (Araucaria cunninghamii Aiton ex D. Don) and Queensland maple (Flindersia brayleyana F Muell.) - and secondary natural rainforests at three sites (i.e. Wongabel State Forest, Gadgarra State Foret, and Danbulla State Forest) in the wet tropics of north Queensland. Some degrees of understorey development had occurred in each of the plantations so that although nominally monocultures, several had other trees present in the canopy layers. Ecosystem structure and functioning on each of the three forest types at the three different sites were assessed in terms of tree biomass and production, litter production and decomposition, fine root dynamics, and nutrient dynamics.
Tree biomass was largest in the Araucaria plantations, whereas those of the Flindersia plantations and natural rainforests were in similar levels. Forest productivity was high in both the natural rainforests and the Araucaria plantations and was lower in the Flindersia plantations. Of the three sites, the wet Gadgarra forests supported the greatest tree biomass and productivity while the more fertile but drier Wongabel forests had the least. The rate of litterfall matched differences in productivity and was higher in the Araucaria plantations and natural rainforests compared to the Flindersia plantations; it was also highest at the Gadgarra sites and lowest at Wongabel sites. Unlike the rate of litter production, the rate of litter decomposition was relatively similar in the environments provided by the three different forest types and sites. However, the decomposition rates were found to differ in the litter from different species. This difference was attributed mainly to differences in their initial C:N and lignin:N ratios. The three forests also differed in below-ground biomass and productivity. While above-ground productivity was generally lowest in the Flindersia plantations (compared to the other two forest types), root development (including fine root biomass, root length density, fine root turnover) was found to be the highest in these plantations. By contrast, the Araucaria plantations exhibited the lowest fine root biomass and root length density but had the highest fine root productivity and specific root length. Across the sites, fine root biomass and root length density were highest at the Danbulla sites and lowest at the Wongabel sites. Meanwhile, the Gadgarra sites supported the highest rates in both fine root production and turnover. Root nutrient contents and turnover were generally higher in both the Flindersia plantations and natural rainforests as opposed to those in the Araucaria plantations, also higher at the Wongabel sites and lower at the Danbulla sites. The degree of mycorrhizal infection was similarly greater in the Araucaria plantations and natural rainforests, and lower in the Flindersia plantations; as was also greater for the forests at Danbulla than those at Wongabel and Gadgarra.
Since each forest was found at each site and soil type, it was possible to make comparisons of the changes to nutrient availability in topsoils at each site as a consequence of the types of forest present. The availability of N and P was higher in the soils supporting the natural rainforests and Flindersia plantations than in soils occupied by the Araucaria plantations. This comparison was based on several measures including foliar nutrient concentration, nutrient retranslocation rates, and nutrient use efficiency There was also strong positive relationship between tree diversity and available nitrogen. In addition, the Araucaria plantations had lower N and P in the topsoil microbial biomass.
The findings of this study indicate that the different forests measured had contrasting patterns of biomass accumulation, productivity, and nutrient cycling; nevertheless, the differences were not based simply on plantation forests versus natural forests. Rather there was a contrast between the conifer and the broadleaf forests. Furthermore, ecosystem functional processes that are directly affected by soil nutrient status such as litter decomposition, nutrient availability and efficiency - were more related to species diversity present at each site. On the other hand, ecosystem attributes such as forest biomass and productivity were related more to age and physical environment, such as water availability. This suggests that initial productivity might be best achieved by monocultures but ecosystem processes that sustain ecosystem function are fostered more effectively when some modest levels of diversity are present. The study also suggests that species choice is crucial in establishing a productive and sustainable forest plantation.