Restoring Rainforest – the Capacities of Three Different Reforestation Pathways to Re-establish Ecosystem Properties

The broad scale destruction of tropical and subtropical rainforests has been one of the largest land-cover conversions taking place on earth, with a wide range of deleterious consequences at local, landscape, and global scales. While the resulting loss of biodiversity and habitat for rainforest-dependent fauna and flora has been well-studied as a major effect, clearing of rainforests also significantly influences soil processes such as biochemical cycles and microbial functions. More recently, there has been a growing public interest in reforestation activities. There are a range of different pathways by which rainforest cover can be restored to cleared land, including autogenic, or „natural‟, regrowth, management of this autogenic regrowth, and tree planting for ecological restoration. However, little is known about the recovery processes of ecosystem properties under different reforestation pathways in the same landscape. The broad objective of this thesis was to assess both the effects of deforestation on a range of ecosystem processes and the potential of different reforestation pathways to restore these processes. The study was conducted in the Big Scrub area in subtropical eastern Australia, a basaltic plateau that once supported the continent‟s largest continuous stand of lowland subtropical rainforest, which was mostly cleared for pasture in the mid to late 19th century. In this landscape, the properties of five site-types were compared, with five replicate sites in each. The site-types consisted of two reference conditions, pasture and intact rainforest, and three different reforestation pathways. These pathways were: autogenic regrowth dominated by the non-native tree species camphor laurel (Cinnamomum camphora); similar regrowth managed in order to remove the camphor laurel and release the growth of recruited rainforest seedlings; and ecological restoration plantations. Camphor laurel is a dominant species in the Big Scrub region, where it readily colonises abandoned pastures and is known to facilitate the recruitment of later successional rainforest tree species. In ecological restoration plantings, a high diversity of native rainforest tree seedlings is planted to restore biodiversity. The main ecosystem properties studied within the sites were as follows: 1. size and composition of viable soil seed banks; and 2. soil physical properties and nutrient cycles. This study also assessed the consequences of altered soil properties and nutrient dynamics for the early growth of rainforest pioneer seedlings. To assess the effects of deforestation and reforestation on soil seed banks, germination experiments were conducted using soil from all five site-types (three reforestation pathways, pasture, and rainforest sites). Germination trays were positioned in a shade-house and seedling emergence was monitored over a period of six months. Germinated seedlings were classified into functional groups according to their life-form, origin, successional stage, and dispersal mode. Additionally, floristic data from a subset of the study sites was used to examine how the seed bank was related to the standing vegetation. Despite a much larger total abundance of seeds in seed banks from pasture sites, these sites contained very few native woody plants and were dominated by grasses and herbaceous species. Further, seed banks of reforested and rainforest sites were mainly composed of pioneer and early secondary species, whereas late secondary and mature phase species appeared almost solely in the standing vegetation. The abundance and diversity of most of the functional groups that were impacted by deforestation showed values similar to rainforest in at least one of the three reforestation pathways; whilst the three pathways differed only slightly in their capacities to restore soil seed banks. In the initial phases of reforestation, however, seed banks play no vital role after long-term pasture establishment. The effects of deforestation and reforestation on the physical and biochemical properties of soil were tested by measuring a range of properties in soil samples from all five site-types. The main emphasis was placed on carbon- and nitrogen-related soil properties, as they are major nutrients in terrestrial ecosystems. Before these data were collected, a study was performed in a subset of the sites to identify the variation of soil properties at different spatial scales (subplot, site, and site-type level) and to develop a spatial design for the collection of soil samples within sites. For each of the five site-types, two replicate sites were measured, with 16 subplots in each site. Subplots were seven cm in diameter and regularly aligned with a spacing of 10 m. The seven soil properties measured in this soil variability study were: gravimetric water content; soil organic matter; pH; total organic carbon; microbial biomass carbon; nitrate-nitrogen, and nitrification rate. Across all sites, water content, soil organic matter, and pH showed a consistently low variability at all three spatial scales. In contrast, soil properties related to microbial processes exhibited higher degrees of spatial variability at the site and the subplot level. However, even in soil properties with a high tendency for spatial variability, the physical mixing of subsamples from subplots within a site, in contrast to analysing subsamples individually, could be validated as a useful technique to reduce analytical effort and cost. In the main study of soil properties under deforestation and reforestation, 19 properties were measured at each of five sites in each of the five site-types (the same 25 sites used for the seedling germination experiment). These properties consisted of: eight nitrogen-related variables (total nitrogen (N), ammonium-N, nitrate-N, total inorganic N, plant-available ammonium-N, plant-available nitrate-N, nitrification rates, and denitrification rates); six carbon-related properties (total carbon (C), total organic carbon, soil organic matter, 13C value, microbial biomass carbon, and soil microbial activity); and five general soil properties (gravimetric water content, pH, bulk density, fine root biomass, and plant-available phosphate). Of the 19 soil properties, nine differed significantly between rainforest and pasture. Nitrate-N levels, plant-available nitrate-N levels, nitrification rates, and fine root biomass were significantly greater in rainforest than in pasture sites, while plant-available ammonium-N levels, 13C values, pH, bulk density, and plant-available phosphate concentrations showed greater levels in pasture sites. Apart from fine root biomass, all of these soil properties were re-established to a level similar to that in rainforest in at least one of the three reforestation pathways. However, the capacity to re-establish soil properties varied among the three reforestation pathways. For example, autogenic regrowth dominated by camphor laurel showed a good recovery of nitrification, ammonium, and phosphate levels, but did not significantly facilitate the re-establishment of nitrate-N and bulk density. The impacts of soil properties – and hence deforestation and reforestation – on early seedling development were tested by measuring the growth of rainforest pioneer seedlings in soils collected from the three different reforestation pathways, as well as from pasture and rainforest soils. Three species, Alphitonia excelsa, Guioa semiglauca, and Omalanthus nutans, all fast-growing pioneer species that are common in the Big Scrub region, were chosen for this study. The seedlings were kept in a shade-house over a period of about seven months, and height and diameter were measured at regular time intervals. Although the three species varied significantly in height and diameter growth, they responded similarly to the five site-types, with generally lower growth rates in untreated autogenic regrowth and higher rates in soils from all other site-types, including pasture. However, there was little evidence that seedling performance was directly influenced by soil properties. Across all three species and all 25 sites, seedling growth rates.