Hoop Pine Plantations Research Articles

Habitat and sex effects on behaviour in fawn-footed mosaic-tailed rats (Melomys cervinipes)

Habitat complexity reflects resource availability and predation pressure – both factors that influence behaviour. We investigated whether exploratory behaviour and activity varied in fawn-footed mosaic-tailed rats (Melomys cervinipes) from two habitats that were categorised differently based on vegetation. We conducted vegetation surveys to determine structural complexity and vegetation cover, confirming that an abandoned hoop-pine (Araucaria cunninghami) plantation forest was structurally less complex, with lower vegetation cover than a variable secondary rainforest. We then tested mosaic-tailed rats from both sites in four behavioural tests designed to assess exploratory and activity behaviours (open field, novel object, light-dark box, acoustic startle), predicting that rats from the less structurally complex habitat would be less exploratory, and show lower activity. Our results provide some evidence for a context-specific trade-off between exploratory behaviour and predation risk in rats from the abandoned hoop pine plantation, as rats were less active, and showed a freezing strategy in the light-dark box. We also found context-specific sex differences in behaviour in response to a novel object and sound. Our results suggest that small-scale variation in habitat structure and complexity, as well as sex differences, is associated with variation in behaviour, most likely through effects on resource availability and/or predation risk.

Long-term impacts of soil compaction and cultivation on soil carbon and nitrogen pools, foliar δ13C and δ15N as well as tree growth in a hoop pine plantation of subtropical Australia

The aim of this study was to examine the long-term impacts of mechanical compaction and cultivation on soil C and N pools, δ13C and δ15N of soil and foliage as well as tree growth. In a hoop pine plantation of subtropical Australia, the diameter at breast height over bark (DBHoB) and tree height; δ13C and δ15N of soil and foliage; and soil C and N pools were determined after 17 years following forwarder compaction (control, light and heavy) and cultivation (control and disc plough) treatments. Light compaction was associated with the significantly higher soil total carbon (TC) and total nitrogen (TN) in the 20–30 cm soil and higher TN in the 30–60 cm soil (p < 0.01). Cultivation was linked to the significantly higher soil TC in the 0–10 cm soil, and TC, TN and hot water-extractable total nitrogen (HWETN) in the 10–20 cm soil (p < 0.01). Higher δ15N in the 0–10 cm soil and in the foliage in the heavily compacted plots was due to previous soil denitrification, while light compaction and cultivation were associated with the lower foliar δ13C due to decreased foliar N concentration and reduced photosynthesis rate with increasing foliar biomass under higher N supply. The δ13C and δ15N of foliage and soil were useful indicators of the C and N processes in plant-soil system in response of management impacts in the plantation. Light compaction and cultivation generally had beneficial impacts on soil C and N pools over long term, whereas there was no difference in the impacts of different compaction and cultivation treatments on tree growth.

Patterns of seed bank and size asymmetry of plant growth across varying sites in the invasive Lantana camara L. (Verbenaceae)

Lantana camara L. (Verbenaceae) is a weed of great significance in Australia and worldwide, but little is known about connections among components of its life history. We document over a 3-year period, the links between L. camara seed-bank dynamics and its above-ground growth, including size asymmetry in four land-use types (a farm, a hoop pine plantation and two open eucalypt forests) invaded by the weed near Brisbane, Queensland Australia. Seed-bank populations varied appreciably across sites and in response to rainfall and control measures, and they were higher (~1,000 seeds/m2) when annual rainfall was 15–30 % below the long-term yearly average. Fire reduced seed-bank populations but not the proportion germinating (6–8 %). Nearly a quarter of fresh seeds remain germinable after 3 years of soil burial. For small seedlings (<10 cm high), the expected trade-offs in two life-history traits—survival and growth—did not apply; rather the observed positive association between these two traits, coupled with a persistent seed-bank population could contribute to the invasiveness of the plant. Relationships between absolute growth rate and initial plant size (crown volume) were positively linear, suggesting that most populations are still at varying stages of the exponential phase of the sigmoid growth; this trend also suggests that at most sites and despite increasing stand density and limiting environmental resources of light and soil moisture, lantana growth is inversely size asymmetric. From the observed changes in measures of plant size inequality, asymmetric competition appeared limited in all the infestations surveyed.

A 13C NMR study of decomposing logging residues in an Australian hoop pine plantation

Residue retention is important for nutrient and water economy in subtropical plantation forests. We examined decomposing hoop pine (Araucaria cunninghamii Ait. Ex D. Don) residues—foliage, branches, and stem wood—to determine the changes in structural chemistry that occur during decomposition. Residues were incubated in situ using 0.05 m2 microplots. We used solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to determine the structural composition of harvest residues in the first 24 months of decomposition. The spectral data for branch and stem residues were generally similar to one another and showed few changes during decomposition. The lignin content of branch and foliage residues decreased during decomposition. When residues were mixed together during decomposition, the O-alkyl fraction of foliage decreased initially then increased up to 24 months, while the alkyl carbon (C) fraction exhibited the opposite pattern. The decomposition of woody hoop pine residues (branch and stem wood) is surprisingly uniform across the major C forms elucidated with 13C NMR, with little evidence of preferential decomposition. When mixed with branch and stem materials, foliage residues showed significant short- and long-term compositional changes. This synergistic effect may be due to the C/N ratio of the treatments and the structure of the microbial decomposer community. Twenty-four months of decomposition of hoop pine residues did not result in substantial accumulation of recalcitrant C forms, suggesting that they may not contribute to long-term C sequestration.

Modeling population growth and site specific control of the invasive Lantana camara L. (Verbenaceae) under differing fire regimes

AbstractIt is at the population level that an invasion either fails or succeeds. Lantana camara L. (Verbenaceae) is a weed of great significance in Queensland Australia and globally but its whole life‐history ecology is poorly known. Here we used 3 years of field data across four land use types (farm, hoop pine plantation and two open eucalyptus forests, including one with a triennial fire regime) to parameterise the weed's vital rates and develop size‐structured matrix models. Lantanacamara in its re‐colonization phase, as observed in the recently cleared hoop pine plantation, was projected to increase more rapidly (annual growth rate, λ = 3.80) than at the other three sites (λ 1.88–2.71). Elasticity analyses indicated that growth contributed more (64.6 %) to λ than fecundity (18.5 %) or survival (15.5 %), while across size groups, the contribution was of the order: juvenile (19–27 %) ≥ seed (17–28 %) ≥ seedling (16–25 %) &gt; small adult (4–26 %) ≥ medium adult (7–20 %) &gt; large adult (0–20 %). From a control perspective it is difficult to determine a single weak point in the life cycle of lantana that might be exploited to reduce growth below a sustaining rate. The triennial fire regime applied did not alter the population elasticity structure nor resulted in local control of the weed. However, simulations showed that, except for the farm population, periodic burning could work within 4–10 years for control of the weed, but fire frequency should increase to at least once every 2 years. For the farm, site‐specific control may be achieved by 15 years if the biennial fire frequency is tempered with increased burning intensity.

Stand dynamics and spatial patterns across varying sites in the invasive Lantana camara L. (Verbenaceae)

As with many invasive plant species, little is known of the population spatial patterns and stand dynamics of Lantana camara L. (Verbenaceae)—a thicket-forming weed of worldwide significance in managed and conservation lands, including coastal and inland habitats of Eastern Australia. Consequently, we mapped and followed annually for 3 years the demographic fate of more than 2000 Lantana individuals at sites with four land-uses (hoop pine plantation, cattle farm, and two eucalyptus forests with occasional grazing and periodic burning regime, respectively) in Queensland, SE Australia. Populations exhibited plant size distributions that were continuous (i.e., of L or symmetric type) and unimodal, except the farm population where bimodality was observed. Newly established plants could be reproductive within one growing season at ~50 cm in height, especially where environmental resources were not limiting. Density had an appreciable effect on the weed’s reproductive capacity and growth, but not on survival. Established and newly recruited individuals were aggregated but the degree of aggregation decreased with plant size. However, in the sites that had experienced burning or mechanical clearing, Lantana seedling/juvenile recruitment assumed negative association (spatial displacement) in relation to established individuals. The findings of this study agree with the notion that ecological processes often leave characteristic spatial signatures, which if interpreted using appropriate hypotheses can help to ascertain factors responsible for the observed spatial patterns and stand dynamics.

Visitors’ Attitudes Towards and Willingness-to-Pay for Hypothetical Hoop Pine Plantations on the Pastoral Southern Atherton Tablelands, Australia

The southern Atherton Tablelands comprises diverse green spaces—meadows, croplands, woodlots and rainforest—which are integrated into a large-scale tablelands landscape. This paper examines visitors’ perceptions of the landscape of the southern Atherton Tablelands as a form of public good and their reaction to a hypothetical alteration to the landscape of the tablelands region by establishing small-scale hoop pine plantations on degraded lands. A survey conducted in 2005 indicated that the hoop pine plantations would not reduce the public enjoyment of the tablelands landscape. From the pairwise comparisons of vegetation types, it was found that the sample of tourists statistically equally preferred the ‘grazing land’ and the ‘hoop pine plantation’. In contrast, the local residents were found to prefer the ‘grazing land’ to the ‘hoop pine plantation’, at the 5% significance level. From a double-bounded contingent valuation study, it was found that the tourists were willing to pay A$13.82 per room per visit on average to support the establishment of hoop pine plantations on the southern Atherton Tablelands whereas local residents were willing to pay A$7.86 per year for the next 5 years for the same project.

Lantana camara L. (Verbenaceae) invasion effects on soil physicochemical properties

Lantana camara is a recognized weed of worldwide significance due to its extensive distribution and its impacts on primary industries and nature conservation. However, quantitative data on the impact of the weed on soil ecosystem properties are scanty, especially in SE Australia, despite the pervasive presence of the weed along its coastal and inland regions. Consequently, mineral soils for physicochemical analyses were collected beneath and away from L. camara infestations in four sites west of Brisbane, SE Australia. These sites (hoop pine plantation, cattle farm, and two eucalyptus forests with occasional grazing and a fire regime, respectively) vary in landscape and land-use types. Significant site effect was more frequently observed than effect due to invasion status. Nonetheless, after controlling for site differences, ~50% of the 23 soil traits examined differed significantly between infested and non-infested soils. Moisture, pH, Ca, total and organic C, and total N (but not exchangeable N in form of NO3-) were significantly elevated, while sodium, chloride, copper, iron, sulfur, and manganese, many of which can be toxic to plant growth if present in excess levels, were present at lower levels in soils supporting L. camara compared to soils lacking the weed. These results indicate that L. camara can improve soil fertility and influence nutrient cycling, making the substratum ideal for its own growth and might explain the ability of the weed to outcompete other species, especially native ones.

Soil microbial biomass, activity and community composition in adjacent native and plantation forests of subtropical Australia

Purpose Soil nitrogen (N) availability is a critical determinant of plantation productivity in subtropical Australia and is influenced by the soil microbial community. The size, structure and function of the soil microbial community can be impacted by land-use change and residue management. The objectives of this study were to examine the impact of land-use change from (1) native forest (NF) to first rotation (1R) hoop pine plantation and (2) 1R hoop pine plantation to second rotation (2R) hoop pine plantation on the soil microbial community. The impact of residue management on the soil microbial community was also investigated in the 2R forest, where soil microbial parameters were measured in tree rows (2R-T) and windrows (2R-W). In addition, relationships between soil microbial parameters and soil N parameters were investigated.

Application of (15NH4)2SO4 to study N dynamics in hoop pine plantation and adjacent native forest of subtropical Australia: the effects of injection depth and litter addition

Purpose Hoop pine (Araucaria cunninghamii) is a nitrogen (N)-demanding native Australian softwood plantation species. Litter quality and its effects on soil mineral N and 15N transformations have not been well studied in the hoop pine plantation and adjacent native forest. The present study was conducted to determine the impact of 15N injection depth and litter additions on the dynamics and fate of mineral 15N and also to compare the difference in litter quality, 15N dynamics, and fate between the hoop pine plantation (HP) and the adjacent native forest (NF).

Assessing management impacts on soil organic matter quality in subtropical Australian forests using physical and chemical fractionation as well as 13C NMR spectroscopy

Successful soil organic matter (SOM) quality assessment is needed to improve our ability to manage forest soils sustainably. Our objective was to use a multivariate data set to determine whether the land use conversion from native forest (NF) to hoop pine plantation and the following rotation and site preparation practices had altered SOM quality at three adjacent sites of NF, first (1R) and second rotation (2R, including tree planting row (2R–T) and windrow of harvest residues (2R–W)) of hoop pine plantations in southeast Queensland, Australia. Cross-polarization magic angle spinning 13C nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy and sequential hot water and acid hydrolysis were conducted on SOM fractions separated by wet-sieving and density fractionation procedures to characterize SOM quantitative and qualitative relevant parameters, including carbon (C) functional groups, C and nitrogen (N) contents, C/N ratios, and C and N recalcitrant indices. Analysis of variance (ANOVA) and principal component analysis (PCA) of these multivariate parameters together indicated a complicated interaction between physical protection and biochemical recalcitrance, making the land use and management induced changes of SOM quality more complex. Knowledge of PCA based on the refined set of 41 SOM quantitative and qualitative parameters identified that principal component 1 (PC1), which explained 55.7% of the total variance, was most responsible for the management induced changes in soil processes. This was reflected by the dynamics of SOM regarding the aspects of total stock, soil basal and substrate induced respirations, gross and net N mineralization and nitrification, and microbial biomass, microbial diversity of C utilization patterns. Further, the macroaggregates ( F 250–2000 μm) and the C/N ratio of acid extracts of SOM physical fractions, which represented the most informative and unique variables loading on PC1, might be the most promising physical and chemical measures for SOM quality assessment of land use and management impacts in subtropical Australian forests.

Carbon storage in a Ferrosol under subtropical rainforest, tree plantations, and pasture is linked to soil aggregation

Soil is a large sink for carbon (C), with the potential to significantly reduce the net increase in atmospheric CO2 concentration. However, we previously showed that subtropical tree plantations store less C into long-term soil pools than rainforest or pasture. To explore reasons for differences in C storage between different land-use systems, we examined the relationships between soil aggregation, iron and aluminium oxide and hydroxide content, and soil organic C (SOC) under exotic C4 pasture (Pennisetum clandestinum), native hoop pine (Araucaria cunninghamii) plantations, and rainforest. We measured SOC concentrations of water-stable and fully dispersed aggregates to assess the location of soil C. Concentrations of dithionite- and oxalate-extractable iron and aluminium were also determined to assess their role in SOC sequestration. Soil under rainforest and pasture contained more C in intra-aggregate particulate organic matter (iPOM, &gt;53 μm) than hoop pine plantations, indicating that in rainforest and pasture, greater stabilisation of SOC occurred via soil aggregation. SOC was not significantly correlated with dithionite- and oxalate-extractable Fe and Al in these systems, indicating that sorption sites of Fe and Al oxides and hydroxides were saturated. We concluded that soil C under rainforest and pasture is stabilised by incorporation within soil aggregates, which results in greater storage of C in soil under pasture than plantations following land-use change. The reduced storage of C as iPOM in plantation soil contributes to the negative soil C budget of plantations compared with rainforest and pasture, even 63 years after establishment. The results have relevance for CO2 mitigation schemes based on tree plantations.

In situ mineral 15N dynamics and fate of added 15NH4 + in hoop pine plantation and adjacent native forest in subtropical Australia

Background, aim, and scope Hoop pine (Araucaria cunninghamii) is a nitrogen (N) demanding indigenous Australia softwood species with plantations in Southeast Queensland, Australia. Soil fertility has declined with increasing rotations and comparison study of N cycling between hoop pine plantations, and adjacent native forest (NF) is required to develop effective forest management for enhancing sustainable forest production and promoting environmental benefits. Field in situ mineral 15N transformations in these two forest ecosystems have not been studied. Hence, the present study was to compare the differences in soil nutrients, N transformations, 15N fluxes, and fate between the hoop pine plantation and the adjacent native forest.

Using light fraction and macroaggregate associated organic matters as early indicators for management-induced changes in soil chemical and biological properties in adjacent native and plantation forests of subtropical Australia

Soil physical structure causes differential accessibility of soil organic matter (SOM), including carbon (C) and nitrogen (N) pools, to decomposer organisms and is an important determinant of SOM storage. Physical fractionation method of SOM in conjunction with elemental as well as isotopic analyses (C, N, δ 13C, δ 15N) of those soil fractions are used in this study to determine the land use and management-induced changes of SOM and associated processes in three adjacent sites of native forest (NF), first (1R) and second rotation (2R, including tree planting row (2R-T) and windrow of harvest residues (2R-W)) of hoop pine plantations in southeast Queensland, Australia. The results suggest that there is a greater accumulation of C and N in the light fraction (LF) of NF site than that of plantation sites (1R and 2R), and the C and N losses following conversion from mixed species NF to the single-species plantation are attributed to the reduction in C and N stocks for all physical fractions separated by wet sieving. In contrast, the C and N losses induced by the rotation practices (e.g., between 1R and 2R-T) are attributed to the C and N decreases in the LF and macroaggregates (250–2000 µm) only. The C and N increases upon the residue management (e.g., between 2R-W and 2R-T) are primarily attributed to the C and N increases in the LF and macroaggregates as well. Coupled with 30 soil chemical and biological parameters obtained in our previous studies, further principal component and multivariable regression analyses were conducted and the results showed that both the LF and macroaggregates were highly related to the status of C and N pools, the processes of N transformation and soil respiration, and the diversity of microbial communities, and thus could serve as diagnostic SOM fractions responsible for the changes of soil properties and processes within the tested ecosystem induced by the land uses and management practices. Knowledge of the interactive relationships between aggregate classes within SOM and soil chemical and biological processes in this study represents a further step towards diagnostically measuring and understanding important soil C and N processes in response to the land use and management changes in the soil ecosystems such as forests in subtropical Australia.

Valuing carbon sequestration benefits: an application to hoop pine plantations in north Queensland, Australia

A timber plantation carbon modelling procedure consistent with the Intergovernmental Panel on Climate Change and Australian greenhouse gas accounting methodologies is described with an application to hoop pine (Araucaria cunninghamii) on the southern Atherton Tablelands of north Queensland, Australia. We also compare the private financial performance (timber value only) and economic performance (timber plus carbon values) of hoop pine plantations. The carbon sequestration value of hoop pine on high quality sites is found to increase social returns to plantation investment substantially above private returns, but it only marginally expands the range of site qualities over which plantations are socio-economically justified. Despite the long rotation of hoop pine (45 years), economic performance is found to be sensitive to carbon modelling parameters, including the decay rate of long-life wood products. This suggests that research effort to quantify these parameters more precisely is warranted.

Soil carbon and nutrient pools, microbial properties and gross nitrogen transformations in adjacent natural forest and hoop pine plantations of subtropical Australia

Background, Aims, and Scope. An improved understanding of important soil carbon (C) and nutrient pools as well as microbial activities in forest ecosystems is required for developing effective forest management regimes underpinning forest productivity and sustainability. Forest types and management practices can have significant impacts on soil C and nutrient pools as well as biological properties in forest ecosystems. Soil C and nutrient pools were assessed for adjacent natural forest (NF), first rotation (1R) (50-year-old), and second rotation (2R) (1year-old) hoop pine (Araucaria cunninghamii Ait. ex D. Don) plantations in southeast Queensland of subtropical Australia. Materials and Methods. Five transects spaced 3 m apart with 9 sampling points along each transect were selected (9.6 m x 12.0 m each site), with 45 soil cores (7.5 cm in diameter) collected and separated into 0–10 and 10–20 cm depths. These soils were analysed for total C, total nitrogen (N), C (δ13C) and N (δ15N) isotope composition. The 0–10 cm soils were analysed for pH, CEC, exchangeable cations, total P and total K, and assayed for microbial biomass C and N, respiration, metabolic quotient, potential mineralizable N (PMN), gross N mineralization (M) and immobilization (I). Results. Total C and N in 0–10 cm soils were higher under NF and 1R plantation than under 2R plantation, while they were highest in 10–20 cm soils under NF, followed by the 1R and then 2R plantation. δ13C was lower under NF than under the plantations, while δ15N was higher under NF than under the plantations. Total P was the highest under NF, followed by the 1R and then 2R plantation, while total K was higher under the 2R plantation. No significant differences were detected for pH, CEC, exchangeable cations, microbial C and N, respiration and metabolic quotient among the 3 sites. PMN and M were higher under NF, while I was the highest under the 2R plantation, followed by the NF and then 1R plantation.

Valuing carbon sequestration benefits: an application to hoop pine plantations in north Queensland, Australia

A timber plantation carbon modelling procedure consistent with the Intergovernmental Panel on Climate Change and Australian greenhouse gas accounting methodologies is described with an application to hoop pine (Araucaria cunninghamii) on the southern Atherton Tablelands of north Queensland, Australia. We also compare the private financial performance (timber value only) and economic performance (timber plus carbon values) of hoop pine plantations. The carbon sequestration value of hoop pine on high quality sites is found to increase social returns to plantation investment substantially above private returns, but it only marginally expands the range of site qualities over which plantations are socio-economically justified. Despite the long rotation of hoop pine (45 years), economic performance is found to be sensitive to carbon modelling parameters, including the decay rate of long-life wood products. This suggests that research effort to quantify these parameters more precisely is warranted.

Sampling Size Required for Determining Soil Carbon and Nitrogen Properties at Early Establishment of Second Rotation Hoop Pine Plantations in Subtropical Australia

Investigations into forest soils face the problem of the high level of spatial variability that is an inherent property of all forest soils. In order to investigate the effect of changes in residue management practices on soil properties in hoop pine ( Araucaria cunninghamii Aiton ex A. Cunn.) plantations of subtropical Australia it was important to understand the intensity of sampling effort required to overcome the spatial variability induced by those changes. Harvest residues were formed into windrows to prevent nitrogen (N) losses through volatilisation and erosion that had previously occurred as a result of pile and burn operations. We selected second rotation (2R) hoop pine sites where the windrows (10{15 m apart) had been formed 1, 2 and 3 years prior to sampling in order to examine the spatial variability in soil carbon (C) and N and in potential mineralisable N (PMN) in the areas beneath and between (inter-) the windrows. We examined the implications of soil variability on the number of samples required to detect differences in means for specific soil properties, at different ages and at specified levels of accuracy. Sample size needed to accurately reflect differences between means was not affected by the position where the samples were taken relative to the windrows but differed according to the parameter to be sampled. The relative soil sampling size required for detecting differences between means of a soil property in the inter-windrow and beneath-windrow positions was highly dependent on the soil property assessed and the acceptable relative sampling error. An alternative strategy for soil sampling should be considered, if the estimated sample size exceeds 50 replications. The possible solution to this problem is collection of composite soil samples allowing a substantial reduction in the number of samples required for chemical analysis without loss in the precision of the mean estimates for a particular soil property.

Soluble organic nitrogen pools in adjacent native and plantation forests of subtropical Australia

Soil soluble organic nitrogen (SON) can play an important role in soil nitrogen (N) cycling in forest ecosystems. This study examined the effect of land-use change from a native forest (NF) to a first rotation (1R) and subsequent second rotation (2R) hoop pine ( Araucaria cunninghamii) plantation on soil SON pools. The impact of residue management on SON pools was also investigated in the 2R forest, where SON was measured in tree rows (2R-T) and windrows (2R-W). Various extraction techniques were used to measure SON pool size in the 0–10, 10–20 and 20–30 cm layers of soil. The results showed that land-use change had a significant impact on soil SON pools. In the 0–10 cm layer, 3.2–8.7, 14–23, 20–28, 60–160 and 127–340 mg SON kg −1 were extracted by water, 0.5 M K 2SO 4, 2 M KCl, hot water and hot 2 M KCl, respectively. The size of the SON pools and the potential production of SON (PPSON) were generally highest in the NF soil and lowest in the 2R-T soil, and in all forest types decreased with soil depth. The larger SON pools in the NF soil coincided with lower soil, litter and root C:N ratios, suggesting that the difference in the size of SON pools between the NF and 1R soil may be related to differences in the quality of organic matter input under the different forest ecosystems. Differences in the size of SON pools between the 1R soil and the 2R soils and between the 2R-T soil and the 2R-W soil may be related to the quantity of organic matter input and time since disturbance. Significant relationships were found between the SON extracted by 0.5 M K 2SO 4 (SON ps) and 2 M KCl (SON KCl), and also among the SON extracted by hot 2 M KCl (SON hKCl), hot water (SON hw) and water (SON w), suggesting that the organic N released by these groups of extracts may be at least partly from similar pools.

Soil carbon turnover and sequestration in native subtropical tree plantations

Approximately 30% of global soil organic carbon (SOC) is stored in subtropical and tropical ecosystems but it is being rapidly lost due to continuous deforestation. Tree plantations are advocated as a C sink, however, little is known about rates of C turnover and sequestration into soil organic matter under subtropical and tropical tree plantations. We studied changes in SOC in a chronosequence of hoop pine ( Araucaria cunninghamii) plantations established on former rainforest sites in seasonally dry subtropical Australia. SOC, δ 13C, and light fraction organic C (LF C<1.6 g cm −3) were determined in plantations, secondary rainforest and pasture. We calculated loss of rainforest SOC after clearing for pasture using an isotope mixing model, and used the decay rate of rainforest-derived C to predict input of hoop pine-derived C into the soil. Total SOC stocks to 100 cm depth were significantly ( P<0.01) higher under rainforest (241 t ha −1) and pasture (254 t ha −1) compared to hoop pine (176–211 t ha −1). We calculated that SOC derived from hoop pine inputs ranged from 32% (25 year plantation) to 61% (63 year plantation) of total SOC in the 0–30 cm soil layer, but below 30 cm all C originated from rainforest. These results were compared to simulations made by the Century soil organic matter model. The Century model simulations showed that lower C stocks under hoop pine plantations were due to reduced C inputs to the slow turnover C pool, such that this pool only recovers to within 45% of the original rainforest C pool after 63 years. This may indicate differences in soil C stabilization mechanisms under hoop pine plantations compared with rainforest and pasture. These results demonstrate that subtropical hoop pine plantations do not rapidly sequester SOC into long-term storage pools, and that alternative plantation systems may need to be investigated to achieve greater soil C sequestration.