Abstract
Forest plantations can access water from some unconfined aquifers that also contain nitrate at concentrations that could support hydroponic culture, but the separate effects of such additional water and nitrogen availability on tree growth have not hitherto been quantified. We demonstrate these effects using simulation modelling at two contrasting sites supporting Eucalyptus globulus Labill. or Pinus radiata D.Don plantations. The APSIM Eucalyptus and Pinus models simulated plantation growth within 2% of observed growth where the water table was at 4 m depth for eucalypts (height 28 m, MAI 32 m3 ha−1 year−1) and at 23 m for pines (height 37 m, MAI 20 m3 ha−1 year−1). In simulations without an aquifer, observed growth could only be matched using unrealistically high surface soil nitrogen (N) supply, suggesting this is an unlikely mechanism. Simulated aquifer N concentrations, evapotranspiration, and net N mineralization and leaching (emergent properties of modelling) were similar to measured values. These results strengthen the plausibility that aquifer N uptake by plantations could be contributing to tree growth. This hypothesis warrants further research that quantifies these processes at multiple sites. Simulations included growth of herbaceous and tree weed species, and pasture, which demonstrated the utility of the process-based APSIM modelling framework for dynamically simulating carbon, water and N of plantations and other mixed-species systems.
Highlights
Simulations were for two sites: (i) ‘Shallow Site’ was a Eucalyptus globulus plantation 53 km north-northwest of Mount Gambier, South Australia, over a shallow water table (4 m), and (ii) ‘Deep Site’ was a Pinus radiata stand 16 km south-west of Mount Gambier over a deep water table (23 m) (Table 1)
Research here linked modelling and observations to strengthen the plausibility that forest plantations at two sites in the Mount Gambier region took up nitrate-N from an aquifer that led to increased tree growth
Several lines of evidence were provided. (i) Nitrate-N was present in the aquifer across the region at concentrations adequate for hydroponic culture. (ii) Roots were known to reach the aquifer and take up water. (iii) With a previously validated process-based model, simulated productivity could match that observed only by either including the possibility of N uptake from the aquifer, or, much less likely, by specifying soil conditions much more favorable for N supply from mineralization than measured. (iv) Emergent properties of the modelling were generally consistent with observations: rates of net N-mineralization and N-leaching, concentrations of nitrate-N in the aquifer, and rates of water uptake from the aquifer
Summary
Eucalyptus and Pinus plantations are globally significant for wood supply. In Australia, plantation forests in southeastern South Australia, colloquially known as the western part of the Green Triangle region, grow above an unconfined aquifer and cover approximately 1350 km. In Australia, plantation forests in southeastern South Australia, colloquially known as the western part of the Green Triangle region, grow above an unconfined aquifer and cover approximately 1350 km2 This area is about 7% of Australia’s total plantation estate, and. 74% supports Pinus radiata D.Don and 26% Eucalyptus globulus Labill. Establishment of pine plantations in the region commenced in 1881 [2], and many of the P. radiata sites support a 3rd rotation that exceeds the productivity of previous rotations [3].
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