Nitrogen and carbon cycling associated with litterfall production in monoculture teak and mixed species teak and flueggea stands
High demand for teak (Tectona grandis L.f.), a species of economic importance, was the reason Solomon Islands experienced a surge in community-wide planting of monoculture teak stands in the last two decades. Mixed species planting of teak and flueggea (Flueggea flexuosa Muell. Arg.) was introduced to overcome the reluctance of growers to thin their stands. However, there is lack of information on the effect of changing from monoculture to mixed species plantings on the cycling of nutrients especially carbon (C) and nitrogen (N).
Materials and methods
This study assessed litter quantity and quality, total C (TC), total N (TN), C:N ratio and C and N isotope compositions (δ13C and δ15N) over 18 months at two sites (Ringgi and Poitete). The treatments included teak planted at 833 stems per hectare (sph) (T1), teak planted in rows with two rows of flueggea at 833 sph (T2), 625 sph (T3) and 416 sph (T4), and teak planted in alternating rows with flueggea at 833 sph (T5).
Results and discussion
Treatment 1 (T1) produced significantly higher total litter than T4 at Ringgi. However, based on individual tree litterfall production, teak in T4 (lowest stocking rate) at both trials produced higher litter per tree than the teak in T3, T2, T5 and T1 while there was no significant difference with litter production of flueggea. An enrichment of litter δ15N was observed over time in either species, which suggested an increased N loss and transformations in both experimental sites. When comparing each treatment and using individual tree productivity, T4 significantly produced and returned higher litter TC and TN than T3, T2, T5 and T1.
Overall, individual tree productivity demonstrated that mixed species stands had a significant potential for cycling higher rates of C and N than monoculture teak stands. Therefore, establishment of mixed species stands, especially T4 and T3, was recommended as a practical measure to address the widely experienced problem of reluctance by growers to thin high value trees while preserving the balance of C and N inputs into the ground.
KeywordsC:N ratio C and N isotope compositions Flueggea flexuosa Mixed species Monoculture Tectona grandis
The research reported here was undertaken through a series of projects supported by the Australian Centre for international Agricultural Research (ACIAR), Canberra, Australia and the Solomon Island Government Ministry of Forestry Research. V. W. Vigulu was supported in this work through a John Allwright Fellowship grant from ACIAR. Kolombangara Forest Products Limited provided staff and transportation and made research sites available for the project. The staff of Munda and Poitete Forestry Offices assisted in the establishment of, and data collection from, the sites. We would like to thank Griffith University for their technical help and support and Rene Diocares for timely analysis of the plant and soil samples.
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