Key words rainforest reforestation, carbon sequestration, cost-effectiveness, old fields, weeds
Introduction. Restoring agricultural landscapes to forest is time-consuming, expensive and often hit-and-miss. Trees take years to show survival and growth rates and effects of planting methods and maintenance. World-wide, there are few large-scale reforestation experiments designed to test the effectiveness of and functional responses to reforestation, especially in the tropical regions for biodiversity and carbon benefits.
In the wet tropics of Australia, far north Queensland, the Thiaki Restoration Research project was established to examine aspects of reforestation (Figs 1-3). The reference model for the project is ‘Simple to complex notophyll vine forest of cloudy wet highlands on basalt, Regional Ecosystem 7.8.4’. Three fully-replicated experiments were established in 2010, 2011 and 2013 to examine different approaches to reforestation. The experimental plots are all replicated, with control plots, to examine different aspects of reforestation. Plot size varies from 25 m square up to 50 m square, and we now manage 90 experimental plots over more than 30 hectares of planted land.
Experimental design. The first experiment is examining the effect of different planting methods; the second is researching three combinations of native rainforest species and two treatments (high and low planting densities); and the third is examining the effects of two different herbicide treatments (blanket spraying and strip spraying). One of the major emphases of the experiments is to analyse planting practices for their cost-effectiveness for the carbon sequestration industry. Reducing establishment and maintenance costs for carbon sink forests is essential, as published and anecdotal costs of establishing forests in the region and elsewhere has been so high as to make the carbon economy unreachable for environmental planting practitioners to ‘make a buck’ from carbon farming. We will publish these findings in the near future, as most of the plantings have reached an (almost) self-maintaining height and size.
Current work, which will be published from the experiments, includes:
- examination of field-based measurements compared with national modelling tools;
- effects of herbicide spraying and grass suppression practices; rates and patterns of natural recruitment;
- functional responses of trees to soil nutrients and characteristics (such as compaction, moisture and organic content); functional responses of dung beetles and mammals to restoration;
- responses of ants to restoration and remnant patches and proximity to remnant forests; and
- the functioning of barriers to recruitment by rainforest fauna.
Weeds also present a significant research component, and examination of the effects and faculty of weeds to restoration is being conducted. We are also examining the effects of different planters on survival rates, which is of vital interest to restorationists.
Results to date. The experiments have resulted in important findings which affect reforestation success, and publications which have contributed some of the first replicated experimental results on: planting methods; allometrics for young trees; functional responses of several taxa to restoration; young tree root:shoot ratios; improved wood density data on young trees; and cost-effectiveness of planting methods. Some of the related research has contributed to better Australian models of carbon sequestration in the tropics.
Lessons learned and future directions. Top priority lessons include the preparation and planting stage, as all else follows and mistakes made at this point ramify later. Vital considerations are: site preparation, especially early weed control; selecting species which will survive the harsh exposed conditions; nurturing and sun-hardening seedlings; ensuring that the soil is very wet and that seedlings are soaked immediately before planting; and, ensuring that planters plant in ways that don’t damage the seedlings.
Collaborators. Charles Darwin, James Cook, Adelaide, Lancaster (UK), and Queensland Universities. Funding: Australian Research Council Linkage project LP0989161, Biome5 Pty Ltd, Terrain NRM, Greening Australia, Stanwell Corporation, Biodiversity Fund.