Optimising Nutrition for Productive and Sustainable Farm Forestry Systems: pasture legumes under shade

With the closure of North Queensland rainforests to logging following World Heritage Listing, timber plantations are being established on degraded and other agricultural land. Experimental evidence indicates that current fertiliser regimes produce suboptimal growth in many species, particularly on the less fertile soils. Further there is public concern about the possibility of fertilisers in agricultural runoff having adverse effects on the downstream environment. Added to this, fertilisers are a significant economic cost to the landowner that affects returns from farm forestry enterprises.\ud \ud Significant acceptance of tree plantation-pasture systems has been impeded by the belief that\ud productive pasture cannot be maintained under tree canopies. The benefits of raising livestock in\ud conjunction with tree plantations include increased and diversified income, better use of land\ud resources, soil stabilisation, and the potential for higher plantation crop yield through better weed\ud control, nutrient cycling and nitrogen accretion. This project identified a suite of pasture legumes that\ud can be used to improve pasture quality in agroforestry systems under differing rainfall regimes in the\ud tropics. Nitrogen fixed by these legumes is also likely to benefit tree growth.\ud \ud The major thrust of the research was to identify pasture legume species which are able to tolerate\ud different levels of shading under tree plantations in the Australian tropics - providing improved\ud forage quality and productivity for grazing in agroforestry situations, and improving soil fertility and\ud reducing fertiliser costs through nitrogen fixation. The benefits to tree growers include enhanced tree\ud growth, weed suppression (reduced costs for herbicides), and reduced erosion on steep or degraded\ud sites (due to mixed ground cover).\ud \ud A total of 35 species and cultivars of tropical pasture legumes were tested for shade tolerance beneath\ud four levels of shade under shadehouse conditions. A range of agronomic and ecophysiological\ud parameters were measured, including biomass production, root:shoot ratio, root nodulation, foliar\ud nutrient content, time to flowering, seed production, and light response curves.\ud \ud Above- and below-ground biomass were both depressed by shading, although species were affected\ud to different degrees. The most promising species tended to yield well under the control treatment and\ud also beneath shade treatments, indicating they may also be suitable for use beneath young plantations\ud where shade levels are relatively low, in addition to older, more heavily shaded plantations. Root\ud nodulation was greatly decreased or entirely absent beneath shaded treatments, implying that fixation\ud of atmospheric nitrogen may be lower under shade than open conditions, however fertilization of\ud pots in this trial was a complicating factor. The concentration of leaf N was affected by shading, with\ud increased concentrations found under shade treatments compared to the control. However no similar\ud changes were discernible in leaf P. Vegetative growth appears to be prolonged in many species by\ud shading, with the time taken to the production of the first flower often increased in plants grown\ud under shade. The yield of seeds was greatly reduced under shade, however seed size appears to have\ud been maintained at the expense of seed number. The seed of several species had an increased fraction\ud of readily germinable seed, speed of seed germination and lower levels of hard seed when grown\ud under shade. Light response curves were not always correlated with the results for dry matter\ud production, with several species that appear to be shade tolerant when viewing dry matter production\ud and relative yields, producing light curves that suggest otherwise.\ud \ud Growth and performance of 15 species/cultivars were also examined under Khaya senegalensis and\ud Eucalyptus pellita / mixed rainforest species plantations. Arachis pintoi was clearly the best\ud performing of six species/accessions of legume tested beneath a five-year-old Eucalyptus pellita /\ud mixed rainforest species plantation. Clitoria ternatea and Centrosema brasilianum proved to be the\ud best performing species beneath both a conventional stand and a Nelder wheel comprised of Khaya\ud senegalensis, however legume growth beneath trees was depressed compared to growth in more open\ud conditions. The production of pasture decreased exponentially with increasing tree density beneath\ud the Nelder wheel plantation. Both Clitoria ternatea and Centrosema brasilianum were observed to\ud climb trees during the trial, highlighting the potential smothering of young or small trees by vigorous\ud climbing species. Due to this, sowing of either species is not recommended in very young\ud plantations, or if they are, then additional management will be required to control climbers. Trees\ud were also found to decrease soil moisture content as planting density increased, suggesting light may\ud not always be the only limiting factor to pasture production beneath trees in seasonally dry areas,\ud which comprise much of the Australian tropical region.\ud \ud This study identified 16 species of potentially useful, shade-tolerant, pasture legume species that\ud show promise for use under trees in farm forestry situations in both the wet and the seasonally dry\ud tropics. The most promising species for the wet tropics were Desmodium intortum, Calopogonium\ud mucunoides, Arachis pintoi, D. ovalifolium, D. canum, Centrosema acutifolium, Pueraria\ud phaseoloides, D. heterophyllum, C. pubescens, D. uncinatum and C. macrocarpum. Clitoria ternatea,\ud Arachis stenosperma, Macroptilium lathyroides, M. atropurpureum and Centrosema brasilianum\ud were the most successful of the species suited for seasonally dry tropical regions.\ud \ud However, many of the species identified as shade tolerant, or being relatively productive under\ud shaded conditions, have a climbing/twining habit that can cause concern in tree plantations,\ud particularly in respect to potential smothering of small trees. When the climbing species are removed\ud the remaining species recommended for the wet tropics are D. intortum, A. pintoi, D. ovalifolium, D.\ud canum, D. heterophyllum and D. uncinatum, while A. stenosperma shows potential for the seasonally\ud dry tropics. Nevertheless climbing species may still be useful beneath older plantations, or where a\ud higher level of pasture management is acceptable in order to help prevent smothering of trees through\ud controlling the frequency and intensity of grazing.\ud \ud Further research should examine the possibility of controlling climbing legumes in tree plantations\ud through grazing management in order to broaden the range of legume species available for use.\ud Research is also required to examine the longer-term persistence and productivity of the species\ud identified in this study, as tree plantations age and shade levels increase. Compatibility with shade\ud tolerant grasses is an important aspect of pasture production beneath tree plantations, which has seen\ud relatively little research and will have an important effect upon the persistence and productivity of\ud both the grass and legume components of the pasture