Pasture yield, quality and species distributions were compared between zones around live and killed eucalypt trees at two woodland sites in northeast Queensland which differed markedly in soil fertility. Trees affected pasture quality and yield on an individual tree basis: N concentration and dry matter digestibility tended to be higher under trees than in inter-tree areas at both sites and pasture yields declined with distance from killed trees at the lower fertility site. However, the distribution of individual species did not vary markedly with distance from trees. Trees also affected pasture yield on a woodland basis: yields were greater where the trees were killed than under intact woodland. Soil under trees had higher levels of organic carbon and greater litter cover than soil in inter-tree areas. However, pasture yields did not generally reflect this fertility gradient since growth was limited by moisture availability due to drought conditions during the study period. Pasture N concentration was higher under trees that in inter-tree areas since plants under trees produced a similar amount of biomass as plants in inter-tree areas, but had access to higher nutrient levels. Trees appeared to have a greater effect on soil nutrient availability at the low fertility site. Live trees depressed pasture yields to a lesser degree at the low fertility site, demonstrating that the effects of trees on soil water availability (on a woodland scale) are less important when soil nutrients are more limiting to growth. These results indicate that, while removing trees may enhance pasture productivity, this benefit may be offset by a reduction in pasture quality. Given the beneficial effect of trees on soil nutrients, tree removal may also have longer term implications for soil nutrient dynamics.

Tree-grass relationships in open eucalypt woodlands of northeastern Australia: influence of trees on pasture productivity, forage quality and species distribution

This review examines the interactions between grasses and trees that are relevant to Queensland's grazing lands. Soils and climate determine the potential amount of woody vegetation within an area and clearing, fire and grazing management can modify that potential. In general, the presence of non-leguminous trees reduces the potential grass production beneath their canopy and within woodland patches. Some non-native leguminous trees (e.g. Indian siris, Albizia lebbeck) enhance production of grasses beneath their canopies, whereas the widespread native leguminous trees, mulga (Acacia aneura) and brigalow (A. harpophylla) have not been reported to enhance grass production in the same way. At the patch scale, pasture production beneath woodlands with moderate to high tree basal areas is generally less than in open patches under the same soil and climatic conditions. At a landscape level, tree density, rainfall amount and distribution, and soil type modify pasture production within forest/woodland/shrubland systems. Grasses can reduce tree seedling survival but have little impact on mature woody plants, apart from providing fuel to carry a fire. Modelling studies have been used to examine some aspects of tree-grass production. Firstly, the relationship between grass production and an increasing amount of trees can vary from linear decrease, to exponential decrease to initial stimulation followed by a decrease, depending solely on the relative strengths of stimulatory and competitive effects of trees on grasses. Secondly, simulated pasture production within woodlands shows that the pasture production may be up to 50% higher in paddocks that have high variability in the distribution of those trees compared with areas where trees are uniformly distributed. This is due to the non-linear (negative exponential) relationship between pasture production and tree density that is commonly observed within Queensland. Lastly, simulation studies show that total aboveground production (trees and pasture) of mulga woodland increases as the number of trees per hectare increases. The pasture production response to tree clearing or tree planting depends on tree species, rainfall, soil type, climatic history and post-clearing management including fire and grazing, and will change with time since clearing. The greatest relative increase in pasture production following the removal of woody vegetation occurs: when the initial tree basal area is highest; where rainfall is evenly distributed though the year and on fertile soils with a low water holding capacity.

Some aspects of tree-grass dynamics in queensland's grazing lands

Brigalow (Acacia harpophylla)–Dawson gum (Eucalyptus cambageana) open forests are predominantly supported by solodic soils in central Queensland. This report describes relations between some physical, chemical and morphological properties of surface soils (0–0.10 m) within a virgin brigalow–Dawson gum forest. Soil property gradients were found to radiate horizontally from tree-dominated to non-vegetated areas, indicative of a vegetation-induction process. These reflect the importance of litterfall organic matter in determining soil fertility and soil physical conditions at this site.

The decomposition of litterfall is probably responsible for significant increases in soil organic carbon (C), total nitrogen (N) and total sulphur (S) concentrations with increasing vegetation canopy cover. The composition of soil organic matter appears constant across the study area, as similar soil C:N:S ratios were recorded across all vegetation canopy classes. Soil salinity, total phosphorus, mineral nitrogen, cation exchange capacity, exchangeable calcium and exchangeable potassium levels also increased with increasing vegetation canopy cover.

Surface soil physical properties were also related to vegetation canopy cover. Bulk density and the < 20 μm dispersion ratio decreased while soil micro-relief and A horizon depth increased with increasing vegetation canopy cover. Organic C, through inverse relations with bulk density and the < 20 μm dispersion ratio, appears to enhance soil porosity and aggregate stability, indicating the role of organic matter in the stabilization of larger virgin soil aggregates.

The close association between virgin forest and surface soil fertility is seen to have implications for changes in land-use, with a decline in nutrient availability, soil aggregate stability and productivity following forest clearing.

Surface soil chemical and physical patterns in a brigalow–Dawson gum forest, central Queensland

Scattered native trees are a significant ecological resource across the agricultural landscape, yet their numbers are declining due to factors such as dieback, senescence and agricultural activity. This study examined the interactions among Eucalyptus melliodora (Cunn. ex Schauer) trees, vegetation composition and selected surface soil chemical properties in grazed and ungrazed paddocks on the Northern Tablelands of New South Wales, Australia. Four farms on granite soils were examined in grazed and ungrazed treatments. Vegetation composition was assessed, and soil samples were collected in plots beneath the canopy and in adjacent open areas in both north and south directions of the tree canopy. Native grasses dominated the vegetation in both beneath the canopy and open areas, at both grazed and ungrazed sites. However, their composition varied between farms. Several C3 and C4 grasses contributed to the groundcover of the canopy and open sites, but C3 grasses were generally more common under the canopy. Significant differences occurred in soil C, N, P and pH, and vegetation composition between canopy and open areas, and between grazed and ungrazed treatments. Soil P, C and N contents in grazed sites were typically similar to or higher than those in ungrazed sites, and soils were less acid in the ungrazed compared with grazed sites. All soil parameters measured were significantly higher under tree canopies, except P. The tree, soil and vegetation factors were strongly related. This study confirms that individual scattered trees create a distinct mosaic of localised soil improvement, and influence vegetation composition so that paddocks with trees are floristically more diverse than paddocks without trees. The results illustrate the potential benefits of retaining trees for both biodiversity values and livestock production in Australia.

The influence of individual native trees and grazing regime on soil properties and groundcover patterns in a temperate landscape of New South Wales, Australia

Effects of grazing, trenching and surface soil disturbance on ground cover in woody encroachment on the Cobar Pediplain, south-eastern Australia

Results are summarized for a number of separate studies which investigated the effects of killing the trees, and of grazing by domestic livestock on herbage biomass, densities of grasses, forbs and woody plants and some surface soil properties in poplar box woodlands. Some temporal changes in vegetation and spatial patterns in soil properties occurred in all treatments. Forb and grass densities increased from 1966 to 1970 and 1972 respectively and then declined. Surface soil pH, micro-relief, salt content and litter biomass were generally higher at the tree bole than between the tree canopies. Herbage biomass was highest where trees had been killed but the killing of trees did not have a direct effect on the densities of either grasses or shrubs. The increased growth of herbage is attributed to an increase in the availability of water and nitrogen. The vegetation changes following tree killing with livestock excluded are summarized in the form of changes in the proportions of the grasses, shrubs and trees; grass proportion increased for the first six years following tree killing after which grasses tended to be replaced by shrubs. The density of shrubs increased following the exclusion of livestock. Shrub densities were less in experimental grazed plots than in ungrazed plots but still increased with time. With grazing by livestock, herbage biomass was highest adjacent to trees and the percentage of bare soil increased away from trees. Changes resulting from interactions between the grazing and killing treat- ments mainly related to soil compaction and water infiltration. It is suggested that shrub encroachment in the poplar box lands may arise because of fluctuations in grazing pressure resulting from large paddock and property sizes and variability in rainfall. Little of the available herbage is grazed during abnormally wet years, thus there is an effective release from grazing at a time when germination of shrubs is likely to occur.

Vegetation change in a Poplar Box (Eucalyptus Populnea) W00dland. Effects of tree killing and domestic livestock.

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Vegetation change in a Poplar Box (Eucalyptus Populnea) W00dland. Effects of tree killing and domestic livestock.