VIth INTERNATIONAL RANGELANDS CONFERENCE
Practical Rangeland Ecology
17 - 18 July, 1999
Queensland Beef Industry Institute,
Department of Primary Industries,
pp. 24 - 41
Tree clearing - rehabilitation or development on grazing land?
Queensland Beef Industry Institute
Tropical Beef Centre
This paper draws on recent quantitative studies, as well as historical and anecdotal records, to conclude that grazed woodlands (tree-shrub communities) have changed substantially since Europeans introduced sheep and cattle to north-east Australia. There is convincing evidence that tree and shrub cover, density and basal area have all increased relative to that present 100 + years ago in most of today's `intact' woodlands. Tree - pasture relationship graphs show that these changes in vegetation structure would have contributed to an appreciable decline in potential pasture and animal production on affected areas.
Clearing these woodlands can therefore be depicted as simultaneous rehabilitation (to a more open grassy structure, akin to that prevailing under pre-European management) and development (since it can commonly lead to a sustainable doubling of pasture production on treated sites). Meanwhile woody plants will continue to thicken up in retained woodland areas until this process is limited by tree - tree competition. At this point there will be little pasture production within these retention zones, while the flora/fauna assemblage supported in them will be considerably different to that existing when livestock were first introduced. Failure to appreciate these points arises from the mistaken belief that woody vegetation communities present in Australia at the time of European settlement were climax for the environment. Rather, these communities were a fire mediated sub-climax developed over 50000 + years of Aboriginal burning, which rapidly altered under changed management coincident with the introduction of domestic livestock.
Asking whether or not clearing woody plants (trees/shrubs/regrowth) on grazing (agricultural) land represents development or rehabilitation is a good question. This is because it focuses attention on firstly, the status of the current tree/shrub cover and, secondly, the purpose for which these lands have been assigned.
An extensive scientific literature details structural changes in the woodlands of Queensland and adjacent regions since the introduction of domestic livestock, as well as the consequence of this to pasture and animal production. This literature is reviewed with emphasis placed on woodland communities relevant to inland south-central Queensland. The question raised in the title of the paper is then addressed in light of the information presented.
Vegetation status in grazed woodlands (`forest' country)
Tree clearing guidelines applicable to leasehold lands in Queensland set retention levels based on the original (pre-European) extent of predefined (by EPA staff) vegetation communities.A For example, communities which are considered to now occupy < 10% of the area on which they occurred at European settlement are classified as `endangered/vulnerable', while those occupying > 30% of the original area are `not of concern'. It is implicit in applying such conservation rules that the vegetation present today is representative (e.g. in structure, composition, density) of what was `originally' present in the defined community - otherwise the conservation rule is meaningless. How certain can we be of that for Queensland's vegetation? The following extracts from scientific papers/reports for a wide geographic area are instructive:-
(a) A continental perspective
(i) Lunt (1998 a, b) - Southern Victoria
"Post settlement changes in vegetation and land use were examined in a reputedly undisturbed woodland remnant at Ocean Gove, southern Victoria, the site of earlier ecological studies. The vegetation has passed through at least three structural phases since European colonisation: an open grassy woodland dominated by Allocasuarina and Eucalyptus species and Banksia marginata with few shrubs; an open scrub of Acacia pycnantha; and a closed scrub of Allocasuarina littoralis which now dominates the reserve. Tree and shrub density has steadily increased from perhaps less than 20 trees/ha in the early 1800's to over 3000 trees/ha in 1996.……… Vegetation changes in the past 200 years can be attributed to the long-term absence of fire. The abundant recruitment of Acacia species in the mid-to late 1800's may have been a rapid response to the curtailment of Aboriginal burning, and the more recent invasion of A. littoralis a longer-term response to fire exclusion" (Lunt 1998a). "The dramatic increase in the density of A. littoralis from 911 trees/ha in 1971 to 3565 trees/ha in 1996 was associated with a continued decline in the once dominant eucalypts, especially E. ovata" (Lunt 1998b).
(ii) Fensham and Fairfax 1996 - Bunya Mountains, Queensland
"The physical setting of 61 grassy bolds on the Bunya Mountains in south-eastern Queensland was surveyed during 1995, and a further 73 balds were assessed from aerial photographs taken in 1951 and 1991. About 26% of the area of balds existing in 1951 had been invaded by forest in 1991. The extent of the invasion was generally higher for balds surrounded by eucalypt forest than balds surrounded by eucalypt and rainforest or rainforest only."
(iii) Hopkins et al. (1996) - Daintree, north Queensland
"The identification and radiocarbon dating of charcoal collected under tropical rainforest indicated that sclerophyll forests dominated by Eucalyptus occupied parts of the wet tropical lowlands in the Daintree region of North Queensland at least intermittently from 12000 year BP (before present) until very recently.……… The process of eucalypt forest replacement by rainforest may have accelerated since the arrival of Europeans and the concomitant decrease in Aboriginal management."
(iv) Neldner et al. (1997) - Cape York Peninsula, far north Queensland
"Data are provided that demonstrate the conversion of some grassland types to woodlands in the last 30 years, and it seems probable that the change is a result of altered fire regimes. Even if adequately reserved, appropriate fire management is required to maintain the grasslands of Cape York Peninsula."
(v) Jacklyn (1998) - Victoria River District, N.T.
The photographic record was utilised "to show that the vegetation has `thickened up' on Bradshaw Station (Victoria River District, N.T.) since 1933 (See Appendix 3). Interestingly, this trend is also seen in other similarly paired photographs assembled by Darrell (Lewis) from locations right across the VRD."
(b) Western NSW
(i) Royal Commission (1901) - Cobar/Byrock
"Generally speaking it was originally open box-forest country with currajong and an occasional pine tree upon it. The overstocking of the country, coupled with the rabbits, prevented the growth of grass to anything like its former extent, and so causes a cessation of bush fires which formerly had occurred periodically. This afforded the noxious scrub a chance of making headway."
(ii) Eric Rolls (1981) - Pilliga Scrub
"The [Cypress] pines came up ten thousand to the hectare. `One year the stockmen saw the little pines just up to the top of the horses `hooves', one man told me. `The next year the pine tops brushed their boots as they rode. And a year or two after that - those old stockmen used to ride at ten past ten, knees cocked up from the saddle like wings - well they had to jam their knees in hard behind the pads or the pines would have pushed them backwards out of the saddle. Soon they just mustered their stock and got out. There was no room for grass to grow'."
(c) Central/Western Queensland
(i) Walker et al. (1981) - Dirranbandi area
"Accounts of the poplar box region by early explorers indicate that the dominant trees had a spacing of between 50 and 100m with little understorey (Leichhardt 1847)" [Contrast with much higher poplar box densities present today].
(ii) Fensham (1998) - Fitzroy Basin, Central Queensland
"Preliminary analysis of the aerial photo comparison suggests substantial thickening has occurred in uncleared remnants in Central Queensland between the mean dates of the earliest photos (1955) and the mean date of the most recent photos". [Tree cover on 7 M ha had increased by > 20% - Rod Fensham pers. comm.].
(iii) Binnington (1997) - White Cypress Pine
"With current proposals to reserve 15% of the area of pre-European vegetation types there is concern about how to interpret the mixed cypress pine ecosystems - because of management changes white cypress pine forests currently cover a greater area than before European settlement." (see Appendix 3).
(iv) Reynolds and Carter (1993) - Mitchell grassland/gidgee
"Surveyed an area aggregating 5.97 ha in Central Western Queensland (315 respondents). Major findings were:
11% of the area of affected properties had been "lost" due to the invasion of woody weeds.
55% of respondents said their naturally timbered area had thickened up and this confirms the drift of grasslands towards woodlands."
[compare Mitchell's (1848) sketch of Salvator Rosa National Park with the recent photograph - Appendix 3]
(v) McCallum (1999) - Moorrinya National Park, Aramac
"Extensive changes in the area of Mitchell grassland have occurred with the woody plant cover (boree, gidgee, blackwood) increasing by 31.8% over the period 1951-1998" [Based on aerial photo interpretation and ground measurements]
(vi) Burrows (1996) - Gidgee, Longreach
Compared 13C values in surface litter and soil organic matter in two adjacent soil profiles (a) under Mitchell grass (Astrebla spp.) grassland and (b) gidgee (Acacia cambagei) juveniles at Strathdarr, Longreach (see Box 1). The 13C values at depth in (b) reflect that of the soil in the open grassland, supportive of grazier contention that gidgee has `recently' invaded.
(vii) Burrows et al. (1998) - Eucalypt woodlands, inland Queensland
"Historical, anecdotal and 13C data, along with permanent transect records (see Appendix 1 for current site distribution) together provide convincing evidence of woody plant thickening (increases in number and size) in the grazed woodlands of north-eastern Australia. In both Australia and elsewhere in the tropics/sub-tropics the woodlands (savannas) were maintained as a fire mediated sub-climax prior to the introduction of domestic livestock. The prime agent changing the structure of these woodlands has been the conversion of land use from hunter-gathering or nomadism to the raising of sheep and cattle - a practice which has often been associated with increased grazing pressure, reduced fuel loads and the containment of fire."
(viii) Purdie 1985 - Queensland's Mulga region
"Because of the widespread pastoral usage, ecosystems in virtually all areas of the mulga region have been affected to some degree by introduced stock and feral animals, and by specific land management practices such as the use of mulga for drought feed, and the thinning or clearing of mulga and other communities to encourage more herbage growth. Such disturbances have been more severe in the eastern part of the region, where there has been widespread introduction of non native pasture species …… or the complete removal of the native vegetation to allow agriculture cropping. As a result of this land use, the mulga region ecosystems can in no way be described as `pristine' i.e. identical with their pre-Aboriginal or pre-European settlement state." (see Appendix 3)
The extracts canvassed here are but a small representation of a much wider Australian and international literature that conclusively demonstrates that the vegetation present in to-day's grazed woodland/savanna communities is very different to that existing at the time domestic livestock were first introduced. To be scientifically credible conservation objectives for such woody vegetation need to be based on a more rigorous and defensible set of rules - than very challengeable assumptions that we can accurately relate the area occupied by vegetation communities as defined to-day, to the area these same communities might have occupied at European settlement.
Is tree clearing rehabilitation?
Blainey (1982) notes that without Aboriginal fires the grassy woodlands that occupied much of the fertile crescent in south-eastern Australian would have been scrubland or forest at the time of European occupation. Yet a period of fifty years was sufficient to change the character of this savanna country when fires were suppressed by Europeans and their livestock. Blainey concludes that "the widespread ringbarking (girdling) that was carried out around the turn of the twentieth century occurred in the regrowth (increased tree density). The landholders were attempting to re-establish the original grazing capacity".
According to the Oxford English Dictionary (OED) rehabilitation = restoration of proper condition. It is clear from the evidence presented here that the proper condition or original (pre-European) status of most of Queensland's grazed woodlands was as a more open grassy savanna - not the closed woodland structure with prominent shrub understoreys so prevalent today. Furthermore tree/shrub - pasture yield curves (Figure 1) clearly demonstrate that pasture yields fall as tree/shrub density, canopy cover or basal area increases. Significantly the most rapid drop in pasture production occurs at comparatively low tree "densities".
To paraphrase Blainey landholders clearing the woodlands in Queensland today are knowingly or unknowingly "attempting to re-establish the original grazing capacity" - as their forbears did in south-east Australia a century ago. In this light and the OED definition such clearing is plainly rehabilitation, especially when applied to land designated for sheep or cattle grazing.
Is tree clearing development?
According to the OED development = a gradual unfolding, fuller working out, growth; a full grown state; stage of advancement. Since a reduction in tree/shrub competition increases pasture production (Figure 1) it is obviously a stage of development with respect to livestock raising. In common usage "development" also implies an economic advantage accrues to the developer. This is quite evident from the analysis (Box 2) of different clearing strategies applied to a poplar box woodland (initial tree basal area c. 10m2/ha).
Even with a 20% tree retention policy the net present value of various clearing strategies over a 15 year time scale was $40-64/ha in comparison with not clearing (trees left intact). Corresponding internal rates of return varied from 16-28%.
On this basis tree clearing therefore certainly represents development. Nevertheless two factors discount the success of development from tree clearing. First, as a management intervention it rarely achieves its objectives of eliminating tree/shrub competition with pasture because of regrowth, enhanced seedling establishment or accelerated growth of woody plants which survive the initial clearing. Second, few take into account or acknowledge the effect of ongoing tree/shrub thickening in retained woodland areas. In these `protected' plots pasture production will continue to decline until tree-tree competition limits further woody plant thickening (Scholes and Archer 197) - the asymptote of the curves in Figure 1. In practice any tree retention, over and above normal management, imposed on grazing land will essentially take that land out of pasture production in the mid-long term. Scanlan and Burrows (1990) also demonstrated that the ground flora changes markedly as tree density/basal area steadily increases in eucalypt woodlands. In other words the vegetation `protected' will no longer be the same as it was at the time it was initially set aside.
The information presented has revealed an apparent paradox where tree clearing on grazing land today can simultaneously lead to both rehabilitation (of an `original' open vegetation structure) and development (increased pasture and livestock production). Can conservation also be accommodated? The data of Appendix 2 supports the case for retention of trees in intact blocks, strips or clumps to maximise pasture production for the same retained tree basal area. But we need to overcome the misconception that what we are conserving to-day is the same vegetation structure and composition as was present at European settlement. Plainly it is not.
Sustainable use of agricultural land does not necessarily require the conservation of all natural organisms, but rather the conservation (maintenance) of ecosystem processes (nutrient and hydrologic cycles, soil integrity etc). For example, in Queensland's Central Highlands ecosystems have been reconstructed to have greater agricultural productivity by replacing a tree with leaves unpalatable to livestock (Acacia harpophylla) by a tree which is highly palatable and nutritious (Leucaena leucocephala). Likewise the low productivity understorey grasses (Paspalidium spp, native Chloris spp.) have been replaced by Cenchrus ciliaris. This `new' system maintains the processes of the old, but is far more productive from an agricultural viewpoint. (see Appendix 4).
None of this is to rule out reasonable conservation objectives being applied to grazing land as were recently addressed, for example, by the Local Tree Clearing Guidelines developed for leasehold land in Queensland. However scientific rigour suggests vegetation retention levels should be based on to-day's vegetation structure and composition, not on questionable estimates of community distributions at the time domestic livestock were first introduced. Further, once retention levels are set it would be unreasonable to then limit the farmer - grazier from accessing the best world organism (plant or animal) which enables him or her to remain competitive (economically viable) with peers in this country or overseas. In the final analysis conservation objectives applied to grazing/ agricultural holdings will always need to be a compromise with the primary purpose of the land - the economic production of agricultural products. As Ainesworth (1989) self evidently observed `the only sustainable agriculture is profitable agriculture'.
Madonna Hoffmann assisted in data preparation. The financial support of Meat and Livestock Australia and the Statewide Landcover and Trees Study (SLATS) for many aspects of QBII's woodland research is much appreciated.
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Lunt, I.D. (1998b). Allocasuarina (Casuarinaceae) invasion of an unburnt coastal woodland at Ocean Grove, Victoria: Structural changes 1971-1996. Australian Journal of Botany 46: 649-656.
McCallum, B.S. (1999). Personal communication - based on "An investigation of native tree incursion into native grassland at Moorrinya National Park, North Queensland" Unpublished B. App. Sci. Hons thesis, JCU, Townsville.
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Scanlan, J.C. (1991). Woody overstorey and herbaceous understorey biomass in Acacia harpophylla (brigalow) woodlands. Australian Journal of Ecology 16: 521-529.
Scanlan, J.C. and Burrows, W.H. (1990). Woody overstorey impact on herbaceous understorey in Eucalyptus spp. communities in central Queensland. Australian Journal of Ecology 15: 191-197.
Scholes, R.J. and Archer, S.R. (1997). Tree-grass interactions in savannas. Annual Review of Ecology and Systematics 28: 517-544.
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Appendix 1 (cont'd)
Appendix 3 (cont'd)