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Summary of full report
By Ross J. Peacock
April 2008
RIRDC Publication No 08/043 RIRDC Project No UMA-18A
Executive Summary
What the report is about
This report examines the
effects of forest thinning, or thinning and grazing, on vegetation biodiversity
and structure in three long-term experiments in intensively managed forests.
The experiments were established in 1954, 1988 and 1991 and repeatedly
monitored until 2005. The report contrasts the initial trends available
in the first years after treatment to the medium-term results evident today,
and discusses their relevance to contemporary private native forest management.
Who is the report targeted
at?
This report is of significance
to policy makers and government regulators who need to assess competing
arguments for active versus passive management of forest landscapes, if
their regulatory regimes are to be defended. Forest managers, private landholders,
forestry researchers and environmentalists will also find this study particularly
relevant.
Background
Forest managers are increasingly
required to demonstrate to the community, government regulators and the
purchasers of forest products, that their activities are environmentally
sustainable. For forestry, thinning of Eucalyptus dominated regrowth forests
is seen as integral to managing wood flows and wood quality in native forests,
as well as providing a short-term cash flow from small poles, mining and
fencing timbers, salvage sawlogs or pulpwood. At the same time landholders
seek or are required by regulators to maintain or improve the biodiversity
value of their forest. However very little information currently exists
on the biodiversity response of forested vegetation in Australia to commercial
thinning or the combination of selective logging/thinning and cattle grazing.
This makes it difficult to assess that management regimes are appropriate
and will meet the varying landowner, site and community demands on forests.
Government reforms since the 1990s have sought to establish a clearer relationship between a secure native forest timber industry and improvements in landscape and site-specific biodiversity. One result of government-negotiated reductions in harvest from public native forests, can be increased intensity of management of regrowth forests – this is commonly a compensatory mechanism to maintain wood flows to industry despite the restriction in area used to calculate a sustainable yield.
There is a need to improve understanding of sustainable forest management practices and biodiversity responses; this also applies if stewardship schemes are implemented. Alongside this, long term responses of forests to disturbance and different silvicultural management must be understood, in order to effect appropriate policy.
Aims/ Objectives
The objective of this study
was to examine the medium-term effects of thinning or selective harvesting
on forest vegetation structure and composition, and therefore the potential
of the forest stand to regenerate following harvesting. It aims to assist
private foresters and land management regulators to assess proposed regrowth
thinning regimes in their region against their biodiversity objectives
or regulatory standards.
Method used
Three long-term experiments
were used to examine the response of vegetation to regrowth thinning or
selective harvesting with cattle grazing. The three experiments all differ
with respect to their location, climate, environmental characteristics,
forest type, silviculture and management inputs. At each experiment, a
variety of response variables were measured describing the forest structure,
its vegetation composition and individual tree characteristics such as
height, diameter and canopy dominance.
The experiment in the Florentine Valley of central Tasmania was initiated in 1991 to assess the response of vegetation to thinning treatments of varying intensity, implemented using either groundbased or cable harvesting techniques. The site was in even-aged 29 year old Eucalyptus regnans wet forest, originating from an intensive regeneration treatment. The climate is cool and wet; the site quality is high.
The experiment in East Gippsland Victoria was initiated in 1988 with the aim of assessing the vegetation response to regrowth thinning in a coastal lowland forest. The Eucalyptus sieberi (silvertop) ? Eucalyptus globiodea (white stringbark) dry forest was un-even aged, originating from a combination of selective harvesting events, and both planned and un-planned fires. The climate is warm and sub-humid; the environment is fire prone and relatively low in nutrients.
The experiment near Grafton in northern NSW was initiated in 1954 with the aim of describing the effect of cattle grazing on the regenerative response in an un-even aged stand of Corymbia henryi (spotted gum) ? Eucalyptus moluccana (grey box) dry open forest following selective harvesting.
The site is typical of a relatively low site quality, coastal forest environment subject to long term grazing, repeated low intensity thinning and logging events, and low to moderate planned and unplanned fires fires. The climate is sub-tropical.
Results/Key findings
Thinning in a single species
even-aged regrowth stand in southern Tasmania had an immediate effect on
the vegetation and biodiversity, correlated with the intensity of the thinning
operation and the degree of soil disturbance resulting from the application
of either cable or ground-based harvesting techniques. Ground-based thinning
created sufficient soil disturbance to encourage a new cohort of overstorey
trees to appear and for ruderal and weed species invasion. Cable thinning,
however, led to an expansion of clonal species such as rhizomatous ground
ferns. Height growth was similar following either of the four thinning
treatments, while the basal area increment varied widely across treatments.
Natural self-thinning in the control treatment of the overstorey trees
after fourteen years had equalled the stem density present on one of the
thinning treatments. Both thinning techniques led to a significant reduction
of the scattered tall shrub and small tree stratum as a result of log yarding
and felling disturbance. Floristic change immediately post-thinning was
of greater significance in the ground-based thinning treatment compared
to the un-thinned control. Differences which were clearly evident between
the thinning treatments in the initial period of monitoring proved to be
of less significance after fourteen years of study.
Thinning in a mixed species un-even aged stand in East Gippsland led to a large increase in total stem density over the seventeen year period, as a result of the in-growth component, despite thinning removing just over half the pre-treatment basal area. In thinned stands the most numerous diameter class was sub-dominant and in-growth stems, while the larger diameter stems retained the dominant crown classification. Changes in the understorey vegetation composition and structure following thinning and its associated site disturbance were abrupt. There was however little evidence indicating that those short term effects would persist after 17 years – the vegetation composition and structure resembled the regrowth forest prior to treatment. Plant species lost in one plot would re-appear in another, indicating the rapid turnover of species in a relatively diverse and nutrient poor coastal lowland forest. Species richness, for example, declined immediately after thinning, but exceeded pretreatment levels 17 years after thinning. Vegetation structure and complexity similarly increased post-thinning.
In northern New South Wales cattle grazing is commonly employed in open forest environments in association with timber harvesting, thinning and frequent low intensity fuel reduction burning. The effect, however, of ongoing cattle grazing following timber harvesting on seedling and coppice regeneration, survival and growth is poorly understood. A long term study examined the relationship between cattle grazing and density, composition and height growth of eleven overstorey tree species following harvesting in 1954. In the unfenced treatment, 65% of the low lignotuberous regeneration stems present after timber harvesting remained in the lowest height category for the first 2-4 years. In the fenced treatment however, these stems rapidly moved into the taller height categories. A lag of four years in tree growth rates in the unfenced treatment compared to the fenced treatment suggested grazing led to slower growth rates for stems under one metre tall. The differences between treatments became less apparent over time and were negligible after 12 years. Although the effect of grazing on forest structure does not appear to be long-term in this particular forest type, due to the lignotuberous regeneration strategy of the dominant tree species, excluding cattle for the first 2?10 years after harvesting can ameliorate the short-term impacts of grazing on regeneration of forest trees and potentially of other forest biota.
Implications for relevant
stakeholders
The results of these studies
provide one of the few examples of medium-term studies examining the vegetation
and plant biodiversity response to thinning in southern Australia. Results
elsewhere from the literature, indicating short-term effects on species
composition and structure, appear simply that ? short term in the vicinity
of years. The medium-term results demonstrate the resilience of the vegetation
to this form of intermediate disturbance, assuming soil disturbance from
thinning is kept to a minimum.
The biodiversity responses described differed between treatments, forest types and with time. The results presented here are considered localized to the vegetation types examined, and to some extent the nature of the thinning operations. The results provided should be assessed along with other influences on vegetation structure and composition, particularly repeated burning and the initial floristic composition at the time of treatment. Nonetheless several trends were apparent which are consistent with the broader literature on forest vegetation responses to harvesting disturbance.
Private foresters and land management regulators can use this knowledge to help them assess proposed regrowth thinning regimes in their region against their biodiversity objectives or regulatory standards.
Recommendations
The following are the key
recommendations provided for private forest managers: Biodiversity, soil
and logging and grazing management
Further study
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