professional papers
Appropriate assumptions: a preliminary assessment of a forest manager's ability to subsume mortality into yield
Sam Zirnhelt Introduction
T
he sustainability of Timberlands West Coast L t d ' s (TWC) management plans for West Coast New Zealand native beech forests is currently under debate. A key aspect ofthe debate surrounds predictions of harvesting influence on residual stand condition. The contrasting results obtained by two competing models are t h o u g h t to rest with the input assumptions controlling mortality (James, pers, comm., 1999). The TWC model, in contrast to that proposed by Murray Efford (Landcare, 1999), assumes that some natural mortality is subsumed into the harvested component (yield1) through selection of mortality-prone trees (Richards, pers, comm., 1999).This version of the model predicts little alteration of the forest due to harvesting. T h e Efford model assumes that the harvested component is additive to the natural mortality and, consequently, predicts a significant reduction in standing volume.
Mortality is a fundamental component of stand dynamics and must be appropriately simulated in models that project stand progression (Buchman et al., 1983). A clear understanding of issues related to pre- empting and subsuming mortality is essential to productive participation in a debate ultimately concerned with the 'sustainability' of native forest management.
As many other ecological, economic and social value issues are central to the comprehensive assessment of sustainability, a detailed evaluation of the sustainability of TWC's management plans is beyond the scope of this paper.
Sam Zirnhelt
School of Forestry, Canterbury University
A review of the international literature supports TWC's assumption that a portion of mortality can be subsumed into the yield. Examples and supporting theory are presented below in which this assumption is supported for specific forest types or management regimes.
A number of issues are identified that must be addressed in attempts to quantify the proportion of mortality that can be subsumed for a specific forest.
Some vital limitations to the validity of inferences that might be drawn from the examples presented to other forest types and management scenarios, and to predictions of sustainability, are briefly discussed.
The benefits to be derived from the ensuing discourse require a clear understanding and concise usage of some key terminology.
Clarifying the terminology
Subsuming mortality here refers to the capture of mortality through a selective harvest of mortality-prone or dead trees. This definition includes salvage2, and requires that mortality in the residual stand be reduced or recovered such that the harvested volume is not additive to other stand losses. Pre-emption of mortality refers to the capture of merchantable volume through harvesting before mortality occurs. This definition does not include salvage.
This distinction is important.
Salvage is not included in the definition of pre-emption in an effort to enhance clarity during the assessment of a manager's capacity to subsume mortality. Different issues and challenges affect pre-emptive capacity because of the requirement for prediction. The effectiveness of salvage operations on the other hand is primarily affected by detection.
T h e reader should also be cognizant of the differences between the two types of mortality; density-
dependent and density-independent mortality. Density-dependent factors include intra-specific competition whereas density-independent factors are predominantly external to the interactions between trees.
Supporting theory
A high proportion of total production in an unmanaged forest will be lost to death and decay. In a typical sawtimber plantation, up to one-half of the potentially merchantable volume may be lost to suppression. Smith (1986) notes that not all of the material removed in anticipation of loss need come from subordinate crown classes. Part ofthe volume can be removed in larger trees thus "forestalling the death and stimulating the growth of their subordinates".3 Smith et al. (1997) maintain that thinnings, designed to anticipate and salvage losses from natural suppression, are a proven means of increasing yield.
Although Smith (1986) or Smith et al. (1997) do not reference data to support their statements, other authors and studies (Drew and Flewelling, 1977; Oliver and Larson, 1996; Reukema and Bruce, 1978;
Smith and Hann, 1984; Westoby, 1984) support similar conclusions.
Thinning can mimic the release of growing space which occurs through natural death, while recovering virtually all merchantable volume that would otherwise be lost, if suppressed trees are harvested shortly before they die (Oliver and Larson, 1996; Reukema and Bruce, 1978).
Biological support for the fact that some predictable, thus pre- emptable, natural mortality occurs in plant communities is best encapsulated in literature devoted to the -3/2 self-thinning rule.
T h e -3/2 self-thinning rule describes the relationship between
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average plant size and density in even- aged, single-species plant populations (Oliver and Larson, 1996). From a management perspective, this theory is often of limited utility in other stand types.
T h e rule is based on intra- specific competition (Drew and Flewelling, 1977) and helps define the upper limit on biomass accumulation (Smith and H a n n , 1984). T h a t mortality is driven by biomass accumulation has been observed and recorded in forests around the world (Westoby, 1984).
T h e -3/2 rule is commonly applied in the development of stand- density management diagrams developed for the even-aged management of single-species plantations. When relative density (the actual number of trees in a stand divided by the maximum number of that average size that could exist) exceeds 0.55, trees begin to succumb to d e n s i t y - d e p e n d e n t mortality (Smith et al., 1997).
The rule thus forms the basis of management strategies designed to optimize yield by timing management interventions to avoid the zone of i m m i n e n t density-dependent mortality. T h e rule implies that targeted intervention such as selective harvesting or commercial thinning can p r e - e m p t mortality through m a n i p u l a t i o n of biomass accumulation based on good inventory data. Such interventions have been implemented successfully and documented worldwide (Omule, 1988; Myers and M a r t i n , 1963;
Kikuzawa, 1979).
Examples of subsumed mortality
A study by Omule (1988) on Douglas-fir in British Columbia found that thinned stands, over 35 years, lost 11% less total volume production to mortality than in comparable unthinned stands. In this case, the merchantable volume of mortality was captured (i.e., subsumed) t h r o u g h commercial thinning.
Kikuzawa (1979) used similar results to support a yield prediction method utilizing the yield-density diagram in which he assumed that tree
mortality after thinning is negligible.
Myers and Martin (1963) showed that mortality was reduced after a partial felling that targeted the oldest and least-vigorous trees.
Although some mortality was pre- empted, the proportion of the pre- empted volume that could be subsumed was restricted by the persistence of lightening, dwarf mistletoe, wind, and insects, which continued to cause unquantified mortality.
Unke (1985) presented two management scenarios designed to ameliorate the ecological and economic consequences of dieback in forests in Baden-Wurtemburg, Germany. Of the two, the more conservative estimated that 50% ofthe prescribed yield would consist of subsumed mortality. This is not inconsistent with a yield regulation system presented by Seydack (1990) in which a 15% reduction factor is used to compensate for irretrievable age-caused mortality.
This allows for imperfect identification of mortality-prone trees during pre-emptive harvest selection and some loss to waste and decay.
Seydack (1990) estimated that 85% of the i m m i n e n t mortality can be subsumed into the yield while maintaining high standing volumes.
The examples presented thus far indicate that at least some mortality can be pre-empted and subsumed into the yield through careful prediction and selection of density-dependent or age-caused mortality. In examples where the mortality was due to other factors such as unanticipated decline (Unke, 1985), no pre-emption was achieved.
However, a significant portion of the mortality was still subsumed into the yield through salvage operations.
Other examples can be cited in which subsumption efforts have failed.
For example, McGarity (1979) recorded similar mortality rates (sph) in both thinned and unthinned plots.
His evidence suggests that the volume of i m m i n e n t mortality removed during commercial t h i n n i n g was additive to that which subsequently occurred. Although the thinned volume represents pre-empted
mortality, it could not be subsumed into the yield. The presence of such examples does not reduce the validity of the thesis that mortality can be subsumed into the yield. Rather, they highlight the importance of examining the specifics of a manager's capacity to subsume mortality in relation to unique sets of circumstances.
Discussion: some key issues
If mortality can be predicted, theoretically, some portion can be pre- empted. The same proportion of the total mortality that can be pre-empted is necessarily subsumed into the yield.
The pre-emptive capacity of a management system is therefore dependent on its predictive capabilities. A clear determination of the likelihood of each tree's survival is essential for effective pre-emption of mortality (Buchman, 1983;
Seydack, 1995).
Some elements of mortality (e.g.
location, quantity or timing) are often largely unpredictable. Such is the case for stochastic events such as fire, flooding, earthquakes or wind. The effects of these disturbances on tree mortality are predominantly density- independent. The unpredictable nature of such events reduces the likelihood of pre-emption.
It should be noted that mortality due to other density-independent factors such as drought or old-age can be predicted to various extents by careful identification and removal of mortality-prone trees (Seydack, 1995).
A closer look at density- dependence
Significant mortality in forests worldwide results from episodic or stochastic density-independent events and should be considered when planning pre-emption and salvage operations (Morrison and Raphael, 1993). The nature of these events renders mortality rates in given stands difficult to measure accurately (James and Norton, 1999).
Although information on mortality is required to demonstrate the long-term sustainability of harvesting, researchers and managers have rarely compiled sufficient data to enable objective determinations
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(Whyte, 1999). As a result, density- independent mortality is difficult to pre-empt. Even if it were pre-empted, it is unlikely to be demonstrable.
Consequently, examples of effective capture of this type of mortality are restricted to salvage operations.
Salvage and sustainability
The ability of forest managers to pre-empt or subsume mortality, although perhaps a prerequisite of sustainable management, does not ensure it. Harvesting may result in mortality exceeding the productive capacity of the residual stand. If that mortality can be identified and captured, it is still subsumed into the yield.
Due to error in prediction or identification of mortality-prone trees incurred in pre-emption efforts, salvaging allows a greater proportion of the total mortality in most stands to be subsumed than can be achieved by pre-emption alone. Key limiting factors to the proportion of mortality subsumed through salvage include:
detection, wood quality, harvesting damage, and access.
Models
Numerous sophisticated yield regulation models exist for even-aged plantations. In these situations, as demonstrated by examples from the literature, mortality can often be successfully pre-empted. Fewer models have been produced for natural, indigenous forests (Vanclay, 1989).
An evaluation of the validity of those that do exist, particularly for mixed-species uneven-aged stands, is hampered by the complexities of forest dynamics, sampling constraints and long-term data requirements (Seydack, 1990; James and Norton, 1999; Whyte, 1999).
Modeling limitations have greater relevance to attempts to quantify the proportion of mortality that can be pre-empted or subsumed and to evaluate the sustainability of the yields generated than to a manager's actual pre-emptive capacity. Pre-emptive capacity is however improved by increased data availability, such as that required to
derive senility criteria enabling accurate selection of trees likely to die within the current cutting cycle (Seydack, 1995).
Modeling, although it may have overlapping data r e q u i r e m e n t s , cannot ensure the accurate selection of imminent mortality required for an effective pre-emptive harvest.
VI. Conclusion
The ability of forest managers to design effective mortality pre-emption strategies and control activities depends largely on their capacities to predict the quantity of mortality in both space and time. Density- dependent mortality is currently predicted and modeled with greater accuracy than density-independent mortality due in part to the near- universal applicability of density- dependent relationships in plant populations.
Pre-emption efforts are likely to be more successful when density- dependent mortality predominates, particularly in managed forests, with comprehensive inventory data.
Despite predictive difficulties associated with some density- independent mortality, in cases where intensive salvage operations are feasible, similar proportions of both density-dependent and density- i n d e p e n d e n t mortality may be subsumed into yield.
Many publications demonstrate pre-emption of mortality. Fewer sources provide or reference the appropriate long-term studies required to quantify the proportion of the total mortality pre-empted or subsumed.
Fewer still provide the data required to prove the sustainability of harvesting operations. Although the ability to pre-empt mortality can be an important component of sustainable yield regulation models or systems, the implications for sustainability are very site-specific and exceedingly complex.
With respect to the current debate surrounding TWC's beech management plan, the existence of international examples of successful
pre-emption and subsumption of mortality are insufficient to evaluate sustainability.
They do, however, help move the debate beyond the basic assumptions regarding the ability of forest managers to pre-empt and subsume a portion ofthe mortality to other p e r t i n e n t issues in the development and evaluation of sustainable management.
Acknowledgements
I thank Dr. Nora Devoe for her encouragement to write this paper and for her efforts in reviewing previous drafts.
References
Buchman, R.G., S.P. Pederson and N.R.Walters. 1983. A tree survival model with application to species of the Great Lakes region.
Canadian Journal of Forest Research. 13: 601-608.
Drew, J. and J.W. Flewelling. 1977.
Some recent Japanese theories of yield-density relationships and their application to Monterey Pine plantations. Forest Science. 23:
517-534.
James, I. 1999. Personal communication. Consultant for TWCL. Hokitika, New Zealand.
James, LL. and D.A. Norton. In Prep.
Sustainable management of a lowland New Zealand conifer forest. New Zealand School of Forstry, Christchurch, New Zealand.
Kikuzwa, K. 1979. A method for yield prediction utilizing the yield- density diagram. Journal of the Japanese Forestry Society 61(12):
429-436.
Landcare. 1999. Cited from: http://
www.landcare.cri.nz/
McGarity, R.W. 1979. Ten-year results of t h i n n i n g and clearcutting in a muck swamp timber type. Southern Journal of Applied Forestry 2 (2): 64-67.
Morrison, M.L. and M.G. Raphael.
1993. Modeling the dynamics of snags. Ecological Applications. 3 (2): 322-330.
Myers, C.A. and E.C. Martin. 1963.
Mortality of Southwestern Ponderosa Pine sawtimber after second partial harvest. Journal of
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Forestry. 61 (2): 128-130.
Oliver, C D . and B.C. Larson. 1996.
Forest Stand Dynamics. John Wiley and Sons, New York.
Omule, S.A.Y. 1988. Growth and yield 35 years after commercially thinning 50-year-old Douglas-fir.
FRDA Report No. 021. British Columbia Ministry of Forests and Lands, Research Branch, Victoria, BC, Canada.
Reukema, D.L. and D. Bruce. 1978.
Effects of thinning on yield of Douglas-fir: concepts and some estimates obtained by simulation.
USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, Portland, Oregon.
Seydack, A.H.W., J.C. van Daalen, D.
van Dijk, D. Reynell, H. Heyns, A.
Jooste, M. Ferguson, W. Pitchford, G. Durrheim and D. Willems.
1990. Yield regulation in the Knysna forests. South African Forestry Journal 152: 50-61.
Seydack, A.W. 1995. An u n c o n v e n t i o n a l approach to timber yield regulation for multi- aged, multispecies forests. Forest Ecology and Management 77:139- 153
Smith, D. 1986. The Practice of Silvilculture. John Wiley and Sons, Brisbane.
Smith, N.J. and D.W. Hann. 1984. A new analytical model based on the -3/2 power rule of self-thinning.
Canadian Journal of Forest Research. 14:605-609
Smith, D.M., B.C. Larson, M.J. Kelty, P. M. S. Ashton. 1997. T h e Practice of Silviculture: Applied Forest Ecology. John Wiley and Sons, Brisbane.
U n k e , T A 1985. Multi-level plan of the Baden-Wurtemberg State Forest Service for dealing with forest dieback. Allgemeine- Forstzeitschrift 14:312-315. Not seen in the original; cited from http :/library.canterbury.ac.nz: 890/
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Vanclay, J.K. 1989. A growth model for north Queensland rainforests.
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1 Yield is the merchantable volume extracted from a stand.
2 Salvage refers to the extraction of merchantable timber volume from trees already dead.
3 Smith (1986) p. 58
Epilogue
Subsequent to the submission of this article in October 1999, papers and presentations by Dr Vanclay, Dr Mason, Dr Whyte, and others have supported and expanded upon some of the key aspects presented.
Although there is still some debate about mortality pre-emption in recent news articles, the debate has moved on in many respects. This article was written to provide evidence for mortality pre-emption in the now- cancelled resource consent process.
Recent debates about the sustainability of indigenous forest management are troubling in their very apparent breaching of community development principles including inclusiveness, consensus, participation and empowerment.
Both sides are engaging in a confrontational approach which forces people into polarized camps from which, should they later choose to move, could not without some loss of dignity.
These issues are important when both sides purport to have the community development needs ofthe West Coast in mind. Current ecological and social development perspectives "emphasise the need for cooperative structures rather than competitive structures." (Ife, 1995 pl 96). Competitive processes, even within educational institutions, can stifle learning.
The manner in which the debate is largely being held restricts opportunities to acknowledge the merits of other's arguments.
Consequently, the beginnings of very good ideas are submerged in rhetoric
rather than being constructively integrated into a public process of learning about sustainability.
The combination of high personal stakes and the apparent need for short-term tactics in order to preserve long-term opportunity may compromise the realization of benefits through truly integrated resource management. The move to deliver an economic development package from a centralized government further violates principles of sustainable development that are broadly recognized in the international community. There is "the need to allow structures and processes to develop organically from the community itself" (Ife, 1995 pl 77).
The diversity of local cultural economic, social, ecological and political factors necessitate unique approaches in each community.
Sustainable development requires that local people are encouraged to take control of projects themselves and the community should seek to utilise its own resources wherever possible.
Difficulties arise when external factors restrict effective management of local resources.
Ife (1995) defines empowerment as "providing people with the resources, opportunities, knowledge and skills to increase their capacity to determine their own future, and to participate in and affect the life of their community." Clearly, pre-empting productive discourse on this issue is an act of disempowerment that marginalises the importance of sustainable resource management.
References
Ife, J. 1995. Principles of community development, in Ife, J., Community development:
creating community alternatives, Melbourne, Longmans: 177-199
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