Differences in maximum resistive bending moments of Pinus radiata trees grown on a range of soil types
Introduction
The plantation forestry estate in New Zealand covers a total area of ≈1.63 million hectares (Ministry of Forestry, 1997) and is distributed over a wide range of soil types. Radiata pine (Pinus radiata D. Don) is the predominant species occupying ≈91% of this area. Since records began, wind damage in the form of stem breakage and uprooting has occurred throughout much of the country (Wendelken, 1955, Wendelken, 1966, Prior, 1959, Chandler, 1968, Irvine, 1970, Wilson, 1976, Somerville et al., 1989, Somerville, 1995).
Keeping in mind the threat posed by wind, a model which uses a combination of fundamental physics and empirical data is being developed to predict the risk of damage (Moore and Somerville, 1998). The model predicts the threshold wind speed necessary to damage the mean tree within a stand of given diameter at breast height (DBH), height and spacing, and which is growing at a particular location. Within the model a tree is assumed to fail if the applied overturning moment exceeds the maximum resistive bending moment (Mb) of the tree. Mb is defined as the maximum resistance of the tree stem to failure or the root system to overturning, with the relative strengths of these determining the mode of failure (Petty and Worrell, 1981).
In order to determine Mb for trees with certain characteristics and growing conditions, a number of authors have used winch and cable systems to apply artificial wind loads to trees. One of the first such studies was carried out by Fraser (1962) who found that a linear relationship existed between Mb and stem weight for Sitka spruce (Picea sitchensis Bong Carr.). Other authors (Fraser and Gardiner, 1967, Smith et al., 1987, Frederickson et al., 1993) have obtained similar results.
In New Zealand two previous tree winching studies have been performed (Somerville, 1979, Papesch et al., 1997). The latter found that strong linear relationships existed between Mb and both stem volume and DBH for the range (DBH=18.2–63.9 cm) of Radiata pine trees measured at Eyrewell Forest, Canterbury. In the same forest, Somerville (1979) investigated the effect of different site preparation techniques on Mb. He found that there was a tendency for trees growing in deep rips to fail by stem fracture rather than by uprooting but that this was not accompanied by a significant increase in Mb. This indicates that root architecture has an influence on the failure mode of a tree. Further observational evidence was provided by Wilson (1976) who observed that the incidence of stem breakage during the 1975 storm in Eyrewell Forest was higher where soils were deeper.
The wind forces acting on tree stems have been the subject of several studies (Petty and Swain, 1985, Milne, 1995, Wood, 1995) while Coutts, 1983, Coutts, 1986 performed some of the first work investigating the mechanics of tree root anchorage. He used static tree pulling tests to identify the four main components of the root anchorage of shallowly rooted Sitka spruce. These were (1) tensile strength of windward roots, (2) weight of the root–soil plate, (3) resistance of leeward roots to bending at the hinge region and (4) the resistance to failure of the soil underneath the root–soil plate. The shallow rooting of the trees in these studies was due to the presence of a high water table; a condition relatively uncommon in New Zealand. However, for hybrid larch (Larix europea×japonica) growing on a free draining soil, Crook and Ennos (1996) found that ≈75% of the anchorage strength was provided by the windward sinkers and tap root. Because the root–soil plate is a compound structure the material properties of the soil are also very important in determining overall root anchorage strength (Mattheck et al., 1997).
The physical properties of forest soil types in New Zealand vary widely and it is therefore possible that the relationship between Mb and tree size will also differ between soil types. More than one function for Mb may therefore be necessary in the model to predict the risk of wind damage to forest stands. This paper investigates the relationship between Mb and tree size by performing a series of tree winching experiments on trees of different sizes growing on a range of New Zealand forest soils types. The effects of tree taper, root plate size and differences in Mb between modes of failure are also investigated.
Section snippets
Site and resource descriptions
Data were collected from seven sites in New Zealand which contained soils with widely varying physical properties (Table 1). Limited data were available on the physical properties of the soils at the seven sites. (DSIR Soil Bureau, 1954, DSIR Soil Bureau, 1968a, and unpublished data). No information was available on rooting depth in soils at Maramarua Forest; however the maximum penetration resistance of the nearby Naike clay soil is 2.54 MPa (DSIR Soil Bureau, 1968b). Radiata pine root
Prediction of Mb from tree characteristics
Since the variance of Mb increased with increasing predicted values, a logarithmic transformation was performed to stabilise this. In order to maintain linear relationships between Mb and each of the potential explanatory variables, logarithmic transformations were also applied to height, DBH and stem volume. Significant linear relationships (p<0.001) existed between Mb, and tree height, DBH and stem volume (Table 3, relations (1)–(3)). The H/DBH ratio of sampled trees ranged between 32 for the
Discussion and conclusions
Failure mode is closely linked to soil type. Ninety-two percent trees failed by uprooting on non-cohesive soils but only 11% failed by this mode on clay soils. Mergen (1954) stated that the distribution and anchoring ability of tree roots are affected by soil texture and consistency. Stronger root anchorage prevents a tree from uprooting and the force is transferred to the tree stem resulting in stem breakage (Coutts, 1986). The factors determining consistency are cohesive and adhesive strength
Acknowledgements
This study was funded by the Ministry of Research, Science and Technology under contract CO4302 and by the Forest Research/Industry Wind Risk Modelling Multi-Client Agreement which comprises CHH Forests Ltd., Fletcher Challenge Forests Ltd., Selwyn Plantation Board Ltd. and Wenita Forest Products Ltd. Todd Cheeseman, Marcel Griffiths and James McEwan helped with the tree winching. Drs. Douglas Maguire, Malcom Skinner, Mr. Piers McLaren and two anonymous reviewers provided useful comments on
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Present address: Department of Forest Resources, Oregon State University, Corvallis, OR 97331, USA.