Rotation-length effects of site preparation

Managers often have access to models of initial growth or results from experiments which quantify short-term gains from site preparation. In order to make good silvicultural decisions, however, they need estimates of the effects of costly treatments throughout a rotation

Snowdon & Waring (1984) identified two alternative long-term responses of plantations to site preparation treatments. Response type I represented an initial gain in productivity which was not sustained throughout the rotation, while response type II was a sustained growth increase. A type I response might result from a treatment such as weed control, which temporarily increased growth inputs to a stand and would be subsequently characterised by parallel yield trajectories, where the growth improving factor ceased to affect growth after a certain time.

The green curve, above, represents yield in an untreated stand. The orange curve is a treated stand exhibiting a type II response, while the grey curve is a treated stand exhibiting a type I, or parallel response.

I have proposed that gains should be measured in time rather than volume, and that a type I response should be defined as one where the time gain brought about by a treatment did not increase after a certain initial period of increased growth (Mason 1992). At least five assumptions would be necessary in order for a treatment to produce a type I response:

(i) the growth input change should be temporary, with site quality in the treated and untreated blocks returning to equivalent potential in due course;
(ii) the site should be capable of supporting more rapid growth, that is, the growth input increase should not result in some other limiting factor at some point during the crop rotation;
(iii) future treatments, such as thinning, should not bring about a resumption of the treatment effect;
(iv) there should be no significant change in allometric relationships caused by the treatment, such as ratios between root and leaf surface areas;
(v) there should be no significant physiological age differences between treated and untreated stands at respective times of equivalent yields.

In other words, a parallel response should occur if treated and untreated stands have identical states at the different ages when they have equivalent yields, and if future influences do not differ between stands.

Assumptions (iv) and (v) are the most likely to be violated. Root to shoot ratios, for example, have been shown to change in response to improvements in fertility (Nambiar 1980), and changes in specific leaf area (Beets & Lane 1987), allocation of carbon (Madgwick et al. 1977, Beets & Pollock 1987), and development of heartwood (Cown et al. 1991) through advancement of physiological age all change throughout a rotation. Nevertheless, apparent type I responses have been reported after weed control (Preest 1977, Mason & Milne 1999, Mason et al. 1997), soil cultivation (Wilhite & Jones 1981, Mason et al. 1988), and fertilisation (Woollons et al. 1988). Snowdon & Waring (1984) reported type I responses after weed control, and type II responses after fertilisation. Type II responses have also been reported for cultivation (Mason et al. 1997) and fertilisation (Mason & Milne 1999).

Eight experiments have been identified where plots are large enough and the experiments are old enough to allow quantification of the effects on growth of site preparation at mid-rotation. These experiments are being remeasured.

References

Beets, P.N., & P.M. Lane, 1987, Specific leaf area of Pinus radiata as influenced by stand age, leaf age, and thinning, New Zealand Journal of Forestry Science 17 (2/3); 283- 291
Beets, P.N., & D.S. Pollock, 1987, Accumulation and partitioning of dry matter in Pinus radiata as related to stand age and thinning, New Zealand Journal of Forestry Science 17 (2/3); 246-271
Cown, D.J., D.L. McConchie, & G.D. Young, 1991, Radiata pine wood properties survey, New Zealand Forest Research Institute Bulletin No. 50 (revised edition); 50 pp
Madgwick, H.A.I., D.S. Jackson, & P.J. Knight, 1977, Above-ground dry matter, energy, and nutrient contents of trees in an age series of Pinus radiata plantations, New Zealand Journal of Forestry Science 7 (3); 445-68
Mason, E.G., A.W.J. Cullen, & W.C. Rijkse, 1988, Growth of two radiata pine stock types on ripped, and ripped/bedded plots at Karioi Forest, New Zealand Journal of Forestry Science 18 (3): 287-96
Mason, E.G., 1992, Decision-support systems for establishing radiata pine plantations in the Central North Island of New Zealand, PhD Thesis, University of Canterbury, Christchurch, New Zealand: 301 pp
Mason, E.G., A.G.D. Whyte, R.C. Woollons & B. Richardson, 1997, A model of the growth of juvenile radiata pine in the Central North Island of New Zealand: Links with older models, and rotation-length analyses of the effects of site preparation, Forest Ecology and management 97: 187-195
Mason, E.G., & P.G. Milne, 1999, Effects of weed control, fertilisation and soil cultivation on the growth of Pinus radiata D.Don at mid-rotation in Canterbury, New Zealand, Canadian Journal of Forest Research, 29 (7): 985-992
Nambiar, E.K.S., 1980, Root configuration and root regeneration of Pinus radiata seedlings, New Zealand Journal of Forestry Science 10 (1); 249-263
Preest, D.S., 1977, Long-term growth response of Douglas fir to weed control, New Zealand Journal of Forestry Science 7 (3); 329-332
Snowdon, P., & H.D. Waring, 1984, Long-term nature of growth responses obtained to fertiliser and weed control applied at planting and their consequences for forest management, IN: Proceedings of the IUFRO symposium on site and site productivity of fast growing plantations, Pretoria and Petermaritzberg, South Africa; 701-711
Wilhite, L.P., & Jones, E.P. Jr., 1981, Bedding effects in maturing slash pine stands, Southern Journal of Applied Forestry 5 (1), pp 24-7
Woollons, R.C., A.G.D. Whyte, & D.J. Mead, 1988, Long-term growth responses in Pinus radiata fertiliser experiments, New Zealand Journal of Forestry Science, 18 (2); 199-209

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