4.1 Productivity change in successive rotations of forest plantations
4.2 Within-rotation yield class/site quality drift
4.1.1 Data limitations
4.1.2 Spruce in Saxony and other European evidence
4.1.3 Pinus radiata in Australia and New Zealand
4.1.4 Pinus patula in Swaziland
4.1.5 Chinese fir in sub-tropical China
4.1.6 Teak in India and Java
4.1.7 Southern pines in the United States
4.1.8 Other evidence
The long cycles in forestry make data collection difficult. Records are rarely maintained from one rotation to the next; funding for long term monitoring is often a low research priority; measurement conventions may change which confound ready comparison; detection of small changes is difficult; and often the exact location of sample plots is poorly recorded (Evans 1984). Moreover, few forest plantations are second rotation, and even fewer third or later rotation, thus the opportunity to collect data has been limited. Unfortunately without data it is difficult to demonstrate whether plantation silviculture is robust and genuinely determine the impacts of successive rotations.
The few comparisons of productivity between rotations have mostly been initiated because of concern over yields, namely ‘second rotation decline’, or about stand health. Thus the focus has been on problems, while the vast extent of plantations where no problems are recorded suggest that managers are not encountering obvious decline problems. Thus data available in the older literature may be biased to problem areas. More recent studies may be less so, such as the European Forestry Institute survey (Spiecker et al. 1996) and CIFOR’s ‘Site management and productivity in tropical forest plantations’, as these incorporate systematic establishment of sample plots.
For forest stands hard evidence of productivity change over successive rotations is meagre with few reliable data. Information is available from only four major studies. There are also some anecdotal evidence or one-off investigations.
The first serious evidence of possible yield decline appeared in Germany in the 1920s but subsequent investigations have not confirmed an extensive problem. Indeed, in Europe today current growth rates generally exceed those of 50 years ago - see 4.2 below.
Significant yield decline in second rotation P. radiata appeared in South Australia in the early 1960s (Keeves 1966) with an average 30 per cent drop. In the Nelson area in New Zealand, on a few impoverished ridge sites there was transitory second rotation yield decline (Whyte 1973). These reports, particularly from South Australia, were alarming and generated a great deal of research. By 1990 it was clear for South Australia that harvesting and site preparation practices, particularly burning, which failed to conserve organic matter and an influx of weeds, especially grasses, in the second rotation were the main culprits. By rectifying these problems and using genetically superior stock second and third rotation pine now grow substantially better than the first crop (Boardman 1988; Nambiar 1996; Woods 1990). Elsewhere in Australia second rotation crops are mostly equal or superior to first rotation (Evans 1999b)
In New Zealand the limited occurrence of yield decline was mostly overcome by site preparation and establishment methods, such as using nursery stock rather than natural regeneration (A.D.G. Whyte and D.J. Mead pers. comm.). The decline could have been partly due to changing weed species. On the great majority of sites successive rotations have grown faster. Dyck and Skinner (1988) conclude that inherently low quality sites that are managed intensively may be susceptible to productivity decline.
Long-term productivity research by the writer in the Usutu forest, Swaziland began in 1968 as a direct consequence of second rotation decline reports from South Australia. For 32 years measurements have been made over three successive rotations of P. patula plantations, grown for pulpwood, from a forest-wide network of long-term productivity plots (Evans 1996, 1999a, Evans and Boswell 1998). Plots have not received favoured treatment, but simply record tree growth during each rotation resulting from normal forest management over 62000 hectares by SAPPI Usutu.
The most recent analyses show second and third rotation growth data obtained from plots on exactly the same sites (Tables 1 and 2). First rotation growth data were derived from stem analysis and from paired plots and are less accurate: some of those data were reported in Evans (1996).
Table 1: Comparison of second and third rotation Pinus patula on granite and gneiss derived soils at 13/14 years of age (means of 38 plots).
|
Rotation |
stocking (stems ha-1) |
Mean height (m) |
Mean DBH (cm) |
Mean tree vol. (m3) |
Stand volume (m3ha-1) |
|
Second |
1386 |
17.5 |
20.1 |
0.205 |
294 |
|
Third |
1248 |
18.7 |
21.2 |
0.233 |
326 |
|
% change |
|
+6.9 |
+5.5 |
|
+11.0 |
Source: updated from Evans (1999a)Table 2: Comparison of second and third rotation Pinus patula on gabbro dominated soils at 13/14 years of age (means of 11 plots)
|
Rotation |
Stocking (stems ha-1) |
Mean height. (m) |
Mean DBH (cm) |
Mean tree vol. (m3) |
Stand volume (m3ha-1) |
|
Second |
1213 |
16.7 |
20.0 |
0.206 |
244 |
|
Third |
1097 |
16.8 |
21.7 |
0.227 |
255 |
|
% change |
|
+0.5 |
+8.5 |
|
+4.5 |
Source: updated from Evans (1999a)These results are from arguably the most accurate data set on biological sustainability. Over most of the forest where granite derived soils occur (Table 1) third rotation height growth is significantly superior to second and volume per hectare almost so. There had been little difference between first and second rotation (Evans 1978). On a small part of the forest (about 13% of area), on phosphate-poor soils derived from slow-weathering gabbro, a decline had occurred between first and second rotation, but this has not continued into the third rotation where there is no significant differences between rotations (Table 2).
No fertiliser addition or other ameliorative treatment has been applied to any of the long-term productivity plots. According to Morris (1987) some third rotation trees are probably genetically superior to the second rotation. However, the 1980s and especially the period 1989-92 have been particularly dry with Swaziland suffering a severe drought (Hulme 1996; Morris, 1993a). This will have adversely impacted third rotation growth.
The results are also important because silviculture was intensive as anywhere but with no thinning or fertilizers, the stands are monocultures, and the rotation of 15-17 years is close to the age of maximum mean annual increment. Large coupes are clearfelled and all timber suitable for pulpwood extracted. Slash is left scattered (i.e. organic matter conserved) and replanting done through it. So far, there is no evidence of declining yield. The limited genetic improvement of some of the third rotation could have disguised a small decline, but evidence is weak. Overall, the evidence suggests no serious threat to sustainability.
There are about 6 million hectares of Chinese fir (Cunninghamia lanceolata) plantations in subtropical China. Most are monocultures and are worked on short rotations to produce small poles, though foliage, bark and even sometimes roots are harvested for local use. Reports of significant yield decline have a long history. Accounts by Li and Chen (1992) and Ding and Chen (1995) report a drop in productivity between first and second rotation of about 10 percent and between second and third rotation up to a further 40 percent. Ying and Ying (1997) quote higher figures for yield decline between first and second rotation of 29 per cent poorer height and 26 per cent less volume. Chinese forest scientists attach much importance to the problem and pursue research into monocultures, allelopathy, soil changes etc. Personal observation suggests that the widespread practices of whole tree harvesting, total removal of all organic matter from a site, and intensive soil cultivation that favours bamboo and grass invasion all contribute substantially to the problem. Ding and Chen (op. cit.) concluded that the problem is “not Chinese fir itself, but nutrient losses and soil erosion after burning (of felling debris and slash) were primary factors responsible for the soil deterioration and yield decline ... compensation of basic elements and application of P fertilizer should be important for maintaining soil fertility, and the most important thing was to avoid slash burning... These (practices) ... would even raise forest productivity of Chinese fir.” (words in parentheses added by writer).
In the 1930s evidence emerged that second rotation teak (Tectona grandis) crops were not growing well in India and Java (Griffith and Gupta 1948). Although soil erosion is widespread under teak and loss of organic matter through burning leaves is commonplace, the research into the ‘pure teak problem’, as it was called in India, did not generally confirm a second rotation problem. However, Chacko (1995) describes site deterioration under teak as still occurring with yields from plantations not coming up to expectation and a generally observed decline of site quality with age. He indicates four main causes: poor supervision of plantation establishment; over-intensive commercial taungya (intercropping) cultivation; delayed planting; and poor after-care. Chundamannii (1998) similarly reports decline in site quality over time and blames poor management. In Java site deterioration is still a problem and “is caused by repeated planting of teak on the same sites” (Perum Perhutani 1992).
Concern about successive teak crops, soil erosion and loss of organic carbon, has also been reported from Senegal (Mahuet and Dommergues 1960).
Plantations of slash (P. elliottii) and loblolly (P. taeda) pines began in mid 1930s as natural stands were logged out (Schultz 1997). With rotations of 30 years or more, some second rotations commenced in the 1970s. In general growth of the second crop was variable (Evans 1999b). A co-ordinated series of experiments, throughout the USA, is currently assessing long-term impacts of management practices on site productivity, but it is too early for results (Powers et al. 1994).
Other evidence is limited or confounded. For example, Aracruz Florestal in Brazil has a long history of continually improving productivity of eucalypts owing to an imaginative tree breeding programme so that regularly new clones are introduced and less productive ones discontinued (Campinhos and Ikemori 1988). The same is true of the eucalypt plantations at Pointe Noire, Congo (P. Vigneron pers comm.). Thus recorded yields may reflect genetic improvement and disguise any site degrade problem. It is clear that greatly increased productivities are being achieved in practice; the Aracruz sites appear capable of supporting productivities up to 60 or 70 m3ha-1y-1.
In India (Das and Rao 1999) claims massive yield decline in second rotation clonal eucalypt plantations which the authors attribute to very poor silviculture.
At Jari in the Amazon basin of Brazil silvicultural practices have evolved with successive rotations since the first plantings between 1968 and 1982. A review of growth data from the early 1970s to present day suggest that productivity is increasing over successive rotations due to silvicultural inputs and genetic improvement (McNabb and Wadouski, in press).
In Venezuela, despite severe and damaging forest clearance practices, second rotation Pinus caribaea shows substantially better early growth than the first rotation (Longart and Gonzalez 1993).
For long rotation (>20 years) crops it is usual to estimate yield potential from an interim assessment of growth rate early in life and then to allocate a stand to a site quality class or yield class. A change from predicted to final yield can readily occur where a crop has suffered check or other damage in the establishment phase or fertiliser application corrects a specific deficiency. However, there is some evidence for very long rotation (>40 years) crops in temperate countries that initial prediction of yield or quality class will underestimate final production. Either the yield models used are now inappropriate or growing conditions are ‘improving’ in the sense of favouring tree growth. Across Europe the latter appears to be the case (Spiecker et al. 1996; Cannell et al. 1998) and is attributed to rises in atmospheric CO2 and nitrogen input in rainfall, better planting stock and cessation of harmful practices such as litter raking.
Closely related is the observation that planting year is often positively related to productivity, with recent crops being more productive than older ones, regardless of inherent site fertility. This shift is measurable and can be dramatic, as seen, for example, in Australia (Nambiar 1998). Attempts to model productivity in Britain on the basis of site factors have often been forced to include planting date as a variable. Maximum mean annual increment of Sitka spruce increased with planting date in successive decades by 1 m3ha-1y-1 (Worrell and Malcolm 1990) and for Douglas fir, Japanese larch (Larix kaempferi) and Scots pine (Pinus sylvestris) by 1.3, 1.6 and 0.5 m3ha-1y-1 respectively in each succeeding decade (Tyler et al., 1996). This phenomenon suggests that some process is favouring present growing conditions over those in the past - perhaps the impact of genetic and silvicultural improvements or cessation of harmful ones, and possibly the atmospheric changes mentioned above. Broadmeadow (2000) confidently predicts an increase in productivity for forests in United Kingdom owing to climate change.
The impact of these observations is that present forecasts of plantation yields are likely to be underestimates, as yields appear to be increasing. The one main exception, as noted earlier, is teak where initial site productivity estimates have been revised downward as stands age.
