Andrew R. Morris

Dry Matter and Nutrients in the Biomass of an Age Series of Pinus patula Plantations in the Usutu Forest, Swaziland

SYNOPSIS The dry matter and nutrient content of 11 stands of Pinus patula, one to 21 years old, were determined by field sampling. Treating these stands as an age series sample of a single yield class the pattern of accumulation with time was examined. At rotation age (17 years) the biomass contained 551 kg/ha N, 73 kg/ha P, 383 kg/ha K, 238 kg/ha Ca and 88 kg/ha Mg. Annual rates of nutrient accretion into biomass peaked at six to eight years, when the rate of canopy development was greatest. Changes in the mass of needles per hectare followed closely the pattern of dry matter accretion in the total biomass, peaking at 10 to 11 years. The influence of biomass accretion and harvest log removal of nutrients on productivity of the stand is discussed.

Forest floor accumulation, nutrition and productivity of Pinus patula in the Usutu Forest, Swaziland

Forest floor mass was determined at 22 sites under 11- to 15-year-old 2R and 3R P. patula that had been reestablished without prior burning of harvesting slash. Forest floor mass increased with site elevation (r=0.593) or with decreasing topsoil exchangeable Ca in particular (r=0.699). The forest floor was found to be greater than reported values for either 1R stands or 2R stands re-established following slash burning at similar age and elevations in the forest. Responses to N fertilizer applied at age 10–12 years, at 9 sites, was closely correlated with forest floor mass (r=0.911). The data suggests that continued accumulation of forest floor under successive rotations in the Usutu Forest, particularly at higher elevations, results in N deficiencies that limit growth in 2R stands. This hypothesis is supported by an analysis of data from sample plots monitoring relative 1R and 2R growth in the forest.

Survival and early growth effects of some re-establishment pratices with Pinus patula

SYNOPSIS The presence of harvesting residues (slash) has previously been found to impact negatively on the early survival of tree seedlings. The aim of the trials discussed in this paper was to investigate ways of improving seedling survival when re-establishing stands with high slash loads. The trials spanned three planting dates throughout the summer planting season of 1997, and investigated whether manual clearing of slash from the pit and the application of insecticide, fungicide and water at planting could improve the survival of Pinus patula seedlings. In both of the trials, slash was found not to affect seedling survival. This could have been due to the good rainfall occurring during the planting season. The application of insecticide was found to improve survival at some planting dates. The main insect pest was Hylastes angustatus. Variation in the efficacy of the insecticide treatment at the various planting dates may have been related to insect population dynamics across the planting period, and to the rainfall occurring after planting. The application of fungicide was found to significantly improve survival in five out of six of the planting dates. There appears to be an inverse relationship between the efficacy of the insecticide and the fungicide depending on the time of planting. When the insecticide was ineffective in improving the survival, the fungicide significantly improved survival and vice versa. Clearing of the slash from around the pit improved seedling growth at 8 months. The application of insecticide or fungicide also improved the early growth rates. The results of these trials indicate that a combined application of insecticide and fungicide would substantially improve survival and reduce the risk of planting failure. Such improvements are sufficiently large to eliminate the need to blank in 80% of the planting dates investigated in these trials.

Site and stand age effects on fertiliser responses in Pinus patula pulpwood plantations in Swaziland

Synopsis Six fertiliser trials were establishedin separate stands in their 4th , 7th or 12th year on both granite and gabbro derived soils intheUsutu Forest, Swaziland. All trials compared the same factorial combination ofN, P and K fertilisers. Tree growth response to fertiliser was measured five years after application. The trials demonstrate that meaningful responses to fertiliser can be obtained whenapplied to established pulpwood stands without thinning. Both stand age at application and soil parent material were found to influence response to fertiliser. With K, positive responses were obtained in the 3 and 6 year old stands but negative responses were evident applied to 11-year-old stands. Positive responses to K were also more pronounced on gabbro derived soils. Tree growth was improved by P applied in 6- and ll-year-old stands and byN applied in Ll-year-old stands. Results are discussed in relationto nutrient cycling, stand development, soil fertility and records oflong-term productivity at Usutu. It is proposed that observed declines in second rotation productivity in block A of the forest are associated with gabbro-derived soils and that this decline can be corrected byfertiliser application.

Culture-independent detection and quantification of Fusarium circinatum in a pine-producing seedling nursery

The primary symptoms associated with Fusarium circinatum infection in pine seedling nurseries are root and collar rot, shoot and tip die-back and seedling mortality. Management of this pathogen in nurseries usually involves the integration of various strategies relating to sanitation, insect control and fungicide treatment. The overall goal of this study was to use quantitative real-time PCR (qPCR) to detect and quantify the airborne inoculum of F. circinatum in a commercial pine seedling nursery. For this purpose, an existing qPCR method was optimised and evaluated for its efficacy to quantify and monitor airborne conidia over a one-year period. Results showed that F. circinatum occurred at relatively low levels in the nursery throughout the year and that its distribution was spatially sporadic. The data suggest that standard nursery sanitation practices in the test nursery maintained the airborne inoculum of F. circinatum at low levels. The uneven distribution of infection also suggests that airborne inoculum does not represent the primary source of inoculum for the F. circinatum-associated seedling disease.

Growth and yield as an indication of sustainable forest management in industrial plantations

Synopsis Evidence from growth and yield measurements of successive rotations of industrial plantations is reviewed. The objectives, methodologies and limitations of the evidence is discussed and suggestions made concerning appropriate aims and methods for such work. Reported comparisons of forest productivity between successive rotations of Pinus radiata, P. patula, P. taeda and P. elliottii demonstrate that yield declines can occur when inappropriate forestry practices are applied to specific sites. Identification and understanding of these effects appear to have been initiated by data from simple growth comparisons between rotations. In these studies the observation of decline in successive rotations was followed by empirical and process studies to establish a cause. This in turn leads to modification offorestry practice to correct the decline. These experiences suggest two important objectives for monitoring of growth and yield in successive rotations; firstly, as a physical check that the actual trend in yield between rotations is consistent with expectation and secondly, as a tool to elucidate the cause of observed change in productivity between rotations. The chosen methodology must relate to the specific objective of the study. The lack of comparative data from successive rotations of fast-grown Eucalyptus plantations is highlighted.

Effect on nursery and field performance of Pinus patula seedlings after inoculation with Fusarium circinatum

Fusarium circinatum is an important fungal pathogen of Pinus species. In South Africa, it is the most significant pathogen of Pinus patula seedlings in forestry nurseries where it presents a substantial constraint to productivity and can continue to cause mortality in-field for up to two years after establishment. This study describes the results from two trials where P. patula seedlings were inoculated with F. circinatum to determine the impact of the pathogen on nursery and field performance. Seedlings were also subjected to water stress treatments to ascertain whether this would trigger the onset of disease symptoms. Inoculum load and timing of inoculation had significant effects on seedling survival in both the nursery and field. High inoculum concentrations caused greater levels of mortality and, where seedlings were inoculated at a young age, they showed higher levels of susceptibility to F. circinatum. Temporary water-stress in the nursery produced smaller plants and improved in-field survival, but this treatment did not trigger higher mortality in inoculated treatments. On the other hand, transplant stress was a major contributor to the higher levels of mortality observed in inoculated treatments. Overall, these studies confirmed that infection in the nursery leads to the disease problems observed during early plant establishment in the field.

Effects of temperature on Pinus patula seedlings growing in pots in a controlled environment

Synopsis The re-establishment of Pinus patula seedlings into sites with high harvesting residue (slash) loads can negatively affect the survival of these plants. Field trials have examined the role that insect pests and fungal diseases play in causing this phenomenon. Research has also indicated that temperatures at ground level tend to be higher in the slash during the day, com pared to areas where slash had been removed. The current investigation aimed to determine whether the high temperatures experienced in the slash are likely to be the sole causal agent for the observed mortality. Pots containing P. patula seedlings were exposed to four different temperatures (26°, 32°, 38° and 44° C) in an illuminated growth chamber. The duration of exposure was 1, 2 or 3 hours. The exposure to the different temperatures was repeated on 7 days in a 10 day period. Respiration rates increased significantly at the high temperatures, while the chlorophyll a, band total chlorophyll concentrations were significantly reduced at the highest temperature. Increasing temperature also significantly reduced both height and new root growth in the seedlings during the experiment. The results indicate that, although no mortality was observed during the experiment, continued exposure to high temperatures over an extended period has the potential to cause seedling mortality. However, it is hypothesised that high temperatures predispose the seedlings to attack by pests and pathogens on site.

Re-use of seedling containers and Fusarium circinatum association with asymptomatic Pinus patula planting stock

Fusarium circinatum is a pathogen causing serious post-planting mortality of Pinus patula seedlings in southern Africa. Containerised planting stock that is asymptomatic but associated with F. circinatum in the nursery is thought to be the cause of this problem. The aim of this study was to determine if re-use of seedling containers could be a source of inoculum resulting in asymptomatic planting stock and increased post-planting mortality of P. patula. Two experiments were conducted in successive years comparing nursery cull of symptomatic seedlings, seedling growth, association of F. circinatum with asymptomatic seedlings and post-planting mortality for crops raised in re-used containers, with and without sanitation, and factory-new containers. Each experiment consisted of a nursery production trial followed by out-planting into pots to assess post-planting mortality. Our results show that re-use of containers without sanitation increases the cull of symptomatic seedlings, incidence of F. circinatum associated with asymptomatic seedlings and post-planting mortality compared with the re-use of containers after steam sanitation or factory-new containers. Growth of asymptomatic seedlings was unaffected by container treatment or association with F. circinatum and in the absence of wilt symptoms the root system did not exhibit typical discolouration. Watering frequency did not influence post-planting mortality in pots. The comparison of two open-pollinated seed mixes of P. patula that, based on seedling stem inoculation screening, represented susceptible and tolerant material did not show differences in nursery cull or post-planting mortality. This work demonstrated that natural contamination of re-used containers can be a primary source of inoculum producing asymptomatic seedlings associated with F. circinatum that will succumb to the pathogen after field planting. The process of seedling infection, apparent latent infection in the seedling and expression of disease after planting needs greater understanding to improve nursery hygiene measures to control this disease.

Rotation-end financial performance of vegetation control on Eucalyptus smithii in South Africa

Control of competing vegetation during eucalypt regeneration is important but the cost-benefit of varying levels of control is not well understood. A trial was initiated to quantify the impact of five vegetation control treatments on Eucalyptus smithii R.T.Baker. to determine the potential loss in growth and differential costs against which this growth loss could be linked. Five treatments were developed to suit the predicted vegetation load at the site and varied according to weeding intensity (high, moderate and low weeding) or area around the tree that was weeded (no vegetation control, a 2 m row weeding and complete vegetation control). The vegetation control treatments were imposed at planting and maintained until canopy closure (16.4 months). Tree growth was monitored throughout the rotation and this, together with the cost of the various establishment and weeding operations, were used to make treatment comparisons. Weed growth was rapid following planting, with competition-induced divergence in tree growth occurring from 52 d. Rotation-end volume for the Weedfree check (414 m3 ha−1) and Row weeding treatment (394 m3 ha−1) were significantly different from the Weedy check (319 m3 ha−1), with the Moderate (374 m3 ha−1) and Low (371 m3 ha−1) weeding intensity treatments intermediate but not significantly different from each other, nor from the other treatments. The financial performance (expressed as bare land value [BLV] with 6% discounting) of the Weedfree check outperformed the other treatments and was 37% higher than the Weedy check. The BLVs of the Moderate and Low weeding intensity treatments were similar to each other, indicating that these treatments were both feasible in terms of financial performance. However, the 2 m Row weeding had c. 10% higher BLV than the Moderate and Low weeding intensity treatments, and could be considered as a viable alternative to the Weedfree check. Sensitivity analysis conducted on the data in terms of local or exogenous risks indicated that between the treatments, differences in response to either local or global market risks were minor. Thus, small changes in cost and price levels, or larger changes in applicable discount rate, would not alter the ranking of the vegetation control treatments, with the Weedfree check (High weeding intensity) being the best in terms of financial performance, and the Weedy check the worst.