Miombo trees and mycorrhizae : ecological strategies, a basis for afforestation

This project has covered one or several aspects of the life cycle of the main miombo tree species, namely Afzelia quanzensis ,Brachystegia microphylla, Brachystegia spiciformis ,Julbernardia globifloraand Pterocarpus angolensis . These aspects included natural and artificial regeneration, fertilization, artificial inoculation of seedlings and natural occurrence of mycorrhizae on field-grown seedlings. Mycorrhizal survey of pine seedlings in various nurseries and inoculation trials on pines with exotic and indigenous fungi had also been conducted. The ultimate goal was to contribute to the development of techniques needed for the silviculture of miombo tree species.Seeds of B. speciformis , J. globiflora and A. quanzensis germinated within 3 weeks in nature provided there was regular rain. Seeds of P. angolensis possessed dormancy. Providing water was not enough to induce germination. In the greenhouse a hot wire scarifier boosted their germination to more than 90% within less than two weeks. Outside rain had a clear positive influence on the germination of scarified surfacesown seeds in the greenhouse. Germination was always initiated by outside rain. All species tested were sensitive to fires. For A. quanzensis germination tests were carried out at 10, 15 and 25°C. Seedlings germinated only at 25°C. Germination failures were also encountered in the field in the cool highlands of Tanzania.Seedlings of the above named indigenous species were raised in the nursery or observed in the miombo woodlands. A number of facts emerged.A big share of carbon is allocated to the development of the taproot. In the nursery, pruning the taproot interfered with the natural survival strategy of seedlings and caused a sharp depression in seedling growth. This was observed on J. globiflora and P. angolensis . The initial growth rate of the miombo tree seedlings covered here is far higher than that achieved by pines. As observed in Morogoro on seedlings of A. quanzensis , more than 70 percent of the height achieved in one year is attained in the first 2 months. Miombo species with big seeds such as A. quanzensis , had an initial higher growth rate than small-seeded species.The effect of drought on A. quanzensis seedling growth and survival during early stage of growth was studied. Inoculated or non- inoculated potted nursery seedlings were irrigated once a day, once every three days or once every four days. Induced drought had a negative effect on biomass accumulation. Non-drought treated seedlings were more than 4 times as heavy than treated ones. Inoculation did not counteract the effect of drought. Inoculated, drought-treated seedlings developed only one mycorrhizal type. The mycorrhizae in this treatment were shrunk and had a very thin sheath. Infection percentage was lower than 10%. Non-drought treated inoculated seedlings developed more than one type including that in treated seedlings. Infection rate was above 50%.A. quanzensis seedlings were observed under natural dry circumstances in the miombo woodlands for survival and mycorrhizal development. All seedlings survived and all formed one single mycorrhizal type similar to that in seedlings watered once every 4 days. All feeder roots were infected. Survival was 100%.Seedlings in the miombo woodlands yearly die back during the long dry season. There was no die-back in nursery raised irrigated seedlings.In the nursery A. quanzensis, B. microphylla, and J. globiflora seedlings were inoculated with mycorrhizal soil and basidiospores of fungi associated with adult trees. As a result of contact with soil inoculum, seedlings of these species formed ectomycorrhizae. only basidiospores of one fungus were able to form mycorrhizae with A. quanzensis in the nursery. In the field observations were made on seedlings of B. spiciformis and A. quanzensis naturally growing or artificially grown in the rhizosphere of their mother trees or other ectomycorrhizal miombo trees. Seedlings got colonised by fungi associated with adult trees. The infection started before germination was completed.Specificity ofAfzeliawas studied by inoculating seedlings with inoculum from other miombo tree species. Afzelia was found to have a broad host range.P. angolensis seedlings were raised in the nursery in the soil from the rhizosphere of adult trees of the same species. Seedlings of P. angolensis formed arbuscular mycorrhizae and nodulated. None of the ectomycorhizal legumes treated in this study nodulated.In the nursery the contribution of the mycorrhizal formation to the biomass accumulation was assessed for B. microphylla. Inoculated seedlings were far taller and had higher biomass than non-inoculated ones.The main features of miombo ectomycorrhizae is that they have many long and thick hyphae and big rhizomorphs. Rhizomorphs as big as 150 μm diameter were recorded.Fertilization in the nursery was done for P. angolensis seedlings. Simultaneous application of nitrogen and phosphorus boosted the growth. Nodulation was, however, eliminated on fertilized seedlings. Iron and zinc did not have any effect. In a nearby nursery, P. angolensis seedlings had iron deficiency symptoms and were dying. Application of iron solution on leaves had a visible effect within 2 days. At the end of the experiment, iron-treated seedlings had significantly grown in height, had produced new leaves and had developed their root system. Non-treated seedlings died or lost their leaves and had their root system very much shortened. In the greenhouse the same symptoms were experimentally reproduced by growing seedlings in compacted and overwatered soil.Mycorrhizal status of P. patula seedlings in various nurseries was assessed in Arusha, Mafiga and Sao Hill, Tanzania. Not all nurseries had mycorrhizae. As a result of lack of mycorrhizae, seedlings in Mafiga nursery were dying. After artificial inoculation, seedlings turned green and grew significantly. The control seedlings deteriorated. Inoculated seedlings formed mycorrhizae. More mycorrhizal types were found in Sao Hill compared to the other 2 regions. Suillus granulatus and Thelephora cf. terrestris were the most common mycorrhizae. The use of earthballs in nurseries instead of pots as practised on trial basis in one nursery, was found inhibitory to mycorrhizal development. Suillus was found sensitive to this practice. Earthballs have a high clay content and are poorly aerated.In Morogoro inoculation trials were conducted. Seedlings were grown in the pots in the greenhouse or directly raised under adult pine trees growing within 100 m from the greenhouse. Seedlings of the greenhouse received inoculum from those same adult trees. Seedlings under adult trees formed Suillus mycorrhizae of adult trees and were associated with sporophores of this fungi while those in the nursery formed Thelephora mycorrhizae and produced Thelephora sporocarps within 5 months. Thelephora sporocarp production stopped from the 6th month onwards while Suillus sporophores production continued under adult trees during the rain seasons. Infection was earlier and higher in the field than in the nursery.In the nursery pine seedlings were raised in fresh soil from the root zone of B.microphylla and Julbernardia globiflora. In the field seedlings were directly raised in the root zone of these miombo trees after addition or non-addition of pine mycorrhizal soil. A similar mycorrhizal type was formed in the nursery and in the field where no pine soil was added. Where pine soil addition was made, seedlings formed Thelephora mycorrhizae earlier observed in the nursery.The ecological strategies of miombo trees can be summarized in the following points. (1) The strategies focus on the underground compartment. (2) They are related to the architecture and dynamics of the root system with its deep and thick taproot. While the shoot in seedlings yearly dies back due to severe dry seasons, the taproot yearly expands. (3) The architecture with thin lateral roots lends itself to mycorrhizal formation and/or nodulation. Ectomycorrhizal legumes are of a broad host range. (4) Mycorrhizal lateral roots participate efficiently in building up the taproot, thus increasing the chances of seedlings to survive in dry and unpredictable environments. (5) The ability of seedlings and trees to develop new shoots using reserves drawn from the taproot towards the end of the dry season, allows them to efficiently use limited showers (6) Recognition of these facts is a basis for a successful afforestation with miombo trees.This research showed indeed that good nursery practices are beneficial for seedling growth and survival in the nursery and later in the field. The following points deserve attention in nursery management. These are (1) seed treatment (for P. angolensis ); (2) nutrient and moisture requirements; (3) (ecto)mycorrhiza; (4) restricted pruning of the taproot and (5) timing of activities.