Prospects And 6challenges Of Poly Cultures In Forest


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Prospects And 6challenges Of Poly Cultures In Forest

  2. 2. Forest plantations : 5% of global forest cover (187 mha) & 35% round wood/timber (FAO,2001) Mixed species plantations Individual plantations containing more than one timber species. The plantation contain few or many tree species planted in rows or at random (Fig.1d-1g)
  3. 3. To maintain an equilibrium between economic and ecological function of the plantations…. Ecological restoration of degraded lands to re establish part of the native diversity Goal is to combine particular species so as to provide specific interaction that increase stand level productivity.
  4. 4. Mixed plantation lay outs to biodiversity(Lamb,1998) d) Mixed species plantation established using 2 Spp. Planted in alternative rows e) Planted at random f) Four species planted in rows g) Four species in random
  5. 5. To improve the productivity in mixed plantations, there are three main processes that need to be considered: competition, competitive reduction and facilitation (Vandermeer, 1989 and Kelty, 1992). Competition: negative effect Competitive reduction, also known as complementarity or the competitive production principle (leading to a more complete use of site resources) Facilitation, also known as the facilitative production principle
  6. 6. The way in which competition is distributed in a stand of trees depends on the resources for which plants are competing (Weiner, 1985,1986 and 1990) Competition for light Competition for belowground resources, such as water and nutrients variations in fine-root architecture, growth rates, distribution and mycorrhizal associations may also lead to asymmetrical competition belowground.
  7. 7. The occurrence of both interspecific and intraspecific competition leads to more complex interactions in mixtures than monocultures.
  8. 8. The yield of mixtures containing species A and B, where the proportion of each species in mixture changes from no individuals (0) up to x individuals and the total number of individuals remains constant (adapted from Harper, 1977). Depending on the nature of the interaction between species, there may be substitution (a and b), a reduction in yield (c), or an increase in yields (d–e).
  9. 9. occur spatially through the stratification of foliage or roots or temporally due to phenological differences *Canopy stratification—competition for light (stratified canopies with a stratification— fast-growing and less shade-tolerant species forming the upper canopy and a more shade-tolerant species forming the lower canopy :light use efficiency of the stand is more) stratification— *Root stratification—competition for nutrients and water The potential reduction of below ground competition occurs through the physical (differences in fine-root distribution that affect exploitation strategies) or chemical stratification (co-occurring species employ different acquisition strategies to obtain nutrients) of roots.
  10. 10. more diverse plantations should have higher net primary production, and in a well-managed plantation, this should translate into larger timber volumes
  11. 11. directly, by the amelioration of harsh environmental conditions or through increased resource availability, indirectly, via the elimination of potential competitors, the introduction of beneficial organisms (mycorrhizae and other soil microbes) or protection from herbivores (Callaway, 1995). Facilitation often occurs simultaneously with competition, and the complex interactions between these mechanisms determine the overall productivity (Callaway, 1995 and Holmgren et al., 1997)
  12. 12. Schematic diagrams of four mixed-species plantation designs, each with two species. Each diagram shows the plantation at the sapling stage and mature stage: (source:Kelty,2006) (A)fine-grained mixture with stratified canopy with equal proportions of species, for complementary resource use (facilitative interaction may also be present); (B) fine-grained mixture similar to (A) but with unequal proportions of species, to promote individual-tree growth of upper canopy species and reduce suppression of lower canopy;
  13. 13. (C) coarse-grained mixture for facilitative interaction, to limit the overtopping of timber species by fast-growing N-fixing species; (D) fine-grained mixture with delayed planting of fast-growing N fixing species, for same objective as in (C)
  14. 14. successful mixed-species plantations require the matching of the species combination with the site so that the competitive interactions are minimised while the competitive reduction and facilitative interactions are maximised.
  15. 15. Relative yields of Eucalyptus globulus and Acacia mearnsii growing in 1:1 mixtures at age 50 weeks in a pot trial, RY > 0.5 indicates that plants were larger in mixture than in monoculture. Modified from Forrester et al., 2006a and Forrester et al., 2006b. indicated that a very fast growing and efficient user of site resources, such as many Eucalyptus spp., planted on a site with high resource availability may not produce higher yields when grown in mixture.
  16. 16. Mixed plantings obviously increase the biodiversity. There is a production gain from using a nitrogen fixing species such as Acacia or Albizia species as the tree species in nitrogen deficient sites. There is production gain from the reduced inter specific competition as a result of mixing trees with contrasting phenologies (seasonal variation) or with different root or canopy architecture(occupation of growing space): complementary resource use/good ecological combining ability Mixed plantation can be less susceptible to serious fungal, insect, or animal damage than plantation monocultures
  17. 17. Financial or social benefit may be derived from harvesting early maturing species ( a mixture of commercially useful, early maturing species with a slow growing (high value species :needing more rotation time) is preferred) Mixing agriculturally or socially important plant might provide a greater range of harvestable products(e.g., fruits and nuts)
  18. 18. A mixture of shallow rooted species with deep rooted species will have less wind damage. More successful on poor sites than monocultures. Mistakes made in the selection of species for artificial regeneration can be easily corrected in mixed stands when compared to monoculture. A mixed stand is more easily transformed or modified to meet the present or probable future demands of the market (reduce risk of market shift).
  19. 19. In some cases net benefits of the mixture may take some time to appear. They are more difficult to manage : (grow at different rates and require different harvesting times) There is a possibility that slow growing species may be damaged when the earlier maturing species is removed (lay out such as 1d and 1f may overcome this by row thinning. Potential benefits have not always been realized in practical situations
  20. 20. Choices of species used in the mixture have a crucial role in deciding the success of a mixed plantations. (Which will form complementary/competitive etc?) More knowledge is required about the attributes of the potential plantation species(growth rate, phenologies, architectural features): More needs to be known about the circumstances in which mixtures actually increase yields, or reduce disease, herbivory or other insect damage over that occur in monocultures).
  21. 21. One of the main obstacle to the commercial uptake of poly cultures in industrial plantations may be lack of operational scale demonstrations coupled with reliable financial analysis. In a financial analysis of mixed species plantations (Nicholas, 2006), it was observed that a yield stimulus of 10% (based on the product and rotation) was needed to offset increased costs associated with planting and management of of mixed species plantations. Allelopathic effect, some times host for potential predators
  22. 22. Management is simplified (silviculture, harvesting, uniform products) The crop can be harvested economically Artificial restocking is simpler The ability to concentrate all site resources on the growth of a species with good vigour and wood quality.
  23. 23. (Lamb,1998)
  24. 24. Can polyculture provide greater yield to outweigh the management simplicity of monocultures??
  25. 25. By mixing species of both ecological and economic combining ability mixed plantation can certainly outweigh the monetary benefit from monoculture but climate change and escalating energy prices may impact on the efficiency of monoculture plantations and stimulate further interest in polycultures. More innovative experiments ( replicated spatially, temporally and with alternative species) is needed.
  26. 26. Dr.H.Pohris Mr. Andrew
  27. 27. Danke