Ecological Principles of Agroforestry Systems: Agroforestry Systems 471/571 September 3, 2009

1. Ecological Principles of Agroforestry Systems:Agroforestry Systems 471/571September 3, 2009

2. Woody plants – unique characteristics • Lignin – rigid structure • Support – elevated canopy, vertical structure • Distribution of resources • Complex types of SOM • Longer residence times (lignin more recalcitrant) • Humus – enhances soil fertility, • Greater longevity / persistence • Competitive advantages & disadvantages

6. Tree-crop competition limits production of simultaneous systems Idealized system in extension manual Reality: severe tree-crop competition

7. Types of interactions • Complementary / facilitative • Competitive • Supplementary

8. Competitive and complementary interactions…

9. Resource capture by single and mixed crop-tree systems (Over-yielding) (Under-yielding)

10. Land Equivalent Ratio (LER) Crop A combined + Crop B combined Crop A alone Crop B alone Example: 75 + 110 = 1.48 100 150 If LER > 1 = positive interaction • Examples from semiarid India (Ong et al. 1991) • Sorghum/pigeon pea: LER = 1.4 biomass, 1.5 grain yield • Leucaena/pigeon pea: LER = 1 • but crop reduced 30-90% • Leucaena alone = 20-37% greater yield

11. Can an agroforestry system out-yield sole crop systems? • Challenge: maximizing complementary interactions and minimizing competitive interactions

12. Competitive-Complementary Interactions in Agroforestry Systems • Give examples of: • Competition (above- & below-ground) • Complementarity (above- & below-ground • What is the limiting resource and how does it affect these interactions? • What are some management interventions to enhance complementary interactions and minimize competitive interactions?

19. Reduced plant stress (ET, radiation) Reduced stress in livestock Can increase protein content & digestibility (pasture) Positive aboveground interactions • Microclimate modification • Water vapor/humidity • Decreased wind • Shading • Decreased temperature

20. Positive aboveground interactions… • Shade can improve crop water use efficiency) • C4 vs C3 plants • C3: light saturated at ~50% maximum radiation • C4: continue to increase photosynthesis

21. Effects on microclimate • Trees can increase water locally by shading or intercepting clouds or wind-driven rain.

22. Weed/Pest Control • Allelopathic effects of tree mulch, prunings (ailanthone from tree of heaven, monoterpenes from eucalyptus, mimosine from L. leucocephala, caffeine from coffee) • Plant-insect interactions • Beneficial insects: soybean aphid control

23. Managing negative aboveground interactions • Shading may lead to reduced yield • Pruning • Thinning plantations - temporarily increase grass growth • Trees have high plasticity – quickly fill in available growing space (leaves, roots)

24. Management: Shade tolerant species • Manage understory crops over time to plant more and more shade tolerant crops as tree canopy closes • Complementarity if products from widely-spaced trees compensate for low yielding crops

25. Managing negative aboveground interactions • Concentration of livestock • Bank erosion near streams • Compaction • Over-grazing

26. Positive belowground interactions • Increased total resource-use efficiency • Niche partitioning • “Safety net role” – deep rooted trees • Greater total productivity per unit land area (LER > 1) • Hydraulic lift – redistribution of water from deep to shallow soil be deeply rooted trees • May influence NPP, nutrient cycling, & water balance

27. Example of Increased Resource-Use Efficiency • Bear Creek Buffer System: Addition of woody species to switchgrass buffers increased removal efficiency of nutrients and sediment • Positive effect on other ecosystems services: water quality, aquatic habitat & biodiversity, soil conservation

28. Bear Creek Buffer SystemSediment & nutrient capture functions: resource use efficiency + safety net concept

29. Positive belowground interactions • Soil fertility enrichment • Litter/prunings, N fixation, P from mycorrhizae • Many reports in tropics, few from temperate zones • Few species: Robinia, Prosopis, Alnus • Herbaceous legumes (alley-cropping, silvopastures) • Radiata pine with subclover • Black Walnut with leguminous forage crops

30. Alley cropping with N-fixing trees (NFTs) + mulch - mulch

31. Nutrients: In the short run, NFTs add N only when leaves are cut and used as mulch

32. Nutrients: In the long run, litterfall from NFTs can add N to the system

33. Negative belowground interactions • Resource competition… • Water • Nutrients

34. The resource in least supply will be limiting Managing competition for resources N deficiency P deficiency

35. Faidherbia albida • Grows in arid climate under broad range of soil fertility conditions. • Unique phenology – drops leaves early in rainy season. • Mixed results: both yield increases & decreases occur in crops growing under trees • Why???? Low nutrients High nutrients (Same water) Trees = no added nutrient benefit + compete for water Trees = Increase nutrients – outweighs negative effects of reduced water

36. Negative belowground interactions • Allelopathy • Juglone in black walnut (but, low concentrations in field conditions, may be oxidized to non-toxic forms) • Tropics: many tree species produce allelochemicals through leaf litter that provide pest control for crops • Exotics

37. Do you agree with these concepts? • Resource capture = productivity of what? • Other values: ecosystem services, market value, preferences, etc. • This is just one way to evaluate AFS! Land Equivalent Ratio (LER) = Crop A combined + Crop B combined Crop A alone Crop B alone * Per unit land area If LER > 1 = positive interaction

38. Complementarity vs. Competition on the landscape scale • Multiple considerations • Profitability / market value • Ecosystem services & sustainability • Contrasting human interests & values • Context • Industrialized vs. developing countries • Costs (labor, other inputs) • Self-sufficiency vs. marketing • Resiliency (diversification) • Policy incentives (subsidies, investments)

39. Bear Creek field trip • List different types of ES that can be obtained from buffers? • What are some important positive (complementary) and negative (competitive) ecological, social, and economic interactions? • How can “current” buffers be managed for maximizing ES and complementary interactions? • What other products or ES values could be incorporated into buffer systems in the future?

40. Groups • 1: Hannah, Jarett, Dan Mahony, Rick • 2: Jessica, Justin, Dan West • 3: Graduate students