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Ecological basis for ginkgo agroforestry systems Cao, Fuliang (Sam)

Abstract

Ginkgo (Ginkgo biloba L.) is a multi-value deciduous tree species grown for the production of nuts, timber and foliar medicinal properties. Understanding the ecological and biological basis for Ginkgo agroforestry systems is essential for the design of optimum Ginkgo-food plant combinations. A field and pot trial compared acclimation of Ginkgo to changing light, moisture, and nutrient levels. These resources had interactive effects on Ginkgo, which tolerated partial shading and soil resource depletion. Variation in light was found to be more important for biomass accumulation and flavonoid production than soil nutrients and moisture resource over the ranges of resource availability that were studied. Pot and field trials tested the influence of planting density on growth, biomass, and leaf flavonoid levels. There was strong Ginkgo belowground competition for soil nutrients, especially for nitrogen. Increasing density resulted in a significant reduction in many measures of individual Ginkgo seedling performance, such as net photosynthetic rate (Pn), biomass, and flavonoid concentration, but increased performance per unit area over the one and two growing seasons examined. A greenhouse pot replacement series examined interactions between Ginkgo and wheat (Triticum aestivum L. cv. "Feng Shou No. 2") and Ginkgo and broad bean (Vicia laba L.). Broad bean and wheat were more competitive than Ginkgo, which was less affected by wheat than by broad bean. However, there was a compensatory interaction between Ginkgo and wheat and Ginkgo and broad bean. There was significant belowground competition for soil N between Ginkgo and crop species in the Ginkgo/crop mixtures. The two mixtures outproduced monocultures of the individual species in the mixtures. The Ginkgo: broad bean (or wheat) ratio 5:1 had the best combined biomass production and the highest flavonoid yield. A Field factorial experiment with three Ginkgo/crop mixture treatments was conducted to extend the greenhouse study of the competitive ability of Ginkgo and crop species and of optimum Ginkgo/crop combinations. Components of crop biomass per m2 increased with increased crop density and decreased with increased Ginkgo density. There was evidence of Ginkgo/crop mixture ecological niche differentiation and overyielding advantage. The integration of all the parameters describing the competitive ability of crop species showed that crop species were more competitive than Ginkgo at low Ginkgo density, but less competitive than Ginkgo at high Ginkgo density and that rapeseed was more competitive than the other two crop species. Root competition was more intense than shoot competition in the Ginkgo/rapeseed mixture; shoot competition in Ginkgo/wheat and Ginkgo/broad bean mixtures was more intense than root competition. Ginkgo grown with broad bean had higher economic biomass (Ginkgo leaf biomass and crop seed yield) and flavonoid yield than with wheat and rapeseed (Brassica napus L.). Ginkgo: wheat ratio 24: 200 and Ginkgo: rapeseed (or broad bean) ratio 24:5 exhibited maximum combined economic yield in respective mixture, and Ginkgo: broad bean ratio 24: 5 had the highest combined economic yield and income. A chronosequence combined with remeasurement provided data from Ginkgo plantation ages 1, 3, 6,10, 13, 17, 20, 21 and 22 years on various parameters. Ginkgo density had significant effects on PAR, spatial change in Ginkgo line root biomass (FRB) and wheat root biomass (WRB), and spatial and temporal change in Ginkgo nut and crop seed yield. Before Ginkgo plantation age 21 in the Ginkgo/broad bean mixture and plantation age 22 in the Ginkgo/wheat and Ginkgo/rapeseed mixture, close Ginkgo spacing had higher nut yield per ha than narrow spacing. Starting from plantation age 21 in Ginkgo/broad bean mixture and plantation age 22 in the other two mixtures, low density stands gained a higher nut yield per ha than high density stands. Ginkgo/broad bean mixture had higher individual tree and per-ha-Ginkgo nut yield than did the other two mixtures. Per-ha-seed yields of the three crop species at all three densities were in the order of wheat > rapeseed >broad bean. The results of these studies were integrated to provide a conceptual model of the tradeoffs between Ginkgo and food crop values, and a tabular model of optimum Ginkgo monoculture and agroforestry systems was constructed based on the knowledge of their growth strategies, and competitive and other interactions. Future work should combine these data into a mechanistic, agroforestry model, such as FORECAST or FORCEE, to test a variety of management strategies.

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