A free email agroforestry journal for practitioners, extension agents, researchers, professionals, students, and enthusiasts. One edition is sent each month focusing on a concept related to designing, developing, and learning more about trees and agroforestry systems. Focuses on trees and their roles in agriculture, natural ecosystems, human culture and economy.

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Overstory #209 - Soil Compaction & Trees


The health and structure of trees are reflections of soil health. The ecological processes which govern tree survival and growth are concentrated around the soilroot interface. As soils, and associated resources change, tree systems must change to effectively utilize and tolerate changing resources quantities and qualities, as well as the physical space available. Soil compaction is a major tree-limiting feature of community forest managers and arborists.

Soil compaction is the most prevalent of all soil constraints on shade and street tree growth. Every place where humans and machines exist, and the infrastructures that support them are built, soil compaction will be present. There are few soil areas without some form or extent of soil compaction. Soil compaction is a fact of life for trees and tree managers. Unfortunately, prevention and correction procedures are not readily used nor recognized for their value.

This paper is a summary of soil compaction processes and tree growth effects. In addition, some general renovation principles are proposed. Understanding how soil compaction occurs, developing more accurate and precise definitions of soil compaction effects, and recognizing tree growth effects stemming from compaction problems will be the primary emphasis here. This paper will concentrate entirely on the negative growth constraints of compaction.

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Overstory #208 - A fresh look at life below the surface


Soil is often described in textbooks as rock and minerals, air, water, living organisms and decaying organic matter. Though an accurate depiction, soil biology often takes a backseat to soil chemistry and physics—soils are classified largely on the presence or absence of certain types and sizes of minerals. However, soil organisms play a huge and underestimated role in the productivity and health of soils. When a rainforest is cleared, burned, and the land subjected to annual tillage and burning, we often see this once highly productive landscape now barely able to support a maize crop. What happened? There is a growing understanding that the answers to this all too common question are found in the abundance and diversity of life hidden below the surface.

Soil foodweb concept

The soil foodweb is essentially the community of organisms that live in the soil. Every agricultural field, forest, prairie, or pasture has its own soil food web with a unique set of soil organisms. Healthy soils contain massive populations of bacteria, fungi, protozoa, nematodes, soil arthropods, and earthworms (Figure 1). A teaspoon (approx. one gram) of productive soil contains between 100 million and 1 billion bacteria. It contains around 25,000 species of bacteria and 8,000 species of fungi!

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Overstory #207 - Agroforester's Library

Thank you for visiting The Overstory journal.

This page is merely a placeholder in the journal's numbered sequence for a previously published edition, informing subscribers of updated reference links and changes in the Agroforester's Library.

Please visit the Agroforester's Library consisting of recommended books, periodicals, species references and other links one may find useful in the agroforestry field.

Overstory #206 - Underutilised crops and invasive species

Plant introduction and invasion

Paradoxical impacts of plant introductions

The introduction of harmful species, including a range of plant pests, both accidentally and intentionally has caused great concern. 'Pests' is used here in its broadest sense, as defined by the International Plant Protection Council of the FAO, as "Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products" (IPPC, 2007). A large number of these species have caused such significant negative environmental and/or economic impacts that they are now collectively called 'alien invasive species'. Alien invasive species, independent of which of the myriad of definitions are used (e.g. Richardson et al., 2000; McNealy et al., 2001; CBD, 2002; Colautti and MasIsaac, 2004), have been widely described as the second greatest threat to biodiversity after habitat loss (e.g. IUCN, 2001). A suitable working definition for invasive species (not necessarily 'alien') used here, is, "organisms that cause, or have the potential to cause, harm to the environment, economies, or human health" (Pasiecznik, 2007).

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Overstory #205 - What smallholder agroforestry systems are appropriate for carbon storage?


Tropical forests have the largest potential to mitigate climate change amongst the world's forests through conservation of existing carbon pools (e.g. reduced impact logging), expansion of carbon sinks (e.g. reforestation, agroforestry), and substitution of wood products for fossil fuels (Schlamadinger et al., 2007; Brown et al. 1996; Brown et al. 2001). In tropical Asia, it is estimated that forestation, agroforestry, regeneration and avoided deforestation activities have the potential to sequester large amounts of carbon.

Tree-based land-use systems – natural forest, forest plantations and agroforestry systems – sequester CO2 through the carbon (C) stored in their biomass. By promoting land-use systems which have higher C contents than the existing plant community, net gains in C stocks (hence sequestration) can be realized. The most significant increases in C storage can be achieved by moving from lower-biomass land-use systems (e.g. grasslands, agricultural fallows and permanent shrublands) to tree-based systems. As many efforts to achieve increased forest C storage may have negative implications for the rural poor, options that support human livelihoods deserve special attention.

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Overstory #204 - Nature is a beauty with perfect taste


Nature is the world's greatest beauty. The German word for nature (die Natur) is a feminine noun, as was also its Latin predecessor natura. Nature has always worn her dress of green-plant life-in the style most appropriately suited to each location and season. In the lowlands of Japan, this means the deep green of evergreen broadleaf forests of castanopsis and evergreen oak species. In the mountains, the deciduous broadleaf forests of Siebold beech (Fagus crenala), mizunara oak (Quercus crispula, formerly Q. mongolica var. grosseserrata) , and daimyo oak (Quercus dentata) change their dress with the four seasons, wearing pale new leaves in spring and bright colors in autumn. The subalpine areas of Honshu wear the dark green of certain fir species and northern Japanese hemlock (Tsuga diversifolia), while in Hokkaido comparable coniferous forests take on the appearance of. boreal forests of Yezo spruce (Picea jezoensis) and Sakhalin fir (Abies sachalinensis). The alpine belts are garbed in creeping pine (Pinus pumila), and in summer, meadow flowers show the primary colors of red, yellow, and blue-violet.

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