- Vegetation layers: the vertical structure of the forest
- Soil horizons: Organic, Assimilation, Eluviation, Banking, Chemical, Durable
- Vegetation density: much like our sense of "personal space" plants have preferences
- Patterning: the forces of edge effects and surface area effects
- Diversity: an attempt to bring more nuance to the discussion of species diversity
Interestingly vegetation layers came up in a different context this week when I went to the orientation for volunteers for the Backyard Habitat Certification Program where they talk in terms of 5 vegetation layers: tall and low trees, tall and low shrubs, and the herbaceous layer. In EFG they add to this list ground covers, and vines. They also introduce relative layers for the overstory, canopy, understory and root zone.
The soil horizons was largely review from the soils class I took last quarter at PCC. Yet it is always good to recall that half the plants and most of the life forms are living in the soil. The Organic or O horizon is often above what we usually think as the soil and includes the leaf litter and other organic mater in various stages of decomposition. Eventually hard working soil organisms incorporate this material into the lower horizon. The Assimilation or A horizon is the topsoil where most of the action happens: root growth, soil life, incorporation of organic matter, etc. Some soils have a distinct Eluviation or E horizon, some don't. This horizon accumulates clays that are carried by water (illuviated) from the A horizon and are deposited (eluviated) in the E horizon, it is usually light colored. The Banking or B horizon is often called the subsoil and tends to have less organic mater. A good way to develop organic matter in this horizon is by planting deeply rooted plants so that as the roots die back they leave organic matter in addition to the decaying soil life from the rhizosphere. They also help improve the soil structure and provide channels for good drainage and while alive a mechanism for pulling up nutrients into higher soil horizons. The Chemical or C horizon is where most of the soil formation activity happens chemically. Finally the Durable or R horizon is where it all got started with the parent material.
The section on vegetation density addressed the issues of vegetative cover and crowding. I found this to be interesting and instructive. Depending on the ecosystem that a plant targets as its niche it may have quite different preferences or tolerances to crowding, contention for root space, shading, etc. It also interacts with the choice of reproductive and dispersal strategies. This will be an important design consideration for me as it will be very tempting to crowd too many plants into too small a space. So the balance is between plant selection to accommodate this density and proper design to provide the plants the space they need to do their thing. At the Backyard Habitat orientation I saw a survey map of the greater columbian region from 1851 where my neighborhood was classified as "woodland" so probably 40-70% tree cover. It might then be reasonable to shoot for that same range in my design.
The patterning section was almost a tease -- the mathematical implications are intoxicating. The author investigated both the forces at play that lead to different sorts of patterns as well as the inferences we can draw to use as tools to weed less (less edges), or maintain easier access (more edges).
The text made the point that many discussions of diversity seem to treat it as an end unto itself rather than a factor to consider in ecological systems. Diversity can also take many forms beyond the simple count of species.
A couple misc. notes:
- Nitrogen fixers need full sun to work.
- Ribes (currants and gooseberries) fruit in shade.
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