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come from tho self-thinning rule'of ecology, which ~tates that
<br />when energy input into a site equals output, there will be no
<br />further growth unless some trees die. As many suppressed trees
<br />die, a much smaller number continue to grow bigger. Simple. Or,
<br />on the basis of the mass-energy ratio law, as some trees on a site
<br />get bigger, many smaller suppressed trees will die. As the
<br />suppressed trees decline, they contribute a higher percentage of
<br />theft SOluble carbohydrates to the rhizosphere. .
<br /> The increase in exudates from a declining,tree with a dsf~n~e
<br />system weakened bY low energy reserves would give root
<br />pathogens an advantage over other soil organisms. When the tree
<br />dies~ its dead wood adds a'great amount.of carbon to the SOil, thus
<br />benefiting all soft organisms. If this scenario is correct, then the
<br />codes for the increase ofexudates as trees decline Would have '
<br />been set in the genes of the forest trees. Then, even after trees are
<br />taken out of their groups in forests and planted.~ individuals, the'
<br />genetic Codes for increasing exudates as tho tree d~clines would
<br />still be in effect. · .. '
<br />· A tree does not "know" why it is dyingl In a crowded,
<br />groWing, young forestJhe self-thlnning nde of ecology does'
<br />benefit tree survivors and all soil o?ganisms; But, when one or
<br />two trees in a ~,ard, ~ity, or p~k start to deCline, their early death.
<br />may benefit only the root pathogens. ,Andeven worse, since the
<br />tree will be cut and removed from tho site, thoro would be no
<br />benefits from, added carbon to the soil..," ' .'
<br />A Closer Lookat Roots.. :'· · .: -
<br />Woody tree'roots are organs that suppOrt th~ tree mechanically,
<br />store energy reserves~, transport ~ater and tho Substances disZ
<br />solved in it,'and synthes/ze substances such as growth regnlators~
<br />amino acids, and vitamins that are essential for growth..
<br /> Trees have different types of root systems. For example,
<br />mangroves along coastlines have stilt ~oots'.'Many ,t~es growing
<br />in tropical areas have aerial roots that become prop roots when
<br />they grow into the SOil. Other trees, have strangling roots that
<br />eventually kill the host tree that first supported their growth.
<br />Trees in sandy soils can have roots that grow downward over 90
<br />feet. Palms have roots that are adventitious and grow from
<br />meristemafic regions in their base. Many tree species have deep
<br />roots when they are young and more shallow roots later. It would
<br />be nearly impossible for the strongest person to pull out young'
<br />saplings of beech, oak, or hickory from forest soft.
<br />Woody roots have cells with walls of cellulose, hem/cellulose,
<br />and lignin. ~ is that natural "cementing" substance that
<br />gives Wood its unique characteristic for strength. Woody roots
<br />also have an outer bark or penderm made up of three layers: 'the
<br />
<br /> roots store energy reserVeS, and root defense is dependent on
<br /> energy reserves. When reserves are low, defense is low. When.
<br /> defense is low, weak or opportunistic pathogens attack. It is
<br /> Nature's way. ,
<br /> Non-WOOdy ROOts ' ..
<br /> Non-woody tree roots are organs that absorb water and elements
<br />dissolved in it. The.two basic types of non-woody roots are: '
<br /> 1, Root hairs on non-woody roots are extensions Of single.
<br />epidermal cells. Common on seedlings, root hairs grow'to
<br />maturity in a few. days. They function for a few Weeks and then
<br />begin to die. On mature trees, they are usually not abundant.
<br />When they do form, they d° so when soil conditiOns are optimum
<br />for absorption of water and elements. I have found root hairs
<br />growing in non-frozen softs beneath fri>zen soils in winter.
<br /> 2.' Mycorrhizae are the other type ofn0nZwoody roots.
<br />Mycorrhizae are organs made up of. tree and fungus tissues thai'
<br />facilitate the absorption of phosphomS~containing ion~? and others
<br />essential, for growth.
<br /> The fungi that infected develoPing non.woody roots to form
<br />' mycorrhizae were very ,biologically smart." Rather than
<br />competing with other micrOOrganisms in the'rhizosphere for
<br />exudates from the tree, the mycorrhizal-forming fungi went right
<br />to the SOurce inSide, tho tree. And, even more to their advantage,
<br />many of the myc°rrhizal fungi grew thread-like strands of
<br />hyphae-qong; vegetative tubes of fungi--°ut from the,
<br />my6Orrhizae, This inside.and outside presence gave the fungi a
<br />, distinct adVantage 9ver_Other microorganisms in. the rhizosphere.
<br />The tree gains efficiency with myc0rrhizae in several ways.
<br />' 1. With their extended hypliae, mycorrbizae not only greatly
<br />eXtend the abSOrbingpotential [of the roots] into the soft, but the'
<br />.hyPhae may connect witl/other hyphae on othe~ trees. In this
<br />· way, the mycorrhlzae, serve to connect trees of the same or...
<br />different species. This leads to the COnjecture· ihat the connections
<br />that developed over long Periods in the natural, forest may have
<br />some survival.v~ue. That is why forest types are often named for
<br />the'grouPs 'of species commonly found grOwing together. For
<br />example, we speak of the birch-beech-maple forest, or the pine:
<br />oak forest. From a Practical standpoint,.wheh tr~eS are planted in
<br />cities and parks, there may. be great survival advantages to
<br />planting grou, Ps of trees made up of the species tha~ are normally
<br /> found together in natural stands.. '
<br /> 2. The myccerhizae tifive been shown tO provide Some' '
<br /> resistance against' root pathogens. It may be that the pathogens
<br /> would have difficulties in building their populations in the
<br /> rhiz0sPhere dornlm!ted by the mycor~hizal fungi.
<br />
<br />phellogen, pheHoderm and phellom. The Phellogen is the bark .Perhaps the most important feature of the mycorrhizal fun, gi is
<br />cambium. The pholloderin is a thin layer of cells on the inner side'' that their boundary' n~aterial is mostly chitin; Chitin differs
<br />Of the PhoHogen. The phellem is the outer cOrky layer. Phellem slightly frbm celluloSe by a chain of atoms that contains a ,
<br />cells are impregnated .w. ith a substance called suberin, Which is'a: nitro~en atom. This slight change in SOme WaY makes chitin a'
<br />
<br />fatty substance that prevents water absorptiOn. .
<br />Some characteristics of woody roots are:.' . ·
<br /> ' · '~ney do not abSOrb water.
<br /> · They have no pith.
<br /> · Their conducting, elements are.usually wider th~n those in
<br />the mink. ......
<br /> · They have a gre~ter p~oportion of parenchyma cells than is
<br />usual for trunks. The living parenchyma store energy reserves,
<br />usually as starch. Asoft cortex without chlorophyll maybe in the
<br />bark. In some tree species that thrive in wet softs or have deep
<br />roots, the cortex may have many open sp~ces that act as channels
<br />for aft to reach the living cells in the roots.
<br /> It is important to remember that the parenchyma in the woody
<br />
<br />material better suited for absorption of elements. Remember that
<br />' the fungal hyphae'giiin all their essentials for life.by absorptiOn
<br />· through.their'boundary subs~ance~ . ·.. . -~ .. .
<br />· Them arepther adVantages to the chitin and the tube-like ..
<br />hyphae that ramify tho soil in the rhizosphare and beyond. ~When
<br />. the hyphae die, they become a nitr0genSOurde for Other ~
<br />organisms. Also, when the hyphae am digested, they .leave
<br />tunnels in the soil that are about eight to 10 microns in diameter.
<br />For the bacteria, these small tunnels may mm the difference.
<br />between life and death2 The bacteria quickly colonize the tnpnels.
<br />The advantage here is that the major threat to their survival'
<br />comes from protozofi that are usually much larger than 10
<br />microns. SO the hungry amoebae are not able to get at the
<br />
<br />26 THE PERMACULTURE AL:I'B/IST ·//40
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