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Little Known or Interesting Factoids About Trees and Tree Physiology

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                     Jim Rediker - Nurseryman -  Arborist  - TDA Certified
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Nitrogen

Nitrogen provides the most universal response to growth because it is the most  limiting nutrient in the soil. It is also the principal plant nutrient, important in production and maintenance of color and vigor in the foliage. Using the proper amounts is most important since over dosage can result in serious root injury while lack of nitrogen will result in poor growth and development. In forest Eco-systems, optimal levels of nitrogen fluctuate during times of leaf litter decomposition. Nitrogen as do all nutrients has to be recycled to an organic state by Mother Nature (soil based organisms - microbial life) in order to be used by plants and trees.

Optimal levels of nitrogen are inconsistent as it is bound up in biomass and ground debris due to seasonal conditions. In spite of leaching from the soil in the soil water, converting back to the gaseous state (volatilization) and de-nitrification by soil organism, nitrogen remains relatively constant because of natural deposition from the atmosphere, release of minerals bound in organic matter, and the nitrogen fixed by soil bacteria and microbes.

 This is the general rule in forest settings and usually light applications at seasonal changes can give Mother Nature a helping hand. In urban landscapes with already depleted nutrient levels due to original topsoil removal, the continual removal of natural leaf litter and organic materials further depletes soil nitrogen, nutrients and soil microbial activity. 

In the landscape settings, it is here that a balanced fertilization program needs to be established to recreate and simulate a forest type Eco-system. (See article Fertilizing Trees and Shrubs). In the Texas Hill Country, phosphorous, potassium and nutrients are usually in the soils but bound up due to the high (pH) alkaline type soils. (See article Soil pH). Potassium levels are usually sufficient in most soils for tree growth. However, potassium is usually deficient in soils of low organic matter.   Applying high nitrogen fertilizer with sulfur will lower the pH and increase the solubility of the other nutrients. Iron supplement may also have to be included in the fertilization program. Although nitrogen levels are usually low in forest settings our native trees have adapted to the soil and environmental conditions (See article on Iron Chlorosis). Trees will grow in their own preferred soil and eco-systems, giving us a variety of trees through our Texas landscapes.

Studies have shown that trees in the natural forest Eco-systems and with natural low-level nitrogen adjust and grow reasonably well. In fact, trees respond to their environmental conditions and slow their growth rate to maintain a healthy canopy and adjust their root: shoot ratio to provide adequate nutrients.

There is no conclusive research indicating that fertilizing trees will prevent, combat or cure bacterial or fungal disease problems or ward off insect attacks. The opposite has more truth, research has shown that the slow to moderate growth is normal and desirable for most trees. In depth studies show that resistance to certain insect and diseases decreases in rapidly growing trees and that such trees are more nutritionally suitable to these predator pests. More of the available energy is shifted to growth in periods of new and increased rapid growth and less energy is diverted to defense. Trees under low to moderate stress and (slow growth, produces higher levels of defensive pest inhibiting chemicals. Cankerworms  or  Leaf  Roller attack Live Oaks immediately after bud break while the new leaves are still palatable, tender and before the insect defense toxins have fully developed. Forcing rapid green growth with large applications of nitrogen is like ringing the dinner bell for the wonderful array of predator insects in love with your trees.

Fertilization decreases insect resistance of trees in two key ways:

     1) By increasing the nutritional value of the plant.

     2) By decreasing concentrations of defense chemicals known as phytochemicals or secondary metalolites.

These include alkaloids, terpenes, phenols and compounds that release cyanide.  Insects adapt to these defense chemicals and usually feed on one or a few closely related species.  Studies have shown that phytochemicals play key roles in protecting  plants from abiotic stress, including drought, heat, air pollution and damaging radiation. Carbon plays an important roll in the production and support of defense chemicals. Fertilization increases tree growth primarily and is dependant on carbon sources which has to trans-located from existing leaves to support active growth of leaves, shoots and stems.  Exported carbon from existing leaves, is unavailable to produce or sustain defensive chemicals in those leaves. Fertilization increased tree growth, with no effect on  photosynthesis, however, foliar nitrogen levels increased and phytochemicals levels became substantially lower in every case thus decreasing the resistance of the tree to insects.      

 Oak wilt infects and moves faster in a healthy tree than a tree that is stressed or already in a weakened condition. It is better to help a tree maintain a somewhat natural health, than to just apply fertilizers at random and with the intention that you’re going to make your tree healthy with vigorous new growth.

Nitrogen deficiency is most prevalent in sandy or silt type soils, poorly drained soils and soils with low organic content. There is a small input of nitrogen from the atmosphere, and certain microorganisms can fix (convert) elemental nitrogen from the air to forms that can be absorbed by trees.  Some nitrogen fixing organisms are free-living in the soil while others occur in specialized root nodules. Appreciable amounts of nitrates (from acid forming nitrogen-based air pollutants) and lighting also fixes smaller quantities of nitrates that are deposited on the soils by the rains. New leaves with sever nitrogen deficiency are typically smaller than normal and appear relatively green. Mature leaves however, are yellow. These symptoms are typical in Post Oaks in early spring, just after leaf out. The mobility of nitrogen in the plant allows it to move from the older leaves to the new, developing foliage. Nitrogen deficiency can be corrected by adding nitrogen-based fertilizer. 

Young trees need increasing supplies of minerals to grow well. Nutrient demand is usually met when root growth and soil volume are unrestricted and soil is relatively fertile or if nitrogen is applied. Older trees, on the other hand, can adapt to reduced soil fertility by slowing growth. However moderate to sever nutrient deficiency can cause abnormalities and poor growth. Moderately slow growth in mature trees is normal and desirable. As trees grow older and larger, demands for minerals, particularly nitrogen, to maintain growth and life functions increases, while availability decreases as minerals are increasingly bound in living and dead tissue. Thus, the nutrient availability may not be able to satisfy the demand of large, old trees. Such trees may benefit from moderate fertilization, but the over stimulation of mature trees with fertilizer can result in excessive growth, reduced drought resistance, and greater susceptibility to predator pests and diseases, and additional maintenance cost. Trees stressed by such problems as drought, poor soil aeration, and inadequate light or root disease usually to not respond to fertilization until these factors are mitigated (See article Trees are Vital).

There is a role for prescription fertilization in a tree health care program, especially in high maintenance landscapes. However, fertilization programs should be implemented with an understanding of potential consequences for pest resistance and stress tolerance. (See article Oak Wilt Treatment/Bio-Stimulants)

“Diseases are created chiefly when we destroy the harmony reigning among mineral substances present in infinitesimal amounts in air, water, food and, most crucially soil”

Dr. Alex Carrel, “Nobel Prize” winning scientist 

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Contact: cell: 830.257.8871
                
email: jim.rediker@usa.net
                     Jim Rediker - Nurseryman -  Arborist  - TDA Certified
SCENIC HILLS NURSERY

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