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.
The small amounts of land where we concentrate many thousands of people do not represent the true carrying-capacity of the natural resources on the site. We are forced to concentrate natural resource inputs and outputs from a large surrounding area in order for our cities to exist. The means of concentrating resources is through building and maintaining engineered infrastructures such as streets, pipes, wires, curbs, buildings, parking lots, water collections and treatment systems, and environmental management devices for building interiors. The infrastructure waste-spaces (not needed for building or maintaining infrastructures) are delegated to "green" things.
Living systems which remain are containerized and walled into small spaces adjacent and intertwined with massive infrastructure systems. The ecology of infrastructures involve resource and process constraints to such a degree that living systems are quickly damaged and exhausted. A summary of the resource attributes around infrastructures are: many humans and machines functioning as sources for disturbance and stress problems (both chronic and acute); fragmented and diminished self-regulating ecological states and processes (declining living things, organic matter, biotic interactions); and, less open soil and ecologically active surfaces.
As infrastructures requirements increase and generate more ecological impacts, the associated building, maintenance, demolition, and renovation processes cause natural resource quality and usability to decline. Key components of this decline are complex soil resource alterations including water, gas exchange, mechanical impedance, and pore space alterations. Soil compaction is a primary measurable feature of the ecological damage with which we are surrounded.
To properly discuss soil compaction as seen in the field which limits and damages tree health, a clearer definition is needed regarding soil compaction. A more precise and accurate definition is needed in order to discuss tree symptoms and managerial solutions. In this discussion the word "compaction" will be used as a composite, generic, negative impact on tree growth and soil health. My composite "compaction" concept will include soil compression, soil compaction, and soil consolidation.
Compression -- The process which damages soil around infrastructures called compaction starts with soil compressibility or loss of soil volume. Soil compression leads to a loss of total pore space and aeration pore space, and a increase in capillary pore space. In other words, large air-filled pore spaces are crushed leading to more small water-filled pores. Compression is most prevalent in soils under wet conditions.
Compaction -- The next process soil undergoes is true compaction. Compaction is the translocation and resorting of textural components in the soil (sand, silt, and clay particles), destruction of soil aggregates, and collapse of aeration pores. Compaction is facilitated by high moisture contents.
Consolidation -- The third primary component of soil compaction is consolidation. Consolidation is the deformation of the soil destroying any pore space and structure, and water is squeezed from the soil matrix. This process leads to increased internal bonding and soil strength as more particle to particle contacts are made and pore space is eliminated.
Tree root survival & growth
Roots utilize space in the soil. The more space controlled the more potential resources controlled. The volume of soil space controlled by tree roots is directly related to tree health. The resources required are water, oxygen, physical space for growth processes, and open soil surface area for replenishment of essential resources. Tree roots occupy the spaces and gaps around, under, and between infrastructures. In heavily compacted sites, roots will be concentrated around the edges of infrastructures and filling any moist air space. The soil matrix is only a significant concern for essential elements, surfaces holding biological cooperators, and frictional and inertial forces for structural integrity.
Tree roots and the soil surrounding them are an ecological composite of living, once-living, and abiotic features facilitating life. Compaction initiates many negative impacts in the soil including: decreases the volume of ecologically active space available; tree rootable space is decreased and made more shallow; the detritus food web, the ecological engine responsible for powering a healthy soil, is disrupted and modified; the diversity of living things decline, beneficial associates are eliminated, and a few ecological niche generalists succeed; and, pests favored by the new conditions (i.e. Pythium & Phytophera) consume organisms and roots not able to defend themselves. Tree roots become more prone to damage and attack at a time when sensor, defense, growth regulation, and carbon allocation processes are functioning at reduced levels.
The ability of primary root tips to enter soil pores, further open soil pores, and elongate through soil pores is dependent upon the force generated by the root and the soil penetration resistance. Root growth forces are generated by cell division and subsequent osmotic enlargement of each new cell . Oxygen for respiration, and adequate water supplies are required. Tree roots can consume large amounts of oxygen during elongation. At 77°F (25°C) tree roots will consume nine times their volume in oxygen each day, at 95°F (35°C) roots can use twice that volume per day. The osmotic costs to cells of resisting surrounding forces and elongating can be significant.
In response to increased compaction, roots thicken in diameter. Compaction also forces roots to generate increased turgor pressures concentrated farther toward the root tip, to lignify cell walls quicker behind the growing root tip, and to utilize a shorter zone of elongation. Thicker roots exert more force and penetrate farther into compacted soil areas. As soil penetration resistance increases in compacted soils, roots thicken to minimize their own structural failure (buckling), to exert increased force per unit area, and to stress soil just ahead of the root cap which allows for easier penetration.
For effective root growth, pore sizes in the soil must be larger than root tips. With compaction in a root colonization area, pore space diameters become smaller. Once soil pore diameters are less than the diameter of main root tips, many growth problems can occur. The first noticeable root change with compaction is morphological. The main axis of a root becomes thicker to exert more force to squeeze into diminished sized pores. As roots thicken, growth slows and more laterals are generated of various diameters. Lateral root tip diameters are dependent upon initiation by growth regulator and the extent of vascular tissue connections. If laterals are small enough to fit into the pore sizes of the compacted soil, then lateral growth will continue while the main axis of the root is constrained. If the soil pore sizes are too small for even the lateral roots, root growth will cease.
Tree species tolerance
There is a great variability in reactions to soil compaction. As there are many different soils and associated responses to compaction, so too are there many gradations of tree responses to compaction. A tree's ability to tolerate compacted soil conditions is associated with four primary internal mechanisms: reaction to mechanical damage is effective and fast; continuation of respiration under chronic O2 shortages; ability to continue to turnover, reorient, and adjust absorbing root systems; and, ability to deal with chemically reduced materials (toxics).
A list of trees meeting the above criteria for soil compaction tolerance can be found in: Coder, Kim D. 2000. Compaction Tolerant Trees. University of Georgia School of Forest Resources Extension Publication FOR00-2. 1pp. (Download at WEB site www.forestry.uga.edu/efr under "tree health care.")
Functional results of compaction
Having reviewed the primary means by which soils become compacted, the results of compaction can be estimated for tree and soil health.
Destruction of soil aggregates and large pore spaces
The pore spaces from cracks, interface surfaces, biotic excavations, organic particle decomposition, and normal soil genesis processes help oxygenate the soil matrix. By definition, compaction results in the destruction of soil aggregates and aeration pore spaces. Pore spaces filled with O2 and interconnected with other aeration spaces exchanging gases with the atmosphere are critical to a healthy soil and tree root system. The destruction of aeration spaces surrounding soil aggregates can be unrecoverable.
Resortingredistribution of particles
(Change in particle distribution) Particles of soils are redistributed into new locations, many of which are open pore spaces in the soil matrix. Through processes of packing, erosion, and cultivation many fine particles can fill-in the spaces surrounding other particles, as well as the spaces between structural aggregates. Some soil types can be compacted more easily through this process than others. Mid-textured soils with a mix of particle sizes can be strongly compacted due to particle size availability to fill any size of pore space.
Total pore space changes
(Change in pore space distribution) Compaction initiates a redistribution of pore sizes within the soil matrix. Large pores are destroyed and small pore are generated. The total pore space of the compacting soil initially increases as more capillary pores are created as aeration pores are lost. With increasing compaction, soil strength increases and pore space declines.
Aeration pore space destruction
The crushing collapse of aeration pores facilitates the upward movement of the anaerobic layer. There are always anaerobic and aerobic micro-sites in and around soils aggregates within the surface layers of soil. The dynamic proportions of each type of micro-site changes with each rainfall event and each day of transpiration. Compaction shifts proportional dominance in the soil to anaerobic sites. With further compaction, aerobic sites are concentrated closer and closer to the surface until little available rooting volume remains. Air pore space less than 15% is severely limiting.
Increased mechanical impedance
Compaction brings soil particles into closer contact with each other (less moisture and/or greater bulk density). Closer contact increases surface friction and soil strength. As soil strength increases and pore sizes decrease, the ability of roots to grow and colonize soil spaces declines rapidly. With compaction, soil strength reaches a level where roots can not exert enough force to push into pore spaces. Pore space average diameters significantly smaller than average root diameters are not utilized by tree roots. The texture and bulk density must be known to estimate compaction impacts.
Connectivity of aeration pores decreased
The aeration pathway (lifeline) from the atmosphere to the root surface through all the interconnected aeration pores declines quickly with compaction. As the tortuosity of the oxygen supply path increases, the closer to the surface the anaerobic layer moves. As pore sizes become smaller with compaction, more of the pore space is filled with water. Water-filled pores diffuse O2 at rates 7,000 to 10,000 times slower than air-filled pores. With all the other aerobes and roots in the soil competing for the same oxygen, oxygen limitations become severe.
Compaction constrains O2 movement in the soil and shifts soil toward anaerobic conditions. Less O2 diffusion into the soil leads to a chemically reducing soil environment (both the soil solution and soil atmosphere). Under these conditions, toxins and unusable essential element forms are generated. In addition, organic matter is not mineralized or decomposed. A soil anaerobic respiration sequence is initiated among bacteria starting with nitrogen and moving through manganese, iron, and sulfur, ending with carbon (fermentation of roots). Tree roots are aerobes as are root symbionts and co-dependent species of soil organisms. Less oxygen prevents growth, defense, and survival in aerobes. Roots use available food 20 times more inefficiently under near anaerobic conditions. Less oxygen also allows common pathogenic fungi which have oxygen demands must less than tree roots to thrive. As O2 concentration falls below 5% in the soil atmosphere, severe root growth problems occur.
Poor gas exchange with atmosphere
Compaction prevent gas exchange with the atmosphere. Compaction prevent O2 from moving to root surfaces, but also prevents CO2 and toxics (both evolved and resident) from being removed from around the roots and vented to the atmosphere. Poor gas exchange allows the anaerobic layer to move closer to the surface and reduce rooting volume. As CO2 comprises more than 5% of the soil atmosphere, problems of aeration become compounded. As CO2 climbs above 15% in soils, root growth dysfunctions accelerate.
Less tree available waterLess water holding capacity
One of the most ignored result of compaction is it effects on soil water availability. Soil compaction reduces the tree available water held in the large capillary pores and increases the volume of small capillary pores which hold water unavailable to trees. With the decreasing number of large capillary pores and increasing number of small capillary pores, the total water holding capacity of the soil declines. Irrigation scheduling and monitoring becomes critical around trees in compacted soils.
Decreased infiltration ratesIncreased surface erosion
Compaction leads to smaller pore spaces and slower infiltration rates. With increasing residency time at the soil surface, water can horizontally move across the surface of the soil initiating erosion. Over the top of compacted soil, water can reach faster velocities (more erosion potential) than in areas where it infiltrates easily.
Poor internal drainage
Compaction prevents effective drainage of soils. Poor internal drainage limits tree available water, prevents O2 movement, and increases production and residence time of CO2 and toxics.
Increased heat conductance
Compaction changes the energy and water balance near the surface of the soil. With more particle to particle contact, heat transfer is greater into the soil. Results include burning- out of organic matter quicker, acceleration of evaporative and transpirational water loss, and increased respiration of roots and soil organisms. As temperature increases, respiration responds along a doubling sequence path - for every 18°F (10°C) increase in temperature, respiration doubles.
Major soil compaction effects on trees are defined below: Reduced elongation growth - As compaction increases, roots are physically prevented from elongating into the soil by lack of O2, by decreasing pore size, and by increased soil strength. As roots are put under greater than 1.2 MPa of pressure, elongation slows and stops.
Reduced radial growth
Trees begin to generate thick and short roots with many more lateral roots as surrounding soil pressure exceeds 0.5 Mpa. O2 shortages and soil strength are major limitations.
Essential element collection and control problems - With less colonizable soil volume, there is less physical space to collect resources from and less resources within that space. With declining respiration processes, energy requiring steps in active element uptake (i.e. N, P, S) fail. Part of the difficulty in collecting essential resources is a buildup of toxics which pollute any existing essential resource supply.
As roots survive in a steadily diminishing aerobic layer, and as the anaerobic layer expands toward the surface, the physical space available for living roots declines. The consequences of having smaller volumes of colonizable space at the surface of the soil means roots and their resources are subject to much greater fluctuation in water, heat loading, and mechanical damage. Drought and heat stress can quickly damage roots in this small layer of oxygenated soil.
Constrained size, reach, and extent of root systems
Compaction limits the depth and reach of tree root systems leading to greater probability of windthrow and accentuating any structural problems near the stem baseroot collar area. Limiting the reach of the root system also prevents effective reactions to changes in mechanical loads on the tree and concentrates stress and strain in smaller areas.
Stunted whole tree form
As resources are limited by soil compaction and more effort is required to seek and colonize resource volumes, trees are stunted. The disruption of growth regulation produces stunting as auxincytokinin ratios shift resource allocations and use. In addition, carbohydrate and protein synthesis rates enter decline cycles interfering with nitrogen and phosphorous uptake, which inturn disrupts carbohydrate and protein synthesis. The result is a tree with a small living mass and with limited ability to take advantage of any short-term changes in resource availability.
Seedling establishment and survival problems
Micro-site variability in compaction levels and a limited resource base constrain young and newly planted trees. Less of a bulk density increase and crusting effect are needed for failure of new trees compared with older, established trees.
Root crushing and shearing-off
The mechanical forces generated in compacting a soil can crush roots, especially roots less than 2 mm in diameter. Larger root can be abraded and damaged. Rutting can shear-off roots as soil is pushed to new locations. The amount of crushing is dependent on root size and depth, weight of the compacting device, soil organic material, and depth to the saturated layer (for rutting).
Soil compaction puts selective pressure against aerobes and favors low O2 requiring organisms, like Pythium and Phytophera root rots, or anaerobes. Because of the destruction of the detritus energy web coupled with successional changes, recovery of soils to precompaction conditions may not be possible. Management must move forward to new solutions for resource availability and deal with new patterns of pest management since returning to the soil microbiology and rhizosphere of pre-compaction is impossible.
Renovation of sites
A summary of this discussion of soil compaction lies with those general principles and renovation techniques managers must use to reclaim a part of the ecological integrity of the site, as well as soil and tree health. General soil compaction renovation principles are listed below in a bullet format:
- Soil compaction should be considered permanent. Studies demonstrate that after one-half century, compaction still afflicts soils under natural forest conditions. Recovery times for significant compaction is at least two human generations. Soils do not "come back" from compaction.
- Every soil used by humankind has a representative compacted layer, zone, area, or crust. Changing management may not change the current compacted zone but may well add an additional compacted zone in a new position.
- Management activities should concentrate on moving forward to increased aeration space and reduced soil strength as best you can, rather than trying to recover past ecological history.
- Measure bulk density, penetration force, O2 diffusion rates, and tree available water. These are the best proxy measures we have to understand soil compaction and its impacts on trees. More careful and direct measures of soil compaction constraints on tree growth are expensive and difficult to make.
- Alleviation of soil compaction is part of a good soil health management plan.
- Use extreme caution in management of water over and in compacted soils. Compaction provides little margin for error for drainage, aeration, infiltration, and water holding capacity of tree available water. (Wet soildry tree problems).
- Seek the assistance of a tree and soil specialist to avoid tree-illiteracy problems on compacted soils.
Once the general principles of working with compacted soils are digested, the next requirement is to identify some techniques for renovating compacted soils. These recommendations are generic across many situations and soil types. General techniques are listed below in a bullet format:
- Restrict site access to the soil surface as soon as possible with fences and fines (legal penalties). Try to be the first one on the site and setup anti-compaction protection.
- Defend the ecological "foot print" of the tree rooting area. Select working conditions (dry, dormant season, surface mulch, etc) that minimizes compaction.
- Restrict where possible vibrational compaction.
- Carefully design tree growth areas using "biology-first" design processes rather than the common (and damaging) "aesthetics-first" design processes.
- Try to soften and distribute compaction forces with temporary heavy mulch, plywood driving pads, and soil moisture content awareness planning.
- Restart or improve the detritus energy web in the soil including addition of organic matter and living organisms, as well as trying to change soil physical properties by increasing aeration pore space.
Soil compaction is a hidden stressor which steals health and sustainability from soil and tree systems. Causes of compaction are legion and solutions limited. Without creative actions regarding the greening of inter-infrastructural spaces in our communities, we will spend most of our budgets and careers treating symptoms and replacing trees. Understanding the hideous scourge of soil compaction is essential to better, corrective management.
Please see the following publication.
Coder, Kim D. 2000. Trees and Soil Compaction: A Selected Bibliography. University of Georgia School of Forest Resources Extension Publication FOR00-1. 2pp. <http://warnell.forestry.uga.edu/service/library/for00-001/for00-001.pdf>
This article was excerpted with the kind permission of the author and publisher from:
Coder, Kim D. 2000. Soil Compaction & Trees: Causes, Symptoms & Effects. University of Georgia School of Forest Resources Extension Publication FOR00-3. 37pp. <http://warnell.forestry.uga.edu/service/library/for00-003/for00-003.pdf>
About the author
Dr. Kim D. Coder is a tree physiologist and forest ecologist. He is professor of tree health care at the Warnell School of Forestry and Natural Resources in Athens, Georgia, USA. Web site: <http://www.warnell.uga.edu/Members/coder>.
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