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BelowGround

Terminology

 

Term Definition Soil Texture The relative proportions of sand, silt, and clay in a soil Soil Water Movement   Soil Physical Properties   Soil Structure Soil Ratio between Pore & Matrix Root turnover The number of times the root biomass is replaced each year.

Basics

Soils make the foundation of urban tree canopy, without good soils

  • Establishment is difficult / poor
  • Growth is  poor
  • Canopy formation is poor

Soils are:

  • A Soil management challenge
  • A Design challenge
  • A Engineering challenge

Urban soils have vastly different challenges from rural soils

  • Principles of soil science are the same, but challenges are different.

Soils directly influence tree phenotype.

Soil Volume

  • Urban forest research has tried to answer / address how to deal with limited soil volume for trees in cities for many years.
  • Cities have only recently introduced soil volume guidelines.
  • How much soil is required for trees?
    • Difficult to answer; one variable is the root architecture of a tree if it weren't restricted

Soil volume standard for Vancouver Parks Board

Canopy Size Minimum Soil Volume

Large (Maple) 30 m³ (single tree) 20 m³ (row of trees)

Medium (Hornbeam) 20 m³ (single tree) 15 m³ (row of trees)

Small (Snowbell) 10 m³ (single tree) 5 m³ (row of trees)

How much soil volume is enough?

Charts exist but they assume that trees will "escape" the confines of these designed or planned soil volumes and send roots out beyond yonder to collect water and nutrient resources.

Considering the basal area that can be supported by 1 ha of land, we discover that the soil volume resource per trunk cross-sectional area is much greater than what is supplied in most urban settings.

Some models for soil volume need is based on water requirements (so that in a hot, dry climate, greater volumes are needed), but this can lead to impossibly large soil volumes. 

Estimating root spread

To estimate root spread of a young tree (approximately 8 inches / 20 centimeters in diameter) where there is no impediment to the roots:

  1. Measure trunk diameter (circumference / pi)
  2. Multiply the number by 38

NOTE: There is not much data on large and mature trees.

NOTE: Roots don't extend very far under intact pavement.

Below-ground competition

There is limited space below ground; two things can't occupy the same place: competition

We need to determine 

  • root location
  • root spread;
  • Root depth;
  • Root interaction with soil
  • Species influence over previous bullet points

NOTE: For very large trees, there is not much actual data available on root spread.

Root systems are massive on large trees, quantifying them is very difficult.

How roots interact with compacted soils

  • Important because:
    • Judgements can't be made under a city context without understanding this concept
    • Necessary to make "judgements" / estimate based upon understanding of species
  • Soil is a structural component
  • Soil Strengths:

0.6 MPa Low Soil Strength 1.75 MPa Moderately High Strength 2.0 MPa High Strength

Roots will follow path of least resistance (POLR)

Allelopathy

Some plants, such as walnut, produces chemicals to gain competitive advantages. 

  • Walnut roots excrete juglone which inhibits the growth of some other plants.
  • Tall fescue (Festuca arundinacea) suppresses the growth of a wide variety of tree species.

Another common allelopathic plant in urban settings is tall fescue. This is a very widely used cool-season turfgrass species (Festuca arundinacea a.k.a. Schedonorus arundinaceus) . This grass seems to have allelopathic interactions with a wide range of tree species, supressing growth. Tall fescue is also host to a mutualistic endophyte that may also contribute to the production of alleopathic chemicals. Some authors (for example, Renne et al. 2004) suggest that biochemical recognition among plants may be adaptive, i,e. that plants may derive an advantage from recognizing the presence of an aggressive competitor like tall fescue and avoid investing resources in growth when success is unlikely. There are lots of different types of tall fescue, there is confusion about the taxonomy and about the infection rate and resulting interactions of the endophyte. However, all in all, there is some evidence that this turf species inhibits growth of many trees beyond simple competition for the same soil resources. Below ground interactions are very complex, however, and new science is rapidly emerging. In particular, our understanding of complex biological interactions is expanding, leading to new paradigms about everything from soil carbon sequestration to plant-to-plant communications.

Conflicts & Challenges between roots & people

In many cities, tripping hazards are massive liabilities for cities.

  • Extremely expensive problem when cities are sued
    • Cities will try to avoid tripping hazards
  • Roots lift sidewalks / cause them to buckle
    • Cities block / try to prevent roots from growing under sidewalks

Roots need to go somewhere to access soil, nutrients, water. 

 

Solutions:

  • Special underground planting spaces
    • Vaulted sidewalks;
      • Sidewalk is suspended about 10 cm's above open soils
        • Open soils that are loose and uncompacted are placed beneath the sidewalk
    • smaller, modular versions of vaulted sidewalks
      • i.e., Silva cells
  • Special "compactable" soils (structural soils, skeletal soils)
    • Modern construction techniques compact soils pretty hard to prevent pavings from sinking or shifting
    • Another example is the Swedish rock system: large rocks are installed and soil is washed in and around the rocks. The rocks support the pavement
  • Access tunnels to larger soil areas
    • Tunnels to access nearby parks?
  • Design solutions
    • Maximize space with design, both above ground and below ground
  • High quality soils provide more resources

 

Root Function

  • Supply water and nutrients to the tree
  • Other physiological functions
  • Building soil structure
    • Fine root turnover (roots die and new ones grow on a weekly and daily basis).
    • Roots push themselves through soil
  • Improve soil drainage
    • Through tunnels created by living and dead roots allow water to move through the soil underground
  • A whole host of ecosystem services including
    • Contaminated soil remediation
    • Storm water filtration
    • Carbon Sequestration

 

Root Growth Periodicity

Root growth periodicity is the timing of root growth. This is important because arborists and urban forest planners must know when and where roots grow every year. 

This root growth is controlled and signaled by the environment, mainly:

  • Soil temperature
  • Soil moisture

Physiological readiness is another critical factor for root growth, as it is species-dependent. If roots on a tree are removed, it takes time (38 days or more depending on species) for them to start growing again. Physiological readiness is essentially how long the tree takes to recover from the shock to start regrowing roots. 

How far will roots grow in a year? It depends on the place. In Chicago, trees were observed to grow around 45 cm/year, while in the Virginia mountains they determined it to be around 1 m/year. In northern Florida, tree roots can grow 2 m/year!