Saturday, July 15, 2017

The Root System of a Tree

Tree roots are often regarded as vexatious – blocking drains, buckling footpaths and roads, etc. However, they do have some redeeming features.

The well known issue of road/gutter buckling
Tree roots help support the tree. They provide the tree with its nutrients from the soil, lower the water table and absorb run-off, prevent erosion and they work in a symbiotic relationship with other plants and fungi to produce a healthy ecosystem. Large roots act as storage for nutrients for when conditions are unfavourable.

A network of fine hairs that grow near the tip of the root absorb water. Although short-lived – they may only last a week or so – these fine hairs are the most important part, capturing water and nutrients from the soil and passing them into the tree via the roots.

The root tip itself is made up of some extra hard cells that are designed to do the tunnelling through the ground, then wear away and be replaced by a new set of cells, thus increasing the length of the root.

Some root systems can be extensive, covering hundreds of square metres and most root growth occurs close to the surface. This makes most tree roots susceptible to temperature changes, soil compaction and drought. (Most scholarly articles on the subject declare that soil compaction is the chief contributor to the poor health of a tree).

The shallow root section of a 30m eucalypt
Although some trees develop a tap root that goes deep into the ground, our native eucalypts generally have a shallow root system and this can make them vulnerable to blowing over in strong winds, especially when the ground is soft. Clusters or groups of eucalypts are usually more stable than lone trees as their mutually entwined root systems sometimes help support the group. The roots of some species will sometimes naturally graft with the roots of a neighbouring tree.

Strength, (stability), in numbers
The root system of many/all(?) eucalypts have swellings near the root crown, just above and below ground level called lignotubers. Lignotubers contain dormant buds that sprout if the tree is damaged by grazing or fire etc, so enabling the tree to survive.

The root zones of trees are rather special places and deserve to be treated with care and respect if we want healthy trees to be part of our urban environment.

PS: There might be numerous possible solutions to the age-old problem of tree roots and footpaths – anti-trip hinges between footpath segments, rubber and plastic footpaths, porous concrete and asphalt footpaths, interlocking pavers and bricks, suspended footpaths, gravel, structural soil, mulch, … (link).

Thursday, July 6, 2017

What is a Tree Worth?

Drouin's trees are valuable for many reasons.

Trees provide habitat for urban wildlife. They reduce energy costs by producing shade in Summer and protection from cold winds in Winter. Properties in treed streets sell more easily and a tree on the nature strip can add thousands of dollars to the sale price of a house. Trees absorb carbon, airborne pollutants and noise. They 'exhale' oxygen, mitigate water run-off and reduce the level of the water table. Well treed suburbs and towns are known to have lower health costs and reduced crime rates.

Fortunately, in Australia and here in West Gippsland, we are catching up with many other places around the world and beginning to think of urban trees as assets rather than liabilities.

But, just how do you calculate the monetary value of a street tree? (If you 'Google' that question, you get hundreds of very technical answers.)

An example relatively close to home is the Urban Forest Tree Evaluation Fact Sheet from the City of Melbourne which provides a formula for providing the monetary amenity value of a tree;
Value(V)=Basic Value(B) x Species(S) x Aesthetics(A) x Location(L) x Condition(C).

Basic value (B) is taken from an internationally acceptable set of figures which provides a monetary figure for each square inch of basal trunk area.

The Species factor (S) is a score determined by analysis of the rate of growth in a particular environment.

The score for Aesthetic value of a tree, (A) is determined by its impact on the landscape if it was removed.

The Locality score (L), is evaluated from a range of localities such as undeveloped bush or open forest to the city centre or main boulevard.

Condition (C) is calculated from a table that lists factors like 'solid and sound trunk' to 'advanced infestation, decay or dieback', etc.

So, let's apply V = B x S x A x L x C to a well-known Drouin tree, (hope you're keeping up?), the Bill Kraft Giant in Albert Rd, (E. cypellocarpa).

    Base Value B
This tree's diameter at breast height is 223cm. The given table for determining the Base Value does not go this high, but extrapolation from the table gives a Base Value B = $248,200.

Species Factor S
Eucalyptus with a long life span, S estimated as 0.9. (could be higher).

Aesthetic Value V
A solitary feature specimen tree, A = 1.0

Locality L
Outer area/residential street, L = 1.5

Condition C
Rating from table C = 1.0

Determining some of the above values is a subjective process but I believe I have been conservative. The basic value figure is an estimate made in 2012 and was extrapolated from a table which did not cover a tree with trunk diameter as large as this.

Thus, the value of the Bill Kraft giant Mountain Grey Gum in Albert Rd is …

V = B x S x A x L x C
   = 248,200 x 0.9 x 1.0 x 1.5 x 1.0
   = $335,070

Monday, July 3, 2017

A Chilly McNeilly Wetlands

The Drouin Amateur Weather Station recorded a minimum of -4.7°C on Sunday morning. Brrrr. Between warming the fingers to press some buttons, I captured a few shots at McNeilly Wetlands, (map, earlier blog post)….
(Click on images to enlarge or open in a separate window)
Plenty of seating available

Even the canines wore coats.

A frosty handrail

A scene that caught my eye

Another scene - sans ducks!

This White-faced Heron sought a sunny perch