# Trees

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## Overview

Trees can be incorporated into bioretention cells with other plant types, or otherwise into their own planting pits.

## Planning considerations

A commonly held view is that a tree's root system will be similar to it's visible crown. For many trees, this is not the case, as roots will more often spread much more widely, but to a shallower depth [1]. For more detailed information on planning (site) considerations see Bioretention

### Planting in slopes

Smooth slopes should be amended into localised terraces by the landscape Architect when planting large trees into slopes > 5 %. [2]

## Design

### Inlets

Multiple methods for distribution and conveyance of runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. into the system are recommended for redundancy and conservative designs. Combinations may be made of:

• tree well flow,
• catch basinsGround depression acting as a flow control and water treatment structure, that is normally dry. and distribution pipes, and
• direct infiltration from permeable pavingAn alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil..

## Performance

### InterceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies.

Tree canopies intercept and store rainfall, thereby modifying stormwater runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. and reducing demands on urban stormwater infrastructure (Xiao et al., 1998; Xiao et al., 2000; Xiao and McPherson, 2002; Xiao et al., 2006). Canopy interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. reduces both the actual runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. volumes, and delays the onset of peak flows (Davey Resource Group, 2008).

The extent of interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. is influenced by a number of factors including tree architecture and it has been estimated that a typical medium-sized canopy tree can intercept as much as 9000 litres of rainfall year. (Crockford and Richardson, 2000).

A study of rainfall interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. by street and park trees in Santa Monica, California found that interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. rates varied by tree species and size, with broadleaf evergreen trees provided the most rainfall interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. (Xiao and McPherson, 2002). Rainfall interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. was found to range from 15.3% for a small jacaranda (Jacaranda mimosifolia) to 66.5% for a mature brush box (Tristania conferta now known as Lophostemon confertus). Over the city as a whole the trees intercepted 1.6% of annual precipitationAny form of rain or snow. and the researchers calculated that the annual value of avoided stormwater treatment and flood control costs associated with this reduced runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. was US$110,890 (US$3.60 per tree).

### Transpiration

Trees suck! (Abstracted from Phyto, by K. Kennen)