Red gum tree
Causes of poor form:
Some species e.g. red gum, are naturally more prone to poor form than others. Wide spacing or gaps in plantations can result in poor form, particularly on exposed sites. Open-grown or edge trees are often shorter with heavier side branching than densely grown trees. Dense planting results in increased shading and competition for light hence increased vertical growth compared to lateral growth, which means better form.
Usually, for this reason, and to increase the opportunity for selection, more trees are initially planted than are needed at final harvest. Within species, genetics also inf uence tree form. Much genetic improvement has been made with Radiata pine since its introduction to Australian forestry. However, little has been done to improve the genetics of most eucalypt species grown for sawlogs. Growers should check the availability of improved seed and recommended provenances before ordering seedlings.
Some species will grow tall and straight on the right site e.g. blackwood on moist sheltered sites, but in the wrong rainfall zone, soil types, or on an exposed windy site, poor form can result in little potential to produce sawlogs. Many factors including insects, disease, strong wind, drought, waterlogging, birds or browsing by stock, rabbits, hares, wallabies or other wildlife can cause damage to growing tips and terminal buds, resulting in temporary loss of the main (apical) leader, and often heavy branching or a bent stem.
Loss of apical leader due to wind damage has ruined this trees form above the break
Some species are particularly susceptible to frost which can kill off the apical leader year after year and may require annual form pruning to maintain a single leader until the target height is reached. Poor nutrition, trace element or mineral def ciencies, in the nursery or the plantation, can result in poor form e.g. boron def ciency in pines causes multiple leaders. Excessive fertiliser can also result in poor form e.g. nitrogen on sandy sites can induce a copper defeciency resulting in a curled stem.
Promoting good form:
Clearly many factors can work against young trees growing straight and tall. The farm forester can inf uence many of these by:
- selecting suitable species and sites
- using good genetic material
- good site preparation and dense tree spacing
- using shelter rows on plantation edges
- excluding stock and browsing animals and giving the tree time to sort out a new dominant leader if damage has been done
- applying timely form pruning
Wallabies can damage growing tips until the tree grows out of their reach
