Effect of thinning and fertilizer on growth and allometry of Eucalyptus marginata

Jarrah (Eucalyptus marginata) forest is one of the most widespread native forest types of south west Western Australia (SWWA) and provides timber, firewood, wildlife habitat, water production, catchment protection and recreation. To inform management aimed at optimizing these values, a thinning trial in even aged regrowth forest was commenced in 1965. A second thinning and N and P fertilizer treatment was applied in 1986. The effect of thinning and fertilizer on tree and stand level growth and allometry was evaluated. Thinning resulted in higher values of under bark diameter at breast height (DBHUB) and tree height, and lower values of height diameter ratio (HDR), stand basal area under bark (BAUB) and stem number per unit area. Growth of DBHUB increased with a decrease in stand density. Growth of stand BAUB peaked at intermediate stand densities. Fertilizer increased growth in DBHUB and stand BAUB. Height growth was significantly greater in thinned plots. Thinning had a significant effect on the allometric relation between diameter at breast height over bark (DBHOB) and each of height, HDR and crown width (CW) while fertilizer had a significant effect on the allometric relation between DBHOB and each of height and HDR. Height and CW increased with an increase in DBHOB but HDR decreased. Larger sized trees resulting from thinning and addition of fertilizer are likely to provide a higher volume of timber, firewood, fruits (important food for threatened cockatoos), and improved visual amenity. As thinning had a positive effect on diameter and height growth and self-thinning in jarrah is relatively low (0.20% yr⁻¹), thinning may be a valuable management tool to accelerate development of larger trees which can be beneficial for both timber production and conservation.