Date of Award

2014

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Health, Engineering and Science

First Advisor

Dr Eddie Van Etten

Second Advisor

Professor William Stock

Third Advisor

Dr Lachie McCaw

Abstract

The establishment of seedling regeneration is a key process in and indicator of ecologically sustainable forest management. The availability of seed and the creation of a suitable seedbed are recognised as important factors limiting seedling recruitment. A silvicultural method commonly used across northern and eastern jarrah forest blocks is shelterwood cutting. The primary objective of treating jarrah forest to shelterwood is to promote seedling regeneration in areas lacking sufficient advanced growth. Despite the widespread and progressive implementation of the shelterwood method, its application in jarrah forest has shown varying degrees of success.

This thesis sought to investigate and better understand the roles of seed supply and seedbed condition in promoting successful seedling regeneration in shelterwood-treated jarrah forest. It addressed two questions from an ecological and management perspective. Firstly, could adequate seed supply and favourable seedbed conditions be effectively managed and produced in shelterwood-harvested coupes? Secondly, could adequate seed supply and suitable seedbed conditions be reliably produced to facilitate successful seedling regeneration following disturbance events, in this case post-harvest burning?

A major effort was dedicated to developing a more accurate and practical method of assessing seed crops in individual trees. The final model produced a high degree of predictability (R² = 0.85), while still maintaining a high level of practicality for field application, with three easily measured variables being used (stem diameter combined with subjective assessments of capsule clump density and capsule clump distribution). The refined model dramatically improved estimates of crown capsule numbers from the previous model, with the R² value increasing from 0.29 to 0.85.

The second major focus of the study was to assess the capacity of prescribed burns, under mild conditions, to produce seedbed conditions suitable for regeneration. Low intensity prescribed burns resulted in the production of suitable conditions for seedling regeneration; that is, leaf litter and understorey vegetation were reduced and ash beds were created. Ash bed production was heterogeneous within sites. This heterogeneity has been attributed to the capacity of low intensity prescribed burns to account for fine-scale variations in fuel quantity, continuity and condition.

Patterns of pre-burn aerial seed crop size and seed fall following low intensity prescribed burning were also assessed. Canopy capsule crops showed a high degree of spatial and temporal variability, both in terms of seed quantity and maturation. Such variability has been attributed to individual trees or groups of trees responding differently to localised climatic events and/or interspecific site factors at each stage of the flowering cycle. The main source of this variability was shown to be the numbers and spatial distribution of super trees; that is, trees defined as having a stem diameter >60 cm and >20 000 capsules. The average rate of seed fall increased substantially following prescribed burning under mild conditions. Postharvest burning under the mild conditions of the current survey did not result in en masse seed fall. Rather, peaks in seed fall were observed in the first few weeks post-burn, followed by low level falls throughout the following year. Sites burnt in spring showed a higher and more consistent release of seed in the first few weeks following fire, whereas seed fall after autumn burning was more sporadic.

The comparative and interactive roles that seed supply and seedbed conditions play in limiting recruitment of jarrah were also studied. Low seedling densities were recorded across all six burnt study sites. The fact that ample levels of post-burn seed fall produced such low seedling numbers suggested that adequate seed supply did not coincide with seedbed conditions suitable for mass seedling regeneration. Conditions favourable for seedling recruitment were highly variable within sites, since both seed supply and seedbed conditions were spatially heterogeneous. Fine-scale areas burnt to mineral soil showed an additive influence to the overwhelmingly dominant factor of seed supply on seedling recruitment. However, the capacity of low intensity burns to produce these seedbed conditions at a broad scale is limited.

Results of this study suggest that successful stocking of shelterwood-treated jarrah forest is not always achievable following a disturbance event, such as post-harvest burning under mild conditions. The chances of a large seed supply coinciding with broad-scale seedbed conditions favourable for mass germination, emergence and establishment appear to be low. Successful stocking of shelterwood-treated jarrah forest is more likely to be a longer term outcome achieved through episodic recruitment, when favourable environmental conditions coincide with optimal seedbed conditions. Such episodic recruitment strategies may be common in resource-limited systems such as jarrah forest and other dry eucalypt forest systems, where conditions controlling the regeneration niche are often variable and unpredictable.

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