Indirect effect of fire on established tuart and peppermint trees

Background

Two studies were conducted to determine the indirect effect of tree canopies to fire. By selecting trees within burnt areas with no or only minor canopy scorch, the direct impact of fire on trees was thought to have been eliminated or reduced. Study 1 assessed the changes in the canopy size and health of tuart trees in a burnt and unburnt area over time. Study 2 involved the measurement of leaf-scale differences between tuart and peppermint trees in burnt and unburnt areas.

Study 1: canopy size and health of tuart trees Hypothesis:

• Canopy area and canopy health ratings are higher for minimally-scorched tuart trees in the year following fire.

Sites

The study occurred at sites Y1 (burnt site), Y2 and Y3 (unburnt sites) in Yalgorup National Park.

Sampling design

One recently burnt area (site Y1) with 22 sub-samples (trees) and one unburnt site (sites Y2 and Y3 pooled) with 26 subsamples were available. Trees with canopy scorch, but with ≤ 10 % of the canopy scorched, comprised the sub-samples in the burnt area. Trees with some scorch were selected to indicate that burning had occurred within their vicinity; the 10 % level was arbitrarily chosen as an upper-limit for minimal direct damage.

Measurements

Pre-fire ratings of CH and CA, and measurements of dbhob, were made from March to May 2004. The fire occurred at site Y1 in December 2004 and estimations of canopy scorch were recorded for each tree later in that month. Post-fire ratings of CA were made in January 2006.

Statistical analysis

Paired-sign tests for pre-fire and post-fire measurements of CH and CA were calculated. Tests were performed separately for the burnt and unburnt samples. In testing the comparability of burnt and unburnt samples, medians for dbhob were compared using the Mann-Whitney test as the data sets could not be normalized.

Study 2: leaf attributes of tuart and peppermint trees Hypotheses:

• The physical and chemical properties of tuart and peppermint leaves are different between burnt and unburnt patches.

• There is an interaction between species and patch (burnt or unburnt) for the physical and chemical properties of leaves.

Sites

The study occurred at sites Y1 (burnt site) and site Y3 (unburnt site) at Yalgorup, but was not confined to within the permanent plots.

Sampling design

There were two factors: species and patch type. The species were tuart and peppermint and the patch-type factor comprised three alternatives: unburnt patches within site Y1, burnt patches within site Y1 and patches within site Y3 (unburnt site). There were eight replicates trees per species per patch. From each tree, 10 youngest-fully-expanded

leaves (sub-samples) were collected. Leaves from from outer, mid-height canopy positions were selected in order to standardize for leaf type between replicates.

Sampling methodology

Replicates were selected by searching for suitable tree patch-type combinations. Suitable combinations had burnt or unburnt ground present within at least the projected canopy area of a tree, and trees between 3 and 15 m tall, in at least moderate health, displaying no evidence of post-fire resprouting (assumed to be indicative of no or minimal direct fire damage). Relatively small, healthy trees were chosen as they were thought to be more able to respond rapidly to environmental changes than larger or less- healthy trees. That the fire was low intensity fortuitously provided burnt and unburnt patches at the site, as well as small trees without scorched canopies within burnt patches.

Sample processing and measurements

Sampling occurred in December 2005, 12 months after the fire. Leaf thickness was measured immediately upon collection using digital calipers aligned parallel to the mid- vein and positioned mid-way between the mid-vein and leaf edge. Leaves were then sealed in plastic bags and stored on ice. On return from the field, they were refrigerated. Within 48 hours they were scanned and leaf area measured using ASSESS software (American Phytopathological Society 2002). After drying at 70°C for 48 hours, they were weighed and specific leaf area (SLA) was calculated. The dried material was then finely ground in a ball mill in preparation for the determination of nitrogen (N) concentration, 15N:14N and 13C:12C. Analysis was carried out at Edith Cowan University (Joondalup) using a continuous flow mass spectrometer (model 20:20, PDZ Europa Crewe, UK). The isotopic ratios, represented in relation to known standards, were calculated as:

δ15_{N or δ}13_{C (‰) = R}

sample/Rstandard – 1) x 1000

where, Rsample and Rstandard are the 15N: 14N or 13C:12C ratios of the leaf sample and the standards, respectively.

Statistical analysis

Data from the unburnt site (Y3) were not compared statistically to the data from the burnt site (Y1) owing to the distinct separation of the sites in space. In contrast, the

samples for the burnt and unburnt patches within site Y1 were compared as they were relatively well dispersed. MANOVA was performed initially. P values from Pillai’s

trace were reported given that this measure is less affected by departures from normality and constant variance compared to the others (Townend 2002). Nevertheless, as serious departures from model assumptions could not be rectified, adjustment of α to 0.01 was made to reduce the probability of Type 1 errors (Tabachnik and Fidell 1996). Following a significant result, ANOVA was then performed for each response variable separately.

4.2.3 Comparison of the anatomical features affecting the capacity of tuart and