Impacts of repeated fuel reduction burning on tree growth, mortality and recruitment in mixed species eucalypt forests of southeast Queensland, Australia

Impacts of repeated fuel reduction burning on tree growth, mortality and recruitment in mixed species eucalypt forests of southeast Queensland, Australia

Forest Ecology and Management 115 (1999) 13±27 Impacts of repeated fuel reduction burning on tree growth, mortality and recruitment in mixed species ...

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Forest Ecology and Management 115 (1999) 13±27

Impacts of repeated fuel reduction burning on tree growth, mortality and recruitment in mixed species eucalypt forests of southeast Queensland, Australia Danilo F. Guintoa, Alan P.N. Houseb,*, Zhi Hong Xub, Paul G. Saf®gnaa a

School of Australian Environmental Studies, Faculty of Environmental Sciences, Grif®th University, Nathan, QLD 4111, Australia b Queensland Forestry Research Institute, MS 483, Fraser Rd, Gympie, QLD 4570, Australia Received 12 October 1997; accepted 7 July 1998

Abstract The long-term effects of repeated prescribed burning on diameter growth of trees in mixed species dry and wet sclerophyll forest sites in southeast Queensland, Australia were assessed. In addition, ®re effects on tree mortality and recruitment in the wet sclerophyll site were evaluated. The results show that growth responses of species to ®re were variable. Nevertheless, for most species, recurrent burning had no deleterious effect on tree growth. At the dry sclerophyll site, annual burning since 1952 did not affect growth rates of Eucalyptus drepanophylla and E. acmenoides. On the other hand, E. tereticornis responded positively to annual burning. Smaller Corymbia variegata (formerly Eucalyptus maculata) trees appeared to respond positively to annual burning, while larger trees appeared to respond negatively but this response lessened over time. Periodic burning (every 2±3 years) since 1973 did not signi®cantly affect the growth rates of any tree species. At the wet sclerophyll site, biennial burning since 1972 has enhanced the diameter growth of Lophostemon confertus but depressed that of Syncarpia glomulifera. Quadrennial burning did not affect the diameter growth of L. confertus but depressed that of S. glomulifera. The diameter growth of E. pilularis, C. intermedia, E. microcorys and E. resinifera was not affected by either burning treatment. Basal area growth of most eucalypts at this site was unaffected by burning. However, basal area growth of both S. glomulifera and L. confertus was adversely affected by burning due to tree mortality. For most species, tree mortality was both diameterdependent and ®re-related, that is, smaller trees have a lower chance of survival than larger trees and frequent burning further reduces this probability. Without ®re, recruitment was dominated by S. glomulifera and to a lesser extent by L. confertus. Recruitment of these species was adversely affected by burning. This result and the greater mortality of smaller trees with frequent burning suggest that if these trends continue, future stand growth and hence productivity of these species could be jeopardized because of the reduction of the regenerative capacity of the forest. Recruitment was negligible for other tree species in this forest regardless of ®re treatment. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Prescribed ®re; Tree growth; Mortality; Recruitment; Eucalyptus spp.

*Corresponding author 0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-1127(98)00434-4

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1. Introduction Although recurrent fuel reduction burning has been widely adopted as an effective tool for protecting both native and plantation forests of Australia from the risk of damaging wild®res, the long-term ecological consequences of such a silvicultural practice remain uncertain (Florence, 1994). In the context of ecologically sustainable forest management, an important issue that needs to be addressed is how does this form of disturbance affect forest ecological processes, particularly those relating to long-term tree growth and forest productivity. Unfortunately, few experimental studies on the long-term effects of this type of ®re on tree growth and forest productivity exist (Peterson et al., 1994). Fire impacts on forest stands may include direct effects such as loss of trees and change in stand structure due to mortality and injury to tree crowns, boles and ®ne roots near the soil surface. Possible indirect effects of ®re include changes in growth due to tree damage, reduced competition, or changes in soil moisture and nutrient availability (Reinhardt and Ryan, 1988). If prescribed burning reduces competition from understorey vegetation or improves soil fertility, growth of trees may be stimulated. On the other hand, if prescribed burning reduces soil fertility substantially, or damages or kills trees, forest stand productivity could decline. Since tree response to ®res is dependent on a number of factors including the time since the last ®re, ®re intensity, site factors (including within-stand competition effects and the in¯uence of understorey), and the physiological condition of trees (Florence, 1996), studies on the effects of prescribed burning on tree growth for a number of forest-types show divergent results ranging from negative responses (reported by Landsberg et al. (1984) for Pinus ponderosa; Boyer (1987, 1994) for P. palustris; Huddle and Pallardy (1996) for Erythrobalanus spp.); no responses (by Hunt and Simpson (1985) for P. elliottii; Waldrop et al. (1987) and Waldrop and Lloyd (1991) for P. taeda; Huddle and Pallardy (1996) for Carya spp./Quercus stellata); to positive responses (by Henry (1961) for C. variegata/E. drepanophylla). Also, differences in experimental design, season of burning, and the length of time of observation further complicate the picture. It is therefore dif®cult to derive general functional relationships between prescribed ®re treatments and tree growth and so it is important to

investigate site-speci®c effects of prescribed burning on forest ecosystems. In a review summarizing the results of prescribed burning studies on pine forests, Landsberg (1994) has indicated that when prescribed ®re is used to thin overstocked stands, the growth of residual trees often improves. However, when prescribed ®re is used in stands that are near the desired stocking, the growth of residual trees is commonly reduced. This growth reduction has been attributed to reduction in the photosynthetic capacity of the tree or damage to ®ne roots. Less information is available for other tree species including eucalypts which dominate the native forests of Australia although for this genus, short-term (3±4 years) studies by Peet and McCormick (1971) and Nicholls (1974) in E. marginata and E. diversicolor forests and long-term (30±50 years) studies by Abbott and Loneragan (1983) in E. marginata forests in Western Australia have shown that prescribed burning has no signi®cant effect on diameter growth. The objective of this study was to evaluate the impact of repeated prescribed burning on long-term tree growth in dry and wet sclerophyll forest sites in southeast Queensland, Australia. For the wet sclerophyll site, an additional objective was to assess if repeated prescribed burning has signi®cantly affected cumulative tree mortality and recruitment. 2. Methods 2.1. Study areas and fire treatments As part of its research program on forest ®re effects (Taylor, 1989), long-term experiments were set up by Queensland's Department of Primary Industries (DPI), and Forestry (formerly Queensland Department of Forestry) in two contrasting native eucalypt forest sites in southeast Queensland to evaluate the ecological and silvicultural impacts of low-intensity (<500 kW mÿ1) recurrent fuel reduction burning. One site is located at Bauple State Forest 260 km north of Brisbane with the other located at Peachester State Forest 100 km north of Brisbane. Bauple State Forest is a dry sclerophyll forest (Beadle and Costin, 1952) or an open forest (Specht, 1970) dominated by spotted gum (Corymbia variegata F. Muell., formerly Eucalyptus maculata Hook.) and

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grey ironbark (E. drepanophylla F. Muell. ex Benth.) Other canopy tree species include forest red gum (E. tereticornis Smith) and white mahogany (E. acmenoides Schauer). The understorey vegetation includes wattles (Acacia spp.), brush box (Lophostemon confertus (R. Br.) P.G. Wilson & Waterhouse), swamp box (L. suaveolens (Solander ex Gaertn.) P.G. Wilson & Waterhouse), red ash (Alphitonia excelsa (Fenzl.) Reisseck ex Benth.) and lantana (Lantana camara L.) (Henry, 1961). The average annual rainfall is 1131 mm with over half of this occurring between December and March, and a marked dry period from July to September. The topography is one of broad ¯at ridges separated by shallow gullies. Brown and red Kurosols (Isbell, 1996) [Soil Taxonomy: Al®sols] cover most of the area with red Kandosols [Soil Taxonomy: Al®sols] appearing on some hill tops and gullies. The soils have loamy surface textures and are generally shallow with clay loam-to-clay textures at 30±40 cm depth. In 1952, two compartments were allocated to monitor the effects of two prescribed ®re treatments, namely, no burning and annual burning in late winter or early spring (August±September). Treatments were applied to plots positioned inside each of the two compartments. In 1973, periodic burning every 2±3 years was added as the third treatment in another compartment to simulate the Department's current routine prescribed burning practice in this forest-type. There are six plots per treatment, with each plot measuring 10040 m2. It is signi®cant to note that the spatial arrangement of treatments (i.e. plots within a compartment receive only one burning treatment) has resulted in a segregated experimental design (Hurlbert, 1984) and is therefore strictly non-randomized, an acknowledged limitation of the experiment. Nevertheless, within a compartment, plot locations were spread evenly and the average distance between two nearest neighbouring plots is about 0.7 km. This situation may weaken any spatial dependence of soil properties that could exist between neighbouring plots. One rationale behind the one compartment±one treatment approach to experimentation in this forest was to ensure protection of the experiment from unscheduled burns (Queensland Department of Forestry, 1975, unpublished establishment report), a major consideration at the time when the technique of controlled burning was still being developed. Due to high fuel moisture con-

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tent and/or unfavourable weather conditions, scheduled burns sometimes could not be achieved. At the time of the latest growth measurements (February 1996), there had been 42 burns in the annually burnt treatment and 10 burns in the periodically burnt treatment (both last burned in September 1994). The unburnt treatment has been protected from ®re since 1946 when this forest land was ®rst acquired by DPI Forestry. Prior to acquisition, however, the site would have probably been burned quite frequently by graziers while it was freehold land as shown by the presence of charcoal on the soil surface. Peachester State Forest is a wet sclerophyll forest (Beadle and Costin, 1952) or a tall open forest (Specht, 1970) dominated by blackbutt (E. pilularis Smith). Other canopy tree species include red bloodwood (C. intermedia R. Baker), tallowwood (E. microcorys F. Muell.), red mahogany (E. resinifera Smith), turpentine (Syncarpia glomulifera (Smith) Niedenzu) and brush box (Lophostemon confertus (R. Br.) P.G. Wilson & Waterhouse). The understorey vegetation is variable and species-rich, in places dominated by grasses (e.g. Imperata cylindrica (L.) Rauschel, Digitaria ciliaris (Retz.) Koeler), ferns (Blechnum cartilagineum Sw.), or shrubs (e.g. Dodonaea triquetra Andr., Hibiscus heterophyllus Vent., Hovea acutifolia Cunn. ex G. Don). Average annual rainfall in the area is 1711 mm. The topography is undulating-to-rolling (2±16% slopes). The soils are deep and sandy having no perceptible increase in clay content to a depth of 60 cm. The soils are classi®ed as red to yellow Kandosols (Isbell, 1996) [Soil Taxonomy: Al®sols]. The experiment was arranged in a split-plot design with ®re regimes as the main plot treatment and slope position (lower and upper slopes) as the subplot treatment. The plots were established in 1969 but the ®rst burn was not conducted until 1972. Since that time, the ®re treatments include: unburnt, biennial burning and quadrennial burning during late winter or early spring (August to September). There are three blocks, two of which are adjacent to each other while the third block is about 6 km away from the others. Plots in the two adjacent blocks measure 3027 m2 while plots in the third block measure 4020 m2. The third block has soils with sandier texture and considerably less organic matter. They are classi®ed as red Kandosols (Isbell, 1996) [Soil Taxonomy: Al®sols]. There is also a marked change in the vegetation with

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the presence of grass trees (Xanthorrhoea spp.), banksias (Banksia spp.) and different grass species (Entolasia stricta (R. Br.) Hughes and Eremochloa bimaculata Hackel) in the understorey. There are 18 plots for the experiment. Like the dry sclerophyll site, high moisture content of the forest ¯oor and/or unfavourable weather conditions have resulted in burns that were wither poor or completely unsuccessful and vary from plot to plot. At the time of the latest growth measurements (February, 1996), there had been, on average, 11 burns in the biennially burnt treatment and 8 burns in the quadrennially burnt treatment (both last burned in August 1994). 2.2. Monitoring of tree growth, mortality and recruitment At both sites, tree species were identi®ed and marked for repeated measurements of diameter at breast height (dbh) over bark at 1.3 m (actually girth at breast height over bark prior to 1974). Field staff had access to earlier dbh records in order to maintain consistency in the measurements. Throughout these long-term experiments, tree measurements were made before burning to avoid apparently negative growth resulting from measurements being taken after ®res when bark may have shed. At the dry sclerophyll site, annual dbh measurements were made between 1952±1972 on all trees taller than 3 m at the beginning of the experiment in each plot of the unburnt and annually burnt treatments (Henry, 1961). In the early 1970s, the experiment was restructured to re¯ect the present-day silvicultural management of this forest-type and this involved thinning of unwanted stems in plots of both control and annually burnt treatments. Consequently, a large proportion of the original sample trees has been removed. After thinning, additional sample trees with dbh of 10 cm or more were included for measurement. Subsequent measurements were made in 1974, 1976, 1978, 1982, 1988, and 1996. Due to this change, studying tree mortality and recruitment for this site was not attempted. For the periodically burnt treatment, dbh measurements were made in 1973, 1974, 1976, 1982, 1988, and 1996. At the wet sclerophyll site, initial dbh measurements were made on all trees with dbh greater than 3 cm on each plot in 1969. Subsequent measurements

were made in 1972, 1974, 1976, 1980, 1991, and 1996. Tree mortality and recruitment were also recorded and this allowed examination of these processes. However, detailed observations on tree damage from burning were not made. Plots at this site have not been logged since the establishment of the experiment. 2.3. Statistical analyses Although periodic growth measurements were made at both sites, evaluation of the long-term effects of ®re on individual diameter growth of each species were con®ned to regressing ®nal versus initial dbh and comparing regression lines of burning treatments. Since species at both sites have a wide range of initial dbh, this approach of comparing regression lines of ®re treatments was adopted to eliminate the confounding effect of tree size differences. Regression lines were compared by coding the ®re categories as indicator or dummy variables, testing for parallel slopes (®re by initial dbh interaction) and comparing intercepts when the slopes were parallel (Kleinbaum et al., 1988). Since the thinning operations undertaken at the dry sclerophyll site may confound the interpretation of ®re effects on long-term tree growth (1952±1996) through the release of trees from competition, the analysis was separated into an early period (1952±1972) when the stands were unthinned and a later period (1973±1996) after the stands were thinned. In both cases, only sample trees that survived until 1996 were used. Exploratory analysis of the data which involved comparison of the dbh±dbh regression lines of the two periods (1952±1972 vs. 1973±1996) indicate that C. variegata and E. acmenoides trees may have exhibited positive responses to thinning. At this site, there was one plot of the unburnt treatment in which growth measurements after 1972 were discontinued because the plot carried a very scrubby forest on a different soil-type. A new plot was then set up in 1973 and subsequent dbh measurements taken, so for each of these there is only a partial record of growth. Due to our interest in long-term growth response, these plots were excluded in the statistical analysis. Thus, the unburnt treatment consists of ®ve replicates only, while the two burnt treatments have six replicates each. For each species at the dry sclerophyll site, 1952± 1972 comparisons were made for the unburnt and

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annually burnt treatments. The 1973±1996 comparisons include the periodically burnt treatment added in 1973. For this analysis, the 1972 and 1974 dbh measurements were averaged in both the control and annually burnt treatments to represent the 1973 initial measure for the periodically burnt treatment. At both sites, minor tree species which occurred only in a few plots were excluded from the analysis. For the dry sclerophyll site, these include red ironbark (E. crebra F. Muell.), grey gum (E. punctata DC.) and grey box (E. moluccana Roxb.). For the wet sclerophyll site, tree species excluded were forest red gum (E. tereticornis) and scribbly gum (E. signata F. Muell.). At the wet sclerophyll site, the 1996 basal area of eucalypts and non-eucalypts was adjusted by covariance analysis using the initial (1969) basal area as covariate in order to evaluate the effect of repeated burning on stand growth (the 1996 measure of basal area incorporates stand mortality). This was not done for the dry sclerophyll site because it was only in the early 1970s when most trees in each plot were measured. At the wet sclerophyll site, the status (dead or alive) in 1996 of each sample tree was recorded and the resulting tree mortality data analyzed by logistic regression (Hosmer and Lemeshow, 1989). Dead trees were assigned code of 0 while live trees were assigned a code of 1. Initial (1969) dbh was treated as a continuous predictor variable and functional relationships between the probability of survival in 1996 and 1969 dbh were developed for each ®re treatment. Comparison of regression lines was made by coding the ®re categories as indicator variables, testing for parallel slopes, and comparing intercepts when the slopes are parallel. The signi®cance of the diameter and ®re terms as well as their interactions were assessed using the Wald 2 statistic. The interaction between diameter and ®re was insigni®cant. Prior to this, a more complete model incorporating the effects of dbh, ®re, slope and block, and all the interaction effects was ®tted. Results of this modelling exercise indicated that the effects of slope and block as well as all other interaction effects were insigni®cant and terms in the model were then progressively dropped resulting in a simpler model described above. Logistic regression was performed using SPSS for Windows (Norusis, 1994).

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For the tree recruitment data, ANOVA was performed on the square root of the 1996 counts (stems haÿ1). Means were then back-transformed and a correction term (i.e. (nÿ1)s2/n, where n is the number of replicates and s2 the error mean square in square roots) added to the transformed means for presentation purposes (Snedecor and Cochran, 1980). 3. Results and discussion 3.1. Diameter growth Previous studies have shown that bark thickness in eucalypts can be affected by ®re. Signi®cant amounts of ®brous bark were burnt off messmate stringybarks (E. obliqua) in Victoria (Tolhurst, 1994): this may be attributed to a relatively long period (30±50 years) since the trees had last been burnt, allowing considerable thicknesses of bark to develop. Similarly, Gill (1980) recorded bark losses from smooth-barked E. dalrympleana after a ®re that had killed all the leaves on the trees. In the present study, the low ®re intensities and high frequencies were considered to have had minimal effect on bark loss, and although it is acknowledged that some bark loss may have occurred, we believe the growth analyses presented give a true re¯ection of the impacts of repeated burning on tree growth. At the dry sclerophyll site, both the 1952±1972 and 1973±1996 comparisons showed that prescribed burning did not signi®cantly affect the growth rates of E. drepanophylla and E. acmenoides (Figs. 1 and 2). Annual burning did not signi®cantly affect the growth rateofE.tereticornisbetween1952and1972(Fig. 3(a)), while the 1973±1996 comparisons (Fig. 3(b)) indicate that annual burning positively affected its growth rate (pˆ0.026). For C. variegata, the annual and unburnt regression lines crossed signi®cantly, indicating a signi®cant-®re-by-initial dbh interaction (pˆ0.001 for the 1952±1972 comparison and pˆ0.041 for the 1973±1996 comparisons) (Fig. 4). This suggests that smaller trees responded positively to annual burning while larger trees responded negatively. It is not clear how this can occur, but judging from the p values in the two periods, this interaction effect lessened with time. Soil fertility assessments at this site (Guinto et al., 1998a, b) have shown that long-term fuel reduction

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Fig. 1. Relationship between final dbh and initial dbh of E. drepanophylla for the (a) 1952±1972 and (b) 1973±1996 comparisons of prescribed burning treatments at the dry sclerophyll site.

burning did not signi®cantly affect topsoil (0±10 cm) total N, potentially mineralizable N and in situ N mineralization rates. However, an increase in topsoil acid-extractable P with annual burning was observed, which could bene®t overstorey trees. It appears, however, that with the exception of the positive response of E. tereticornis to annual burning, this increase in available P (6.6 mg P kgÿ1 soil for annually burnt versus 3.8 mg P kgÿ1 soil for unburnt) has not enhanced tree growth. The increase may not have been suf®cient to cause biologically signi®cant growth enhancement, or the larger trees may be too old to respond to such soil fertility change due to internal recycling of nutrients within trees. Periodic burning had no measurable effect on the growth rates of any tree species studied (Fig. 1(b), Fig. 2(b), Fig. 3(b), Fig. 4(b)).

Results of the ®rst 8 years of this experiment was reported by Henry (1961), who observed that annual burning increased the diameter increments of C. variegata and E. drepanophylla especially during the period 1953±1957. After 1957, however, increments were about the same for both treatments and this latter result agrees with our current assessment of long-term growth. Henry (1961) further explored this early growth response by subsequently measuring the diameter of trees at 4.67 m above ground level, subtracting this from the diameter at 1.3 m, and comparing the difference for the two treatments to test if the observed growth response can be attributed to ®reinduced butt swell. He found that only a slight increase in stem form had occurred with annual burning and this was not statistically signi®cant. A more plausible

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Fig. 2. Relationship between final dbh and initial dbh of E. acmenoides for the (a) 1952±1972 and (b) 1973±1996 comparisons of prescribed burning treatments at the dry sclerophyll site.

explanation for this early growth advantage could be that since the site had been protected from ®re 6 years before the initial burn, it had abundant and well-cured fuel which, when burned, could have caused a ¯ush of nutrients to be released in the ash and subsequently taken up by the vegetation. At the wet sclerophyll site, frequent burning signi®cantly decreased topsoil (0±10 cm) total N, mineralizable N and cumulative N mineralization which could in turn lead to a long-term reduction in tree growth (Guinto et al., 1998a, b). However, given the adaptability of sclerophyllous vegetation to ®re (Gill, 1981) and its ability to cope with low nutrients (Bowen, 1981), signi®cant negative long-term effects on tree growth may not easily be demonstrable. In fact, both quadrennial and biennial burning did not

signi®cantly affect the growth rates of E. pilularis, C. intermedia, E. microcorys, and E. resinifera (Figs. 5±8). This agrees with the results of Keith (1991) and Keith and Raison (1992), who found that frequent burning had no effect on the growth rate of E. pauci¯ora Sieber ex Sprengel despite a dramatic reduction in soil N availability. However, continued burning has begun to have an effect on the growth of E. pauci¯ora as the N capital within the trees is being spent (J. Raison pers. comm.). The growth of S. glomulifera was negatively affected by both burning treatments (p<0.001) (Fig. 9). This species, which occurs at rainforest margins and in wet sclerophyll forests, appears to be more sensitive than eucalypts to a reduction in soil fertility. Surprisingly, the growth of L. confertus was enhanced

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Fig. 3. Relationship between final dbh and initial dbh of E. tereticornis for the (a) 1952±1972 and (b) 1973±1996 comparisons of prescribed burning treatments at the dry sclerophyll site.

by biennial burning (P<0.001) (Fig. 10) and this may be because this burning regime caused high mortality of this species releasing the residual trees from competition (Fig. 16, discussed under tree mortality section).

3.2. Basal area growth at the wet sclerophyll site Table 1 shows the adjusted 1996 basal area of eucalypts and non-eucalypts. With the exception of E. resinifera, prescribed burning had no deleterious

Table 1 Influence of prescribed burning on adjusted 1996 basal area of eucalypts and non-eucalypts at the wet sclerophyll sitea Fire frequency

Basal area (m2 haÿ1) E. pilularis

E. intermedia

E. microcorys

E. resinifera

S. glomulifera

L. confertus

Unburnt Quadrennial Biennial

28.82a 26.81a 27.30a

3.96a 3.36a 3.60a

1.38a 1.23a 1.80a

2.01a 1.43b 1.62a,b

6.12a 4.80a,b 4.16b

1.03a 0.99a ÿ0.08b

a

Adjusted means from covariance analysis using the 1969 basal area as covariate. Within a column, means with the same letter are not significantly different at pˆ0.05.

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Fig. 4. Relationship between final dbh and initial dbh of C. variegata for the (a) 1952±1972 and (b) 1973±1996 comparisons of prescribed burning treatments at the dry sclerophyll site.

Fig. 5. Relationship between final dbh and initial dbh of E. pilularis for the comparison of prescribed burning treatments at the wet sclerophyll site.

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Fig. 6. Relationship between final dbh and initial dbh of C. intermedia for the comparison of prescribed burning treatments at the wet sclerophyll site.

Fig. 7. Relationship between final dbh and initial dbh of E. microcorys for the comparison of prescribed burning treatments at the wet sclerophyll site.

impacts on stand growth of eucalypts. In contrast, frequent burning reduced the basal area of S. glomulifera and L. confertus because of ®re-enhanced tree mortality (Figs. 15 and 16, Section 3.3). Covariance analysis produced a slightly negatively adjusted basal area for L. confertus in the biennially burnt treatment. Such a strange result is attributable to excessive mortality of this species under biennial burning (Fig. 16, Section 3.3). 3.3. Tree mortality at the wet sclerophyll site Compared with the dif®culty of predicting the effects of ®re on tree growth, the effects of ®re on

mortality are more accessible given a tree's initial size characteristics and vigor. In eucalypts, a tree's resistance to ®re is very highly correlated with the thickness and extent of protective bark tissue on the stem and upper branches which insulates the cambium from heat (and may be lost during ®res), and generally increases directly with the size of the individual. Large tree size may also confer an advantage by distancing the unprotected foliage from the source of heat (Bell et al., 1989). However, rough-barked species are at a slight disadvantage since their bark can catch ®re adding to localized heating (Noble, 1986). The ®tted logistic regression curves for all species at the wet sclerophyll site, based on the binary data of

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Fig. 8. Relationship between final dbh and initial dbh of E. resinifera for the comparison of prescribed burning treatments at the wet sclerophyll site.

Fig. 9. Relationship between final dbh and initial dbh of Syncarpia glomulifera for the comparison of prescribed burning treatments at the wet sclerophyll site.

dead/live trees in 1996, show that trees with small initial diameters have a lower probability of survival until 1996 than trees with larger initial diameters. Also, in general, tree survival was highest in unburnt, followed by quadrennial and least in the biennially burnt treatment (Figs. 11±16). For E. pilularis, biennial burning caused a slight (pˆ0.078) reduction in its survival (Fig. 11). Both burning treatments signi®cantly reduced the survival of C. intermedia (Fig. 12), while quadrennial burning caused signi®cant reduction in the survival of E. microcorys (Fig. 13). The survival curve for the biennially burnt treatment was not ®tted because parameter estimates failed to con-

verge. In this treatment, all trees with initial dbh below 10 cm died while those with initial dbh above 10 cm remained alive. The statistical program therefore detected a perfect ®t (step function). For E. resinifera, none of the burning treatments signi®cantly reduced its survival (Fig. 14), whereas biennial burning caused a signi®cant reduction in the survival of S. glomulifera (Fig. 15). In L. confertus, both burning regimes adversely affected its survival, particularly biennial burning (Fig. 16). Thus, for most species, survival is both diameter- and ®re-dependent, that is, smaller trees have a lower probability of survival and frequent burning further reduces this probability. This trend

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Fig. 10. Relationship between final dbh and initial dbh of Lophostemon confertus for the comparison of prescribed burning treatments at the wet sclerophyll site.

in mortality was expected. Since bark thickness increases with diameter, larger trees are better protected than smaller ones, and the higher the frequency of burning the higher the chances of smaller trees getting killed. It is also signi®cant to note that there is evidence that ®res in the biennial burn plots tend to be more intense than ®res in the quadrennial burn plots. For example, during the 1994 burns, average ®re intensity in biennial burn plots was 315 kW mÿ1 compared with 177 kW mÿ1 in the quadrennial burn plots. This anomaly is explained by differences in the nature and degree of curing of ®ne fuels between the treatments, where a ®re interval of 4 years is suf®cient

for the shrubby understorey to shade out the grassy elements in the ground ¯ora and prevent rapid curing of fuels. These results are consistent with the study of Keith (1991) and Keith and Raison (1992) on the E. pauci¯ora forest in the Brindabella Ranges in southeast Australia, where frequent fuel reduction ®res killed or damaged trees with dbh below 16 cm. The study by Huddle and Pallardy (1996) on prescribed burning in an oak±hickory forest in Missouri, USA, however, showed that less frequent burning (burning every 4 years) actually reduces the survival of Carya and Erythrobalanus species compared with annual burning. They attributed this trend to higher fuel loads

Fig. 11. Fitted logistic regression curves showing the probability of survival until 1996 of E. pilularis at the wet sclerophyll site as a function of initial dbh. (Both quadrennial and biennial curves are not significantly different from the unburnt curve at pˆ0.05.)

Fig. 12. Fitted logistic regression curves showing the probability of survival until 1996 of C. intermedia at the wet sclerophyll site as a function of initial dbh. (Both quadrennial and biennial burn curves are significantly different (pˆ0.035 and pˆ0.001, respectively) from the unburnt curve.)

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Fig. 13. Fitted logistic regression curves showing the probability of survival until 1996 of E. microcorys at the wet sclerophyll site as a function of initial dbh. (The quadrennial burn curve is significantly different from the unburnt curve (pˆ0.015). The biennial burn curve was not fitted because parameter estimates did not converge.)

Fig. 15. Fitted logistic regression curves showing the probability of survival until 1996 of S. glomulifera at the wet sclerophyll sit as a function of initial dbh. (The biennial burn curve is significantly different from the unburnt curve (pˆ0.010)).

in the periodically burnt treatment resulting in more intense burns that caused greater tree mortality.

Appreciable recruitment in the wet sclerophyll forest was observed only beginning with the 1991 inventory. As the number of dominant tree species recruited were similar in 1991 and 1996, only the latter enumeration is reported here. In the absence of ®re, recruitment (1969±1996) was dominated by the noneucalypts S. glomulifera and to a lesser extent by L.

confertus. With burning, however, recruitment of these two species was greatly reduced (Table 2). Also, for S. glomulifera, there was a wider range of diameter classes for the unburnt treatment (range: 4.4±23.5 cm) compared with the burnt treatments (range: 5.5± 16.8 cm). It appears that recurrent burns do not allow recruited trees to grow into larger individuals through its effect on seedling/sapling mortality. This result and the greater mortality of the smaller trees under a regime of low-intensity but frequent ®res imply that if these trends continue, future stand growth and hence productivity of these species could be jeopar-

Fig. 14. Fitted logistic regression curves showing the probability of survival until 1996 of E. resinifera at the wet sclerophyll site as a function of initial dbh. (Both quadrennial and biennial curves are not significantly different from the unburnt curve at pˆ0.05.)

Fig. 16. Fitted logistic regression curves showing the probability of survival until 1996 of L. confertus at the wet sclerophyll site as a function of initial dbh. (Both quadrennial and biennial burn curves are significantly different (p<0.001) from the unburnt curve.)

3.4. Tree recruitment at the wet sclerophyll site

26

D.F. Guinto et al. / Forest Ecology and Management 115 (1999) 13±27

Table 2 Influence of prescribed burning on recruitment until 1996 of S. glomulifera and L. confertus at the wet sclerophyll sitea Fire frequency Unburnt Quadrennial Biennial

Cumulative recruitment (stems haÿ1) S. glomuliferab

L. confertus

305.4a 55.4b 29.8b

13.5a 1.1b 1.1b

a

Original means were in square roots. Values given are backtransformed means. b Within a column, means with the same letter are not significantly different at pˆ0.05.

frequent ®res implies that if these trends continue, future stand growth and hence productivity of these species could be jeopardized because of the reduction of the regenerative capacity of the forest. In the longer term there may be some decline in growth rates of trees due to reduced availability of soil P and N, and possible failure of the nutrient reserves of the trees themselves to buffer against these losses, but this has not yet been demonstrated by these experiments Acknowledgements

dized because of the reduced regenerative capacity of the forest. Recruitment of eucalypt species including E. pilularis was negligible regardless of the ®re treatment (very low counts of species that occurred only on a few plots) and this precluded any meaningful statistical analysis. Florence (1965) has pointed out that where E. pilularis grows in association with S. glomulifera, L. confertus and rainforest species, natural regeneration of the species may be inadequate in the absence of canopy-removing destructive ®res or logging.

We thank Paul White and Damian Cotter for doing the latest diameter measurements in the two forest sites, Dave Osborne for assistance with the new Australian soil classi®cation and Marks Nester for guidance in the statistical analysis and interpretation of the data. We also thank staff of the former Queensland Department of Forestry who were involved in the establishment and early maintenance of these long-term ®re experiments. D.F. Guinto was supported by QFRI and a postgraduate scholarship grant from the Australian Agency for International Development.

4. Conclusions

References

At both sites, although growth responses of tree species to ®re were quite variable, repeated lowintensity fuel reduction burning over 27-(wet site) and 45-(dry site) year periods had no deleterious effects on the long-term diameter growth of dominant trees of most species. In the wet sclerophyll forest, basal area growth of most eucalypts was unaffected by burning. However, basal area growth of both S. glomulifera and L. confertus was adversely affected by burning due to tree mortality. At the wet sclerophyll site, for most species, tree mortality was both diameter-dependent and ®re-related, that is, smaller trees have a lower chance of survival than larger trees and frequent burning further reduces this probability. Without ®re, recruitment was dominated by non-eucalypts S. glomulifera and to a lesser extent by L. confertus. Recruitment of these species was adversely affected by burning and this result coupled with the greater mortality of the smaller trees under a regime of

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