Dynamics of Forests with Araucariaceae in the Western Pacific - 793
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Journal of Vegetation Science 10: 793-804, 1999 © IAVS; Opulus Press Uppsala. Printed in Sweden - Dynamics of forests with Araucariaceae in the western Pacific - 793 Dynamics of forests with Araucariaceae in the western Pacific Enright, N.J.1*, Ogden, J.2 & Rigg, L.S.1 1Department of Geography & Environmental Studies, University of Melbourne, Parkville, Australia 3052; 2School of Environmental and Marine Science, Tamaki Campus, University of Auckland, Auckland, New Zealand *Corresponding author; Fax +61393444972; E-mail [email protected] Abstract. Several species of Araucaria and Agathis (Arau- Introduction cariaceae) occur as canopy emergents in rain forests of the western pacific region, often representing major components The family Araucariaceae comprises 40 species of of total stand biomass. New data from permanent forest plots conifers from three genera: Araucaria (19 spp.), Agathis (and other published work) for three species (Araucaria (20 spp.) and the recently described Wollemia (1 spec.) hunsteinii from New Guinea, A. laubenfelsii from New Cal- edonia, and Agathis australis from New Zealand) are used to (Enright et al. 1995; Jones et al. 1995). The majority of test the validity of the temporal stand replacement model these species are large trees, often emergent (reaching proposed by Ogden (1985) and Ogden & Stewart (1995) to heights of 40 - 90 m), in rain forests of the Asia-Pacific explain the structural and compositional properties of New region stretching from peninsular Malaysia to northern Zealand rain forests containing the conifer Agathis australis. New Zealand (Fig. 1). Forest stands containing Araucaria Here we propose the model as a general one which explains and Agathis vary greatly in dominance of the conifer the stand dynamics of rain forests with Araucariaceae across relative to the angiosperm tree species component. Sev- a range of sites and species in the western Pacific. eral authors have sought to understand the population Forest stands representing putative stages in the model and stand dynamic processes operating within such were examined for changes through time in species recruit- ment, growth and survivorship, and stand richness, density forests which might explain conifer-angiosperm coex- and basal area. Support for the model was found on the basis istence and the variations in abundances observed of: 1. Evidence for a phase of massive conifer recruitment (Whitmore 1966; Havel 1971; Gray 1975; Enright 1982a; following landscape-scale disturbances (e.g. by fire at the Ogden 1985; Ahmed & Ogden 1987; Enright & Ogden Huapai site, New Zealand for Agathis australis); 2. Increasing 1995; Ogden & Stewart 1995). species richness of angiosperm trees in the pole stage of forest Results of studies on Araucaria hunsteinii in New stand development (i.e. as the initial cohort of conifers reach Guinea (Enright 1982a - c, 1992, 1995), Agathis australis > tree size; 10 cm DBH); 3. A high turnover rate for angiosperms in New Zealand (Ogden 1985; Ogden & Stewart 1995) (< 100 yr), and low turnover for conifers (>> 100 yr) in the pole and, more recently, Araucaria laubenfelsii in New Cal- stage, but similar turnover rates for both components (50 - 100 yr) as forests enter the mature to senescent phase for the initial edonia (Rigg et al. 1998), suggest many common fea- conifer cohort; 4. Very low rates of recruitment for conifers tures in the stand level dynamics of these species. This within mature stands, and projected forest compositions which paper seeks to bring together the evidence across the show increasing dominance by angiosperm tree species; 5. A western Pacific region, for selected species within the low probability of conifer recruitment in large canopy gaps Araucariaceae, which supports a common explanation created by conifer tree falls during the initial cohort senescent of stand level dynamics in relation to disturbance re- phase, which could produce a second generation low density gimes and competitive interactions between conifers stand in the absence of landscape scale disturbance; 6. Evi- and angiosperms. In particular, we highlight results dence that each of the three species examined required open based on monitoring of mortality and recruitment rates canopy conditions (canopy openness > 10 %) for successful recruitment. and trends in species richness in permanent plots, and The evidence presented here supports the temporal stand characterization of canopy light environments. replacement model, but more long-term supporting data are We place the interpretation of species and stand needed, especially for the phase immediately following land- dynamics in context of the temporal stand replacement scape level disturbance. model for Agathis australis (Fig. 2) described by Ogden (1985) and Ogden & Stewart (1995). This model identi- Keywords: Araucaria; Agathis; Conifer-angiosperm forest; fies infrequent landscape level disturbance (e.g. by fire), Long-term dynamics; Recruitment; Stand replacement model. great longevity of conifer relative to angiosperm forest species, and varying (but generally declining) opportuni- Nomenclature: Allan (1961), De Laubenfels (1972) and Silba ties for conifer recruitment through time in the absence (1986). of such disturbance, as the key factors determining 794 Enright, N.J. et al. Fig. 1. Distribution of species of Agathis (broken line) and Araucaria (solid line) in the western Pacific, and location of permanent study sites in Papua New Guinea, New Caledonia and New Zealand. conifer persistence and abundance in these mixed coni- 3. Landscape scale disturbance: There should be fer-angiosperm forests. According to the model, an initial evidence for stand level disturbances of types sufficient dense cohort of conifers establishes after stand level to provide the opportunity for dense, initial conifer disturbance (Fig. 2, stage a), self-thins as it matures over a cohort recruitment. period of 200 - 400 yr, and is invaded by angiosperm tree species which grow, mature and are replaced beneath the emergent conifer stratum (Fig. 2, stage b). Cohort senes- Species and sites cence (and tree fall) of the emergent conifers creates large canopy gaps which may allow gap-phase recruitment of a Three species of Araucariaceae from widely sepa- second, but less dense, cohort of conifers (Fig. 2, stage c). rated regions of the western Pacific were examined The density of conifers declines progressively as oppor- (Table 1). Duration of sample stand remeasurement tunities for parent replacement decrease over 1000 - 2000 varies from 2 to 25 yr, with studies ongoing in New yr in the absence of any new stand level disturbance (Fig. 2, Zealand and New Caledonia. The longest possible pe- stage d). Periods of recruitment become more blurred with riod of remeasurement was used in each case since the each succeeding cohort until some catastrophe destroys the aim here is to identify overall directions of change in forest and starts the cycle again. In order to support such a stand properties. model, we attempt here to identify the different stages of Data for Araucaria hunsteinii are from three stands the cycle (illustrated in Fig. 2), and show that change within in lower montane rain forest at Mt. Susu and Middle these putative stages is occurring in the hypothesized direc- Creek near Bulolo, Papua New Guinea. They are plots tion. The stages, and hypothesized directions of change, 13, 14 and 15 (of sizes 0.5, 1.5 and 0.5 ha respectively) examined here are: previously described in ordination studies by Enright 1. Dense initial cohort stage: Stands should be (1982b). Mean annual rainfall at Bulolo (670 m a.s.l.) is strongly dominated by conifers recruited only within a 1600 mm/yr with a ‘dry’ season from May to September short period following disturbance relative to their po- (although rainfall is never < 75 mm/month), while mean tential longevity. They should show increasing evi- annual temperature is 23.7 ºC. Information for Araucaria dence of invasion by angiosperm tree species (increas- laubenfelsii is from three montane rain forest patches ing richness) with time. (0.25 to > 1 ha) on Mt. Do, New Caledonia (Rigg et al. 2. Cohort senescence stage: Dense, mature stands 1998). Mean annual rainfall is 1690 mm/yr with Septem- should show high rates of conifer mortality leading to ber the driest month (45 mm) and February the wettest many large tree-fall gaps. There may be evidence for (255 mm). Mean annual temperature is ca. 17 ºC. Data for occasional recruitment of conifers within such gaps, Agathis australis are from a 1.34 ha monitored catchment creating a low density second generation stand which is in temperate lowland rain forest about 30 km north of dominated by angiosperm trees. Angiosperm tree spe- Auckland, New Zealand (Enright & Ogden 1987). The cies should be faster-growing but shorter-lived than the catchment supports temperate rain forest patches of three co-occurring conifers. distinct ages; a young post-fire stand now 47 yr old, an - Dynamics of forests with Araucariaceae in the western Pacific - 795 advanced seedlings (≥ 30 cm height to < 1 cm DBH), and saplings (1 - 5 cm DBH) were tagged in subplots and their height and DBH (as appropriate) measured. Seed production was estimated on the basis of canopy cone counts and/or seed counts in litter fall traps and replicate 1-m2 quadrats (Enright 1982e, 1999; Rigg 1998). Indi- viduals were remeasured (and new recruits tagged) an- nually in most cases (but sometimes less frequently) so that rates of recruitment, growth, survival and seed production could be determined. Data on growth and survival for angiosperm tree species were collected within subplots up to 0.5 ha in size for periods of 1 to 14 Fig. 2. Hypothetical temporal stand replacement model for yr. Recruitment rate was calculated as number of new Araucaria and Agathis rainforest emergents in the western stems added to a given size class per year, relative to all Pacific. The sum of the areas across cohorts remains constant stems (conifer plus angiosperm) for that size class in the over about the first thousand years (i.e.