Ecology of palms (Arecaceae) in reponse to cyclonic disturbances in North Queensland, Australia

Extensive ecological research has been conducted on vegetation changes following cyclones in northern Australia, particularly in recent times (de Gouvenain and Silander Jr 2003; Dowe 2009; Hopkins and Graham 1987; Lugo 2008; Lugo et al. 1983; Metcalfe et al. 2008; Murphy et al. 2008; Turton 2008; Turton 1992; Unwin et al. 1988; Webb 1958). However, there have been few studies on palms although they are important components of many rainforests. Dowe (2009) studied population changes of Archontophoenix alexandrae following Cyclone Larry and recommended that further studies were needed on the long term impacts of cyclones on palms and their recovery throughout their life cycle. In the current study, field observations were made of population structure, regeneration strategies and reproductive phenology; germination responses were examined under controlled conditions; and a glasshouse study examined the responses of seedling growth to different levels of shading.\ud \ud Thus, the aims of this research were:\ud \ud (1)to investigate whether the population structure of five palms, Arenga australasica, Calamus australis, C. moti, Hydriastele wendlandiana and Licuala ramsayi, as shown by size class, reflect mass recruitment after a major cyclonic disturbance (Cyclone Larry), and determine the relationship between height, dbh (diameter at breast height) and wind resistance;\ud \ud (2) to investigate the effects of canopy gaps following cyclonic disturbance (Cyclone Larry) on regeneration, reproductive phenology (flowering, fruiting and seed production), seed dispersal and predation of these palms;\ud \ud (3)to determine the effects of seed coat, light and temperature on germination; and\ud \ud (4) to investigate the effects of different light intensities on seedling growth.\ud \ud The field research was carried out in four plots at each of three study sites (Tam O’Shanter/Djiru National Park, Clump Mountain National Park and Kurrimine Beach Conservation Park) located near Mission Beach and Kurrimine Beach, in north Queensland. The study plots were characterised based on comparisons of canopy openness (light intensities) and damage categories. The canopy openness ranged from 27 to 80% and damage categories ranged from 'minor to moderate' to 'severe'. Observations were made of life stage distribution, relationships between height, dbh and wind resistance, recruitment, growth rate, leaf turnover and life history.\ud \ud The field research demonstrated that increased canopy openness and damage, following Cyclone Larry, affected population structure, regeneration and reproductive phenology. Population structure was dominated by seedlings. Most of the palms were susceptible to wind stress; however, they had regeneration or life history strategies such as a multi-stemmed, clustered growth habit in A. australasica and H. wendlandiana or "tilting" in L. ramsayi, where half-fallen individuals survived. Seedlings of some palm species were more abundant in less shaded areas, suggesting that recruitment may be favoured by increased openness in canopy gaps created by cyclones. Seedlings of C. australis, C. moti and H. wendlandiana showed greater recruitment and survival where canopy openness was higher; however, A. australasica and L. ramsayi favoured less open (or more shaded) areas. The growth of A. australasica, C. australis, C. moti and L. ramsayi seedlings were not significantly different between sites, while H. wendlandiana seedlings grew more rapidly under smaller canopy gaps.\ud \ud Germination experiments were conducted in a constant temperature room to examine imbibition, dormancy, and effects of light intensity, light quality and temperature. Germination times from first to final germination were recorded. Germination of A. australasica, C. australis and L. ramsayi was improved by abrasion of their hard seed coats (scarification) and far red light inhibited germination. This suggests that canopy gaps created by cyclonic disturbances favour the germination of these four palm species. The seeds of A. australasica, C. australis and H. wendlandiana were able to germinate in darkness, suggesting that burial of the seed does not inhibit germination; however, darkness inhibited the germination of L. ramsayi seeds.\ud \ud Seedling growth experiments (pot trials) were conducted in a glass house using shade cloth providing four different levels of shading: 59, 29, 17 and 6 % sunlight. The growth rate, leaf turnover, leaf area, total chlorophyll, chlorophyll a:b ratio, vigour, above-ground and below-ground biomass and growth indices (LAR, SLA and LWR) of palm seedlings were measured. As a result of these measurements the relative shade tolerance of the five species was determined. The seedlings of A. australasica were classified as intermediate-shade intolerant species. H. wendlandiana seedlings were shade-intolerant. Calamus australis and C. moti seedlings are intermediate-shade intolerant. Licuala ramsayi seedlings were found to be shade-tolerant. These experimental results extended the field observations, because light intensities in the field ranged from 27 to 80% sunlight, indicating disturbance to the canopies and the lack of very shaded niches.\ud \ud The field observations and the frequency of cyclones in the region suggest that disturbed canopies and the lack of deeply shaded environments are a persistent feature of the rainforests at Mission Beach and Kurrimine Beach, and therefore the vegetation must be adapted to disturbance. Restoration efforts may need to be undertaken in cyclone-prone rainforests to accelerate natural regeneration that may otherwise take hundreds of years. When palms are selected to be used in rainforest restoration, sowing seeds or transplanting seedlings are suitable methods. The palm seeds can be sown in medium-sized canopy gaps in the wet season to prevent drying out (desiccation). Despite some variation in results between field research and the glass house experiments, overall trends showed that C. australis, C. moti and H. wendlandiana seedlings appeared to be suitable species for revegetating medium to large, low-shade canopy gaps and areas with 'minor' to 'severe' damage. Arenga australasica may be planted in medium-sized canopy gaps with 'minor-moderate' to 'severe' damage. Licuala ramsayi appears best suited for subsequent revegetation, being planted after the formation of the canopy