Rain forest invasion of eucalypt-dominated woodland savanna, Iron Range, north-eastern Australia: I. Successional processes

Aim To explore successional processes associated with rain forest expansion in Eucalyptus‐dominated woodland savanna vegetation in north‐eastern Australia. Location Iron Range National Park and environs, northeast Queensland, Australia. This remote region supports probably the largest extent of lowland (< 300 m) rainforest remnant in Australia. Rainfall (c. 1700 mm p.a.) occurs mostly between November and June, with some rain typically occurring even in the driest months July–October. Methods (1) Sampling of rain forest seedling distributions, and other vegetation structural attributes, in fifteen 10 × 10 m quadrats distributed equi‐distantly between mature rain forest margins (range: 70–840 m), at each of 10 sites which were open‐canopied vegetation in 1943. (2) Assessment of relationships between rain forest seedling densities and structural characteristics, including distance‐to‐rain forest‐margin, canopy height, stem density. (3) Assessment of lifeform and dispersal spectra for defined vegetation structural types. Results Rates of rain forest invasion were found to be substrate‐mediated. Transects established on hematite schist, diorite, riverine alluvium, and granite developed closed canopies (termed phase III sites) by 1991. The remainder (four transects on poorly drained colluvial/alluvial sediments; one on dune sands) continued to occur either as grassy woodland (phase I), or with developing rain forest understoreys (phase II). Rain forest seedlings were observed at maximum sampled distances from mature rain forest margins at all sites. Lifeform and dispersal spectra data illustrated that: (1) the proportions of woodland trees, shrubs and graminoids declined with successional phase, with concomitant increases in rain forest primary trees and all other lifeform categories save rain forest trees; (2) the proportions of major dispersal syndromes did not vary between successional phases, neither for rain forest nor woodland taxa. Main conclusions Rain forest seedling distribution data for phases I and II sites illustrate three successional processes: margin extension – seedling density significantly negatively correlated with distance from mature rain forest margins at two sites; nucleation – seedling densities significantly positively correlated with tall trees at two sites; and irruption – seedling densities at two sites neither correlated with distance from mature rain forest margins, nor with measured vegetation structural features. The observation of irruptive rain forest regeneration at these sites, combined with decadal‐scale rain forest canopy development at the five remaining sites, illustrates that under conditions conducive to growth (moisture, substrate), low fire disturbance, and maintenance of diverse dispersal processes (high frugivore richness), rain forest can rapidly invade regional landscapes.     

Monsoon rain forest seedling dynamics, northern Australia: contrasts with regeneration in eucalypt-dominated savannas

Aim To explore: (1) the relative influences of site conditions, especially moisture relations, on pathways and rates of monsoon rain forest seedling and sapling regeneration, especially of canopy dominants, in northern Australia; and (2) contrasts between regeneration syndromes of dominant woody taxa in savannas and monsoon rain forest. Location Four monsoon rain forest sites, representative of regional major habitat and vegetation types, in Kakadu National Park, northern Australia. Methods A decadal study involved: (1) initial assessment over 2.5 years to explore within‐year variability in seed rain, dormant seed banks and seedling (< 50 cm height) dynamics; and (2) thereafter, monitoring of seedling and sapling (50 cm height to 5 cm d.b.h.) dynamics undertaken annually in the late dry season. On the basis of observations from this and other studies, regeneration syndromes of dominant monsoon rain forest taxa are contrasted with comparable information for dominant woody savanna taxa, Eucalyptus and Corymbia especially. Results Key observations from the monsoon rain forest regeneration dynamics study component are that: (1) peak seed rain inputs of rain forest taxa were observed in the wet season at perennially moist sites, whereas inputs at seasonally dry sites extended into, or peaked in, the dry season; (2) dormant soil seed banks of woody rain forest taxa were dominated by pioneer taxa, especially figs; (3) longevity of dormant seed banks of woody monsoon rain forest taxa, including figs, was expended within 3 years; (4) seedling recruitment of monsoon rain forest woody taxa was derived mostly from wet season seed rain with limited inputs from soil seed banks; (5) at all sites rain forest seedling mortality occurred mostly in the dry season; (6) rain forest seedling and sapling densities were consistently greater at moist sites; (7) recruitment from clonal reproduction was negligible, even following unplanned low intensity fires. Main conclusions By comparison with dominant savanna eucalypts, dominant monsoon rain forest taxa recruit substantially greater stocks of seedlings, but exhibit slower aerial growth and development of resprouting capacity in early years, lack lignotubers in mesic species, and lack capacity for clonal reproduction. The reliance on sexual as opposed to vegetative reproduction places monsoon rain forest taxa at significant disadvantage, especially slower growing species on seasonally dry sites, given annual–biennial fires in many north Australian savannas.     

Contrasting demographics of tropical savanna and temperate forest eucalypts provide insight into how savannas and forests function. A case study using Corymbia clarksoniana from north‐eastern Australia

Eucalypts (Eucalyptus spp. and Corymbia spp.) dominate many communities across Australia, including frequently burnt tropical savannas and temperate forests, which receive less frequent but more intense fires. Understanding the demographic characteristics that allow related trees to persist in tropical savannas and temperate forest ecosystems can provide insight into how savannas and forests function, including grass–tree coexistence. This study reviews differences in critical stages in the life cycle of savanna and temperate forest eucalypts, especially in relation to fire. It adds to the limited data on tropical eucalypts, by evaluating the effect of fire regimes on the population biology of Corymbia clarksoniana, a tree that dominates some tropical savannas of north‐eastern Australia. Corymbia clarksoniana displays similar demographic characteristics to other tropical savanna species, except that seedling emergence is enhanced when seed falls onto recently burnt ground during a high rainfall period. In contrast to many temperate forest eucalypts, tropical savanna eucalypts lack canopy‐stored seed banks; time annual seed fall to coincide with the onset of predictable wet season rain; have very rare seedling emergence events, including a lack of mass germination after each fire; possess an abundant sapling bank; and every tropical eucalypt species has the ability to maintain canopy structure by epicormically resprouting after all but the most intense fires. The combination of poor seedling recruitment strategies, coupled with characteristics allowing long‐term persistence of established plants, indicate tropical savanna eucalypts function through the persistence niche rather than the regeneration niche. The high rainfall‐promoted seedling emergence of C. clarksoniana and the reduction of seedling survival and sapling growth by fire, support the predictions that grass–tree coexistence in savannas is governed by rainfall limiting tree seedling recruitment and regular fires limiting the growth of juvenile trees to the canopy.     

Seed Dynamics in Relation to Gaps in a Tropical Montane Rainforest of Hainan Island, South China： （I） Seed Rain

Seed dynamics is an important part of stand dynamics in forest ecosystems. In this paper, 26 gaps were randomly selected to study the influence of gaps on the spatial and temporal patterns of seed rains in a tropical montane rainforest of Hainan Island, South China. Three zones for each gap, including outside gap zone （Non-gap）, transitional gap zone （EG-CG）, and central gap zone （CG）, were designed, and fourseed traps （each lm x lm in size） were placed in each zone. Seed rains were collected by these traps every 10 days from June 2001 to May 2002. Seed rain varied greatly with season and generally exhibited a pattern of unimodal change during the study period： seed abundance and species richness were both greater in the wet season than in the dry season. Gaps significantly influenced the temporal patterns of both species richness and density of seed rains. Gaps had no significant influences on the spatial distribution patterns of seed rain species richness, but significantly affected the spatial distribution pattern of seed rain densities. Among the three different zones of gaps, the outside gap zone generally received more seeds inputs than the two other gap zones.     

海南尖峰岭热带林土壤动物群落——群落结构的季节变化及其气候因素

对土壤原生动物、线虫和其它大中型土壤动物等3个部分,分别描述群落的季节变化,然后,综合分析气候因素对群落的影响。①原生动物群落的季节化表现为：种数在1、2月份最高,达43、44种;个体数量则在8月份和12月份（14万多和12万多/g.干土）。②线早的种数也在1月份最高,达67种;个体数则在9月份（88.3万/m^2）;DG指数显示出有9月份的大高峰（19.9）和1月份的小高峰（19.7）。③大中型土壤动物群落的季节变化幅度很大：类群数的大高峰在7月份（31个）,小高峰在12月份（17种）;个体数量则在6月份和11月份（5.8万个和1.05万个/m^2）。④气温与地表温度对群落变化的影响较小：对线虫的种数变化有显著负相关;对原生动物、大中型土壤动物种数变化也只有阶段性的相关。⑤土壤含水量对群落变化的影响较大：与原生动物丰度呈非常显著的相关（r=0.896,d?=10,r0.01=0.708）;与其它大中型土壤动物的类群数、总个体数及DG指数的影响则具有季节性差异,即在湿季呈显著的负相关。⑥降水量年内和年间的变化均具很大的不稳定性,暴雨又是土壤动物（除原生动物外）灾难性的因素。因此降水量的变化是造成群落跳跃式波动和大起大落的主要因素。     

Comparison of regeneration following burning,clearing or mineral sand mining at Tomago,NSW: II. Succession of ant assemblages in a coastal forest

Abstract The dry sclerophyll forest community of the Tomago Sandbeds, near Newcastle in New South Wales, has been subject to regular disturbances due to fire, clearing and strip mining for over 18 years. In this study we use chronosequence analysis to examine whether the structure of the ant community varies with the type of disturbance and the time since disturbance. We treat the recovery trajectory after fire as a control trajectory because fire is an endogenous disturbance. The main analyses were based on an ant fauna comprising 72 species sampled from 44 sites surveyed in December 1992. Comparison with samples taken in April and December 1991, and for cumulative records for all sites over this 20 month period, all show quantitatively similar responses. Results suggest that while fire has a minor effect on the composition of the ant community over time, the impact of clearing and mining is much more severe. Ant species richness at cleared and mined sites recovers rapidly, overshoots controls in mid-succession and returns to control levels by 18 years after disturbance. The cumulative number of species recorded over all sites (from the total recorded fauna of 82 species) for each different disturbance type were: burned, 61; cleared, 55; and mined 56. Species composition at cleared or mined sites, after 18 years, approaches but does not match controls. The recovery trend for mined sites lags slightly behind that for cleared sites, which have reached 49% similarity with the oldest burned sites, while mined sites have not exceeded 39% similarity of species composition. The main patterns in the ant community appear to be related to habitat variables. These results provide further evidence that the ant community may be used as a reliable bio-indicator for evaluating the extent of habitat damage and recovery after disturbance in these Australian forests.