Plant recruitment bottlenecks in temperate forest fragments: seed limitation and insect herbivory

Numerous studies have documented declines in plant diversity in response to habitat loss in fragmented landscapes. However, determining the mechanisms that lead to species loss is challenging using solely a correlative approach. Here we link correlative assessments of plant community composition with seed additions for a focal species to test the hypothesis that distributions of forests plants within a fragmented landscape are limited by seed dispersal. Woody plant species richness of fragments declined as fragments (n=26) became more isolated by agricultural fields. We predicted that if these isolation effects were driven by poor dispersal rather than other effects associated with habitat loss, then plants should vary in their response to isolation in relation to their seed size (i.e., stronger effects for plants with larger seeds). As predicted under this dispersal limitation hypothesis, sensitivity of bird-dispersed shrubs to isolation was related to their seed mass, with species with heavy seeds (e.g., Lindera benzoin) exhibiting stronger declines in presence across isolation gradients than species with light seeds. Seed addition experiments were performed for Lindera benzoin in two high isolation forest fragments (nearest neighbor mean distance=803 m) where Lindera was naturally absent, and two low isolation fragments (nearest neighbor mean distance=218 m) with naturally occurring Lindera populations. Seed addition and control plots (n=50 1 m² plots per fragment) were monitored for 13 censuses over 3 years. Across all four fragments, seed additions resulted in significant increases in Lindera seedling recruitment with no differences in final seedling establishment among fragments. However, insect herbivory was higher on Lindera seedlings in high isolation compared to low isolation fragments and was negatively correlated with seedling survival over some years. Consistent with prior work, our results confirm that seed dispersal plays a significant role in affecting plant diversity in fragmented landscapes. However, results also suggest the need for a better understanding of how additional processes, such as herbivory, may be altered as habitat is lost and what effects such changes have for forest plants.     

Dynamics of seedling populations and tree species coexistence in a forest: a simulation study

Theoretically, temporal variation of reproduction promotes species coexistence of sessile and polycarpic organisms when the reproduction is synchronized within species but independent among species. Monopoly of vacant sites and high relative population growth rate of minor species in the absence of propagules of other species is the essence of this mechanism. The mechanism is expected to work in forests, but persistent populations of seedlings may affect the promotion of coexistence. Using a tree-based simulation model of forest dynamics, it was demonstrated that the number of coexisting tree species was sensitively affected by the seedling establishment rate. The coexistence was not enhanced by temporal variation of reproduction when seedling establishment rate was low. This is because the reproducing minor species fail to monopolize vacant sites and allow the establishment of seedlings of other species in later years. High mortality of established seedlings under shade also suppressed coexistence. This is likely to be the result of a reduced storage effect of the population of seedlings. A forest structure and dynamics pattern that appears when tree species coexistence is promoted by fluctuating reproduction was searched for, and the number of coexisting species was varied by manipulating the seedling establishment rate. No distinct difference other than the species number itself was found between species-rich and species-poor forests. For example, the seedling population size varied, reflecting the temporal variation of reproduction, irrespective of the seedling establishment rate. Further strategy development is needed to validate the proposed mechanisms of species coexistence.     

Light-dependent changes in the relationship between seed mass and seedling traits: a meta-analysis for rain forest tree species

Seed mass is considered to be an important attribute for the establishment success of plant species being linked with their seed production, establishment, and survival. This meta-analysis shows that seed mass is also closely correlated to growth-related species attributes of the established phase of rain forest tree species, and that the strength of this relationship varies with light conditions. Seed mass is an especially good predictor of species traits under high-light conditions, when the species attain their full growth potential. At high irradiance (>20% of full light) seed mass is negatively correlated with RGR, NAR, LAR, SLA and LMF. At low irradiance (<5% of full light), seed mass is only negatively correlated with LAR and SLA. Correlations between seed mass and morphological traits are therefore strongest at low irradiance where light interception is important. Conversely, correlations between seed mass and a physiological trait are strongest at high irradiance, where maximisation of photosynthetic rates is important. The strength of the correlation between growth parameters and seed mass declines over time, and disappears after 1–4 years. Seed mass appears to be a good proxy for the shade tolerance of tropical tree species, especially at the younger stages of the life cycle.     

Seed rain and advanced regeneration in a tropical rain forest

By comparing seed rain, seedling and sapling community structures we assessed the possible role played by vertebrate seed dispersal as a structuring factor in advanced regeneration of closed-canopied sites in the tropical rain forest of Los Tuxtlas, Mexico. Seed weight, initial morphology of seedlings and species abundance were also analyzed to determine if these traits influenced the probability of establishment in the shade.About half of the seed species falling in five closed forest sites (25×25 m) during one year came from fruiting trees growing within the sites (local seeds) and half from fruiting trees found outside the sites (immigrant seeds). Seeds of liana and upper-canopy species were over-represented among immigrant seeds compared with seeds of understory tree species. This probably reflects the activity of frugivorous arboreal mammals, bats, and birds. Species with immigrant seeds had both a lower abundance and a narrower spatial distribution than locally produced seeds. Therefore, immigrant seeds showed higher diversity values than locally produced seeds.Average seed size and the proportion of epigeous seedlings were similar in local and immigrant species. Under closed-canopied sites, factors affecting community organization seem to operate selectively, favoring the establishment of large-seeded, local abundant species in the advanced regeneration. However, the fact that some saplings of immigrant species were found in the plots suggest that a slow species infiltration may be occurring leading to a slow shift in the advanced regeneration species composition. We propose that the influence of seed dispersal on advanced regeneration structure depends on the disturbance history of the patches where seeds land.     

The response of forest floor vegetation and tree regeneration to deer exclusion and disturbance in a riparian deciduous forest,central Japan

The response of forest floor vegetation and regeneration of major treespecies to deer exclusion in a riparian forest were studied for 3 years withtheinteractive effects of natural disturbances. At the start of this study, deerdensity had quickly increased to a fairly high level (29–31 individualskm^–2) during the last decade and had severely reduced theamount of forest floor vegetation in the study area. Dwarf bamboos, which weredominant before, had almost diminished from the forest floor. During the studyperiod, aboveground biomass increased steadily but species diversity did notchange much in the exclosures. Outside the exclosures, the seedlings of alltreespecies were damaged greatly by deer browsing, especially the taller ones. Deerbrowsing had little effect on the emergence of tree seedlings, but deertrampling might have accelerated emergence indirectly by disturbing the soilsurface for some species. Differences in plant responses were also observed fordeer browsing and the presence of dwarf bamboo that strongly inhibits therecruitment of tree seedlings. The nine major species were classified intothreegroups according to the response of seedlings to deer browsing and disturbance.Group 1 consists of the species whose seedling survival is affected bybrowsing,but seldom by disturbances (Phellodendron amurense,Kalopanax pictus, Quercus crispulaandMalus toringo). Groups 2 and 3 consist of species adaptedto tree-fall disturbance (Betula spp.) and ripariandisturbance (Alnus hirsuta var.sibirica, Ulmus davidiana var.japonica, Populus maximowiczii andSalix sachalinensis), respectively, and seedling survivalof these two groups is principally affected by light conditions. The effect ofdeer browsing on seedling survival and growth was greater for Group 2 than forGroup 3.     

Effects of natural disturbance on the regeneration of riparian forests in a Chichibu Mountains,central Japan

Disturbances in the riparian area had a large effect on each stage of regeneration in riparian forests dominated by Fraxinus platypoda in the Chichibu Mountains, central Japan. F. platypoda adapted well to various disturbances in frequency and size such as landslides and canopy gap formation.The spatial distribution and age structure of F. platypoda were studied in relation to the disturbance regime of the riparian zone. The bell-shaped size and age distributions of F. platypoda suggest that F. platypoda trees were established synchronously in a large disturbance site caused by an earthquake landslide about 200 years ago. For the past 200 years, the topography has been stable and the canopy gap has been recovered by advance regenerated saplings.The distribution of F. platypoda saplings was restricted to an abandoned channel and part of a floodplain. These topographical sites formed by gravel provided safe sites for saplings because stream disturbances did not occur for a long time. Channel bars were under low shade stress because of the lack of herbs and a litter layer, which represents a safe site for seedling establishment. However, the seedling bank could have been destroyed by high frequent flooding caused by large typhoons and the establishment of seedlings might have been prevented for a long time.The regeneration process of F. platypoda was explicated based on the gap dynamics theory during the stable period of topography. On the other hand, an even-aged forest was established in a large scale disturbance site.Nomenclature: Ohwi & Kitagawa (1992)     

Decreased frugivory and seed germination rate do not reduce seedling recruitment rates of <Emphasis Type=Italic>Aristotelia chilensis</Emphasis> in a fragmented forest

Habitat fragmentation reduces frugivorous bird abundance. Such a reduction may lead to a reduction in seed dispersal, thereby compromising seedling recruitment rate with far reaching consequences for plant population persistence. We assessed frugivory, seed germination, and seedling recruitment rates in a fragmented forest of central Chile by comparing a continuous forest with four forest fragments surrounded by pine plantations. Frugivory was 2.4 times higher in continuous forest than in forest fragments. Seeds eaten by birds germinated 1.7 and 3.7 times higher than non-eaten seeds from continuous forest and fragments respectively. Non-eaten seeds from continuous forest germinated 2.2 times higher than those from forest fragments, suggesting inbreeding depression. However, seedling recruitment rates at forest fragments were far higher than in continuous forest where no seedling recruited in the five years analysed. Therefore, despite forest fragmentation negatively affected frugivory, it did not translate into a decreased fitness of plants, thus highlighting the importance of considering the overall processes leading the reproductive success of plants following anthropogenic disturbances.     

The influence of soil cover organization on the floristic and structural heterogeneity of a Guianan rain forest

The impact of soil cover organization on the forest community has been studied in a 19-ha tract at Piste de St Elie station in French Guiana. 195 species each represented by at least 10 individuals were chosen from records of the position, diameter at breast height (dbh) and precise identification by botanical sampling of 12104 ligneous plants (dbh 10 cm).Spatial variations in the soil were mapped using the method proposed by Boulet et al. (1982). The soil mapping units correspond to the successive stages of evolution of a currently unbalanced ferralitic cover. These stages describe firstly the thinning by erosion of the microaggregated upper horizon and secondly the mineralogical changes under more or less extended hydromorphic conditions. The degree of evolution of ferralitic cover is also related to the hydrodynamic functioning and chemical properties of the soil. Geological substrate, topography and slope have also been taken into account.Analysis of the influence of environmental variables on plant cover has been performed using the Ecological Profiles method and Correspondence Analysis (CA) of the table of ecological profiles.The forest community seems to be dependent on the soil and the topographical features that govern it. There are significant, exclusive soil-species links for each soil functioning mapping unit. However, the highest proportion of significant positive links is connected with a thick microaggregated horizon (25%). Several species are of real value as indicators and more particularly enable differentiation between the forest stands of typical ferralitic soil and the ones of thinned out, transformed and hydromorphic soils. The CA of the species by environmental variables matrices reveals two significant factorial axes. The first can be associated with the drainage mainly related to the thinning of the soil and the second with the hydromorphic conditions related to the topography. The vegetation ordination of the stands ( 0.25 ha) delimited in the various soil domains clearly shows that changes in ferralitic cover and in particular the transition from soil with deep vertical drainage to soil with superficial lateral drainage is accompanied by substantial changes in the forest community.     

Effects of flooding regime and woody bamboos on tree community dynamics in a section of tropical semideciduous forest in South-Eastern Brazil

The influence of a population of the understorey woody bamboo Merostachys riedeliana and different flooding regimes on tree community dynamics in a section of tropical semideciduous forest in South-Eastern Brazil was examined. A forest section with an area of 1.6 ha composed of 71 adjacent plots was located on a slope ending at the river margin. The section was divided into five topographical sectors according to the mean duration of river floods. In 1991 and 1998 all trees with a diameter at the base of the trunk ≥ 5 cm were measured, identified and tagged, and all live bamboo culms were counted. Annualised estimates of the rates of tree mortality and recruitment, gain and loss of tree basal area, and change in bamboo density were calculated for each of the 71 plots and five topographical sectors as well as for diameter classes and tree species. To segregate patterns arising from spatially autocorrelated events, geostatistical analyses were used prior to statistical comparisons and correlations. In general, mortality rates were not compensated by recruitment rates but there was a net increase in basal area in all sectors, suggesting that the tree community as a whole was in a building phase. Tree community dynamics of the point bar forest (Depression and Levée sectors) differed from that of the upland forest (Ridgetop, Middle Slope and Lower Slope sectors) in the extremely high rates of gain in basal area. The predominant and specialised species, Inga vera and Salix humboldtiana, are probably favoured by relaxed competition in an environment stressed by long-lasting floods. In the upland forest, mortality rates were highest at the Middle Slope, particularly for smaller trees, while recruitment rates were lowest. As bamboo clumps were concentrated in this sector, the locally higher instability in the tree community probably resulted from the direct interference of bamboos. The density of bamboo culms in the upland forest was negatively correlated with the rates of tree recruitment and gain in basal area, and positively correlated with tree mortality rates. Bamboos therefore seemed to restrict the recruitment, growth and survival of trees. This revised version was published online in June 2006 with corrections to the Cover Date.     

热带森林乔木种群分布格局及其研究方法的比较

应用样地个体数为基础的方差／均值比率、Ｍｏｒｉｓｉｔａ分散指标,以及以个体距离为基础的最近邻体法、复合个体距离分析法对海南岛吊罗山山地雨林８个主要种群分布格局进行了比较研究．结果表明,８个种群为聚集或随机分布,４种测定方法中以复合个体距离分析法较好．     

The effect of canopy gaps on growth and morphology of seedlings of rain forest species

Summary Growth and morphology of seedlings of ten tropical rain forest species were studied at Los Tuxtlas, Mexico. Seedlings were grown in three environmental conditions: the shaded forest understorey (FU, receiving 0.9–2.3% of the daily photosynthetic photon flux, PF, above the canopy), a small canopy gap of approx. 50 m² (SG, receiving 2.1–6.1% of daily PF), and a large canopy gap of approx. 500 m² (LG, receiving 38.6–53.4% of daily PF). The growth of all species was enhanced in gaps, and in LG the effect was stronger than in SG. Plants grown in LG had a sunplant morphology, with a high root-shoot ratio (R/S), a high specific leaf weight (SLW) and a low leaf area ratio (LAR). Plants grown in SG or FU showed a shade-plant morphology, with a low R/S, a low SLW and a high LAR. Growth responses varied from species unable to grow in the shade but with strong growth in the sun, to species with relatively high growth rates in both shade and sun conditions. Shade tolerant species were able to grow in the shade because of a relatively high unit leaf rate. The pioneerCecropia had a high growth rate in LG because of a high LAR. Most species showed a complex growth response in which they resembled the shade intolerant extreme in some aspects of the response, and the shade tolerant extreme in other aspects.     

Changes in avian species composition following surface mining and reclamation along a riparian forest corridor in southern Indiana

Data on the response of bird communities to surface mining and habitat modification are limited, with virtually no data examining the effects of mining on bird communities in and along riparian forest corridors. Bird community composition was examined using line transects from 1994 to 2000 at eight sites within and along a riparian forest corridor in southwestern Indiana that was impacted by an adjacent surface mining operation. Three habitats were sampled: closed canopy, riparian forest with no open water; fragmented canopy, riparian forest with flood plain oxbows; and reclaimed mined land with constructed ponds. Despite shifts in species composition, overall bird species richness, measured as the mean number of bird species recorded/transect route, did not differ among habitats and remained unchanged across years. More species were recorded solely on mined land than in either closed forest or forested oxbow habitats. Mined land provided stopover habitat for shorebirds and waterfowl not recorded in other habitats, and supported an assemblage of grassland-associated bird species weakly represented in the area prior to mining. A variety of wood warblers and other migrants were recorded in the forest corridor throughout the survey period, suggesting that, although surface mining reduced the width of the forest corridor, the corridor was still important habitat for movement of forest-dependent birds and non-resident bird species in migration. We suggest that surface mining and reclamation practices can be implemented near riparian forest and still provide for a diverse assemblage of bird species. These data indicate that even narrow (0.4 km wide) riparian corridors are potentially valuable in a landscape context as stopover habitats and routes of dispersal and movement of forest-dependent and migratory bird species.     

Comparison of an aquatic invertebrate assemblage between an old-growth natural forest and planted coniferous forest basins in a Japanese temperate region: the Kuroson stream in the Shimanto River basin

The purpose of this study is to clarify the differences in the aquatic invertebrate assemblage between the two basins that have the most common types of Japanese headstream forests – an old-growth natural forest (ONF) and a planted coniferous forest (PCF) – in the Kuroson stream of the Shimanto River basin and to find the groups that tend to inhabit either the ONF basin or PCF basin. Abundance, the number of families and -diversity were not different between the two basins. However, a cluster analysis of similarity showed differences in aquatic invertebrate assemblages between the ONF and PCF basins and that this difference was brought about by specific characteristics of some families, such as the Taeniopterygidae and Athericidae. We suggest that differences in the aquatic invertebrate assemblages between the two basins are a reflection of differences in the composition of the forest vegetation.     

Seedling recruitment patterns in a Belizean mangrove forest: effects of establishment ability and physico-chemical factors

A field study was conducted to evaluate the relative importance of factors affecting seedling establishment and survival on a mangrove-dominated island in Belize. An examination of spatial patterns of seedling relative densities in relation to reproductive adults and physico-chemical conditions provided correlative information on factors affecting mangrove regeneration patterns. Distance from reproductive adults explained 89–94% of the variation in relative density of Rhizophora mangle seedlings, whereas availability of resources (light and NH₄) explained 73–80% of variation in Avicennia germinans seedling relative density. Just after dispersal (December), 89% of the variation in Laguncularia racemosa seedling relative density was attributable to distance from reproductive adults, but 7 months later (July) 74% of the variation was explained by intensity of flooding- and salinity-related stresses. Survivorship (after 2.5 years) of propagules and seedlings of R. mangle and A. germinans transplanted to zones of contrasting physico-chemical conditions demonstrated that: (1) mortality was highest during the establishment phase and major causes were failure to strand before viability was lost, consumption by predators and desiccation; and (2) after establishment, differences in sensitivity to physicochemical stress factors such as flooding (A. germinans) and initial orientation of the seedling axis (R. mangle) exerted a further influence on seedling survival. The results indicate that seedling recruitment in these neotropical forests is strongly influenced by dispersal patterns, differential establishment abilities and effects of physico-chemical factors that vary with elevation and distance from the shoreline.     

Reproductive investment and seedling survival of the mast-fruiting rain forest tree,Microberlinia bisulcata A. chev.

In the southern part of Korup National Park, Cameroon, the mast fruitingtree Microberlinia bisulcata occurs as a codominant ingroves of ectomycorrhizal Caesalpiniaceae within a mosaic of otherwisespecies-rich lowland rain forest. To estimate the amount of carbon andnutrientsinvested in reproduction during a mast fruiting event, and the consequentialseed and seedling survival, three related field studies were made in 1995.Theseprovided a complete seed and seedling budget for the cohort. Seed productionwasestimated by counting woody pods on the forest floor. Trees produced on average26,000 (range 0–92,000) seeds/tree, with a dry mass of 16.6kg/tree. Seeds were contained in woody pods of mass 307kg/tree. Dry mass production of pods and seeds was 1034 kgha^–1, equivalent to over half (55%) of annual leaflitterfall for this species, and contained 13% of the nitrogen and 21% of thephosphorus in annual leaf litterfall. Seed and young-seedling mortality wasinvestigated with open quadratsand cages to exclude vertebrate predators, at two distances from the parenttree. The proportion of seeds on the forest floor which disappeared in thefirst6 wk after dispersal was 84%, of which 26.5% was due to likelyvertebrate removal, 36% to rotting, and 21.5% to other causes. Vertebratepredation was greater close to the stem than 5 m beyond the crown(41 vs 12% of seeds disappearing) where the seed shadow was less dense.Previousstudies have demonstrated an association between mast years at Korup and highdry-season radiation before flowering, and have shown lower leaf-litterfallphosphorus concentrations following mast fruiting. The emerging hypothesis isthat mast fruiting is primarily imposed by energy limitation for fruitproduction, but phosphorus supply and vertebrate predation are regulatingfactors. Recording the survival of naturally-regeneratingM. bisulcata seedlings (6-wk stage) showed that 21% ofseedlings survived to 31 mo. A simple three-stage recruitment model wasconstructed. Mortality rates were initially high and peaked again in each ofthenext two dry seasons, with smaller peaks in the two intervening wet seasons,these latter coinciding with annual troughs in radiation. The very poorrecruitment of M. bisulcata trees in Korup, demonstratedinprevious investigations, appears not to be due to a limitation in seed oryoung-seedling supply, but rather by factors operating at theestablished-seedling stage.     

The Atlantic forest in the Volta Velha Reserve: a tropical rain forest site outside the tropics

With its very broad latitudinal extension (from 5° to 30°S), Brazil's Atlantic Rain Forest includes sites that wouldnot be classified as tropical by commonly used climatic classificationsystems. This situation particularly holds for the sites nearthe southern limits of this ecosystem. The goal of this study was to assess forest floristic composition, physiognomic aspects (deciduousspecies, leaf types, leaf area index) and reproductive biology (monoecious/dioeciousspecies, pollination syndromes and dispersal syndromes) of the Atlantic forest in the Volta VelhaReserve, southern Brazil (26°04 S, 48°38 W Gr). The research focuses on theaffinities of this site, which lies outside generally accepted boundaries for tropical forest. Theresults demonstrate that Volta Velha has a typically tropical floristic composition, apparentlymaintained by local climatic conditions. The differences between the characteristics of the VoltaVelha forest and those of other tropical stands are within the range of differences observed amongclassically defined tropical sites.     

Correspondence analysis of functional groups in a riparian landscape

We used multivariate analysis and ordinations to characterize thecomposition and distribution of woody vegetation within the Ozark NationalScenic Riverways (ONSR), Missouri, USA. The objectives of the study were to: 1)evaluate patterns of woody species distributions along existing environmentalgradients; 2) determine if different classes of woody plants (i.e., dominantoverstory trees, all trees, understory trees, and shrubs) responded similarlytothe same suite of environmental variables; and 3) determine if discreteecotonaland/or ecoclinal vegetation patterns were present across the landscape. Woodyvegetation was sampled from 94 plots along 35 transects positioned at rightangles to the river channel. Sample plots were analyzed with DetrendedCorrespondence Analysis (DCA), Canonical Correspondence Analysis (CCA), andTWINSPAN. Overall, woody vegetation was correlated with several environmentalgradients, including elevation of the plot above the river, soil pH, soilmoisture, and soil particle size. Responses to secondary gradients differedamong the four classes of plants analyzed, however. CCA biplots of understorytrees indicated that patterns of those species were strongly correlated withslope through the plot and sand content of soil. CCA biplots of shrubs showedthat CCA axes were most strongly correlated with soil organic matter content,soil moisture, and silt content. Further, there was limited evidence fordiscrete assemblages of woody species, with the exception of streamsidevegetation. Instead, mixing of woody species was observed across a broadtransition zone. Because there is little correspondence between vegetationlayers, our results demonstrate including plant classes other than a subset ofcanopy dominant trees can provide additional resolution in characterizingvegetation responses along complex environmental gradients.     

Flood-deposited wood debris and its contribution to heterogeneity and regeneration in a semi-arid riparian landscape

We investigated whether large woody debris (LWD) piles create nodes of environmental resources that contribute to the recovery of riparian vegetation and that also augment the heterogeneity and resilience of the riverine system. River and riparian systems are typified by a large degree of heterogeneity and complex interactions between abiotic and biotic elements. Disturbance such as floods re-distribute the resources, such as LWD, and thereby add greater complexity to the system. We examined this issue on a semi-arid savanna river where a ~100-year return interval flood in 2000 uprooted vegetation and deposited substantial LWD. We investigated the micro-environment within the newly established LWD piles and compared this with conditions at adjacent reference sites containing no LWD. We found soil nutrient concentrations to be significantly higher in LWD piles compared with the reference plots (total N +19%, available P +51%, and total C +36%). Environmental variables within LWD piles and reference sites varied with landscape position in the river–riparian landscape and with LWD pile characteristics. Observed differences were generally between piles located in the terrestrial and riparian areas as compared to piles located on the macro-channel floor. After 3 years the number and cover of woody species were significantly higher when associated with LWD piles, regardless of landscape position or pile type. We conclude that LWD piles formed after large floods act as resource nodes by accumulating fine sediments and by retaining soil nutrients and soil moisture. The subsequent influence of LWD deposition on riparian heterogeneity is discerned at several spatial scales including within and between LWD piles, across landscape positions and between channel types. LWD piles substantially influence the initial developmental of riparian vegetation as the system regenerates following large destructive floods.     

Variability in flood disturbance and the impact on riparian tree recruitment in two contrasting river systems

The vegetation within the riparian zone performs animportant ecological function for in-stream processes.In Australia, riparian zones are regarded as the mostdegraded natural resource zone due to disturbancessuch as river regulation and livestock grazing. Thisstudy looks at factors influencing vegetation dynamicsof riparian tree species on two contrasting riversystems in Western Australia. The Blackwood River insouth-western Australia is influenced by aMediterranean type climate with regular seasonalwinter flows. The Ord River in north-western Australiais characterized by low winter base flows andepisodic, extreme flows influenced by monsoon rains inthe summer. For both rivers, reproductive phenology ofstudied overstory species is timed to coincide withseasonal river hydrology and rainfall. An evendistribution of size classes of trees on the BlackwoodRiver indicated recruitment into the population iscontinual and related to the regular predictableseasonal river flows and rainfall. In contrast, on theOrd River tree size class distribution was clustered,indicating episodic recruitment. On both rivers treeestablishment is also influenced by elevation abovethe river, microtopography, moisture status and soiltype. In terms of vegetation dynamics riparianvegetation on the Ord River consists of long periodsof transition with short lived stable states incontrast to the Blackwood river where tree populationstructure is characterized by long periods of stablestates with short transitions.