Reproductive patterns of selected understorey trees in the Malaysian rain forest: the apomictic species

HA, C. O., SANDS, V. E., SOEPADMO, E. & JONG, K., 1988. Reproductive patterns of selected understorey trees in the Malaysian raia forest: the apomictic species. Garcinia is the predominant representative of the Clusiaceae in the understorey of the lowland rain forest of Peninsular Malaysia. In Pasoh Forest Reserve the dioecious species G. paruifolia was investigated since no male trees, but only trees with structurally hermaphodite flowers which later set fruit, were found there. Pollination and embryological studies indicated a condition of non-pseudogamous agamospermy, with no viable pollen grains being formed in the staminodes, and the unfertilized egg cell giving rise to the embryo. Supportive studies were made of microspore development in G. forbesi , and of embryogenesis and seed development in G. malaccensis and G. scortechinii which also appear to be agamospermous. The role of either facultative or obligate apomixis in gene fixation in these dioecious species is discussed, and the possible adaptive significance of reproductive versatility among certain rain forest trees is examined.     

Reproductive allometry in Malaysian rain forest trees: Biomechanics versus optimal allocation

Summary Quantitative predictions of reproductive allometry in iteroparous plants may be derived from two bodies of theory: biomechanics and optimal allocation theory. Biomechanical theory predicts allometric scaling exponents between reproductive (R) and vegetative (V) biomass in the range of 0.44–1.33, while very general models of life history evolution predictR–V exponents > 1 in all cases. These predictions are examined in light of allometric patterns of flower and fruit production in 32 species of Malaysian rain forest trees. Among these species the mean estimatedR–V exponents are in the range 1.8–2.0 for staminate flower, pistillate flower and fruit production. This range of exponent values provides unambiguous support for some of the general predictions of optimal allocation models, but not for biomechanical theory. Optimal allocation models also predict a positive relationship between species size andR–V slope and a positive relationship between species size andR–V intercept parameters. The latter, but not the former prediction is supported by the data.R–V allometries in sexes of dioecious species were also found to differ in intercept, though not slope, reflecting smaller sizes at reproductive onset in staminate trees. Further critical examinations of reproductive allometry are encouraged as a relatively unexplored avenue for increasing the contact of theory and data in studies of life history evolution in long-lived organisms such as tropical trees.     

Distribution patterns of tree species in a Malaysian tropical rain forest

Abstract. Spatial patterns of tree species were studied in a 50-ha tropical rain forest plot in the Pasoh forest, Malaysia. This forest is characterized by a high diversity and very high number of rare species. Out of the 745 species occurring with > five individuals, 80.4 % had an aggregated distribution, 19.5% were randomly distributed and one species had a regular distribution. The spatial patterns of rare vs. common species, juvenile vs. adult trees, and coarse vs. fine scales were compared. Rare species are generally less aggregated than common ones and most of the randomly distributed species are rare. Spatial patterns shift from high clumping to looser intensity or random distribution when moving from juveniles to adults for the same species. No adult tree species display a regular pattern, however. Regular distributions were rarely found; this is probably due to intraspecific competition at a local scale. There is a negative correlation between per capita death rate and population density. This study suggests that the Pasoh forest and its high diversity are subjected to multiple controlling factors, e.g., topography, spacing effect, density-dependent processes and species rarity. The importance of any factor changes across spatial and temporal scales.     

Crown development in tropical rain forest trees in gaps and understorey

Emergence and establishment of Tsuga sieboldii seedlings were monitored for six years in an old-growth mixed forest in Japan, to clarify the reason why adult T. sieboldii trees are distributed only on convex slopes. T. sieboldii seedlings germinated on exposed soils without broad-leaved litter cover on both convex and concave slopes, but they died out from concave slope during the first two years after germination. Along a gradient from convex to concave slope, litter cover became higher and surface soil erosion became severer, and T. sieboldii seedlings died at the plots with severe surface soil erosion. Other environmental variables, such as relative illuminance, vegetation cover of seedling layer and soil water potential were not significantly different between convex and concave slopes. We concluded that the restricted seedling establishment of T. sieboldii to convex slope is determined by stability of surface soils and broad-leaved litter cover.     

Growth strategies differentiate the spatial patterns of 11 dipterocarp species coexisting in a Malaysian tropical rain forest

We examined relationships between mortality rate, relative growth rate (RGR), and spatial patterns of three growth stages (small, medium, and large trees) for 11 dipterocarp species in the Pasoh 50-ha plot. Mortality rates for these species tended to be positively correlated with RGRs, although the correlation was significant only at the small-tree stage. Seven species with high growth and mortality rates exhibited peaks in spatial aggregation at small distances (<100 m) in small trees, but this aggregation disappeared in medium and large trees. In contrast, the other four species with low growth and mortality rates aggregated at large distances (>200 m) throughout the three growth stages in all but one species. Negative associations between different growth stages were observed only for the high-mortality species, suggesting density-dependent mortality. The high-mortality species showed habitat associations with topography, soil type, and the forest regeneration phase after gap formation, whereas the three low-mortality species only had associations with the forest regeneration phase. A randomization procedure revealed that these habitat associations explained little of their spatial aggregation. Our results suggest that the growth strategy has a large effect on the structuring of the spatial distribution of tree species through mortality processes.     

Drought avoidance and the effect of local topography on trees in the understorey of Bornean lowland rain forest

The water relations of two tree species in the Euphorbiaceae werecompared to test in part a hypothesis that the forest understorey plays anintegral role in drought response. At Danum, Sabah, the relatively commonspecies Dimorphocalyx muricatus is associated with ridgeswhilst another species, Mallotus wrayi, occurs widely bothon ridges and lower slopes. Sets of subplots within two 4 -hapermanent plots in this lowland dipterocarp rain forest, were positioned onridges and lower slopes. Soil water potentials were recorded in1995–1997,and leaf water potentials were measured on six occasions. Soil water potentialson the ridges (–0.047 MPa) were significantly lower than onthe lower slopes (–0.012 MPa), but during the driest periodin May 1997 they fell to similarly low levels on both sites (–0.53MPa). A weighted 40-day accumulated rainfall index was developedtomodel the soil water potentials. At dry times, D.muricatus(ridge) had significantly higher pre-dawn (–0.21 v.–0.57 MPa) and mid-day (–0.59 v.–1.77 MPa) leaf water potentials than M.wrayi (mean of ridge and lower slope). Leaf osmotic potentials ofM. wrayi on the ridges were lower (–1.63MPa) than on lower slopes (–1.09 MPa), withthose for D. muricatus being intermediate (–1.29MPa): both species adjusted osmotically between wet and dry times.D. muricatus trees were more deeply rooted thanM. wrayi trees (97 v. 70cm). M. wrayi trees had greaterlateral root cross-sectional areas than D. muricatus treesalthough a greater proportion of this sectional area for D.muricatus was further down the soil profile. D.muricatus appeared to maintain relatively high water potentialsduring dry periods because of its access to deeper water supplies and thus itlargely avoided drought effects, but M. wrayi seemed to bemore affected yet tolerant of drought and was more plastic in its response. Theinteraction between water availability and topography determines these species'distributions and provides insights into how rain forests can withstandoccasional strong droughts.     

Studies in Garcinia, dioecious tropical forest trees: agamospermy

RICHARDS, A. J., 1990. Studies in Garcinia , dioecious tropical forest trees: agamospermy . In Garcinia , agamospermy is known or suspected for ten species. Most are facultative agamosperms, with males occurring, but males are probably lacking in G. mangostana . In G. mangostana and G. hombroniana , adventitious embryony occurs autonomously, and haploid parthenogenesis may also occur rarely. In G. parvifolia , it is reported previously that gametophytic agamospermy occurs, and adventitious embryos are suppressed. Autonomous endospermy is found in G. mangostana and G. parvifolia . In G. hombroniana , endosperm probably only develops after PEN fertilization. In G. hombroniana , some proembryos are formed precociously, and further proembryo formation is inhibited by sexual embryos. Asexual proembryos tend to occur in large ovules in small ovaries, and sexual embryos tend to occur in small ovules in large ovaries. It is considered that facultative agamospermy renders the genus Garcinia particularly suitable for the development of new types of fruit. Agamospermy in a dioecious genus, adventitious embryony in the absence of fertilization and/or pseudogamy, and the co-occurrence of gametophytic and sporophytic agamospermies in the same genus are unusual phenomena, which are discussed in the context of the lifestyle of a tropical forest tree. For G. hombroniana, n = 24, and for G. cowa, n = 26 are reported.     

Reproductive skew drives patterns of sexual dimorphism in sponge-dwelling snapping shrimps

Sexual dimorphism is typically a result of strong sexual selection on male traits used in male–male competition and subsequent female choice. However, in social species where reproduction is monopolized by one or a few individuals in a group, selection on secondary sexual characteristics may be strong in both sexes. Indeed, sexual dimorphism is reduced in many cooperatively breeding vertebrates and eusocial insects with totipotent workers, presumably because of increased selection on female traits. Here, we examined the relationship between sexual dimorphism and sociality in eight species of Synalpheus snapping shrimps that vary in social structure and degree of reproductive skew. In species where reproduction was shared more equitably, most members of both sexes were physiologically capable of breeding. However, in species where reproduction was monopolized by a single individual, a large proportion of females—but not males—were reproductively inactive, suggesting stronger reproductive suppression and conflict among females. Moreover, as skew increased across species, proportional size of the major chela—the primary antagonistic weapon in snapping shrimps—increased among females and sexual dimorphism in major chela size declined. Thus, as reproductive skew increases among Synalpheus, female–female competition over reproduction appears to increase, resulting in decreased sexual dimorphism in weapon size.     

Effects of successional status, habit, sexual systems, and pollinators on flowering patterns in tropical rain forest trees

Based on data from observations of 302 tree species at La Selva, Costa Rica, we tested a range of hypotheses about the relationship between flowering parameters such as time, frequency, and duration and ecological features such as successional status, habit, sexual systems, and pollen vectors with and without considering the effect of family membership. We predicted that early successional species would flower any time of the year, but species pollinated by different vectors as well as dioecious species would flower nonrandomly across seasons. However, there was little evidence that flowering time varied with successional status, pollen vectors, and sexual systems. As we predicted, supra-annual flowering was proportionately less common in early successional species as compared to late ones, in understory species as compared to canopy species, and in dioecious species as compared to those with hermaphroditic flowers. When considering phylogeny, however, supra-annual flowering in the understory was not as rare as predicted. Our prediction of longer flowering in the early successional species as compared to late successional species was also supported. Predictions about longer flowering of dioecious species as compared to hermaphroditic species and of species pollinated by generalist vectors as compared to the specialists were not supported, though there was a trend in the expected direction.     

Congruence between floristic patterns of trees and lianas in a southwest Amazonian rain forest

The congruence in floristic patterns between different life-forms of woody plants remains poorly understood in tropical rain forests. We explored whether the floristic patterns of woody plants, divided into small trees 2.5–10 cm dbh, large trees ≥10 cm dbh, and lianas ≥2.5 cm dbh were associated with each other or with patterns in soil properties, elevation, and geographical distances between sample plots. We also tested whether ecological amplitudes in relation to environmental variables differed among the plant groups. Trees and lianas were inventoried in 44 0.1-ha plots, distributed among three lowland and two submontane sites in the Madidi National Park, Bolivia. Soil samples were analysed for physico-chemical properties. Floristic differences between sites (as measured with each plant group separately) yielded significant Mantel correlations with each other, and with pH, Ca, Mg, elevation and geographical distance. Mantel correlations with edaphic distances were higher for large trees than small trees, but for Mantel correlations with geographical distance the situation was reversed. Environmental and geographical distances explained 31% of the variation in floristic differences for large trees, 22% for small trees, and 10% for lianas. The ecological amplitudes of lianas were wider than those of all trees for pH, Mg and elevation. The amplitudes of the two size classes of trees did not differ. In principal coordinates ordination, the three plant groups produced similar overall floristic patterns that were explainable by environmental factors.     

Roots of pioneer trees in the Amazonian rain forest

. On low fluvial terraces of the Caquetá River, Araracuara region, Colombia, tree root systems were examined with regard to their structure, underground stratification and biomass distribution. Excavations of skeleton roots and microscopic observation of terminal fine roots included ten species belonging to the Cecropia, Vismia, Miconia, Goupia, Clathrotropis and Brosimum genera. Roots of particular species varied in the external features of their periderm and structure of freshly cut slash. Coarse skeleton roots differed in the shape and position of the plagiotropic branches and positively geotropic taproots or sinkers. None of the observed species could be identified as a shallow-rooted tree, in spite of the general fine root concentration in the upper soil horizon. In the two seasons of study, the terminal roots displayed prevailingly secondary anatomical structure and did not maintain much primary anatomical tissue in their apices. Neither ectomycorrhizas nor endomycorrhizas were detected in the samples. In a set of regeneration stages the amount of tree roots linearly increased with the age of growth. However, fine roots below 2 mm in diameter shared 80-90% of all roots in the upper 20 cm layer of all sample plots. In an old-growth forest, the total tree root biomass amounted to 39 ton/ha, thus being comparable to the underground biomass observed in similar tropical forests.     

Crown light environments of saplings of two species of rain forest emergent trees

Summary The crown light environments of saplings of two Costa Rican rain forest tree species were simultaneously compared. The species, Dipteryx panamensis (Pitt.) Record & Mell., a relatively shade-intolerant species, and Lecythis ampla Miers, a shade-tolerant species, have contrasting growth and branching patterns. Quantum sensors were placed throughout the crowns of saplings up to 2.5 m tall and quantum fluxes were recorded with microloggers for seven-day periods. The shade-intolerant species had total quantum flux densities 35% larger than those of the shade-tolerant species, but totals for both species were less than 2% of full sun. More than 90% of the quantum flux densities measured within the crowns of both species were less than 25 mol m^-2s^-1. Lateral light was an important component of daily quantum flux totals; for saplings of both species, the half-hour with the maximum average irradiance for the day frequently occurred in mid-morning or midafternoon. Despite dissimilar crown and leaf display, there was no difference in the overall variability of irradiance within the crowns of the two species. However, quantum fluxes received within the crowns differed substantially in both species. Within-crown locations differed significantly from day to day because of variation in weather conditions. Daily total quantum flux densities and totals expressed as a percent of full sun were significantly correlated with height growth over the previous 12 months.     

Photosynthetic induction in saplings of three shade-tolerant tree species: comparing understorey and gap habitats in a French Guiana rain forest

The photosynthetic induction response under constant and fluctuating light was examined in naturally occurring saplings (about 0.5-2 m in height) of three shade-tolerant tree species, Pourouma bicolor spp digitata, Dicorynia guianensis, and Vouacapoua americana, growing in bright gaps and in the shaded understorey in a Neotropical rain forest. Light availability to saplings was estimated by hemispherical photography. Photosynthetic induction was measured in the morning on leaves that had not yet experienced direct sunlight. In Dicorynia, the maximum net photosynthesis rate (A_max) was similar between forest environments (ca 4 µmol m^-2 s^-1), whereas for the two other species, it was twice as high in gaps (ca 7.5) as in the understorey (ca 4.5). However, the time required to reach 90% of A_max did not differ among species, and was short, 7-11 min. Biochemical induction was fast in leaves of Pourouma, as about 3 min were needed to reach 75% of maximum carboxylation capacity (V_cmax); the two other species needed 4-5 min. When induction continued after reaching 75% of V_cmax, stomatal conductance increased in Pourouma only (ca 80%), causing a further increase in its net photosynthesis rate. When fully induced leaves were shaded for 20 min, loss of induction was moderate in all species. However, gap saplings of Dicorynia had a rapid induction loss (ca 80%), which was mainly due to biochemical limitation as stomatal conductance decreased only slowly. When leaves were exposed to a series of lightflecks separated by short periods of low light, photosynthetic induction increased substantially and to a similar extent in all species. Although A_max was much lower in old than in young leaves as measured in Dicorynia and Vouacapoua, variables of the dynamic response of photosynthesis to a change in light tended to be similar between young and old leaves. Old leaves, therefore, might remain important for whole-plant carbon gain, especially in understorey environments. The three shade-tolerant species show that, particularly in low light, they are capable of efficient sunfleck utilization.     

Species richness patterns in the understorey of Pyrenean Pinus sylvestris forest

Abstract. Species richness was studied in the understorey of natural Pinus sylvestris forest in the eastern Pyrenees. Understorey plant species were grouped in three structural groups as woody species, herbs and mosses. The response curves of total species richness and species richness of each structural group were fitted against environmental and stand-structural parameters, using Generalized Linear Models. The results suggested that, to predict species richness, environmental parameters were more important than tree-canopy structural parameters, in particular incoming radiation and soil nutrient concentration. The species richness response curve was often humped in relation to soil nutrient concentration. Different patterns of species richness were found for each structural group.     

Variation in leaf toughness and phenolic content among five species of Australian rain forest trees

Several leaf characteristics, including toughness and total phenols and condensed tannins, were measured in Australian rain forest leaves of different ages and related to observed herbivory rates. In most cases, toughness and chemical toxicity increased as leaves aged, and corresponding insect grazing decreased. Herbivory losses ranged from 4.8% to 32.5% leaf area losses, and were more positively correlated with toughness than with phenolics. It is suggested that a suite of factors, including physical and chemical characteristics of leaves as well as spatial and temporal factors, interact to create variation in grazing intensities.