Dry Forest Species Research Articles

Biogeography and composition of dry forest bird communities in Bolivia

In Bolivia dry forests are distributed extensively in the southeastern lowlands and extend into the country's northern and southern Andes in rain-shadowed intermontane valleys, where they often form habitat islands. We examined the biogeography and composition of bird communities at 14 Andean and seven lowland dry forest localities in Bolivia. To minimise biases, analyses considered only core speciessensu Remsen (1994) of zonal vegetation types. Of a total of 608 recorded species, 454 were zonal forest core species and 99 (22 %) typical dry forest species. Core species richness varied from 65 to 156 at individual sites. One species occurred at all sites and 154 (34 %) at a single locality each; this tendency was most evident in the northern Bolivian Andes where 46 % of core species were recorded only once. Comparisons of species composition showed that sites in the northern valleys were the most heterogeneous and distinct. Localities in the southern valleys were the most homogeneous with affinities to lowland dry forests; the latter separated into Velasco forest and Chaco woodland sites, respectively. A continuous ordination of sites revealed a steep gradient from large habitat areas in the dry southeastern lowlands to small, relatively humid Andean habitat islands in the northwest. Typical dry forest species were most prevalent in large habitat areas in the dry south, whereas typical humid forest species predominated in the relatively humid northern Andean habitat islands. Frugi-granivores and nectarivores such as pigeons and doves, hummingbirds, and emberizid finches were most prevalent at Andean sites, whereas insectivores made relatively greater contributions to lowland bird communities. The biogeographical affinities of Bolivian dry forest birds are varied. Overall, lowland birds gradually decreased and Andean species gradually increased with elevation, which was most pronounced in the southern Bolivian Andes; we found no elevational threshold of increased avifaunal turnover. An examination of range limits of typical dry forest species revealed a pronounced northwestward reduction of the dry forest avifauna from Chaco lowlands to small, isolated valleys in the northern Bolivian Andes. With seven species restricted to the Andean dry forests of Bolivia, this area has a considerably higher level of endemism than the Chaco. Andean dry forests were colonised by lowland species in two different ways. In the southern valleys and upper northern valleys, dry forest species ascended directly from the adjacent Chaco. The lower northern valleys were probably colonised by dry forest species during drier and cooler periods of the Pleistocene that resulted in expansions of seasonally dry tropical forest in Amazonia.

Density‐dependence in common tree species in a tropical dry forest in Mudumalai, southern India

Abstract. Density‐dependence in tree population dynamics has seldom been examined in dry tropical forests. Using long‐term data from a large permanent plot, this study examined 16 common species in a dry tropical forest in southern India for density‐dependence. Employing quadrat‐based analyses, correlations of mortality, recruitment and population change with tree densities were examined. Mortality in 1–10 cm diameter trees was largely negatively correlated with conspecific density, whereas mortality in > 10 cm diameter trees was positively correlated. Mortality was, however, largely unaffected by the basal area and abundance of heterospecific trees. Recruitment was poor in most species, but in Lagerstroemia microcarpa (Lythraceae), Tectona grandis (Verbenaceae) and Cassia fistula (Fabaceae), species that recruited well, strong negative correlations of recruitment with conspecific basal area and abundance were found. In a few other species that could be tested, recruitment was again negatively correlated with conspecific density. In Lagerstroemia, recruitment was positively correlated with the basal area and abundance of heterospecific trees, but these correlations were non‐significant in other species. Similarly, although the rates of population change were negatively correlated with conspecific density they were positive when dry‐season ground fires occurred in the plot. Thus, the observed positive density‐dependence in large‐tree mortality and the negative density‐dependence in recruitment in many species were such that could potentially regulate tree populations. However, repeated fires influenced density‐dependence in the rates of population change in a way that could promote a few common species in the tree community.

Effects of microsite, water, weeding, and direct seeding on the regeneration of native and alien species within a Hawaiian dry forest preserve

Tropical dry forests are among the most endangered ecosystems in the world in general and in Hawaii in particular. To investigate the regeneration ecology of native and alien dry forest species on the island of Hawaii, we used a factorial experiment with microsite (sub-canopy vs. inter-canopy), water (supplemental vs. ambient), and weeding (alien species removed vs. not removed) treatments, and also seeded six native woody species into each plot at the start of the experiment. At the end of the 21-month study, the biomass of the volunteer native and alien species (i.e. unplanted species consisting mainly of relatively fast-growing shrubs) was nearly three and 13 times that of the seeded species, respectively. The biomass of the native volunteers was greater in the inter-canopy plots, greater for the seeded species in the sub-canopy plots, and did not differ significantly within this treatment for the alien species. Few species survived in the ambient water plots, resulting in greater biomass in the watered plots for all species. There were no significant differences in the biomass of the native species within the weeded vs. non-weeded plots; on the contrary, we found consistently positive correlations between the abundance of the seeded species and the volunteer native and alien species. Thus it may be possible to restore Hawaii's degraded dry forests by manipulating these naturally recruiting species to create microsites favorable for the eventual re-establishment of the endangered native canopy tree flora.

The effects of drought and fire on seed and seedling dynamics in a tropical seasonal forest in Thailand

Seed production and seedling dynamics were studied for 6 years(1992–1997) with particular emphasis on the effects of the drought andfire in a natural tropical seasonal forest at Mae Klong Watershed ResearchStation, Kantchanaburi, western Thailand. The number of seed produced andemerged seedlings varied greatly among years and species. Most tree speciesproduced very few seeds in 1994 and many in 1995 and 1996, particularly,Shorea siamensis Miq. which displayed the most seedinghabit. Some seedlings, Shorea, Dipterocarpus alatus Roxb.,and Dipterocarpus turbinatus Gaertn. f., emerged at theendof the dry season (late April, after the first rain but when soils still have alow soil moisture content), while the majority of seedlings emerged at thebeginning of the rainy season. After a fire disturbed the plot in April 1996,many species increased their seedling emergences, especially Berryaammonilla. Seedlings of most species less than a year old showed arelatively low mortality in the rainy season, but those ofPterocarpusmacrocarpus Kurz had high mortality. The mean survival rate for allseedling species in this forest was quite low (24.1% y−1), anddifferent values existed for the rainy and the dry season (11.5, and 6.1%month−1, respectively). This suggests that different specieshave adaptations related to the season of seedling emergence and resistance todrought in this tropical seasonal forest community. The traits of tree speciesare classified in terms of their traits in the early stages of regeneration;seed and/or seedling bank, resprouting, and drought resistance. Most specieshave adapted to fire and/or drought by resprouting, seed bank, and/or seedlingbank, although the few species which occur mainly in mesic evergreen forestshave less adapted to these environments. The demographic variations in seed andseedling stages may contribute to the coexistence of these species in thisspecies-rich dry tropical forest.

Density‐dependence in common tree species in a tropical dry forest in Mudumalai, southern India

Density-dependence in tree population dynamics has seldom been examined in dry tropical forests. Using long-term data from a large permanent plot, this study examined 16 common species in a dry tropical forest in southern India for density-dependence. Employing quadrat-based analyses, correlations of mortality, recruitment and population change with tree densities were examined. Mortality in 1-10 cm diameter trees was largely negatively correlated with conspecific density, whereas mortality in > 10 cm diameter trees was positively correlated. Mortality was, however, largely unaffected by the basal area and abundance of heterospecific trees. Recruitment was poor in most species, but in Lagerstroemia microcarpa (Lythraceae), Tectona grandis (Verbenaceae) and Cassia fistula (Fabaceae), species that recruited well, strong negative correlations of recruitment with conspecific basal area and abundance were found. In a few other species that could be tested, recruitment was again negatively correlated with conspecific density. In Lagerstroemia, recruitment was positively correlated with the basal area and abundance of heterospecific trees, but these correlations were non-significant in other species. Similarly, although the rates of population change were negatively correlated with conspecific density they were positive when dry-season ground fires occurred in the plot. Thus, the observed positive density-dependence in large-tree mortality and the negative density-dependence in recruitment in many species were such that could potentially regulate tree populations. However, repeated fires influenced density-dependence in the rates of population change in a way that could promote a few common species in the tree community.

Carbon uptake, growth and resource-use efficiency in one invasive and six native Hawaiian dry forest tree species.

Photosynthetic gas exchange, nitrogen- and water-use efficiency, leaf water potential and seasonal patterns of leaf production were studied in seven, dominant dry-forest species from the island of Lana'i, Hawaii, including the rapidly colonizing, non-native Schinus terebinthifolius (Raddi). We evaluated whether unique physiological characteristics of the invasive species explain its capacity to rapidly invade dry forests throughout the Hawaiian Islands. Apparent anomalies in stable carbon isotope data (delta13C) relative to other results led us to study effects of environmental conditions and physiological performance during leaf expansion on delta13C. Species that expanded all their foliage at the beginning of the wet season had more negative leaf delta13C values during the dry season than species with continuous leaf expansion. Among species, S. terebinthifolius had a strong seasonal pattern of leaf production and the most negative delta13C (-29 per thousand). With respect to almost every trait measured, S. terebinthifolius fell at an end of the range of values for the native species. Rapid growth of S. terebinthifolius in this ecosystem may be partially explained by its high maximum CO2 assimilation rates (15 micromol m-2 s-1), low leaf mass per area, high photosynthetic nitrogen-use efficiency per unit leaf mass or area and large decrease in stomatal conductance during the dry season. Relative to the native species, the invasive species exhibited striking phenotypic plasticity, including high rates of stem growth and water and CO2 uptake during the wet season, and maintenance of leaves and high leaf water potentials, as a result of reduced water loss, during the dry season, enabling it to utilize available resources effectively.

Ecology of seed and seedling growth for conservation and restoration of tropical dry forest : a review

Dry forests are among the most threatened ecosystems and have been extensively converted into grasslands, secondary forest, savanna or agricultural land. Knowledge of seed germination and seedling establishment is required for the success of efforts on restoration of these forests. This review focuses on the ecological requirements at seed and seedling stages, and collates the current knowledge of seed viability, dormancy, germination pattern and seedling behaviour of dry tropical tree species. The spatio-temporal variations within the tropical dry forest biome in soil moisture, light, temperature, nutrients and intensity of predation, significantly affect the seed and seedling traits of component species. The majority of dry tropical species possess orthodox seeds which are characterized by dormancy, while a few have recalcitrant seeds which possess little or no dormancy. Seed coat dormancy, which can be overcome by mechanical or acid scarification or sometimes by transit through animal guts, is most prevalent in the dry tropical forest species. Persistent species dominating the undisturbed portions of the forest have bigger seeds compared to those that mostly occur in disturbed regions and require shade for the survival of their seedlings. Shade demand is associated with drought endurance, and may be absolute in species such as Guettarda parviflora and Coccoloba microstachya, or facultative as in Plumeria alba and Bursera simaruba. The fluctuation in temperature significantly affects seed germination in several species of dry Afromontane forest trees of Ethiopia. Seedling mortality is primarily a function of moisture stress during the dry period. Adaptive responses of seedlings to drought stress include increased chlorophyll content, for example in Acacia catechu, and root biomass, as in several dry forest species (for example Drypetes parvifolia, Teclia verdoornia) of Ghana. Mulching, application of fertilizers, interplanting of leguminous species and mycorrhizal inoculation are useful tools for promoting seedling establishment in nutrient-poor dry tropical soils. Periodic forest fires, and predation affect recruitment and seedling development according to their intensity. Many species experiencing frequent fires have evolved thick seed coats, produce fire-hardy seedlings, or escape the effect by temporal separation of seed dispersal and fire events. Predation may result in abortion of fruits or may enhance germination and recruitment by scarification and dispersal, as in most species of the Guanacaste dry forest. Exposure to elevated CO2 has increased relative growth rate, total leaf area and water use efficiency in most of the dry tropical seedlings tested, but the magnitude of the effect has varied markedly among species. Due to the availability of a large source of energy, large seeds show higher germination percentage, greater seedling survival and increased growth. Seeds originating from different provenances exhibit differences in germination and seedling growth (for example Prosopis cineraria, Albizia lebbeck, Eucalyptus camaldulensis and Acacia mangium), efficiency of nodulation (for example Acacia nilotica, A. auriculiformis), and stress resistance (for example Populus deltoides, Dalbergia sissoo). The review points out the need for coordinated, long-term, field-based studies for identification of multiple cues and niches for germination, on seed and seedling dynamics in response to fire, and on within-species genetic variability for selection of suitable provenances. Field-based studies at species and community levels are also needed to permit manipulations of biotic components to augment the recruitment of desired species and to suppress that of undesirable species.

Foliar Demand and Resource Economy of Nutrients in Dry Tropical Forest Species

Foliar Demand and Resource Economy of Nutrients in Dry Tropical Forest Species

Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot‐dry period when soil water is limiting, while the subsequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during the dry summer period, however, is substantial in these species. We tested the hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a significant extent. We examined the leaf traits (with parameters such as leaf life span, leaf nutrient content and retranslocation of nutrients during senescence) in eight selected tree species in northern India. The concentrations of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a substantial amount of nutrient resorption before senescence and a ‘tight nutrient budgeting’. The resorbed N‐mass could potentially support 50 to 100% and 46 to 80% of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 100% and 20 to 91%, for K 29 to 100% and 20 to 57%, for Na 3 to 100% and 1 to 54%, and for Ca 0 to 32% and 0 to 30%. The species differed significantly with respect to their efficiency in nutrient resorption. Such interspecific differences in leaf nutrient economy enhance the conservative utilization of soil nutrients by the dry forest community. This reflects an adaptational strategy of the species growing on seasonally dry, nutrient‐poor soils as they tend to depend more or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new foliage biomass in summer when the availability of soil moisture and nutrients is severely limited.

Patterns of amphibian and reptile diversity at Berara Forest (Sahamalaza Peninsula), NW Madagascar

Amphibians and reptiles were surveyed at Berara, a forest on the Sahamalaza Peninsula, NW Madagascar. Visual methods and pitfalls were used, leading to the discovery of 12 amphibian and 30 reptile species. The herpetofaunal community appeared as a mosaic of dry forest species and species from the more humid Sambirano Domain. The comparatively low amphibian diversity may be correlated with the ecological characteristics of Berara, in particular with the scarcity of permanent water bodies and strong seasonality. The survey provided new records of Aglyptodactylus securifer and Heterixalus luteostriatus, further north than hitherto known. The encountered specimens of the treefrog Boophis albi‐labris displayed characters typical for the subspecies occidentalis, resulting in a substantial range extension. Boophis jaegeri proved to be abundant at Berara, which is the second known locality for this treefrog. The reptile fauna included taxa of special interest, such as a new Pseudoacontias burrowing skink, a new Amphiglos‐sus, the aquatic skink Amphiglossus reticulatus, and many geckos. Presence of several taxa known only from restricted western localities (e.g., Aglyptodactylus securifer, Amphiglossus reticulatus) or from a few protected areas (e.g., B. jaegeri), as well as of several regional endemics emphasises the importance for conservation of the Sahamalaza transitional forest, which should urgently be included in the network of protected areas in Madagascar.

Temporal and spatial partitioning of water resources among eight woody species in a Hawaiian dry forest.

Lowland dry forests are unique in Hawaii for their high diversity of tree species compared with wet forests. We characterized spatial and temporal partitioning of soil water resources among seven indigenous and one invasive dry forest species to determine whether the degree of partitioning was consistent with the relatively high species richness in these forests. Patterns of water utilization were inferred from stable hydrogen isotope ratios (δD) of soil and xylem water, zones of soil water depletion, plant water status, leaf phenology, and spatial patterns of species distribution. Soil water δD values ranged from -20‰ near the surface to -48‰ at 130 cm depth. Metrosideros polymorpha, an evergreen species, and Reynoldsia sandwicensis, a drought-deciduous species, had xylem sap δD values of about -52‰, and appeared to obtain their water largely from deeper soil layers. The remaining six species had xylem δD values ranging from -33 to -42‰, and apparently obtained water from shallower soil layers. Xylem water δD values were negatively correlated with minimum annual leaf water potential and positively correlated with leaf solute content, an integrated measure of leaf water deficit. Seasonal patterns of leaf production ranged from dry season deciduous at one extreme to evergreen with near constant leaf expansion rates at the other. Species tapping water more actively from deeper soil layers tended to exhibit larger seasonality of leaf production than species relying on shallower soil water sources. Individuals of Myoporum sandwicense were more spatially isolated than would be expected by chance. Even though this species apparently extracted water primarily from shallow soil layers, as indicated by its xylem δD values, its nearly constant growth rates across all seasons may have been the result of a larger volume of soil water available per individual. The two dominant species, Diospyros sandwicensis and Nestegis sandwicensis, exhibited low leaf water potentials during the dry season and apparently drew water mostly from the upper portion of the soil profile, which may have allowed them to exploit light precipitation events more effectively than the more deeply rooted species. Character displacement in spatial and temporal patterns of soil water uptake was consistent with the relatively high diversity of woody species in Hawaiian dry forests.

Phenology of Tree Species in Bolivian Dry Forests

ABSTRACTPhenological characteristics of 453 individuals representing 39 tree species were investigated in two dry forests of the Lomerío region, Department of Santa Cruz, Bolivia. The leaf, flower, and fruit production of canopy and sub–canopy forest tree species were recorded monthly over a two–year period. Most canopy species lost their leaves during the dry season, whereas nearly all sub–canopy species retained their leaves. Peak leaf fall for canopy trees coincided with the peak of the dry season in July and August. Flushing of new leaves was complete by November in the early rainy season. Flowering and fruiting were bimodal, with a major peak occurring at the end of the dry season (August–October) and a minor peak during the rainy season (January). Fruit development was sufficiently long in this forest that fruiting peaks actually tended to precede flowering peaks by one month. A scarcity of fruit was observed in May, corresponding to the end of the rainy season. With the exception of figs (Ficus), most species had fairly synchronous fruit production. Most canopy trees had small, wind dispersed seeds or fruits that matured during the latter part of the dry season, whereas many sub–canopy tree species produced larger animal– or gravity–dispersed fruits that matured during the peak of the rainy season. Most species produced fruit annually. Lomerio received less rainfall than other tropical dry forests in which phenological studies have been conducted, but rainfall can be plentiful during the dry season in association with the passage of Antarctic cold fronts. Still, phenological patterns in Bolivian dry forests appear to be similar to those of other Neotropical dry forests.

Host Specificity of Some Wood-Inhabiting Fungi in a Tropical Forest

Sporocarps of 82 species of poroid and stereoid wood-inhabiting fungi (Aphyllophorales) were recorded on 44 tree species in a Neotropical dry forest, Santa Rosa National Park, Costa Rica. Only thre...

Host specificity of some wood-inhabiting fungi in a tropical forest

Sporocarps of 82 species of poroid and stereoid wood-inhabiting fungi (Aphyllophorales) were recorded on 44 tree species in a Neotropical dry forest, Santa Rosa National Park, Costa Rica. Only three of the 32 species with 3 or more records showed signs of host tree specialization. For the others, number of host species increased with increasing number of colonized logs. The results support the assumption that most wood rotting fungi have broad host ranges in tropical areas.

Neotropical seasonally dry forests and Quaternary vegetation changes

AbstractSeasonally dry tropical forests have been largely ignored in discussions of vegetation changes during the Quaternary.We distinguish dry forests, which are essentially tree‐dominated ecosystems, from open savannas that have a xeromorphic fire‐tolerant, grass layer and grow on dystrophic, acid soils. Seasonally dry tropical forests grow on fertile soils, usually have a closed canopy, have woody floras dominated by the Leguminosae and Bignoniaceae and a sparse ground flora with few grasses. They occur in disjunct areas throughout the Neotropics. The Chaco forests of central South America experience regular annual frosts, and are considered a subtropical extension of temperate vegetation formations.At least 104 plant species from a wide range of families are each found in two or more of the isolated areas of seasonally dry tropical forest scattered across the Neotropics, and these repeated patterns of distribution suggest a more widespread expanse of this vegetation, presumably in drier and cooler periods of the Pleistocene.We propose a new vegetation model for some areas of the Ice‐Age Amazon: a type of seasonally dry tropical forest, with rain forest and montane taxa largely confined to gallery forest. This model is consistent with the distributions of contemporary seasonally dry tropical forest species in Amazonia and existing palynological data.The hypothesis of vicariance of a wider historical area of seasonally dry tropical forests could be tested using a cladistic biogeographic approach focusing on plant genera that have species showing high levels of endemicity in the different areas of these forests.

Stem water storage capacity and efficiency of water transport: their functional significance in a Hawaiian dry forest

ABSTRACTWe investigated the contribution of internal water storage and efficiency of water transport to the maintenance of water balance in six evergreen tree species in a Hawaiian dry forest. Wood‐saturated water content, a surrogate for relative water storage capacity, ranged from 70 to 105%, and was inversely related to its morphological correlate, wood density, which ranged between 0·51 and 0·65 g cm−3. Leaf‐specific conductivity (kL) measured in stem segments from terminal branches ranged from 3 to 18 mmol m−1 s−1 MPa−1, and whole‐plant hydraulic efficiency calculated as stomatal conductance (g) divided by the difference between predawn and midday leaf water potential (ΨL), ranged from 70 to 150 mmol m−2 s−1 MPa−1. Hydraulic efficiency was positively correlated with kL (r2 = 0·86). Minimum annual ΨL ranged from − 1·5 to − 4·1 MPa among the six species. Seasonal and diurnal variation in ΨL were associated with differences among species in wood‐saturated water content, wood density and kL. The species with higher wood‐saturated water content were more efficient in terms of long‐distance water transport, exhibited smaller diurnal variation in ΨL and higher maximum photosynthetic rates. Smaller diurnal variation in ΨL in species with higher wood‐saturated water content, kL and hydraulic efficiency was not associated with stomatal restriction of transpiration when soil water deficit was moderate, but avoidance of low minimum seasonal ΨL in these species was associated with a substantial seasonal decline in g. Low seasonal minimum ΨL in species with low kL, hydraulic efficiency, and wood‐saturated water content was associated with higher leaf solute content and corresponding lower leaf turgor loss point. Despite the species‐specific differences in leaf water relations characteristics, all six evergreen tree species shared a common functional relationship defined primarily by kL and stem water storage capacity.

Comparative regeneration ecology of three leguminous timber species in a Bolivian tropical dry forest.

A comparative study of the regeneration ecology ofthree lesser-known leguminous timber species wasconducted in the seasonally dry forests of SantaCruz, Bolivia to determine species regenerationstrategies and make silvicultural recommendationsfor these species. The study included arepresentative from each subfamily of Leguminosae:Anadenanthera colubrina (Vell. Conc.) Benth.(Mimosaceae), Copaifera chodatiana Kunth.(Caesalpinaceae), and Centrolobium microchaeteC. Martius (Fabaceae). After production in themid-late dry season, seeds of all species sufferedhigh (>30%) rates of predation. For seedssurviving predation, Anadenanthera germinatedwithin three days after the first rains and a highgermination capacity (82%), but most seedlings diedfrom inadequate light or during subsequent periodsof drought. Copaifera germinated more slowlybut had high germination capacity (85%). Centrolobium had very low germination capacity(4%) and germinative energy. Most successfulregeneration of Centrolobium occurred viasprouting from damaged roots on or near loggingroads where it had a density of 261 root sprouts/ha.Anadenanthera regenerated best from seedin areas with soil disturbance or burning. Theregeneration of these species will likely increaseunder more intensive logging and/or post-harvestcompetition control treatments in logging gaps. Themore shade-tolerant Copaifera is most suitedto the current regime of light selective logging,but all three species are likely to be responsive topost-harvest competition control treatments. Impacts of controlled and natural fire were mixed,but generally seedling regeneration and growth wereeither not significantly affected or were increasedby fire.

Effects of Canopy Gaps, Topography, and Soils on the Distribution of Woody Species in a Central Brazilian Deciduous Dry Forest1

ABSTRACTThe interrelationships between the distribution of woody species and environmental variables were investigated in an area of deciduous dry forest in Santa Vitöria, central Brazil. This is the first study of a vanishing type of dry forest which grows on base‐rich soils originating from the basalt bedrocks of southern Goiás and western Minas Gerais. A survey of topography, soil properties, canopy gaps and woody plants (≥5 cm diameter at the base of the stem) was conducted in 50–15 × 15 m quadrats. The soils were classified into the following soil series: Hapludolls → Haplustolls → Haplustolls → Ustropepts → Rhodustalfs. This series corresponded to a gradient of increasing elevation and effective soil depth and decreasing slope gradient, soil organic matter and total exchangeable bases. A canonical correspondence analysis and a detrended correspondence analysis indicated that plant species’abundance distribution was significantly correlated with both the relative area of canopy gaps in the quadrats and the soil‐topography gradient. Presumably, the critical factors involved in these two gradients are, respectively, light and ground water regimes. The influence of canopy gaps (i.e., light) was surprising and has not been documented previously for tropical deciduous dry forests.

Seedling populations and regeneration of woody species in dry Afromontane forests of Ethiopia

The composition and density of seedlings, as well as height class distribution of woody species, were studied at two dry Afromontane forests in Ethiopia. A total of 40 species were recorded at Gara Ades and 41 at Menagesha forests. The density was 16 290 and 32 650 seedlings/ha at Gara Ades and Menagesha, respectively. A few species were not represented in the seedling bank, although mature trees were present. This may suggest that the species are under threat of local extinction or ‘that they recruit after major disturbances’. The trend in the height class distribution varied from species to species. However, about 50% of the species showed a negative exponential distribution at both sites, while there were indications of intermittent regeneration in the others. Seedling survival and growth of four species were also investigated, both under shade and in the open, at Gara Ades. The number of surviving seedlings declined progressively during the study period in all the species. Damage and uprooting by animals and people, drought, defoliation by insects and burial by siltation are among the apparent causes of seedling mortality. Mean annual height increment was generally low (< 15 cm) in the four species. Only one of the species showed more rapid height growth in gaps than under shade while the other three species had almost similar growth rates under shade and in gaps. The seedling growth analysis does not support the view that climax species quickly commence rapid height growth after canopy gap develops above them. The results show that in dry Afromontane forests, formation of seedling banks under the forest canopy is the major regeneration route of woody plants, especially climax species.

Omphalea (Euphorbiaceae) in Madagascar: A New Species and a New Combination

Omphalea ankaranensis, a shrub endemic to limestone karst hills of northern Madagascar, is described and illustrated. The species is closely related to Omphalea palmata Leandri of western Madagascar but differs in its entire to coarsely crenate-dentate leaf blade margins, broader inflorescence bracts, lax cymules, and shorter pistillate flower pedicels. The new combination Omphalea oppositifolia (Willdenow) L. J. Gillespie is made for the widely distributed rainforest species of eastern Madagascar, previously known as Omphalea biglandulosa Persoon. A synopsis including new lectotypifications and a key to the four Madagascan species of Omphalea is provided. Omphalea L. is a genus of ca. 17 species belonging to the Euphorbiaceae. Six species are canopy lianas, while the remaining species range in habit from shrubs to large canopy trees. The genus is known from the Neotropics, southeast Asia to northeast Australia, Tanzania, and Madagascar, with centers of diversity and endemism in the Caribbean (6 species, 5 endemic) and in Madagascar (4 species, all endemic). While several species are widespread, the majority have limited ranges and some are quite rare. Species occupy a diversity of habitats from lowland rainforest, wet montane forest, to dry forest and thicket on limestone and sandy soils. The genus is characterized by an androecium of two or three stamens with connate filaments, styles that are completely fused into a usually massive stylar column, large foliaceous inflorescence bracts, and a complex inflorescence architecture. Inflorescences are technically thyrses and consist of spicate, racemose, or paniculate main axes that bear numerous cymules. These cymose units may be glomerate to lax and each is subtended by a usually foliaceous bract (note that bracteoles are defined as subtending axes within a cymose unit). Other features common in the genus but rarely found elsewhere in the family include red latex, liana habit with tendril-like climbing stems, mushroom-shaped androecia, and large fruits. Omphalea has long been considered a well-defi ed genus and one that is rather isolated in the Euphorbiaceae. The genus has been allied with genera that belong to all three uniovulate subfamilies in Webster's classification (1975, 1994). In the past Omphalea was usually placed in either tribe Hippomaneae (Baillon, 1858, as tribe Excaecarieae; Pax & Hoffmann, 1912), which is included in Webster's subfamily Euphorbioideae, or tribe Gelonieae (Mueller, 1866; Pax & Hoffmann, 1931; Hutchinson, 1969), a heterogeneous grouping of genera belonging to all three uniovulate subfamilies. More recently, Webster (1994) treated the genus as the monogeneric tribe Omphaleae, close to tribe Plukenetieae in subfamily Acalyphoideae. In contrast, Airy Shaw (1980) included the genus in his tribe Jatropheae (which comprises genera placed by Webster in subfamily Crotonoideae) in his tentative scheme for natural grouping of genera. Recent preliminary evidence from molecular and specialist herbivore phylogenetic studies appears to support a relationship with members of subfamily Crotonoideae (Wurdack & Chase, 1996; Lees & Smith, 1991). Its isolated position and unusual combination of plesiomorphic (e.g., extrastaminal disc, imbricate staminate sepals) and derived (e.g., presence of laticifers, foliaceous bracts, few connate stamens, entirely connate styles) character states suggest that Omphalea may represent a relatively early evolutionary branch within the uniovulate euphorbs (Gillespie, 1988b), perhaps near the base of subfamily Crotonoideae. Three species of Omphalea were previously known from Madagascar. The east coast rainforest tree 0. oppositifolia (Willdenow) L. J. Gillespie was first collected, illustrated, and described as the genus Hecatea by Du Petit-Thouars (1804a, 1804b, 1805) and formally named as a species by Willdenow (1805) in the early 1800s. Much later, Leandri (1938) described two dry forest species, 0. occidentalis and 0. palmata, from the west coast of Madagascar. A fourth species was recently discovered by the author on limestone karst in the Reserve Speciale Ankarana in northern Madagascar. A specimen of this new species from Montagne des NovoN 7: 127-136. 1997. This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 04:55:17 UTC All use subject to http://about.jstor.org/terms