Tropical Semideciduous Forest Research Articles

Short dry spells in the wet season increase mortality of tropical pioneer seedlings

Variation in plant species performance in response to water availability offers a potential axis for temporal and spatial habitat partitioning and may therefore affect community composition in tropical forests. We hypothesized that short dry spells during the wet season are a significant source of mortality for the newly emerging seedlings of pioneer species that recruit in treefall gaps in tropical forests. An analysis of a 49-year rainfall record for three forests across a rainfall gradient in central Panama confirmed that dry spells of > or = 10 days during the wet season occur on average once a year in a deciduous forest, and once every other year in a semi-deciduous moist and an evergreen wet forest. The effect of wet season dry spells on the recruitment of pioneers was investigated by comparing seedling survival in rain-protected dry plots and irrigated control plots in four large artificially created treefall gaps in a semi-deciduous tropical forest. In rain-protected plots surface soil layers dried rapidly, leading to a strong gradient in water potential within the upper 10 cm of soil. Seedling survival for six pioneer species was significantly lower in rain-protected than in irrigated control plots after only 4 days. The strength of the irrigation effect differed among species, and first became apparent 3-10 days after treatments started. Root allocation patterns were significantly, or marginally significantly, different between species and between two groups of larger and smaller seeded species. However, they were not correlated with seedling drought sensitivity, suggesting allocation is not a key trait for drought sensitivity in pioneer seedlings. Our data provide strong evidence that short dry spells in the wet season differentially affect seedling survivorship of pioneer species, and may therefore have important implications to seedling demography and community dynamics.

Multi-species fruit and seed removal in a tropical deciduous forest in Mexico

We determined patterns of post-dispersal fruit and seed removal for 11 common native plants, as well as sunflower seeds ( Helianthus annuus L.), within a tropical deciduous forest in Jalisco, Mexico. Removal values were high in Delonyx regia (Bojer) Raf. (90%), Crescentia alata Kunth. (87%), H. annus (81%), Pithecellobium dulces (Roxb.) Benth (81%), Albizia occidentalis Brandegee (80%), Coccoloba barbadensis Jacq. (80%), Recchia mexicana DC. (80%), Caesalpinia pulcherrima (L.) Sw. (79%), Enterolobium cyclocarpum (Jacq.) Griseb. (73%), moderate in Guazuma ulmifolia Lam. (42%) and Celtis iguanaeus Sarg. (44%), and low in Amphipterygium adstringens (Schlechtend.) Schiede ex Standl. (17%). Low removal rates (29%) in experimental patches open only to arthropods suggest that arthropods have a minor role in removing fruits and seeds of these species. Removal values were high in experimental patches open to all potential agents (77%) and semipermeable patches open to forest-dwelling rodents (76%), suggesting that the latter group, the most abundant terrestrial mammal in this forest, was an important agent for removing the fruits and seeds of study plants. Removal values were higher in experimental patches located in tropical deciduous forest than in tropical semideciduous forest, as well as higher in high density (patches of 30 fruits or seeds) than in low density (patches of five fruits or seeds) experimental patches for most study plants. Mice appear to selectively remove and hoard fruits and seeds according to their energy and nutritional content and the presence of secondary metabolites, and from from high-density food patches and preferred habitats. Nonindependent effects of species, habitat, and density suggest that a complex interplay of factors determines fruit and seed removal for the plants examined from the Chamela tropical forest.

Prediction of desiccation sensitivity in seeds of woody species: a probabilistic model based on two seed traits and 104 species.

Seed desiccation sensitivity limits the ex situ conservation of up to 47 % of plant species, dependent on habitat. Whilst desirable, empirically determining desiccation tolerance levels in seeds of all species is unrealistic. A probabilistic model for the rapid identification of woody species at high risk of displaying seed desiccation sensitivity is presented. The model was developed using binary logistic regression on seed trait data [seed mass, moisture content, seed coat ratio (SCR) and rainfall in the month of seed dispersal] for 104 species from 37 families from a semi-deciduous tropical forest in Panamá. For the Panamanian species, only seed mass and SCR were significantly related to the response to desiccation, with the desiccation-sensitive seeds being large and having a relatively low SCR (i.e. thin 'seed' coats). Application of this model to a further 38 species, of known seed storage behaviour, from two additional continents and differing vegetation types (dryland Africa and temperate Europe) correctly predicted the response to desiccation in all cases, and resolved conflicting published data for two species (Acer pseudoplatanus and Azadirachta indica). This model may have application as a decision-making tool in the handling of species of unknown seed storage behaviour in species from three disparate habitats.

Nitrogenous compounds, phenolic compounds and morphological aspects of leaves: comparison of deciduous and semideciduous arboreal legumes

In general, perennial species contain higher concentrations of certain secondary compounds, such as phenolics, lower levels of nitrogenous compounds, and greater specific leaf mass (SLM) than deciduous species. The aim of this study was to verify whether the comparative data reported in the literature regarding deciduous and evergreen species, also applies to four selected species of a semideciduous tropical forest (a remnant of the Atlantic Forest). The four species chosen for this study, each with apparently different leaf life spans, were two semideciduous (Hymenaea courbaril L. var. stilbocarpa (Hayne) Lee et Lang., and Lonchocarpus guilleminianus (Tul.) Malme, and two deciduous (Enterolobium contortisiliquum (Vell.) Morong., and Peltophorum dubium (Spreng.) Taub.). Plants were grown in a greenhouse in plastic pots containing soil taken from their natural habitat. Mature leaves were harvested for determination of the SLM and contents of phenolic compounds, tannins, nitrate, protein, free amino acids, chlorophyll, and nitrogen. Data registered are in agreement with those of the literature. Leaves with the longest life span had the highest content of phenolics and the lowest levels of tannins, nitrate, protein, amino acids, chlorophyll and nitrogen, as well as presenting the greatest SLM.

Effects of topographic position, leaf litter and seed size on seedling demography in a semi-deciduous tropical forest in Panamá

This study examined whether topography-induced gradients in water potential and leaf litter depth contribute to species coexistence in tropical forests through species-specific effects on seedling emergence and mortality. Seedling emergence and mortality were followed for a period of 12 months in 36 (1 × 2 m) plots on Barro Colorado Island (BCI), Panama. Plots with and without litter were distributed on slope and plateau sites in three catchments. In the absence of manipulations, the lower litter depth on slopes resulted in approximately four times as many emergent seedlings than on plateau sites. However, litter depth had little effect on seedling community composition. By the end of the first dry-season, post-emergence, there were no significant differences in surviving seedling numbers between any treatments. There were differences in the emergent seedling community between slope and plateau sites within the same catchment as well as differences in composition between catchments, suggesting that both niche partitioning and dispersal limitation might play a role in structuring seedling community composition. During the wet-season seedling mortality was highest on slope sites although this pattern was reversed during the dry-season. In both seasons mortality was higher for small-seeded species. These results demonstrate that gradients in water potential related to topography impact on patterns of seedling emergence and mortality although processes in the first year after emergence may be insufficient to explain observed habitat preferences of adult plants.

Tree population and community dynamics in a Brazilian tropical semideciduous forest

Tree population and community dynamics in a Brazilian tropical semideciduous forest). The dynamics of the tree community and 30 tree populations were examined in an area of tropical semideciduous forest located on the margin of the Rio Grande, SE Brazil, based on surveys done in 1990 and 1997 in three 0.18 ha plots. The main purpose was to assess whether variations in dynamics were related to topography and the effects of a catastrophic flood in 1992. Rates of mortality and recruitment of trees and gain and loss of basal area in two topographic sites, lower (flooded) and upper (non-flooded), were obtained. Projected trajectories of mean and accelerated growth in diameter were obtained for each species. In both topographic sites, mortality rates surpassed recruitment rates, gain rates of basal area surpassed loss rates, and size distributions changed, with declining proportions of smaller trees. These overall changes were possibly related to increased underground water supply after the 1992 flood as well as to a c. 250-year-old process of primary succession on abandoned gold mines. Possible effects of the 1992 flood showed up in the higher proportions of dead trees in the flooded sites and faster growth rates in the flood-free sites. Species of different regeneration guilds showed particular trends with respect to their demographic changes and diameter growth patterns. Nevertheless, patterns of population dynamics differed between topographic sites for only two species.

Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE—Brazil III. Three legume trees in a semi-deciduous dry forest

Three legume tree species (Fabaceae) occurring abundantly in a semi-deciduous tropical dry forest of the Atlantic forest complex in southeastern Brazil were subjected to a comparative ecophysiological study at the end of the dry season/beginning of the wet season. The trees chosen were morphologically very similar: Caesalpinia echinata Lam. and Caesalpinia ferrea Mart. ex. Tul., both 10–20 m of height, of the sub-family Caesalpinioideae, and the somewhat smaller, 2–4 m tall, Machaerium obovatum Kuhlm. & Hoehne of the sub-family Faboideae. Despite their similarities with respect to their geographic distribution restricted to Brazilian dry forests, their comparable abundance in the study site and their phylogenetic proximity, the three species display distinctly different ecophysiological behaviour. Compared to the other two species, C. ferrea had the highest photosynthetic capacity (maximum apparent photosynthetic electron transport rate, ETRmax) and higher saturation light-intensity, was less subject to photoinhibition as indicated by potential quantum yield of photosystem II (F v/F m) and had the lowest bulk N content of which soluble non-protein N compounds were only 1.5%. It showed stronger sun plant characteristics. C. echinata had lower photosynthetic capacity, was under chronic photoinhibition and had high bulk N content of which 6.1% were soluble N compounds with high concentrations of proline. In addition to proline, high concentrations of sugars may serve as osmoprotectants. M. obovatum also showed lower photosynthetic capacity and was under chronic photoinhibition. Here, arginine may have a function as osmoprotectant. The ecophysiological differences between the three species are not related to local abundance. However, the observations presented highlight a contrasting behaviour of the otherwise very similar compatriot species.

Water storage capacitance and xylem tension in isolated branches of temperate and tropical trees

Trees of tropical semi-deciduous forests range from "drought-avoiding" stem-succulent species with low-density wood (< 0.5 g cm(-3)), which maintain high stem water potentials (psi(STEM) > -0.7 MPa) throughout the year, to "drought-tolerant" deciduous hardwood species (wood density > 0.75 g cm(-3)), which dehydrate strongly during seasonal drought (psi(STEM) < -6 MPa). In stem-succulent and other drought-avoiding species, xylem vessels are surrounded by extensive parenchyma providing intracellular water storage, whereas in deciduous species stem water storage is mainly extracellular. Thirteen tropical and two temperate tree species, representing different functional types, were studied. The contribution of stem water storage to these species' water use during water stress was determined by time-series analysis of dehydration and rehydration of excised leaf-bearing branches of these trees. During dehydration, stem water potential slowly declined 1-2 MPa in drought-avoiding species, but in deciduous species it rapidly fell 4-5 MPa, suggesting that water storage capacitance was related to xylem anatomy. After immersion of dehydrated, leafless branches in water, the decline in xylem tension and rate of water uptake during rehydration were linearly related, as predicted by application of Ohm's law to water flux. The decline of xylem tension during rehydration was biphasic, with a phase of rapid water uptake into extracellular spaces being followed by a prolonged phase of slow water uptake into living cells. The rate of water uptake during rehydration and the minima of leaf water potential observed in the field during the dry season were highly correlated with water storage capacitance, indicating that wood anatomy is a major determinant of drought adaptation.

Non-structural carbohydrate pools in a tropical forest

The pool size of mobile, i.e. non-structural carbohydrates (NSC) in trees reflects the balance between net photosynthetic carbon uptake (source) and irreversible investments in structures or loss of carbon (sink). The seasonal variation of NSC concentration should reflect the sink/source relationship, provided all tissues from root to crown tops are considered. Using the Smithsonian canopy crane in Panama we studied NSC concentrations in a semi-deciduous tropical forest over 22 months. In the 9 most intensively studied species (out of the 17 investigated), we found higher NSC concentrations (starch, glucose, fructose, sucrose) across all species and organs in the dry season than in the wet season (NSC 7.2% vs 5.8% of dry matter in leaves, 8.8/6.0 in branches, 9.7/8.5 in stems, 8.3/6.4 in coarse and 3.9/2.2 in fine roots). Since this increase was due to starch only, we attribute this to drought-constrained growth (photosynthesis less affected by drought than sink activity). Species-specific phenological rhythms (leafing or fruiting) did not overturn these seasonal trends. Most of the stem volume (diameter at breast height around 40 cm) stores NSC. We present the first whole forest estimate of NSC pool size, assuming a 200 t ha(-1) forest biomass: 8% of this i.e. ca. 16 t ha(-1) is NSC, with ca. 13 t ha(-1) in stems and branches, ca. 0.5 and 2.8 t ha(-1) in leaves and roots. Starch alone (ca. 10.5 t ha(-1)) accounts for far more C than would be needed to replace the total leaf canopy without additional photosynthesis. NSC never passed through a period of significant depletion. Leaf flushing did not draw heavily upon NSC pools. Overall, the data imply a high carbon supply status of this forest and that growth during the dry season is not carbon limited. Rather, water shortage seems to limit carbon investment (new tissue formation) directly, leaving little leeway for a direct CO2 fertilization effects.

Litter decomposition and nutrient release in a bamboo plantation

Decomposition and nutrient release patterns of bamboo litter were studied in a bamboo plantation. Litter decay rates were assessed using the litterbag technique. The monthly decomposition rate (K) ranged from 0.03 (June, September, October) to 0.08 (November) for leaves, 0.02 (June) to 0.09 (November) for branches and 0.01 (May, June) to 0.06 (November) for culms. Rainfall, temperature and soil moisture content were significantly correlated with the decomposition rates of litter components. Resident times (years) of litter components, calculated by dividing the litter standing crop by annual litter fall, were ranked as follows: culms (0.85) > leaves (0.48) > branches (0.24). The turnover coefficients (per year) were: branches (4.11) > leaves (2.07) > culms (1.17). Nutrient element residence times in the litter standing crops were ranked: Ca (1.0) > P (0.92) > Mg (0.64) > N (0.36) > K (0.31). The results from this study indicate that litter decomposition and nutrient element losses in bamboo plantations are slower than typical lowland rainforests and more similar to tropical semi-deciduous forests.

Vegetation and sedimentation in the Lake Edward Basin, Uganda–Congo during the late Pleistocene and early Holocene

New palynological and sedimentological data obtained from the basal 3 m of core E96-2P from Lake Edward, Uganda–Congo record conditions wetter than present in the Edward basin from ∼11 000 to 6700 yr BP, in phase with other climate and vegetation records of northern hemispheric East Africa. Dominant pollen taxa include Celtis spp., Alchornea spp., Olea spp., and Moraceae indicating a moist semi-deciduous tropical forest. More xeric indicators such as Amaranthaceae and Asteraceae together with Poaceae comprise less than 5% of the pollen sum throughout this interval as compared with between 44 and 50% during a lake lowstand at ∼2000 cal yr BP and at the core top (near modern). The differences between these two assemblages suggest a ∼25 to 60% increase in annual precipitation during the early- to mid-Holocene as compared to modern (1500–2000 vs. 1200 mm/yr today). Early Holocene sediments in E96-2P are composed of laminated diatom oozes with moderately high total sulfur concentrations (2.8–4.7%) and no authigenic calcite, also indicative of conditions wetter than present. Between ∼9000 and 6700 yr BP, palynological and sedimentary proxies indicate sub-millennial-scale events related to changes in riverine discharge and runoff in the Edward basin. We attribute the variability in runoff, and hence precipitation, to Holocene variability in Indian or Atlantic Ocean SSTs or to shifts in the relative contribution of Indian and Atlantic moisture sources to the western Rift of equatorial Africa.

Variations in tree community composition and structure in a fragment of tropical semideciduous forest in southeastern Brazil related to different human disturbance histories

In an area of tropical seasonal semideciduous forest, the soil characteristics, floristic composition, physiognomic structure, and the distribution of three regeneration and three dispersal guilds were studied for four stands within the forest that had documented histories of varying degrees of human disturbance. The aim was to study forest regeneration in areas of preserved forest and secondary forest, with parts of both types of forest experiencing either ‘intensive’ or ‘occasional’ cattle trampling. The study was carried out in the Sebastião Aleixo da Silva Ecological Station, Bauru, São Paulo State, Brazil. Two stands were called ‘secondary’ because they corresponded to forest tracts that were felled and occupied by crops and pastures in the past and then abandoned to forest regeneration ca. 40 years before this study. The other two stands, called ‘preserved’, corresponded to areas of the fragment where the forest has been maintained with only minor human impacts. The arboreal component of the tree community (diameter at breast height or dbh≥5 cm) was sampled in 20 plots of 40 m×40 m, and the subarboreal component (diameter at the base of the stem or dbs<5 cm and height≥0.5 m) in subplots of 40 m×2 m. Physiognomic features, such as canopy height and density of climbing plants, were registered all over a 5 m×5 m gridline laid on the sample plots. Soil bulk samples were collected for chemical and textural analyses. Most detected differences contrasted the secondary to the preserved forest stands. The soils of the secondary stands showed higher proportions of sand and lower levels of mineral nutrients and organic matter than those of the preserved stands, probably due to higher losses by leaching and erosion. Compared to the secondary stands, the preserved ones had higher proportions of tall trees, higher mean canopy height, lower species diversity, higher abundance of autochorous and shade-tolerant climax species, and lower abundance of pioneer and light-demanding climax species. Despite the high proportion of species shared by the preserved and secondary stands (108 out of 139), they differed consistently in terms of density of the most abundant species. On the other hand, the secondary and preserved stands held similar values for tree density and basal area, suggesting that 40 years were enough to restore these features. Effects of cattle trampling on the vegetation were detected for the frequency of trees of anemochorous and zoochorous species, which were higher in the stands under occasional and intensive cattle trampling, respectively. The density of thin climbers was lower in the stands with intensive trampling.

Fruiting trees as dispersal foci in a semi-deciduous tropical forest.

Quantification of seed rain patterns is an initial step toward explaining variation in plant recruitment, and consequently, organization of forest communities. Spatially contagious patterns of seed deposition, where seeds are patchily dispersed with some sites receiving relatively high densities and others receiving low densities of seeds, may be a common phenomenon for which we have very little knowledge. For example, prior feeding events by frugivores (monkeys and birds) combined with transport and dispersal of seeds to other fruiting trees may result in the contagious deposition of non-conspecific seeds below them. Here, we examined whether fruiting trees act as dispersal foci in the semi-deciduous tropical rainforest of the Dja Reserve, Cameroon. Seed rain was sampled below the canopies of nine tree species: three typically dispersed by large, frugivorous birds, three dispersed by monkeys, and three dispersed by wind. We found no evidence that monkeys generate spatially contagious patterns of seed rain under fruiting trees at which they feed. However, we found that rates of deposition of non-conspecific seeds and species richness of seeds delivered by birds (hornbills and turacos) were significantly greater during fruiting than non-fruiting periods, and significantly greater under fruiting individuals of bird-dispersed tree species than under fruiting individuals of monkey- or wind-dispersed tree species. Additionally, during fruiting periods, the composition of non-conspecific seed rain under bird-dispersed tree species was more similar to other bird-dispersed trees than to monkey- or wind-dispersed tree species. The contagious dispersal of non-conspecific seeds to fruiting, bird-dispersed trees leads to higher seed densities under fruiting trees than those caused by local seed production. Non-conspecific seeds deposited in high densities may experience increased seed mortality even far from parent trees if predators are generalists. Alternatively, in the absence of complete density-dependent mortality, contagious seed dispersal could result in associations among species dispersed by the same dispersal agent.

The Role of Termites in Soil Formation in the Tropical Semi-Deciduous Forest Zone, Ghana

Soil Macrofauna especially insects have been recognized as being an important factor in regulating soil processes and thereby the soil profde development. The influence of earthworms is well documented and it is the dominant member of the soil macrofauna influencing the soil formation processes in the temperate zone. In the tropics, termites and ants play the major role in the nutrient recycling and the movement and transportation of soil material. This paper describes the influence of termites on the soil formation in the tropical wet semi-deciduous forest in Ghana at the north coast of the Gulf of Guinea. The termites form distinct soil horizons especially at the summits of the landscapes where gravel free soil horizons are formed upon gravel rich horizons, and in some cases soil heaps are formed after the decomposition of huge trees.

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 Levee 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.

Interactions of gap size and herbivory on establishment, growth and survival of three species of neotropical pioneer trees

Summary 1 The difference in gap size requirements observed for neotropical pioneer species is a classic example of habitat partitioning among tropical tree species. Previous research has shown that the three species Miconia argentea , Cecropia insignis and Trema micrantha differ in the minimum gap size that they occupy as adult plants in semi-deciduous tropical forest in Panama. Here we test whether these differences result from variation in seed germination and seedling establishment requirements, or from differences in susceptibility to herbivores. 2 We planted seeds of Miconia argentea and Trema micrantha into artificially created small (25 m 2 ), medium (64 m 2 ) and large (225 m 2 ) gaps in secondary forest, and transplanted seedlings of all three species into small and large gaps inside and outside mesh cages that excluded invertebrate herbivores. 3 Miconia seedlings emerged and established successfully across a broad range of gap sizes, and transplanted seedlings grew slowly but survived well in all gap sizes. By contrast, emergence and survival of Trema seedlings occurred only in the large gaps and transplants died in small gaps. Survival of Cecropia seedlings was intermediate between Trema and Miconia in the small gaps and growth was greater than either species in the large gaps. 4 Herbivore exclosures reduced foliar herbivory on all three species. The median lifespan of Trema seedlings in small gaps was reduced when the seedlings were exposed to herbivores, but survival of the other species was not affected by the exclosure treatment in either gap size. The growth of Miconia seedlings was lower outside exclosures in both gap sizes. 5 Differences in germination and establishment could generate the contrasting gap size distributions observed between Miconia and Trema , but they are partially offset by measured differences in seed bank density. However, patterns of seedling growth and survival reinforce differences among species at the establishment phase and provide a mechanism for the observed differences in gap size distribution at the sapling stage. ‘Cross-overs’ in species growth rates result in specialization to different environments and reduce their survival in unfavourable conditions. We found no evidence for a definitive influence of herbivory as a mechanism driving differences in gap size distribution among species.

Flora arbustivo-arbórea do fragmento de floresta estacional semidecidual do Ribeirão Cachoeira, município de Campinas, SP

Foi realizado inventário florístico das árvores e arbustos da mata Ribeirão Cachoeira (233,7ha, altitude 650m, coordenadas 46°55'58''W, 22°50'13''S), o segundo maior e mais bem conservado fragmento de floresta estacional semidecidual do município de Campinas, SP. O solo característico é o Podzólico Vermelho Amarelo e o clima é Cwag' de Köppen. As coletas foram feitas durante o período de agosto/1996 a setembro/1997. Foram incluídos apenas indivíduos férteis com perímetro a altura do peito igual ou superior a 9cm. Foram encontradas 175 espécies de 119 gêneros e 49 famílias. As famílias mais ricas foram Myrtaceae (14 espécies), Rutaceae e Fabaceae (13), Caesalpiniaceae (11), Solanaceae (9) e Rubiaceae (8). Algumas espécies foram encontradas pela primeira vez na região: Tachigali multijuga Benth. e Schoepfia brasiliensis A.DC. A floração foi maior entre agosto e outubro. A frutificação foi maior nos meses de agosto a novembro. A maioria das espécies encontradas é zoocórica (58%), as anemocóricas foram 23% e as autocóricas 19%. Compararam-se as relações florísticas desta mata com outros 20 levantamentos do Estado. Os resultados obtidos indicaram a formação de dois grupos distintos. O mais homogêneo deles inclui as florestas do município de Campinas, mostrando que, possivelmente, são remanescentes de uma vegetação originalmente contínua.

Variações da fisionomia, diversidade e composição de guildas da comunidade arbórea em um fragmento de floresta semidecidual em Lavras, MG

Este estudo teve como objetivo analisar diferenças na estrutura fisionômica, na diversidade de espécies e na composição de guildas da comunidade arbórea de setores de um fragmento de floresta semidecidual que se encontram em diferentes fases de regeneração e graus de exposição à borda. O fragmento, que possui 5,8ha de área e está situado no município de Lavras, MG (21°13'40''S, 44°57'50''W), foi dividido em quatro setores e submetido a um censo das árvores com DAP &gt; 5cm. Os setores foram comparados quanto à densidade, área basal, distribuição de alturas e de diâmetros, diversidade de espécies e freqüência de árvores por guildas de regeneração, estratificação e dispersão. Os setores menos perturbados no passado, Interior Alto e Borda Alta, apresentaram maiores áreas basais e menores densidades de árvores (sobretudo das de menor tamanho), maiores proporções de árvores de espécies clímax tolerantes à sombra e de espécies de grande porte que os mais perturbados, Interior Baixo e Borda Baixa. Estas diferenças realçam que os setores Altos estão em uma fase mais avançada do processo de regeneração. Os setores de Borda tiveram maior freqüência de indivíduos de espécies com dispersão anemocórica que os de Interior. Cada setor de Borda teve maior diversidade de espécies que seu parceiro de Interior. Concluiu-se que os parâmetros analisados podem, em conjunto, servir como bons avaliadores da severidade da perturbação sofrida no passado, da fase regenerativa atual e do efeito borda.

Carbon limitation in trees

Summary The ongoing enrichment of the atmosphere with CO2 raises the question of whether growth of forest trees, which represent close to 90% of the global biomass carbon, is still carbon limited at current concentrations of close to 370 p.p.m. As photosynthesis of C3 plants is not CO2‐saturated at such concentrations, enhanced ‘source activity’ of leaves could stimulate ‘sink activity’ (i.e. growth) of plants, provided other resources and developmental controls permit. I explore current levels of non‐structural carbon in trees in natural forests in order to estimate the potential for a carbon‐driven stimulation of growth. The concentration of non‐structural carbohydrates (NSC) in tree tissues is considered a measure of carbon shortage or surplus for growth. A periodic reduction of NSC pools indicates either that carbon demand exceeds con‐current supply, or that both source and sink activity are low. A steady, very high NSC concentration is likely to indicate that photosynthesis fully meets, or even exeeds, that needed for growth (surplus assimilates accumulate). The analysis presented here considers data for mature trees in four climatic zones: the high elevation treeline (in Mexico, the Alps and Northern Sweden), a temperate lowland forest of central Europe, Mediterranean sclerophyllous woodland and a semideciduous tropical forest in Panama. In all four climatic regions, periods of reduced or zero growth show maximum C‐loading of trees (source activity exceeding demand), except for dry midsummer in the Mediterranean. NSC pools are generally high throughout the year, and are not significantly affected by mass fruiting episodes. It is concluded that, irrespective of the reason for its periodic cessation, growth does not seem to be limited by carbon supply. Instead, in all the cases examined, sink activity and its direct control by the environment or developmental constraints, restricts biomass production of trees under current ambient CO2 concentrations. The current carbohydrate charging of mature wild trees from the tropics to the cold limit of tree growth suggests that little (if any) leeway exists for further CO2‐fertilization effects on growth.

Variáveis ambientais e a distribuição de espécies arbóreas em um remanescente de floresta estacional semidecídua montana no campus da Universidade Federal de Lavras, MG

Este trabalho teve como objetivo detectar padrões de distribuição espacial da comunidade arbórea de um fragmento de floresta estacional semidecídua montana e suas relações com variáveis ambientais, com ênfase nas variações catenárias do solo e efeito das bordas. O remanescente florestal, conhecido como Mata da Subestação, cobre uma área de 8,75ha e está localizado no campus da Universidade Federal de Lavras, MG, nas coordenadas de 21º13'17''S e 44º57'47''W e com altitudes variando de 910 a 940m. A amostragem foi realizada por meio de duas transeções (A e B) separadas por 80m e compostas, cada uma, por 21 e 31 parcelas contíguas de 20 x 20m de dimensões. As transeções foram lançadas no sentido da declividade maior da encosta e ligando bordas opostas do fragmento. Todos os indivíduos arbóreos com diâmetro à altura do peito (DAP) &lt;FONT FACE=Symbol&gt;³&lt;/FONT&gt; 5cm presentes nas parcelas foram identificados e medidos (DAP e altura). Foi feito um levantamento topográfico detalhado da área amostrada. Amostras do solo superficial (0-20cm) foram coletadas em cada parcela para análise química e textural. O solo das parcelas foi classificado, sendo identificadas duas catenas que continham, da baixa para a alta encosta, Nitossolos e Latossolos (transeção A) e Nitossolos, Cambissolos e Latossolos (transeção B). O levantamento florístico geral, feito nas parcelas e no fragmento como um todo, registrou 238 espécies pertencentes a 143 gêneros e 57 famílias. Nas parcelas, os 3120 indivíduos amostrados pertenciam a 175 espécies, 115 gêneros e 49 famílias. O índice de diversidade de Shannon foi de H' = 4,19 nats.indivíduo-1, com equabilidade J' = 0,81, valores altos no contexto de levantamentos semelhantes na região. O elevado índice de espécie raras (densidade relativa &lt; 1 árvore.ha-1) de 34% sugere que o isolamento genético devido à fragmentação pode prejudicar a conservação biológica de muitas espécies a longo prazo. Os Latossolos de ambas as transeções mostraram-se mais ácidos, pobres em bases trocáveis e ricos em alumínio que o Nitossolo e o Cambissolo. A composição e estrutura da comunidade arbórea diferiram entre os subgrupos de solo. Uma análise de correspondência canônica indicou correlações significativas entre variáveis topográficas e edáficas e a distribuição da abundância das espécies, mas nenhuma correlação clara com as bordas. As variáveis ambientais mais fortemente correlacionadas foram cota média, saturação por bases e teores de areia, silte e argila. Portanto, a distribuição das espécies arbóreas no fragmento parece ser primariamente influenciada pelo status nutricional e regime de água dos solos. A heterogeneidade edáfica e topográfica da área pode ter contribuído para tornar não detectável pelo método usado qualquer efeito da borda na composição e estrutura da comunidade.