Do Dipteryx alata Vogel seedlings recover the quality and the photosynthetic and antioxidant responses in the post-flooding? / Mudas de Dipteryx alata Vogel recuperam a qualidade e as respostas fotossintéticas e antioxidantes no pós-alagamento?

Dipteryx alata Vogel is a tree species widely found in Cerrado, settling preferentially in well drained soils. Studies related to ecophysiology of D. alata may contribute to the decision making about using seedlings of this species in projects aimed at the recovery of degraded areas where seasonal flooding happens. This study aimed to assess the effects of flooding on photosynthetic and antioxidant metabolism and quality of D. alata seedlings cultivated or not under flooding during four assessment periods (0, 20, 40, and 60 days), followed by 100 days after the end of each assessment period (0+100, 20+100, 40+100, and 60+100 days), allowing verifying the potential for post-flooding recovery. Flooded plants showed lower photosynthetic efficiency than non-flooded plants, regardless of the periods of exposure. However, this efficiency was recovered in the post-flooding, with values similar to that of the non-flooded seedlings. Moreover, the damage to FV/FM was evidenced by an increase in the period of exposure to flooding, but recovery was also observed at this stage of the photosynthetic metabolism. Seedling quality decreased under flooding, not varying between periods of exposure, but remained lower although the increase observed in the post-flooding period, with no recovery after flooding. The occurrence of hypertrophied lenticels associated with physiological changes and an efficient antioxidant enzyme system might have contributed to the survival and recovery of these seedlings. Thus, this species is sensitive to flooding stress but capable of adjusting and recovering metabolic characteristics at 100 days after the suspension of the water stress, but with no recovery in seedling quality. Thus, we suggested plasticity under the cultivation condition and determined that the time of 100 days is not enough for the complete resumption of growth.

Dipteryx alata Vogel é uma arbórea de ampla ocorrência no Cerrado, se estabelecendo preferencialmente em solos bem drenados. Estudos referentes à ecofisiologia de D. alata em podem contribuir para a tomada de decisão sobre o uso de mudas dessa espécie em programas de recuperação de áreas degradadas sujeitas a alagamento temporário. Objetivamos com essa pesquisa avaliar os efeitos do alagamento no metabolismo fotossintético e antioxidante, além da qualidade de mudas dessa espécie, cultivadas ou não sob alagamento durante quatro períodos de avaliação (0, 20, 40 e 60 dias) seguidos de 100 dias após o término de cada período (0+100, 20+100, 40+100, 60+100 dias), possibilitando verificar o potencial de recuperação pós-alagamento. Observamos que as plantas alagadas apresentaram menor eficiência fotossintética e danos em FV/FM entretanto houve recuperação dessas características no pós alagamento. A qualidade das mudas reduziu sob alagamento não variando entre os períodos de exposição e embora tenha aumentado no pós-alagamento manteve-se menor não se recuperando. A ocorrência de lenticelas hipertrofiadas associadas a alterações fisiológicas e um eficiente sistema enzimático antioxidante devem ter contribuído para a sobrevivência e recuperação metabólica dessas mudas. Diante disso, sugerimos que a espécie é sensível ao estresse por alagamento, mas capaz de se ajustar e recuperar as características metabólicas 100 dias após a suspensão deste estresse hídrico, no entanto a qualidade da mudas não apresentou recuperação, assim, sugerimos plasticidade diante da condição de cultivo e ressaltamos que o tempo de 100 dias não é suficiente para a completa retomada do crescimento.

Evidence of drought memory in Dipteryx alata indicates differential acclimation of plants to savanna conditions.

The remarkable phytogeographic characteristics of the Brazilian savanna (Cerrado) resulted in a vegetation domain composed of plants with high structural and functional diversity to tolerate climate extremes. Here we used a key Cerrado species (Dipteryx alata) to evaluate if species of this domain present a mechanism of stress memory, responding more quickly and efficiently when exposed to recurrent drought episodes. The exposure of D. alata seedlings to drought resulted in several changes, mainly in physiological and biochemical traits, and these changes differed substantially when the water deficit was imposed as an isolated event or when the plants were subjected to drought cycles, suggesting the existence of a drought memory mechanism. Plants submitted to recurrent drought events were able to maintain essential processes for plant survival when compared to those submitted to drought for the first time. This differential acclimation to drought was the result of orchestrated changes in several metabolic pathways, involving differential carbon allocation for defense responses and the reprogramming and coordination of primary, secondary and antioxidant metabolism. The stress memory in D. alata is probably linked the evolutionary history of the species and reflects the environment in which it evolved.

Dipteryx alata, a tree native to the Brazilian Cerrado, is sensitive to the herbicide nicosulfuron.

The expansion of land use for agricultural interests and the excessive use of herbicides are among the causes of biodiversity losses in the Brazilian Cerrado biome. Therefore, we aimed to test the hypothesis that Dipteryx alata Vogel, a common species in this biome, is sensitive to nicosulfuron because of its high phytotoxicity. We evaluated physiological, biochemical and morphological responses in D. alata plants exposed to increasing doses of the herbicide. Young plants were transplanted to 10 L pots containing substrate composed of soil and sand (2:1) after fertilization. After an acclimation period, the following doses of nicosulfuron were applied: 0 (control), 6, 12, 24, 48, and 60 g a.e. ha-1. The experiment was conducted in a randomized block design factorial scheme with six doses of nicosulfuron, three evaluation times, and five replicates per treatment. The effects of the herbicide were assessed by measuring gas exchange, chlorophyll a fluorescence, photosynthetic pigments, membrane permeability, antioxidant enzymes and acetolactate synthase. Nicosulfuron altered the photosynthetic machinery and enzymatic metabolism of D. alata. Reductions in physiological traits, increased catalase and ascorbate peroxidase activities, enhanced malondialdehyde concentrations rate of electrolyte leakage and decreased acetolactate synthase activity in response to nicosulfuron all suggest that D. alata is sensitive to this herbicide.

Presence of the anther gland is a key feature in pollination of the early-branching papilionoids Dipteryx alata and Pterodon pubescens (Leguminosae).

The presence of glandular appendages in the anthers is a rare condition in angiosperms. In Leguminosae it occurs in species of the Mimosoid clade and in early-branching clades of papilionoids such as Dipterygeae. In Dipterygeae such appendages surprisingly exhibit a secretory cavity instead of secretory emergences as is the case for the Mimosoid clade. Thus, the objective of this study was to elucidate the function of anther glands in Dipteryx alata and Pterodon pubescens, species in the Dipterygeae clade that exhibit a pollen release mechanism that is intermediate between the explosive and valvular types. Flower buds and flowers were processed for surface, anatomical, histochemical and ultrastructural analyses. Anther glands consist of a cavity secreting sticky substances (oleoresins and polysaccharides) that play a key role during the flower's lifespan by aggregating pollen grains and attaching them to the floral visitor's body. Other floral features that are important for understanding the pollen release mechanism that is intermediate between the valvular and the explosive types are: (i) keel petals intertwined with tector trichomes; (ii) glandular appendages in the abaxial and lateral sepals and in petals composed of secretory ducts; and (iii) a continuous secretion process of the anther glands followed by an asynchronous dehiscence of anthers. The well-adapted papilionoid flag blossom with anther glands and keel petals intertwined with trichomes provided the foundation for a successful canalisation toward a pollen release mechanism intermediate between the explosive and valvular types inside early-branching papilionoids.

Seed removal of Dipteryx alata Vog. (Leguminosae: Faboidae) in the edge and interior of Cerrado / Remoção de sementes de Dipteryx alata Vog. (Leguminosae: Faboidae) na borda e interior do Cerrado

Abstract Post-dispersal seed agents of mortality include pathogens, invertebrates, and vertebrates, which may shape tropical tree populations and communities. In this study I experimentally evaluated Dipteryx alata endocarp removal both in the interior and edge of Cerrado vegetation. Specifically, I simulated primary dispersion of endocarps by bats and evaluated secondary removal intensity according to habitat, season, and endocarp number. This study was developed in the Pombo Natural Municipal Park, a large Cerrado remnant with an area of 9,000 ha, located in the Western Brazil. In each of the two habitat types, I set down 45 points in which D. alata piles of 1, 3, 7, 15, and 40 endocarps were positioned. On average, endocarp removal in the interior was higher than in the edge, so that the intensity of removal varied from positive density-dependent (interior: rains), to negative density-dependent (edge: both seasons). Also, at both habitat types and seasons non removed endocarps were intensely attacked by fungus. Indeed, seed mortality by fungus infestation was positive density-dependent. The results indicate high propensity of endocarps to escape from removal in the edge, in principle, a defaunated area. As Dipteryx species strongly rely on caviomorph rodents for their dispersal, the scarcity of these vectors might reduce recruitment chances of D. alata in remnants of Cerrado. This imply in future changes in the vegetation structure from the edge to the interior towards low tree diversity.

Resumo A mortalidade pós-dispersão de sementes é causada por agentes como patógenos, invertebrados e vertebrados, os quais influenciam a dinâmica de populações e comunidades arbóreas. Nesse estudo, avaliei experimentalmente intensidade de remoção, por roedores, de endocarpos de Dipteryx alata na borda e interior da vegetação do Cerrado. Simulando a dispersão primária por morcegos, posicionei pilhas com diferentes números de endocarpos nos dois hábitats do cerrado e avaliei a remoção secundária, tanto na estação seca quanto na chuvosa. Os experimentos foram desenvolvidos no Parque Natural Municipal do Pombo, um grande remanescente com 9000 ha (Três Lagoas, MS, Brasil). Em cada um dos habitat defini 45 pontos nos quais pilhas com 1, 3, 7, 15 e 40 endocarpos foram posicionadas. A remoção de endocarpos foi significativamente maior no interior em relação à borda, tal que a intensidade de remoção variou de positivamente dependente da densidade (interior: chuvas), a negativamente dependente da densidade (borda: ambas as estações). Além disso, tanto na borda, quanto interior, bem como nas duas estações os endocarpos não removidos foram intensamente infestados por fungos. Consequentemente, a mortalidade causada por esses patógenos foi densidade dependente. Os resultados sugerem que os endocarpos de D. alata teriam maiores chances de escapar à remoção na borda, em princípio, depauperada em termos de roedores dispersores. A escassez desses vetores reduziria as chances de recrutamento de D. alata, favorecendo mudanças futuras na estrutura da vegetação, a partir da borda, em termos de menor diversidade arbórea.

Seed removal of Dipteryx alata Vog. (Leguminosae: Faboidae) in the edge and interior of Cerrado.

Post-dispersal seed agents of mortality include pathogens, invertebrates, and vertebrates, which may shape tropical tree populations and communities. In this study I experimentally evaluated Dipteryx alata endocarp removal both in the interior and edge of Cerrado vegetation. Specifically, I simulated primary dispersion of endocarps by bats and evaluated secondary removal intensity according to habitat, season, and endocarp number. This study was developed in the Pombo Natural Municipal Park, a large Cerrado remnant with an area of 9,000 ha, located in the Western Brazil. In each of the two habitat types, I set down 45 points in which D. alata piles of 1, 3, 7, 15, and 40 endocarps were positioned. On average, endocarp removal in the interior was higher than in the edge, so that the intensity of removal varied from positive density-dependent (interior: rains), to negative density-dependent (edge: both seasons). Also, at both habitat types and seasons non removed endocarps were intensely attacked by fungus. Indeed, seed mortality by fungus infestation was positive density-dependent. The results indicate high propensity of endocarps to escape from removal in the edge, in principle, a defaunated area. As Dipteryx species strongly rely on caviomorph rodents for their dispersal, the scarcity of these vectors might reduce recruitment chances of D. alata in remnants of Cerrado. This imply in future changes in the vegetation structure from the edge to the interior towards low tree diversity.

orchard: Paternity program for autotetraploid species.

Advances in molecular marker technology have provided new opportunities to study the population genetics of polyploid taxa. Paternity analysis using microsatellite markers can be used in detection of gene flow between individuals and populations, in mating system analysis, to identify factors that influence fecundity and fertility, to identify behaviour of parent-offspring relationships and in the analysis of the reproductive success of different ecological groups. As there is no specific program for carrying out paternity analysis in tetraploid species, specialized software was designed for the assignment of paternity for autotetraploid species. orchard is a novel implementation of exclusion and likelihood statistics for carrying out paternity analysis of autotetraploids. First, the program performs an exclusion method, and then, a likelihood statistic is used with nonexcluded candidate fathers. Optional features include estimation of allele dosage of known mother trees and the estimation of pollen flow distances. orchard was tested using a data set of microsatellite data of Dipteryx odorata, a tetraploid Amazonian tree species.

Ecological consequences of primary and secondary seed dispersal on seed and seedling fate of Dipteryx oleifera (Fabaceae)

The relative contributions of primary and secondary seed dispersal to plant demography have received little investigation. Evidence on these seed dispersal types, on seed fate and seedling recruitment of the tropical rain forest tree Dipteryx oleifera, is presented. The study was conducted in a 6.37ha permanent plot where seeds and seedlings were located and tagged for the 2007 cohort. A total of 2 814 seeds were threaded and their fate was followed one year after germination. Primary seed dispersal by bats protected seeds from insect larval predation below the adult tree. Bats congregated seeds in bat seed piles located at a mean distance of 40.94±1.48m from the nearest adult individual of D. oleifera. Terrestrial vertebrates congregated seeds in caches located 41.90±2.43m from the nearest adult individual of D. oleifera. The results of the fitted proportional hazard model suggested that primary seed dispersal decreased seed hazard probability by 1.12% for each meter from the adult conspecific (p<0.001) and that secondary seed dispersal decreased it by 23.97% (p<0.001). Besides, the odds ratio regression models results showed that the overall effect of unviable seeds was a reduction in viable seed predation rate. For each unviable seed deposited by bats into the seed piles, the rate of seed predation by terrestrial vertebrates decreased 6% (p<0.001). For each damaged seed by terrestrial vertebrates in the seed piles, the rate of germination decreased 4% (p<0.001). For each germinated seed in the seed piles, the rate of recruitment increased 16% (p=0.001). Seedling survival of seeds that emerged after secondary seed dispersal events, showed no statistically significant difference in arthropod herbivory, in relation to seedlings that came from seeds that were dispersed only primarily by bats (F=0.153, p=0.697, df=1.98). Thus both primary and secondary dispersal contributed to higher seedling survival away from the nearest adult D. oleifera (r2=0.713, n=578, p=0.004). The distribution of D. oleifera seedlings is consistent with the Janzen-Connell Hypothesis and depends on primary dispersal by bats, secondary dispersal by terrestrial vertebrates, a seed masking effect and, the constant threat of insect herbivores on seedlings. Rev. Biol. Trop. 58 (3): 991-1007. Epub 2010 September 01.

Presentamos evidencia de la contribución relativa de la dispersión primaria y secundaria de semillas en la suerte que corren las mismas y en el reclutamiento de plántulas de la especie de bosque húmedo tropical Dipteryx oleifera. Este estudio fue realizado en una parcela permanente de 6.37 ha en la que semillas y plántulas del cohorte del 2007 fueron localizadas y mapeadas. Etiquetamos 2 814 semillas para evaluar su suerte un año después de la germinación. Encontramos que la dispersión primaria por murciélagos libera a las semillas de la depredación bajo la copa del árbol adulto causada por la fase larval de un insecto. Los murciélagos dispersan semillas bajo la copa de palmas usadas como comederos, localizados a 40.94±1.48m del árbol de D. oleifera más cercano y vertebrados terrestres congregaran las semillas en caches localizados aproximadamente a 41.90±2.43m del árbol de D. oleifera. Un modelo proporcional de riesgos fue ajustado a los datos de semillas y los resultados sugieren que la dispersión decrece la probabilidad de mortalidad de las semillas 1.16% por cada metro del árbol conspecífico más cercano (p<0.001) y la dispersión secundaria de semillas decrece la probabilidad de mortalidad 23.94% por cada metro (p<0.001). Los modelos de regresión de quebrados muestran que las semillas inviables reducen la tasa de depredación de las viables. Por cada semilla inviable depositada por los murciélagos en las agrupaciones de semillas, la tasa de depredación por vertebrados terrestres decreció 6% (p<0.001). Por cada semilla dañada por vertebrados en las agrupaciones, la tasa de germinación decreció 4% (p<0.001). Por cada semilla germinada en las agrupaciones, la tasa de reclutamiento incrementó 16% (p=0.000). La sobrevivencia de plántulas con dispersión secundaria fue estadísticamente diferente a la de aquellas que únicamente tuvieron dispersión primaria (F=0.153, p=0.697, df=1.98). Ambos tipos de dispersión contribuyeron con una alta sobrevivencia de plántulas lejos del árbol congénere más cercano (r²=0.713, n=578, p=0.004). Concluimos que la distribución de las plántulas de D. oleifera es consistente con la Hipótesis de Janzen- Connell y depende de la dispersión primaria por murciélagos, dispersión secundaria por vertebrados terrestres, el efecto de enmascaramiento y el constante ataque de insectos herbívoros a las plántulas.

Ecological consequences of primary and secondary seed dispersal on seed and seedling fate of Dipteryx oleifera (Fabaceae).

The relative contributions of primary and secondary seed dispersal to plant demography have received little investigation. Evidence on these seed dispersal types, on seed fate and seedling recruitment of the tropical rain forest tree Dipteryx oleifera, is presented. The study was conducted in a 6.37ha permanent plot where seeds and seedlings were located and tagged for the 2007 cohort. A total of 2 814 seeds were threaded and their fate was followed one year after germination. Primary seed dispersal by bats protected seeds from insect larval predation below the adult tree. Bats congregated seeds in bat seed piles located at a mean distance of 40.94 +/- 1.48m from the nearest adult individual of D. oleifera. Terrestrial vertebrates congregated seeds in caches located 41.90 +/- 2.43m from the nearest adult individual of D. oleifera. The results of the fitted proportional hazard model suggested that primary seed dispersal decreased seed hazard probability by 1.12% for each meter from the adult conspecific (p<0.001) and that secondary seed dispersal decreased it by 23.97% (p<0.001). Besides, the odds ratio regression models results showed that the overall effect of unviable seeds was a reduction in viable seed predation rate. For each unviable seed deposited by bats into the seed piles, the rate of seed predation by terrestrial vertebrates decreased 6% (p<0.001). For each damaged seed by terrestrial vertebrates in the seed piles, the rate of germination decreased 4% (p<0.001). For each germinated seed in the seed piles, the rate of recruitment increased 16% (p=0.001). Seedling survival of seeds that emerged after secondary seed dispersal events, showed no statistically significant difference in arthropod herbivory, in relation to seedlings that came from seeds that were dispersed only primarily by bats (F=0.153, p=0.697, df=1.98). Thus both primary and secondary dispersal contributed to higher seedling survival away from the nearest adult D. oleifera (r2=0.713, n=578, p=0.004). The distribution of D. oleifera seedlings is consistent with the Janzen-Connell Hypothesis and depends on primary dispersal by bats, secondary dispersal by terrestrial vertebrates, a seed masking effect and, the constant threat of insect herbivores on seedlings.

Metodologia para teste de tetrazólio em sementes de Amburana cearensis (Allemão) A.C. Smith / Methodology for tetrazolium test in Amburana cearensis (Allemão) A.C. Smith seeds

Objetivou-se com este trabalho estudar a metodologia do teste de tetrazólio para sementes de Amburana cearensis (Allemão) A.C. Smith, determinando as melhores condições de pré-umedecimento, bem como temperatura, período de coloração e concentração da solução de tetrazólio. Avaliou-se a coloração das sementes intactas e escarificadas (lixadas), ambas com tegumento, diretamente na solução de tetrazólio nas concentrações de 0,025; 0,050; 0,075 e 1 por cento a 35 e 40ºC, por 12 e 24h no escuro. Verificou-se o pré-umedecimento para retirar o tegumento utilizando-se sementes intactas e escarificadas, as quais foram imersas diretamente em água e submetidas a embebição em papel toalha e por 12, 24, 36 e 48 horas em temperaturas de 35 e 40ºC. Após a determinação do método para retirar o tegumento os embriões foram submetidas à coloração em solução de tetrazólio nas concentrações de 0,025; 0,050; 0,075 e 1 por cento. Com os resultados obtidos verificou-se a necessidade da retirada total do tegumento para que ocorra a coloração e que a temperatura de 40ºC é prejudicial para a embebição no pré-umedecimento. O teste de tetrazólio nas sementes de A. cearensis deve ser realizado com o pré-umedecimento das sementes escarificadas (lixadas) e imersas diretamente em água por 24 horas, na temperatura de 35ºC, para posterior retirada total do tegumento. E para atingir a coloração ideal os embriões devem ser imersos em solução de tetrazólio a 0,05 por cento por 3 horas, a 40ºC.

This work aimed to evaluate the methodology of tetrazolium test in Amburana cearensis (Allemão) A.C. Smith seeds, establishing the best pre-moistening conditions, as well as temperature, coloration period and tetrazolium solution concentration. The coloration of intact and scarified (sanded) seeds, both with tegument, was directly evaluated in tetrazolium solution at the concentrations of 0.025; 0.050; 0.075; and 1 percent, at 35 and 40ºC, for 12 and 24 h in the dark. Pre-moistening to remove the tegument was evaluated by using intact and scarified seeds, which were directly immersed in water and subjected to imbibition in paper towel for 12, 24, 36, and 48 h at 35 and 40ºC. After establishing the method to remove the tegument, embryos were subjected to coloration in tetrazolium solution at the concentrations of 0.025; 0.050; 0.075; and 1 percent. Based on the obtained results, the complete tegument removal is needed for coloration; besides, the temperature of 40ºC is harmful for imbibition during pre-moistening. The tetrazolium test in A. cearensis seeds should be performed including pre-moistening through direct immersion of scarified (sanded) seeds in water for 24h at 35ºC for later complete tegument removal. To achieve the ideal coloration, embryos must be immersed in tetrazolium solution at 0.05 percent for 3h at 40ºC.

Recruitment dynamics of the tropical rainforest tree Dipteryx oleifera (Fabaceae) in eastern Nicaragua.

Seed production, seed dispersal and recruitment are critical processes in population dynamics, because they are almost never completely successful. We recorded the recruitment dynamics for the population of Dipteryx oleifera in a tropical rainforest in eastern Nicaragua (12 degrees 05' N., 83 degrees 55' W.) from March 2002 to August 2006. Seeds and seedlings had highly clumped distributions, while sapling distributions appeared to be random. Seedling survival increased away from the nearest conspecifc adult tree, where seedling density is lower. Since relative growth rates of seedlings are not correlated with the distance to the nearest conspecific adult, seedling survival appears to be independent of seedling growth. Seedling density is inversely correlated with seedling insect herbivory damage. Seedling survival correlated negatively with the number of saplings per sub-plot (10x10 m), suggesting that insect herbivore may also cue in on saplings rather than only on adult D. oleifera trees in order to locate seedlings. Seedling establishment is significantly clumped with respect to the nearest adult tree. Larger clumps of seedlings seems more ephemeral than isolated smaller clumps located away from the nearest D. oleifera tree. These results support current empirical evidence presented earlier for the Janzen-Connell hypothesis for Dipteryx oleifera at seed and seedling stages and, the Recruitment Limitation hypothesis at the sapling stage, because sapling individuals might have recruited after random light-gap formation.

Recruitment dynamics of the tropical rainforest tree Dipteryx oleifera (Fabaceae) in eastern Nicaragua

Seed production, seed dispersal and recruitment are critical processes in population dynamics, because they are almost never completely successful. We recorded the recruitment dynamics for the population of Dipteryx oleifera in a tropical rainforest in eastern Nicaragua (12°05’ N., 83°55’ W.) from March 2002 to August 2006. Seeds and seedlings had highly clumped distributions, while sapling distributions appeared to be random. Seedling survival increased away from the nearest conspecifc adult tree, where seedling density is lower. Since relative growth rates of seedlings are not correlated with the distance to the nearest conspecific adult, seedling survival appears to be independent of seedling growth. Seedling density is inversely correlated with seedling insect herbivory damage. Seedling survival correlated negatively with the number of saplings per sub-plot (10x10m), suggesting that insect herbivore may also cue in on saplings rather than only on adult D. oleifera trees in order to locate seedlings. Seedling establishment is significantly clumped with respect to the nearest adult tree. Larger clumps of seedlings seems more ephemeral than isolated smaller clumps located away from the nearest D. oleifera tree. These results support current empirical evidence presented earlier for the Janzen-Connell hypothesis for Dipteryx oleifera at seed and seedling stages and, the Recruitment Limitation hypothesis at the sapling stage, because sapling individuals might have recruited after random light-gap formation. Rev. Biol. Trop. 57 (1-2): 321-338. Epub 2009 June 30.

Resumen Estudiamos la dinámica de regeneración de la población de Dipteryx oleifera en un bosque húmedo tropical del este de Nicaragua. Semillas y plántulas se encuentran altamente agregadas, pero la distribución de vástagos podría ser al azar. La supervivencia de plántulas aumenta con la distancia al congéner más cercano, donde la densidad de plántulas es más baja. Como las tasas de crecimiento relativo de plántulas no se correlacionan con la distancia al congénere más cercano, la supervivencia de las plantas pareciera no estar determinada por el crecimiento de plántulas. La densidad de plántulas está inversamente correlacionada con los niveles de daño de herbivoría insectívora. La supervivencia de plántulas se correlaciona negativamente con el número de vástagos, lo cuál sugiere que los insectos herbívoros podrían estar localizando vástagos y árboles y de allí, las plántulas. La supervivencia de plántulas presentó una distribución significativamente agrupada lejos de los congéneres más cercanos. Los parches de plántulas más grandes fueron más efímeros que los parches pequeños y aislados ubicados lejos de los árboles adultos. Los resultados coinciden con la hipótesis de Janzen-Connell sobre estados de semilla y plántula. Además, ofrecen evidencia en favor de la hipótesis de Limitación de Reclutamiento para los vástagos, ya que los vástagos podrían estar estableciéndose en claros de luz pequeños que se forman al azar.

Relative importance of photosynthetic physiology and biomass allocation for tree seedling growth across a broad light gradient.

Studies of tree seedling physiology and growth under field conditions provide information on the mechanisms underlying inter- and intraspecific differences in growth and survival at a critical period during forest regeneration. I compared photosynthetic physiology, growth and biomass allocation in seedlings of three shade-tolerant tree species, Virola koschynii Warb., Dipteryx panamensis (Pittier) Record & Mell and Brosimum alicastrum Swartz., growing across a light gradient created by a forest-pasture edge (0.5 to 67% diffuse transmittance (%T)). Most growth and physiological traits showed nonlinear responses to light availability, with the greatest changes occurring between 0.5 and 20 %T. Specific leaf area (SLA) and nitrogen per unit leaf mass (N mass) decreased, maximum assimilation per unit leaf area (A area) and area-based leaf N concentration (N area) increased, and maximum assimilation per unit leaf mass (A mass) did not change with increasing irradiance. Plastic responses in SLA were important determinants of leaf N and A area across the gradient. Species differed in magnitude and plasticity of growth; B. alicastrum had the lowest relative growth rates (RGR) and low plasticity. Its final biomass varied only 10-fold across the light gradient. In contrast, the final biomass of D. panamensis and V. koschynii varied by 100- and 50-fold, respectively, and both had higher RGR than B. alicastrum. As light availability increased, all species decreased biomass allocation to leaf tissue (mass and area) and showed a trade-off between allocation to leaf area at a given plant mass (LAR) and net gain in mass per unit leaf area (net assimilation rate, NAR). This trade-off largely reflected declines in SLA with increasing light. Finally, A area was correlated with NAR and both were major determinants of intraspecific variation in RGR. These data indicate the importance of plasticity in photosynthetic physiology and allocation for variation in tree seedling growth among habitats that vary in light availability.