Milton N. Garcia

Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 x 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.

On the nature of facultative and constitutive CAM: environmental and developmental control of CAM expression during early growth of Clusia, Kalanchoë, and Opuntia

The capacity to induce crassulacean acid metabolism developmentally (constitutive CAM) and to up-regulate CAM expression in response to drought stress (facultative CAM) was studied in whole shoots of seven species by measuring net CO(2) gas exchange for up to 120 day-night cycles during early growth. In Clusia rosea, CAM was largely induced developmentally. Well-watered seedlings began their life cycle as C(3) plants and developed net dark CO(2) fixation indicative of CAM after the initiation of the fourth leaf pair following the cotyledons. Thereafter, CAM activity increased progressively and drought stress led to only small additional, reversible increases in dark CO(2) fixation. In contrast, CAM expression was overwhelmingly under environmental control in seedlings and mature plants of Clusia pratensis. C(3)-type CO(2) exchange was maintained under well-watered conditions, but upon drought stress, CO(2) exchange shifted, in a fully reversible manner, to a CAM-type pattern. Clusia minor showed CO(2) exchange reponses intermediate to those of C. rosea and C. pratensis. Clusia cretosa operated in the C(3) mode at all times. Notably, reversible stress-induced increases of dark CO(2) fixation were also observed during the developmental progression to pronounced CAM in young Kalanchoë daigremontiana and Kalanchoë pinnata, two species considered constitutive CAM species. Drought-induced up-regulation of CAM was even detected in young cladodes of a cactus, Opuntia ficus-indica, an archetypal constitutive CAM species. Evidently, the defining characteristics of constitutive and facultative CAM are shared, to variable degrees, by all CAM species.

Spatial heterogeneity of soil chemical properties in a lowland tropical moist forest, Panama

We evaluated spatial heterogeneity for pH and a comprehensive set of nutrient and trace elements in surface (0–0.1 m depth) and subsurface (0.3–0.4 m depth) soils across 26.6 ha of old-growth, lowland, tropical moist forest, established on a highly weathered soil in Panama. Little is known about spatial heterogeneity patterns of soil properties in tropical forest soils. Soil was moderately acidic (pH 5.28) with low concentrations of exchangeable base cations (13.4 cmolc/kg), Bray-extractable PO4 (2.2 mg/kg), KCl-extractable NO3 (5.0 mg/kg), and KCl-extractable NH4 (15.5 mg/kg). The coefficient of variation for soil properties ranged from 24% to >200%, with a median value of 84%. Geostatistical analysis revealed spatial dependence at a scale of 10–100 m for most of the soil properties; however, pH, NH4, Al, and B had spatial dependence at a scale up to 350 m. Best-fit models to individual variograms included random, exponential, spherical, Gaussian, linear, and power functions, indicating many different spatial patterns among the set of soil properties. Correlation among individual elements was poor, indicating independent patterns. Our results show complex spatial patterns in soil chemical properties and provide a basis for future investigations on soil–plant relationships and soil nutrient niche differentiation.

High-temperature tolerance of a tropical tree, Ficus insipida: methodological reassessment and climate change considerations.

In view of anthropogenic global warming, heat tolerance of a neotropical pioneer tree, Ficus insipida Willd., was determined. Sections of sun leaves from a mature tree and from seedlings cultivated at ambient and elevated temperatures were heated to 42–53°C. Leaves from a late-successional tree species, Virola sebifera Aubl., were also studied. Widely used chlorophyll a fluorescence methods based on heat-induced rise of initial fluorescence emission, Fo, and decrease in the ratio of variable to maximum fluorescence, Fv/Fm, were reassessed. Fv/Fm determined 24 h after heat treatment was the fluorescence parameter most suitable to assess the lethal temperature causing permanent tissue damage. Thermo-tolerance was underestimated when Fo and Fv/Fm were recorded immediately after the heat treatment. The limit of thermo-tolerance was between 50 and 53°C, only a few °C above peak leaf temperatures measured in situ. The absence of seasonal changes in thermo-tolerance and only marginal increases in thermo-tolerance of plants grown under elevated temperatures suggest little capacity for further heat acclimation. Heat-stress experiments with intact potted seedlings also revealed irreversible leaf damage at 51–53°C, but plants survived and developed new leaves during post-culture.

Pervasive effects of a dominant foliar endophytic fungus on host genetic and phenotypic expression in a tropical tree

It is increasingly recognized that macro-organisms (corals, insects, plants, vertebrates) consist of both host tissues and multiple microbial symbionts that play essential roles in their hosts ecological and evolutionary success. Consequently, identifying benefits and costs of symbioses, as well as mechanisms underlying them are research priorities. All plants surveyed under natural conditions harbor foliar endophytic fungi (FEF) in their leaf tissues, often at high densities. Despite producing no visible effects on their hosts, experiments have nonetheless shown that FEF reduce pathogen and herbivore damage. Here, combining results from three genomic, and two physiological experiments, we demonstrate pervasive genetic and phenotypic effects of the apparently asymptomatic endophytes on their hosts. Specifically, inoculation of endophyte-free (E−) Theobroma cacao leaves with Colletotrichum tropicale (E+), the dominant FEF species in healthy T. cacao, induces consistent changes in the expression of hundreds of host genes, including many with known defensive functions. Further, E+ plants exhibited increased lignin and cellulose content, reduced maximum rates of photosynthesis (Amax), and enrichment of nitrogen-15 and carbon-13 isotopes. These phenotypic changes observed in E+ plants correspond to changes in expression of specific functional genes in related pathways. Moreover, a cacao gene (Tc00g04254) highly up-regulated by C. tropicale also confers resistance to pathogen damage in the absence of endophytes or their products in host tissues. Thus, the benefits of increased pathogen resistance in E+ plants are derived in part from up-regulation of intrinsic host defense responses, and appear to be offset by potential costs including reduced photosynthesis, altered host nitrogen metabolism, and endophyte heterotrophy of host tissues. Similar effects are likely in most plant-endophyte interactions, and should be recognized in the design and interpretation of genetic and phenotypic studies of plants.

Responses of Legume Versus Nonlegume Tropical Tree Seedlings to Elevated CO2 Concentration

We investigated responses of growth, leaf gas exchange, carbon-isotope discrimination, and whole-plant water-use efficiency (WP) to elevated CO2 concentration ([CO2]) in seedlings of five leguminous and five nonleguminous tropical tree species. Plants were grown at CO2 partial pressures of 40 and 70 Pa. As a group, legumes did not differ from nonlegumes in growth response to elevated [CO2]. The mean ratio of final plant dry mass at elevated to ambient [CO2] (ME/MA) was 1.32 and 1.24 for legumes and nonlegumes, respectively. However, there was large variation in ME/MA among legume species (0.92–2.35), whereas nonlegumes varied much less (1.21–1.29). Variation among legume species in ME/MA was closely correlated with their capacity for nodule formation, as expressed by nodule mass ratio, the dry mass of nodules for a given plant dry mass. WP increased markedly in response to elevated [CO2] in all species. The ratio of intercellular to ambient CO2 partial pressures during photosynthesis remained approximately constant at ambient and elevated [CO2], as did carbon isotope discrimination, suggesting that WP should increase proportionally for a given increase in atmospheric [CO2]. These results suggest that tree legumes with a strong capacity for nodule formation could have a competitive advantage in tropical forests as atmospheric [CO2] rises and that the water-use efficiency of tropical tree species will increase under elevated [CO2].

Responses of model communities of two tropical tree species to elevated atmospheric CO2 : growth on unfertilized soil

Summary Biomass accumulation of model seedling communities of two tropical tree species was studied at ambient and elevated CO 2 levels (four replicates per CO 2 concentration), using open-top chambers situated in a cleared area at the edge of a tropical forest near Panama City, Republic of Panama. Each chamber (diameter about 2 m) contained a mixture of 18 plants of Ficus insipida , a fast-growing pioneer species, and 18 plants of Virola surinamensis , a slow-growing late-successional species. Plants grew in well-drained, well-watered, loose, unfertilized soil. During the 30 week experimental period, biomass accumulation by F. insipida , which increased in height from about 20 to 180 cm, predominated, whereas V. surinamensis grew only slowly in the strongly shaded understorey. Neither community biomass accumulation (above- plus belowground) nor the biomass ratio ( F. insipida: V. surinamensis ) of the two species was significantly affected by elevated CO 2 , although community biomass accumulation was on average slightly higher at elevated than at ambient CO 2 , and the biomass of F. insipida increased on average relative to V. surinamensis at elevated CO 2 . Plants grown at elevated CO 2 contained greater levels of starch and lower levels of nitrogen per unit leaf dry mass, and, therefore, showed higher ratios of C: N than plants at ambient CO 2 . Specific leaf area, the area per unit leaf dry mass; and the leaf area ratio (LAR), the total leaf area per total plant dry mass, decreased in response to elevated CO 2 . This decrease in LAR counteracted the potential effect on biomass gain of increases in both net assimilation rate (the rate of dry mass accumulation per unit leaf area) and rates of photosynthetic leaf net CO 2 uptake at elevated as compared to ambient CO 2 , particularly in Ficus insipida .

Marked growth response of communities of two tropical tree species to elevated CO2 when soil nutrient limitation is removed

Summary As part of an ongoing project to understand the effects of elevated atmospheric CO 2 on plants in complex, tropical communities, we studied biomass accumulation in a simplified model seedling community consisting of two species of tropical trees ( Ficus insipida , a fast growing pioneer species, and Virola surinamensis , a slow-growing, shade-tolerant late successional species). The plants were grown at ambient and elevated (about two times ambient) CO 2 concentrations using open-top chambers at a field site in Panama. Communities grew in heavily fertilized soil. Compared to a previous experiment with model communities of F. insipia and V. surinamensis grown on unfertilized soil (Winter et al., Flora [2000] 195, 289) application of soil fertilizer markedly accelerated community growth rates at ambient CO 2 , and biomass accumulation was enhanced by an additional 52% at elevated CO 2 . In contrast, elevated CO 2 had no significant effect on biomass accumulation in unfertilized communities. Communities growing on fertilized soil showed greater biomass allocation into leaves, i.e. higher leaf weight ratios (LWRs) than did communities on unfertilized soil. Higher LWRs were related to lower root: shoot ratios and together with greater specific leaf areas (area per unit leaf mass), largely a consequence of lower leaf starch contents, resulted in higher leaf area ratios (LARs). While elevated CO 2 caused the relatively low LARs in unfertilized communities to decrease further, by strongly increasing leaf starch levels and decreasing specific leaf areas, these leaf characteristics changed only slightly in fertilized communities exposed to elevated CO 2 . Thus, by maintaining relatively high LARs at elevated CO 2 , fertilized plants were able to effectively use enhanced CO 2 concentrations for increased carbon gain and growth. Leaves of plants on fertilized soil were characterized by relatively low C: N ratios which were largely unaffected by CO 2 concentration. In contrast, C: N ratios in leaves of unfertilized plants were higher than those of fertilized plants and increased in response to elevated CO 2 .

Growth irradiance effects on photosynthesis and growth in two CO-occurring shade-tolerant neotropical perennials of contrasting photosynthetic pathways

Dieffenbachia longispatha (C3) and Aechmea magdalenae (Crassulacean acid metabolism, CAM) are syntopic, neotropical forest perennials in central Panama that are restricted to shaded habitats. This is of particular interest for A. magdalenae because, like other understory CAM bromeliad species, it appears functionally and structurally to be better suited to life in full sun. Growth irradiance (GI) effects on photosynthesis and growth in both species were explored in the context of sun/shade trade-off concepts largely derived from studies of C3 plants. Potted plants were grown outdoors in 1, 55, and 100% full sun for 5 mo under well-watered conditions. While both species grew faster in high compared to low light, maximum relative growth rates (RGR) in full sun were still extremely slow with A. magdalenae showing a RGR approximately half that of D. longispatha. Photosynthetic capacity increased with GI in D. longispatha but not in A. magdalenae. Aechmea magdalenae responded to GI with shifts in the activity of the different CAM phases. Both species were photoinhibited in full sun, but more so in A. magdalenae. Despite possessing many traits considered adaptive in high light, these results suggest that A. magdalenae is unlikely to attain sufficient growth rates to thrive in productive, high-light habitats.

WHOLE-PLANT CONSEQUENCES OF CRASSULACEAN ACID METABOLISM FOR A TROPICAL FOREST UNDERSTORY PLANT

We examined leaf and whole-plant characteristics in mature individuals of several herbaceous species growing in the understory of a tropical moist forest in central Panama. Our objective was to see if contrasts in leaf physiology among Crassulacean acid metabolism (CAM) and C3 plants were associated with differences in whole-plant structure or performance in a habitat that is considered atypical for CAM. Foliage of Aechmea magdalenae, an understory CAM bromeliad, has a higher maximum photosynthesis rate, and greater nitrogen, chlorophyll, and water contents on a leaf-area basis compared to three sympatric C3 species. Leaf characteristics of two other understory CAM bromeliads, Ananas comosus and Bromelia plumieri, were similar to that of Aechmea. Aechmea, compared to three sympatric C3 species, allocates less biomass to roots and more to foliage. The annual aboveground relative growth of Aechmea was lower than it was for the C3 species, despite Aechmea’s higher photosynthetic capacity. This is consistent ...