Growth Of Tropical Tree Seedlings Research Articles

Negative density dependence in the mortality and growth of tropical tree seedlings is strong, and primarily caused by fungal pathogens

Abstract Natural enemies have been implicated as agents of negative density dependence (NDD) in tropical forests, but their relative contributions to NDD, and thus to the maintenance of diversity, are largely unknown. We monitored the rates of survival and relative growth rates on seedlings for 10 years in tropical moist forest in Manu National Park, Peru. We then experimentally manipulated the plots to exclude fungal pathogens, insects, small mammals and large mammals for an additional 31 months to assess the influence of these natural enemies on density‐dependent interactions among tropical seedlings. Fungal pathogens made the most important contribution to NDD. The application of fungicide led to lower mortality rates, faster growth rates and decreased species diversity. Other taxa of natural enemies had at most minor effects on seedling performance. Synthesis. We conclude that fungal pathogens are the strongest contributors to the widely observed NDD that occurs among seedlings. Moreover, the presence of fungal pathogens augments the species diversity of seedlings, indicating their critical contribution to the maintenance of species coexistence and the structure of tropical tree communities.

Influence of Mean Leaf Angles and Irrigation Volumes on Water Capture, Leaching, and Growth of Tropical Tree Seedlings

Research Highlights: The mean leaf angle and crown projection area can be used as criteria for grouping tree seedling species in different irrigation zones in tree nurseries with overhead microsprinkler systems, preventing water and fertilizer waste, and increasing growth. Background and Objectives: There are important gaps in current functional knowledge about how plant architecture, especially the mean leaf angles of tree seedlings, affect water and nutrient solution capture in overhead microsprinkler systems. These gaps contribute to water and fertilizer waste in tree nurseries. This research aimed to ascertain how mean leaf angles affect irrigation water capture, leaching, and the growth of tree seedlings given different volumes of irrigation. Materials and Methods: Nine species of tree seedlings with different mean leaf angles were submitted to four irrigation volumes (8, 10, 12, and 14 mm) applied daily by overhead microsprinklers in a split-plot design completely randomized. The variables leaching fraction, height, stem diameter, shoot, root, and total dry mass, Dickson quality index, crown projection area, root system quality, and leachate electrical conductivity were evaluated. Results: For species with mean leaf angles of −54, 31, 38, 42, 55, 57, and 58°, the 8 mm irrigation volume was sufficient to produce greater growth and less leaching. For species with angles of −56 and −14°, the 14 mm irrigation volume was required to produce greater growth. Conclusions: The tree seedling species with positive mean leaf angles facilitate irrigation water and nutrient solution capture, allowing the application of lower irrigation volume. On the other hand, some tree seedling species with negative mean leaf angles hinder irrigation water and nutrient solution capture, requiring the application of higher irrigation volume. When the tree seedling species have a negative mean leaf angle, but the crown projection area is small, the difficulty of water and nutrient solution reaches directly the substrate is attenuated.

Are photosynthetic leaf traits related to the first-year growth of tropical tree seedlings? A light-induced plasticity test in a secondary forest enrichment planting

Understanding light-induced plasticity of growth and its relationship with photosynthetic trait plasticity is central to the identification of the mechanisms associated with tropical tree seedling establishment in enrichment plantings. In a secondary forest submitted to silvicultural treatments – that manipulate light environment – and enriched with six tropical tree species we tested three main hypothesis. First, the growth of the six species are related to photosynthetic leaf traits; second, there is a relationship between the light-induced plasticity of growth and photosynthetic leaf traits and, finally, that growth plasticity is a subjacent mechanism of the growth-survival trade-off. Growth was analyzed as a biomass index that combines collar and height growth. Photosynthetic leaf traits were selected considering their significance in the photosynthetic process: light interception (specific leaf area, SLA), light absorption (chlorophyll a and b ratio, Chla/b), light use (fluorescence parameters, Fv/Fm; PIabs, PItotal), biochemical limitations (light-saturated CO2 assimilation, Asat) and diffusive limitations (stomatal conductance, gs). The plasticity was represented by the slope of the relationship between the canopy light transmittance and growth and the photosynthetic leaf traits. At the end of the first year after planting, growth was related to almost all photosynthetic leaf traits, and the strongest relationships (high slope and R2) were observed with Asat, SLA and PItotal. Light-induced growth plasticity varied three times between species. The interspecific growth plasticity was related to SLA and Fv/Fm plasticity, which varied 4.9 and 2.4 times between species, respectively. Seedling mortality in the shaded understory and the growth in a high light environment were associated with growth plasticity. Photosynthetic leaf traits effectively represent the growth response mechanisms to silvicultural treatments that manipulate the light environment in enrichment plantings, and the subjacent mechanism of growth plasticity has an intrinsic effect on the growth-survival trade-off.

Growth of tree seedlings in a tropical dry forest in relation to soil moisture and leaf traits

Aims The growth of plant species in tropical dry forest (TDF) is expected to be largely governed by the availability of soil moisture. In this study we attempt to identify mechanisms by which seedlings of dry tropical trees cope with water stress by adjusting their leaf characteristics to water availability and micro environments, and address following questions: How are leaf traits and relative growth rate (RGR) of the dominant seedling species of TDF affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional traits with each other? Can leaf traits singly or in combination predict the growth rate of seedling species of TDF? The study was conducted in situ on four sites (viz., Hathinala, Gaighat, Harnakachar and Ranitali, listed in order of decreasing SMC) within the tropical dry deciduous forest in northern India. Methods Five leaf traits viz., specific leaf area (SLA), leaf dry matter content (LDMC), concentrations of leaf nitrogen (leaf N), phosphorus (leaf P) and chlorophyll (Chl) and two physiological processes, viz., stomatal conductance (Gsnet) and photosynthetic rate (Anet), and RGR, of four dominant tree seedling species of a TDF (viz., Buchanania lanzan, Diospyros melanoxylon, Shorea robusta and Terminalia tomentosa) on four sites were analysed for species, site and season effects over a 2-year period. Step-wise multiple regression was performed to predict RGR from mean values of SMC, leaf traits and physiological processes. Principal component analysis (PCA) was performed to observe the extent of intra- vs. inter-specific variability in the leaf traits and physiological rates. Important Findings All the traits and physiological rates were interrelated and showed significant positive relationship with RGR except for the correlation of LDMC with RGR which was not significant. Further, relationships of SMC with all leaf traits, physiological rates and RGR were significant, except for that between SMC and SLA for B. lanzan and D. melanoxylon. The slope of seedling trait:SMC relationship, a measure of phenotypic plasticity in response to soil moisture gradient, varied among species. Among the four species, T. tomentosa was the most plastic and S. robusta the least. In conclusion, leaf traits and physiological processes were strongly related to soil water availability on the one hand and seedling growth on the other. Gsnet is the most important variable which accounted for the greatest amount of variability (62%) in RGR, emphasizing the role of stomatal conductance in shaping growth patterns across spatial and temporal gradients of soil water availability. Gsnet and SMC together explained 64% variability in RGR, indicating that other traits/factors, not studied by us are also impor tant in modulating the growth of tropical tree seedlings.

Differential growth responses in seedlings of ten species of Dipterocarpaceae to experimental shading and defoliation

Abstract:The responses of plants to shade and foliar herbivory jointly affect growth rates and community assembly. We grew 600 seedlings of ten species of the economically important Dipterocarpaceae in experimental gradients of shading (0.3–47.0% of full sunlight) and defoliation (0, 25%, 50% or 75% of leaf area removed). We assessed stem diameters initially, after 2 and 4 mo, and calculated relative growth rates (RGR) with a linear model. Shading interacted with defoliation, reducing RGR by 21.6% in shaded conditions and 8.9% in well-lit conditions. We tested three hypotheses for interspecific trade-offs in growth responses to shading and defoliation. They could be positively related, because both reduce a plant's access to carbon, or inversely related because of trade-offs between herbivore resistance and tolerance. We observed, however, that species varied in their response to shading, but not defoliation, precluding an interspecific trade-off and suggesting that plants tolerate shade and herbivory with differing strategies. Shading most strongly reduced the growth of species with less-dense wood and larger seeds. The strong and variable growth responses to shade, contrasted with the weak and uniform responses to defoliation, suggest that variation in light availability more strongly affects the growth of tropical tree seedlings, and thus community assembly, than does variation in herbivory.

Conspecific plant–soil feedbacks reduce survivorship and growth of tropical tree seedlings

Summary 1. The Janzen–Connell (J–C) Model proposes that host‐specific enemies maintain high tree species diversity by reducing seedling performance near conspecific adults and promoting replacement by heterospecific seedlings. Support for this model often comes from decreased performance for a species at near versus far distances from conspecific adults. However, the relative success of conspecific versus heterospecific seedlings recruiting under a given tree species is a critical, but untested, component of the J–C Model. 2. In a shade‐house experiment, we tested plant–soil feedbacks as a J–C mechanism in six tropical tree species. We assessed effects of conspecific versus heterospecific cultured soil extracts on seedling performance for each species, and we compared performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. Additionally, we tested whether soil microbes were creating these plant–soil feedbacks and whether low light increased species vulnerability to pathogens. 3. Among 30 potential comparisons of survival and mass for seedlings grown in conspecific versus heterospecific soil extracts, survival decreased in seven and increased in two, whereas mass decreased in 13 and increased in 1. To integrate survival and growth, we also examined seedling performance [(mean total mass × mean survival time)/(days of experiment)], which was lower in 16 and higher in 2 of 30 comparisons between seedlings grown with soil extract cultured by conspecific versus heterospecific individuals. Based on performance within a soil extract, conspecific seedlings were disadvantaged in 15 and favoured in 7 of 30 cases relative to heterospecific seedlings. 4. Species pairwise interactions of soil modification and seedling performance occurred regardless of sterilization, suggesting chemical mediation. Microbes lacked host‐specificity and reduced performance regardless of extract source and irradiance. 5. Synthesis. These results, along with parallel research in temperate forests, suggest that plant–soil feedbacks are an important component of seedling dynamics in both ecosystems. However, negative conspecific feedbacks were more prevalent in tropical than temperate species. Thus, negative plant–soil feedbacks appear to facilitate species coexistence via negative distance‐dependent processes in tropical but not temperate forests, but the feedbacks were mediated through chemical effects rather than through natural enemies as expected under the J–C Model.

Application of mycorrhizal roots improves growth of tropical tree seedlings in the nursery: a step towards reforestation with native species in the Andes of Ecuador

Most tree species in tropical mountain rain forests are naturally associated with arbuscular mycorrhizal fungi. Previous studies in southern Ecuador of 115 tree species revealed that only three species were not associated with arbuscular mycorrhizal fungi. Seedlings of tropical tree species raised in the nursery may need to be associated with arbuscular mycorrhizal fungi to survive transplantation shock in higher numbers. Methods for establishing plantations with native tree species are not yet established for Ecuador. Thus, we investigated the feasibility of using mycorrhizal roots of seedlings of Inga acreana, Tabebuia chrysantha, Cedrela montana and Heliocarpus americanus that had trapped mycorrhizal fungi from forest humus in the nursery to inoculate C. montana and H. americanus with native arbuscular mycorrhizal fungi. Inoculation with either a mixture of mycorrhizal roots from the four species or only with mycorrhizal roots from the same tree species were compared with effects of moderate fertilization. Assessment of plant growth and mycorrhizal status of 6-months-old Cedrela montana and Heliocarpus americanus revealed an improvement in growth and diverse associated fungi through mycorrhizal root inoculation in comparison with moderate fertilization. Moderate fertilization did not suppress mycorrhization.

Mycorrhizal Fungi Increased Early Growth of Tropical Tree Seedlings in Adverse Soil

The rate of reforestation has increased throughout the countries in Southeast Asia region during the last 20 years. At the same time, inconvenient situations such as forest destruction, forest exploitation, illegal logging, clear-cut forest areas, old agricultural lands, post-wildfire areas, conversion of natural forests into plantations, resettlement areas, mine lands, and amended adverse soils have also been increasing significantly. Mycorrhizas, hovewer, play important role to increase plant growth, enrich nutrient content and enhance survival rates of forest tree species in temperate and sub-tropical regions. Unfortunately, a little information so far is available regarding the effect of mycorrhizas on growth of tree species growing in tropical forests. In relevant, several experiments were carried out to determine whether ectomycorrhizal (ECM) fungi and arbuscular mycorrhizal (AM) fungi can enhance mycorrhizal colonization, nutrient content, and plant growth of some tropical rain forest tree species in Indonesia under nursery and field conditions. The families of tropical tree species used in the experiment were Thymelaeaceae ( Aquilari a crassna ), Leguminosae ( Sesbania grandifolia) , Guttiferae ( Ploiariu m alternifolium and Calophyllum hosei ), Apocynaceae ( Dyera polyphylla and Alstonia scholaris ), and Dipterocarpaceae ( Shore a belangeran ). These families are important as they provide timber and non-timber forest products (NTFPs). This paper discusses the role of mycorrhizal fungi in increasing early growth of tropical tree seedlings in adverse soil.

Photoprotection, photosynthesis and growth of tropical tree seedlings under near-ambient and strongly reduced solar ultraviolet-B radiation

Seedlings of two late-successional tropical rainforest tree species, Tetragastris panamensis (Engler) O. Kuntze and Calophyllum longifolium (Willd.), were field grown for 3-4 months at an open site near Panama City (9 degrees N), Panama, under plastic films that either transmitted or excluded most solar UV-B radiation. Experiments were designed to test whether leaves developing under bright sunlight with strongly reduced UV-B are capable of acclimating to near-ambient UV-B conditions. Leaves of T. panamensis that developed under near-ambient UV-B contained higher amounts of UV-absorbing substances than leaves of seedlings grown under reduced UV-B. Photosynthetic pigment composition, content of alpha-tocopherol, CO(2) assimilation, potential photosystem II (PSII) efficiency (evaluated by F(v)/F(m) ratios) and growth of T. panamensis and C. longifolium did not differ between seedlings developed under near-ambient and reduced solar UV-B. When seedlings were transferred from the reduced UV-B treatment to the near-ambient UV-B treatment, a pronounced inhibition of photosynthetic capacity was observed initially in both species. UV-B-mediated inhibition of photosynthetic capacity nearly fully recovered within 1 week of the transfer in C. longifolium, whereas in T. panamensis an about 35% reduced capacity of CO(2) uptake was maintained. A marked increase in UV-absorbing substances was observed in foliage of transferred T. panamensis seedlings. Both species exhibited enhanced mid-day photoinhibition of PSII immediately after being transferred from the reduced UV-B to the near-ambient UV-B treatment. This effect was fully reversible within 1d in T. panamensis and within a few days in C. longifolium. The data show that leaves of these tropical tree seedlings, when developing in full-spectrum sunlight, are effectively protected against high solar UV-B radiation. In contrast, leaves developing under conditions of low UV-B lacked sufficient UV protection. They experienced a decline in photosynthetic competence when suddenly exposed to near-ambient UV-B levels, but exhibited pronounced acclimative responses.

Light gradient partitioning by tropical tree seedlings in the absence of canopy gaps.

To explore the importance of light availability for seedling growth in low light environments, we examined light-dependent growth, biomass allocation and mortality of tree seedlings growing in sites with 0.2-6.5% full sun, the range of light commonly encountered in the understory of closed canopy, lowland tropical forests. We transplanted seedlings of the canopy tree species, Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum into second-growth forest and native tree plantations at La Selva Biological Station, Costa Rica. We assessed seedling survival, growth, and seedling light environments bimonthly for 14months. Plants were harvested at the end of the study to assess leaf area, total biomass, biomass partitioning and root growth. Survivorship of all species exceeded 60% across all microsites, although both D. panamensis and B. alicastrum had lower probabilities of survival in the darkest microsites. All species showed a strong positive relationship between light availability and growth, increasing in total biomass as light increased. However, the strength of the growth response differed among species causing a change in the rank order of species growth rates as light availability increased. Although D. panamensis showed the lowest growth rates in the darkest microsites, a strong response to increasing light led to a cross-over in performance, such that D. panamensis had the highest growth rate at the highest light levels studied. These data suggest that resource gradient partitioning could occur even in low light environments (0.2-6.5%). Given the limited range of light regimes sampled (i.e., non-gap microsites), our data demonstrate that growth of tropical tree seedlings beneath closed canopies is highly sensitive to light availability and that shade-tolerant species vary in these responses. Our results show that understory light heterogeneity, in the absence of canopy gaps, can significantly affect recruitment processes for shade-tolerant tree species.

Effects of nitrogen source and aluminum on growth of tropical tree seedlings adapted to low pH soils

The effects of N-source and Al on the growth of seedlings of Melastoma malabathricum, Acacia mangium, and Melaleuca cajuputi, which are tropical woody plants and are very tolerant to Al, and barley (Hordeum vulgare), which is a typical Al-sensitive plant, were investigated. The Al and N treatments consisted of the application of either 0 or 0.5 mM Al, and 2 mM NH4 + or N03 -, respectively. Growth of the tropical plants was enhanced by Al and NH4 application. In all the plant species, the pH of the culture solution decreased and the concentrations of soluble Al and P increased with the + NH4 treatment, which positively affected the growth of the tropical plant species. Excised roots of M. malabathricum dissolved insoluble Al with NH4 application and absorbed Al mainly from root tips. Al did not affect the leaf N concentration except in the case of barley. Roots of M. cajuputi exuded a large amount of citrate, which slightly increased by the + Al treatment. In A. mangium, the reactivity of soluble Al to PCV (pyrocatecholviolet) decreased in the culture solution of the + Al + NH4. treatment and Al concentration of roots in this treatment was very low. Roots of M. malabathricum released H+ along with Al uptake as well as NH4 + uptake. It is concluded that Al and NH4 + exert beneficial effects on the growth of tropical tree seedlings.