Performance of tropical forest seedlings under shade and drought: an interspecific trade-off in demographic responses

Tree demographic strategies largely overlap across succession in Neotropical wet and dry forest communities

Secondary tropical forests play an increasingly important role in carbon budgets and biodiversity conservation. Understanding successional trajectories is therefore imperative for guiding forest restoration and climate change mitigation efforts. Forest succession is driven by the demographic strategies-combinations of growth, mortality and recruitment rates-of the tree species in the community. However, our understanding of demographic diversity in tropical tree species stems almost exclusively from old-growth forests. Here, we assembled demographic information from repeated forest inventories along chronosequences in two wet (Costa Rica, Panama) and two dry (Mexico) Neotropical forests to assess whether the ranges of demographic strategies present in a community shift across succession. We calculated demographic rates for >500 tree species while controlling for canopy status to compare demographic diversity (i.e., the ranges of demographic strategies) in early successional (0-30 years), late successional (30-120 years) and old-growth forests using two-dimensional hypervolumes of pairs of demographic rates. Ranges of demographic strategies largely overlapped across successional stages, and early successional stages already covered the full spectrum of demographic strategies found in old-growth forests. An exception was a group of species characterized by exceptionally high mortality rates that was confined to early successional stages in the two wet forests. The range of demographic strategies did not expand with succession. Our results suggest that studies of long-term forest monitoring plots in old-growth forests, from which most of our current understanding of demographic strategies of tropical tree species is derived, are surprisingly representative of demographic diversity in general, but do not replace the need for further studies in secondary forests.

Litter Deposition and Nutrient Cycling of Invaded Environments by Cryptostegia madagascariensis at Tropical Cambisols from Northeastern Brazil

Cryptostegia madagascariensis is an invasive plant species that covers 11% of the Brazilian northeastern territory, but its role on the litter trait in tropical ecosystems remains unclear. Here, we analyzed and compared the litter deposition, litter nutrient content, soil organic matter, and the litter decay rate from invaded and non-invaded environments by C. madagascariensis at a tropical Cambisol. The PCA analysis revealed that litter deposition, litter quality, and soil organic matter were correlated with the invaded environment. We grew plant species in greenhouse conditions to obtain a standard litter material to use in our litter bags in field conditions. We found that litter decay rate was higher in the invaded environment than in the non-invaded one. Our results suggest that C. madagascariensis changes litter traits in tropical ecosystems that in turn create negative plant–soil feedback to the native species by creating a physical barrier on soil surface and to promote its own rhizosphere.

Shade moderates the drought stress on saplings of Beneh (Pistacia atlantica Desf. subsp. mutica) in semiarid areas of Iran

Pistacia atlantica Desf. (Beneh) is an important woody species that has been facing significant challenges to its natural regeneration and reforestation in Iran. This study investigates the interaction of soil moisture and shade on growth, chemical contents, and morphological and physiological characteristics of Beneh saplings. One-year-old Beneh saplings were treated with varying amounts of soil moisture (20, 50, and 100% of field capacity) and shade (0, 30, and 50% of full sunlight) in a split-plot experiment of a randomized complete block design in semiarid conditions of the Alborz Research Station of the Research Institute of Forests and Rangelands (RIFR) in Iran. The results indicate that soil moisture significantly affects the water content of the leaf, total chlorophyll, proline content, activity of catalase enzyme, leaf dry biomass, leaflet area, and dry stem biomass in the leaf. Shade significantly affected total chlorophyll, catalase enzyme activity, specific leaflet area, relative water content of the leaf, proline content, dry root biomass, and leaflet area. The interaction of shade and soil moisture significantly affected seedling height, catalase enzyme activity, specific leaflet area, and nitrogen and potassium content of the leaf. Shade moderates the stress of drought on Beneh saplings, but shading of Beneh saplings is not recommended in conditions where there is no concern about soil moisture. These conclusions can be used to improve the production of Beneh saplings in nurseries.

Exploring trait-performance relationships of tree seedlings along experimentally manipulated light and water gradients

A foundational assumption of trait-based ecology is that individual performances should be predicted by its functional traits. However, the trait-performance relationships reported in literature were typically weak, probably due to the ignorance of interactions between traits and environments, intraspecific trait variability and hard traits (directly linked to physiological processes of interest). We conducted an experiment of planting 900 seedlings of six tree species separately (one seedling per pot) along experimentally manipulated light and water gradients, monitored their survival and growth, and measured their morphological, photosynthetic and hydraulic traits. Most trait-performance relationships depended on the environments, either marginally changing (weak trait × environment interaction) or even reversing (strong trait × environment interaction) along light or water gradients in our experiment. Such trait × environment interactions were more likely to be detected in growth models using individual-level traits than models using species mean traits, but seedling growth was not better modelled with individual-level traits than species mean traits. Additionally, none of the hard traits (photosynthetic and hydraulic traits) were better predictors than soft traits (morphological traits) modeling seedling growth and survival along light and water gradients. Our study highlights the necessities of considering trait × environment interactions when predicting response of plants to changing environments. The benefits of using individual-level traits or hard traits to predict plant performance might be reduced or even cancelled if their measurement errors are not well controlled.

Effects of Shade and Planting Methods on the Growth of Heracleum moellendorffii and Adenophora divaricata in Different Soil Moisture and Nutrient Conditions

In this study, the interacting effects of shade and planting methods on the growth and competitive ability of two understory plants Heracleum moellendorffii Hance and Adenophora divaricata Franch. & Sav. were investigated under different soil moisture and nutrient conditions. One-year-old seedlings were subjected to different light levels (0%, 35%, and 55% shade) and planting methods (monoculture and mixed) under contrasting soil moisture (1.2 L/m² and 2.3 L/m² of water) and soil nutrient conditions (unfertilized and fertilized). Here, shading significantly improved the height growth of H. moellendorffii (10–20 cm increase) in unfertilized and fertilized plots and at high soil moisture conditions. Contrarily, A. divaricata seedlings planted singly at full sunlight yielded a higher aboveground biomass growth (8–17 g plant⁻¹), compared with those shaded and intercropped seedlings (0.9–3.9 g plant⁻¹). The increased competitiveness of H. moellendorffii suppressed the growth of A. divaricata across different light conditions when planted together. The amount of light, soil moisture, and nutrients and their interactions significantly affected the growth of the seedlings, resulting in asymmetric interspecific competition between the two species. Results provide us with a better understanding of the environmental factors affecting plant growth for forest farming in the understory.

The triangular space of abiotic stress tolerance in woody species: a unified trade-off model

Tolerance of abiotic stress in woody plants is known to be constrained by biological trade-offs between different forms of stress, especially shade and drought. However, there is still considerable uncertainty on the relationship between tolerances and the limits on tolerance combinations. Using the most extensive database available on shade, drought, waterlogging and cold tolerance for 799 northern hemisphere woody species, we determined the number of dimensions needed to summarise their tolerance combinations, and the best trade-off model among those currently available, for description of the interdependence between tolerances. Two principal component analysis (PCA) dimensions summarised stress tolerance combinations. They defined a triangular stress tolerance space (STS). The first STS dimension reflected segregation between drought-tolerant and waterlogging-tolerant species. The second reflected shade tolerance, which is independent of the other tolerances. Cold tolerance scaled weakly with both dimensions. Tolerance combinations across the species in the database were limited by boundary-line trade-offs. The STS reconciles all major theories about trade-offs between abiotic stress tolerances, providing a unified trade-off model and a set of coordinates that can be used to examine how other aspects of plant biology, such as plant functional traits, change within the limits of abiotic stress tolerance.