Mineral nutrient status of coastal hill dipterocarp forest and adinandra belukar in Singapore: bioassays of nutrient limitation

Tropical root responses to global changes: A synthesis

Tropical ecosystems face escalating global change. These shifts can disrupt tropical forests' carbon (C) balance and impact root dynamics. Since roots perform essential functions such as resource acquisition and tissue protection, root responses can inform about the strategies and vulnerabilities of ecosystems facing present and future global changes. However, root trait dynamics are poorly understood, especially in tropical ecosystems. We analyzed existing research on tropical root responses to key global change drivers: warming, drought, flooding, cyclones, nitrogen (N) deposition, elevated (e) CO2, and fires. Based on tree species- and community-level literature, we obtained 266 root trait observations from 93 studies across 24 tropical countries. We found differences in the proportion of root responsiveness to global change among different global change drivers but not among root categories. In particular, we observed that tropical root systems responded to warming and eCO2 by increasing root biomass in species-scale studies. Drought increased the root: shoot ratio with no change in root biomass, indicating a decline in aboveground biomass. Despite N deposition being the most studied global change driver, it had some of the most variable effects on root characteristics, with few predictable responses. Episodic disturbances such as cyclones, fires, and flooding consistently resulted in a change in root trait expressions, with cyclones and fires increasing root production, potentially due to shifts in plant community and nutrient inputs, while flooding changed plant regulatory metabolisms due to low oxygen conditions. The data available to date clearly show that tropical forest root characteristics and dynamics are responding to global change, although in ways that are not always predictable. This synthesis indicates the need for replicated studies across root characteristics at species and community scales under different global change factors.

Nitrogen addition promotes terrestrial plants to allocate more biomass to aboveground organs: A global meta-analysis

A significant increase in reactive nitrogen (N) added to terrestrial ecosystems through agricultural fertilization or atmospheric deposition is considered to be one of the most widespread drivers of global change. Modifying biomass allocation is one primary strategy for maximizing plant growth rate, survival, and adaptability to various biotic and abiotic stresses. However, there is much uncertainty as to whether and how plant biomass allocation strategies change in response to increased N inputs in terrestrial ecosystems. Here, we synthesized 3516 paired observations of plant biomass and their components related to N additions across terrestrial ecosystems worldwide. Our meta-analysis reveals that N addition (ranging from 1.08 to 113.81 g m-2  year-1 ) increased terrestrial plant biomass by 55.6% on average. N addition has increased plant stem mass fraction, shoot mass fraction, and leaf mass fraction by 13.8%, 12.9%, and 13.4%, respectively, but with an associated decrease in plant reproductive mass (including flower and fruit biomass) fraction by 3.4%. We further documented a reduction in plant root-shoot ratio and root mass fraction by 27% (21.8%-32.1%) and 14.7% (11.6%-17.8%), respectively, in response to N addition. Meta-regression results showed that N addition effects on plant biomass were positively correlated with mean annual temperature, soil available phosphorus, soil total potassium, specific leaf area, and leaf area per plant. Nevertheless, they were negatively correlated with soil total N, leaf carbon/N ratio, leaf carbon and N content per leaf area, as well as the amount and duration of N addition. In summary, our meta-analysis suggests that N addition may alter terrestrial plant biomass allocation strategies, leading to more biomass being allocated to aboveground organs than belowground organs and growth versus reproductive trade-offs. At the global scale, leaf functional traits may dictate how plant species change their biomass allocation pattern in response to N addition.

Nutrient-uptake and -use efficiency in seedlings of rain-forest trees in New Caledonia: monodominants vs. subordinates and episodic vs. continuous regenerators

The upper canopy of some rain forests in New Caledonia is dominated by single species. These monodominants are commonly secondary species, their dominance not persisting without disturbance. We tested whether dominance is associated with efficient uptake and use of nutrients (N, P and K), comparing between seedlings of monodominants (Nothofagus spp., Arillastrum gummiferum and Cerberiopsis candelabra) and 14 subordinates, grown in a nursery house. We also tested whether this trend applies more broadly to shade-intolerant trees that regenerate episodically (ER species) versus shade-tolerant trees that regenerate continuously (CR species). In the sun treatment, monodominants had higher photosynthetic nutrient-use efficiency and productivity for N and K, and uptake efficiency for N, P and K, than subordinates; ER species had higher photosynthetic nutrient-use efficiency for N, P and K, and uptake efficiency for N and P, than CR species. Uptake efficiency and productivity per nutrient mass were uncorrelated across species, yet Nothofagus spp., A. gummiferum and C. candelabra combined high levels of both traits for N, and Nothofagus spp. and A. gummiferum combined moderate to high levels for P, in sun-grown seedlings. This trait combination may contribute substantially to competitiveness and post-disturbance dominance on these nutrient-poor soils.

Mineral nutrient status of coastal hill dipterocarp forest and adinandra belukar in Singapore: analysis of soil, leaves and litter

The soil of remnant primary rainforest on granite in Singapore is very acidic (pH mostly 3.5–4.2 at 0–10 cm) and has unusually low total concentrations of nitrogen and phosphorus, but the mean concentrations of N and P in the living leaves and freshly fallen leaves of the more shade-tolerant species are within the ranges found for other lowland forests on infertile oxisols and ultisols. The concentration of Ca in freshly fallen leaves is very low. The soil under secondary forest (bselukar) on sandstone dominated by Adinandra dumosa (Theaceae) was degraded during use for agriculture. It has the same pH range but even lower values of total N and P. The mean concentrations of N, P, K, Ca and Mg in the living leaves are slightly higher than in leaves of the more shade-tolerant trees of primary forest, but lower than in the leaves of the species which require canopy gaps for establishment or early onward growth. The few species sampled in both primary forest and belukar show no consistent trend in foliar concentrations. The degraded soil has selected species with inherently lower foliar concentrations.

Fine litter input to terrestrial humus forms in Colombian Amazonia

A comparative litter fall study was made in five rain forest stands along a gradient of humus form development and soils in the Amazon lowlands of eastern Colombia. The total fine litter fall was highest in a plot on a well drained soil of the flood plain of the Caquetá River (1.07 kg · m^-2 · y^-1), lower in three plots on well drained upland soils (0.86, 0.69, and 0.68 kg · m^-2 · y^-1), and lowest in a plot on a poorly drained, upland podzolised soil (0.62 kg · m^-2 · y^-1). In the four upland plots, leaf litter fall patterns were highly associated, which points at climatic regulation. Litter resource quality, as represented by nutrient concentrations and area/weight ratio of the leaf litter fall, was comparatively high in the flood plain plot. In the upland plots, concentrations and fluxes of Ca, Mg, K, and P were as low as in oligotrophic central Amazonian upland forests. This questions generalisations that the western peripheral region of the Amazon basin should be less oligotrophic than central Amazonia. The upland plot on the podzolised soil showed the lowest concentrations and fluxes of N. Mean residence times of organic matter and nutrients in the L horizons hardly differed between the five plots, suggesting that edaphic properties and litter resource quality are of little importance in the first step of decomposition. Mean residence time of organic matter in all ectorganic horizons combined (estimated on the basis of litter input and necromass on the forest floor, and uncorrected for dead fine root input) varied from 1.0 y in the flood plain forest, 1.1-3.3 y in the well drained upland forests, and 10.2 y in the forest on the podzolised soil.