Summary
Mature neotropical lowland forests have relatively lower symbiotic nitrogen fixation (SNF) rates compared with secondary forests. Canopy gap formation may create transient SNF hotspots in ...mature forests that increase overall SNF rates in these ecosystems, as canopy gaps are pervasive across the landscape and increasing in frequency. However, what environmental conditions are driving SNF upregulation in canopy gaps is unknown.
In a field experiment to test these potential environmental controls on SNF, we grew 540 neotropical nitrogen‐fixing legume seedlings (Pentaclethra macroloba, Zygia longifolia, and Stryphnodendron microstachyum) under manipulated light and soil nitrogen availability in canopy gaps and intact forests at La Selva Biological Station, Costa Rica.
Seedling biomass, nodule biomass, and SNF (g N seedling−1 h−1) were 4‐, 17‐ and 42‐fold higher, respectively, in canopy gaps than in the intact forest. Nitrogen additions decreased SNF, but light had a stronger positive effect. Upregulation of SNF in canopy gaps was driven by increased plant growth and not a disproportionate increased SNF allocation.
These data provide evidence that canopy gap SNF hotspots are driven, in part, by light availability, demonstrating a potential driver of SNF spatial heterogeneity. This further suggests that canopy gap dynamics are important for understanding the biogeochemistry of neotropical forests.
Plants and their soil microbial symbionts influence ecosystem productivity and nutrient cycling, but the controls on these symbioses remain poorly understood. This is particularly true for plants in ...the Fabaceae family (hereafter legumes), which can associate with both arbuscular mycorrhizal fungi (AMF) and nitrogen (N) -fixing bacteria. Here we report results of the first manipulated field experiment to explore the abiotic and biotic controls of this tripartite symbiosis in Neotropical canopy gaps (hereafter gaps). We grew three species of Neotropical N-fixing legume seedlings under different light (gap–full light, gap–shadecloth, and understory) and soil nitrogen (20 g N·m−2·yr−1 vs. 0 g N·m−2·yr−1) conditions across a lowland tropical forest at La Selva Biological Station, Costa Rica. We harvested the seedlings after 4 months of growth in the field and measured percent AMF root colonization (%AMF), nodule and seeding biomass, and seedling aboveground:belowground biomass ratios. Our expectation was that seedlings in gaps would grow larger and, as a result of higher light, invest more carbon in both AMF and N-fixing bacteria. Indeed, seedlings in gaps had higher total biomass, nodule biomass (a proxy for N-fixing bacteria investment) and rates of AMF root colonization, and the three were significantly positively correlated. However, we only found a significant positive effect of light availability on %AMF when seedlings were fertilized with N. Furthermore, when we statistically controlled for treatment, species, and site effects, we found %AMF and seedling biomass had a negative relationship. This was likely driven by the fact that seedlings invested relatively less in AMF as they increased in biomass (lower %AMF per gram of seedling). Taken together, these results challenge the long-held assumption that high light conditions universally increase carbon investment in AMF and demonstrate that this tripartite symbiosis is influenced by soil nutrient and light conditions.
Estimating leaf area index (LAI) and assessing spatial variation in LAI across a landscape is crucial to many ecological studies. Several direct and indirect methods of LAI estimation have been ...developed and compared; however, many of these methods are prohibitively expensive and/or time consuming. Here, we examine the feasibility of using the free image processing software CAN-EYE to estimate effective plant area index (PAIeff) from hemispherical canopy images taken with an extremely inexpensive smartphone clip-on fisheye lens. We evaluate the effectiveness of this inexpensive method by comparing CAN-EYE smartphone PAIeff estimates to those from drone lidar over a lowland tropical forest at La Selva Biological Station, Costa Rica. We estimated PAIeff from drone lidar using a method based in radiative transfer theory that has been previously validated using simulated data; we consider this a conservative test of smartphone PAIeff reliability because above-canopy lidar estimates share few assumptions with understory image methods. Smartphone PAIeff varied from 0.1 to 4.4 throughout our study area and we found a significant correlation (r = 0.62, n = 42, p < 0.001) between smartphone and lidar PAIeff, which was robust to image processing analytical options and smartphone model. When old growth and secondary forests are assumed to have different leaf angle distributions for the lidar PAIeff algorithm (spherical and planophile, respectively) this relationship is further improved (r = 0.77, n = 42, p < 0.001). However, we found deviations in the magnitude of the PAIeff estimations depending on image analytical options. Our results suggest that smartphone images can be used to characterize spatial variation in PAIeff in a complex, heterogenous tropical forest canopy, with only small reductions in explanatory power compared to true digital hemispherical photography.
Summary
Tropical forest root characteristics and resource acquisition strategies are underrepresented in vegetation and global models, hampering the prediction of forest–climate feedbacks for these ...carbon‐rich ecosystems. Lowland tropical forests often have globally unique combinations of high taxonomic and functional biodiversity, rainfall seasonality, and strongly weathered infertile soils, giving rise to distinct patterns in root traits and functions compared with higher latitude ecosystems. We provide a roadmap for integrating recent advances in our understanding of tropical forest belowground function into vegetation models, focusing on water and nutrient acquisition. We offer comparisons of recent advances in empirical and model understanding of root characteristics that represent important functional processes in tropical forests. We focus on: (1) fine‐root strategies for soil resource exploration, (2) coupling and trade‐offs in fine‐root water vs nutrient acquisition, and (3) aboveground–belowground linkages in plant resource acquisition and use. We suggest avenues for representing these extremely diverse plant communities in computationally manageable and ecologically meaningful groups in models for linked aboveground–belowground hydro‐nutrient functions. Tropical forests are undergoing warming, shifting rainfall regimes, and exacerbation of soil nutrient scarcity caused by elevated atmospheric CO2. The accurate model representation of tropical forest functions is crucial for understanding the interactions of this biome with the climate.
Resumen
Las características de las raíces de los bosques tropicales y las estrategias de adquisición de recursos están subrepresentadas en modelos de vegetación, lo que dificulta la predicción del efecto de cambio de clima para estos ecosistemas ricos en carbono. Los bosques tropicales a menudo tienen combinaciones únicas a nivel mundial de alta biodiversidad taxonómica y funcional, estacionalidad de precipitación, y suelos infértiles, dando lugar a patrones distintos en los rasgos y funciones de las raíces en comparación con los ecosistemas de latitudes más altas. Integramos los avances recientes en nuestra comprensión de la función subterránea de los bosques tropicales en modelos de vegetación, centrándonos en la adquisición de agua y nutrientes. Ofrecemos comparaciones de avances recientes en la comprensión empírica y de modelos de las características de las raíces que representan procesos funcionales importantes en los bosques tropicales. Nos centramos en: (1) estrategias de raíces finas para adquisición de recursos del suelo, (2) acoplamiento y compensaciones entre adquisición del agua y de nutrientes, y (3) vínculos entre funciones sobre tierra y debajo del superficie en bosques tropicales. Sugerimos vías para representar estas comunidades de plantas extremadamente diversas en grupos computacionalmente manejables y ecológicamente significativos en modelos. Los bosques tropicales se están calentando, tienen cambios en los regímenes de lluvias, y tienen una exacerbación de la escasez de nutrientes del suelo causada por el elevado CO2 atmosférico. La representación precisa de las funciones de los bosques tropicales en modelos es crucial para comprender las interacciones de este bioma con el clima.
Symbiotic nitrogen (N) fixation is the largest non-anthropogenic N input to many terrestrial ecosystems. The energetic expense of symbiotic N fixation suggests soil phosphorus (P) availability may ...regulate symbiotic nitrogen fixation directly through nodule development and function, and/or indirectly through plant growth. Since P availability is heterogenous in the landscape, we sought to understand if symbiotic nitrogen fixation responds to both P availability and heterogeneity. To test how P availability affects symbiotic nitrogen fixation, we grew
Robinia pseudoacacia
seedlings under high (8.1 g P m
−2
) and low (0.2 g P m
−2
) conditions. Soil P heterogeneity was simulated by splitting roots into soil patches receiving P or no-P fertilizer. At the whole plant level, P availability limited seedling and nodule biomass. However, the low P treatment had higher nitrogenase efficiency (acetylene reduced (AR) g
−1
nodule; a nodule efficiency proxy). High P seedlings had significantly more root and nodule biomass in the patches directly receiving P fertilizer, but patch proliferation was absent in the low P treatment. AR g
−1
seedling did not differ between P treatments, suggesting P indirectly limited symbiotic nitrogen fixation through plant growth, rather than directly limiting symbiotic nitrogen fixation. This relatively consistent AR g
−1
seedling across treatments demonstrates the ability of seedlings to respond to low P conditions with increased nitrogenase efficiency.
Purpose
Although congenital anomalies of the kidney and urinary tract (CAKUT) are among the leading causes of end-stage kidney disease (ESKD) in children and young adults, kidney transplantation ...access for this population has not been well studied in the US. We compared transplantation access in the US based on whether the etiology of kidney disease was secondary to CAKUT, and additionally by CAKUT subgroups (anatomic vs. inherited causes of CAKUT).
Methods
Using the United States Renal Data System, we conducted a retrospective cohort study of 80,531 children and young adults who started dialysis between 1995 and 2015. We used adjusted Cox models to examine the association between etiology of kidney disease (CAKUT vs. non-CAKUT, anatomic vs. inherited) and receipt of kidney transplantation, and secondarily, receipt of a living vs. deceased donor kidney transplant.
Results
Overall, we found an increased likelihood of kidney transplantation access for participants with CAKUT compared to those without CAKUT (HR 1.23; 95% CI 1.20–1.27). Among the subset of individuals with CAKUT as the attributed cause of ESKD, we found a lower likelihood of kidney transplantation in those with anatomic causes of CAKUT compared to those with inherited causes of CAKUT (adjusted HR 0.85; 0.81–0.90).
Conclusion
There are notable disparities in kidney transplantation rates among CAKUT subgroups. Those with anatomic causes of CAKUT started on dialysis have significantly reduced access to kidney transplantations compared to individuals with inherited causes of CAKUT who were initiated on dialysis. Further studies are needed to understand barriers to transplantation access in this population.
Carbon cycle perturbations in high-latitude ecosystems associated with rapid warming can have implications for the global climate. Belowground biomass is an important component of the carbon cycle in ...these ecosystems, with, on average, significantly more vegetation biomass belowground than aboveground. Large quantities of dead root biomass are also in these ecosystems owing to slow decomposition rates. Current understanding of how live and dead root biomass carbon pools vary across high-latitude ecosystems and the environmental conditions associated with this variation is limited due to the labor- and time-intensive nature of data collection. To that end, we examined patterns and factors (abiotic and biotic) associated with the variation in live and dead fine root biomass (FRB) and FRB carbon (C), nitrogen (N) and phosphorus concentrations for 23 sites across a latitudinal gradient in Alaska, spanning both boreal forest and tundra biomes. We found no difference in the live or dead FRB variables between these biomes, despite large differences in predominant vegetation types, except for significantly higher live FRB C:N ratios in boreal sites. Soil C:N ratio, moisture, and temperature, along with moss cover, explained a substantial portion of the dead:live FRB ratio variability across sites. We find all these factors have negative relationships with dead FRB, while having positive or no relationship with live FRB. This work demonstrates that FRB does not necessarily correlate with aboveground vegetation characteristics, and it highlights the need for finer-scale measurements of abiotic and biotic factors to understand FRB landscape variability now and into the future.
Climate change is expected to increase temperature and temporal precipitation variability leading to higher evapotranspiration and more frequent and severe droughts. While advancements are being made ...in our understanding of how plants will respond to these changes, gaps remain in our knowledge of species-specific drought response. This is especially true among herbaceous plant communities, including ferns and other seed-free vascular plants. Previous hydraulic work on ferns has almost exclusively concentrated on the leaves, with very little information on the rhizome, which is surprising given that the rhizome is the long-lived perennial organ (making it more costly and important in species survival). Only recently have rhizome hydraulics been explored in the context of drought stress. Similar to observations in many woody trees, fern leaves tend to desiccate and hydraulically disconnect before the perennial stem experiences significant levels of drought-induced embolism, suggesting strong vulnerability segmentation. These findings have significant implications for fern survival during drought. In this review we expand on these observations, integrating information from previous work on plant hydraulics and ecophysiology, to understand the implications of vulnerability segmentation on the response of ferns to future climate change.
Several plant species in the legume family, including peas, have evolved partnerships with bacteria that can capture (also termed fix) nitrogen from the atmosphere and convert it into molecules that ...are the building blocks for amino acids and DNA. Nitrogen-fixing trees are more competitive than are trees that don't fix nitrogen during periods in which light availability and demand for nutrients are high8-10, such as the early stages of forest development, or when a large tree falls and creates a gap in the canopy that can be filled. Barker and colleagues tested whether nitrogen-fixing trees experience higher levels of herbivory than do non-fixers, and modelled the carbon cost of this herbivory. in a field study, the authors examined diverse tree species (23 species capable of fixing nitrogen and 20 non-fixers) growing as understorey seedlings in mature tropical forests in Barro Colorado island, Panama.
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