Spatial heterogeneity in composition and function enables ecosystems to supply diverse services. For soil microbes and the ecosystem functions they catalyze, whether such heterogeneity can be ...maintained in the face of altered resource inputs is uncertain. In a 50-ha northern California grassland with a mosaic of plant communities generated by different soil types, we tested how spatial variability in microbial composition and function changed in response to nutrient and water addition. Fungal composition lost some of its spatial variability in response to nutrient addition, driven by decreases in mutualistic fungi and increases in antagonistic fungi that were strongest on the least fertile soils, where mutualists were initially most frequent and antagonists initially least frequent. Bacterial and archaeal community composition showed little change in their spatial variability with resource addition. Microbial functions related to nitrogen cycling showed increased spatial variability under nutrient, and sometimes water, additions, driven in part by accelerated nitrification on the initially more-fertile soils. Under anthropogenic changes such as eutrophication and altered rainfall, these findings illustrate the potential for significant changes in ecosystem-level spatial heterogeneity of microbial functions and communities.
Tree Transpiration and Urban Temperatures WINBOURNE, JOY B.; JONES, TAYLOR S.; GARVEY, SARAH M. ...
BioScience/Bioscience,
07/2020, Letnik:
70, Številka:
7
Journal Article
Recenzirano
Odprti dostop
The expansion of an urban tree canopy is a commonly proposed nature-based solution to combat excess urban heat. The influence trees have on urban climates via shading is driven by the morphological ...characteristics of trees, whereas tree transpiration is predominantly a physiological process dependent on environmental conditions and the built environment. The heterogeneous nature of urban landscapes, unique tree species assemblages, and land management decisions make it difficult to predict the magnitude and direction of cooling by transpiration. In the present article, we synthesize the emerging literature on the mechanistic controls on urban tree transpiration. We present a case study that illustrates the relationship between transpiration (using sap flow data) and urban temperatures. We examine the potential feedbacks among urban canopy, the built environment, and climate with a focus on extreme heat events. Finally, we present modeled data demonstrating the influence of transpiration on temperatures with shifts in canopy extent and irrigation during a heat wave.
Carbon accumulation in tropical secondary forests may be limited in part by nitrogen (N) availability, but changes in N during tropical forest succession have rarely been quantified. We explored N ...cycle dynamics across a chronosequence of secondary tropical forests in the Mata Atlântica of Bahia, Brazil in order to understand how quickly the N cycle recuperates. We hypothesized that N fixation would decline over the course of succession as N availability and N gaseous losses increased. We measured N fixation, KCl-extractable N, net mineralization and nitrification, resin-strip sorbed N, gaseous N emissions and the soil δ
N in stands that were 20, 35, 50, and > 50 years old. Contrary to our initial hypothesis, we found no significant differences between stand ages in any measured variable. Our findings suggest that secondary forests in this region of the Atlantic forest reached pre-disturbance N cycling dynamics after just 20 years of succession. This result contrasts with previous study in the Amazon, where the N cycle recovered slowly after abandonment from pasture reaching pre-disturbance N cycling levels after ~50 years of succession. Our results suggest the pace of the N cycle, and perhaps tropical secondary forest, recovery, may vary regionally.
Limestone tropical forests represent a meaningful fraction of the land area in Central America (25%) and Southeast Asia (40%). These ecosystems are marked by high biological diversity, CO2 uptake ...capacity, and high pH soils, the latter making them fundamentally different from the majority of lowland tropical forest areas in the Amazon and Congo basins. Here, we examine the role of bedrock geology in determining biological nitrogen fixation (BNF) rates in volcanic (low pH) vs. limestone (high pH) tropical forests located in the Maya Mountains of Belize. We experimentally test how BNF in the leaf-litter responds to nitrogen, phosphorus, molybdenum, and iron additions across different parent materials. We find evidence for iron limitation of BNF rates in limestone forests during the wet but not dry season (response ratio 3.2 ± 0.2; P = 0.03). In contrast, BNF in low pH volcanic forest soil was stimulated by the trace-metal molybdenum during the dry season. The parent-material induced patterns of limitation track changes in siderophore activity and iron bioavailability among parent materials. These findings point to a new role for iron in regulating BNF in karst tropical soils, consistent with observations for other high pH systems such as the open ocean and calcareous agricultural ecosystems.
Accurately quantifying rates and patterns of biological nitrogen fixation (BNF) in terrestrial ecosystems is essential to characterize ecological and biogeochemical interactions, identify mechanistic ...controls, improve BNF representation in conceptual and numerical modelling, and forecast nitrogen limitation constraints on future carbon (C) cycling.
While many resources address the technical advantages and limitations of different methods for measuring BNF, less systematic consideration has been given to the broader decisions involved in planning studies, interpreting data, and extrapolating results. Here, we present a conceptual and practical road map to study design, study execution, data analysis and scaling, outlining key considerations at each step.
We address issues including defining N‐fixing niches of interest, identifying important sources of temporal and spatial heterogeneity, designing a sampling scheme (including method selection, measurement conditions, replication, and consideration of hotspots and hot moments), and approaches to analysing, scaling and reporting BNF. We also review the comparability of estimates derived using different approaches in the literature, and provide sample R code for simulating symbiotic BNF data frames and upscaling.
Improving and standardizing study design at each of these stages will improve the accuracy and interpretability of data, define limits of extrapolation, and facilitate broader use of BNF data for downstream applications. We highlight aspects—such as quantifying scales of heterogeneity, statistical approaches for dealing with non‐normality, and consideration of rates versus ecological significance—that are ripe for further development.
Free-living heterotrophic nitrogen fixation (FNF) is a widespread nitrogen input pathway in terrestrial ecosystems. However, questions remain over the relative influence of co-occurring controls on ...patterns of heterotrophic FNF activity, especially across generalized stages of primary succession, from biomass accumulation to retrogressive phases. Here, we experimentally test two alternative hypotheses regarding FNF rates during ecosystem development: (H1) site (i.e., changes in soil fertility during succession) is the primary driver of leaf-litter FNF rates, vs. (H2) leaf-litter chemistry is the primary determinant of FNF activity across a broad range of ecosystem conditions. We evaluated these hypotheses across a well-studied soil chronosequence in California (i.e., the Ecological Staircase), which spans ∼1 million years of ecosystem development and displays extreme ranges in plant-soil nutrient conditions, culminating in the nutrient depleted and stunted Pygmy forest. Across this successional gradient, we implemented a reciprocal leaf-litter transplant and a common garden litter bag decomposition experiment with senesced needles of Pinus muricata. Our results support H1: rates of FNF were similar for all leaf-litter types decomposed at the same site regardless of initial leaf-litter C and nutrient contents. FNF rates sharply declined from the maximal to retrogressive stage of succession. Trends in P dynamics during decomposition suggest an important role of P in regulating FNF. For example, P. muricata litter collected from the infertile Pygmy site displayed substantially higher FNF rates when decomposed at the fertile site, in part by immobilizing significant quantities of P from the soil at the fertile site. Conversely, P. muricata litter collected from the fertile site decomposed more slowly at the Pygmy site, with concomitant declines in FNF rates that matched those of Pygmy site litter decomposed in situ. These results are consistent with the idea that, over millennia, long-term declines in P availability feedback to constrain FNF rates, in part explaining the emergence of extremely nutrient-poor and retrograded ecosystems.
Trees play an important role in both the water and carbon cycles. Sensors that measure the flow rate of sap in individual trees provide critical insight into water use dynamics, and these data are ...vital for the understanding of ecosystem functions. Current sap flux sensors typically require the installation of multiple needle‐like probes into the sapwood of the tree. Despite the scientific value of high‐density measurements, the high cost and complexity of existing methods prevents the deployment of large networks of sensors. This study presents a new sap flow sensor design where all sensor components are on a single integrated circuit board (ribbon) thereby reducing cost and complexity. This system also eliminates misalignment errors, which are a problem inherent in some sap flow sensor designs. The new ribbonized sap flow (RSF) sensor was compared to gravimetric measurements of water flow to quantify the accuracy of the method and the tree wounding response. In a pair of field experiments, we tested the general performance of the sensor design over a growing season, and among several tree species. The laboratory calibration showed that the RSF sensor has similar accuracy and wounding response as traditional three‐probe designs. At a flow rate of 15 cm/h, the sensor underreported rates by 68% with a 19‐d wounding response, compared to other sensor designs, which underreport rates by 20–60%. Collectively, our field experiments show that the RSF sensor can produce reliable field data in a variety of species across full growing seasons. The RSF sensor platform is compact, robust, and versatile. Ribbonized sap flow sensors can be quickly and easily manufactured at low cost, which makes them ideal for the dense network of observations needed for the study of complex forest ecosystems and their response to global 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.
PDF
