Weijun Shen

Empirical patterns of the effects of changing scale on landscape metrics

While ecologists are well aware that spatial heterogeneity is scale-dependent, a general understanding of scaling relationships of spatial pattern is still lacking. One way to improve this understanding is to systematically examine how pattern indices change with scale in real landscapes of different kinds. This study, therefore, was designed to investigate how a suite of commonly used landscape metrics respond to changing grain size, extent, and the direction of analysis (or sampling) using several different landscapes in North America. Our results showed that the responses of the 19 landscape metrics fell into three general categories: Type I metrics showed predictable responses with changing scale, and their scaling relations could be represented by simple scaling equations (linear, power-law, or logarithmic functions); Type II metrics exhibited staircase-like responses that were less predictable; and Type III metrics behaved erratically in response to changing scale, suggesting no consistent scaling relations. In general, the effect of changing grain size was more predictable than that of changing extent. Type I metrics represent those landscape features that can be readily and accurately extrapolated or interpolated across spatial scales, whereas Type II and III metrics represent those that require more explicit consideration of idiosyncratic details for successful scaling. To adequately quantify spatial heterogeneity, the metric-scalograms (the response curves of metrics to changing scale), instead of single-scale measures, seem necessary.

Effects of Urbanization-Induced Environmental Changes on Ecosystem Functioning in the Phoenix Metropolitan Region, USA

A bstractUrban ecosystems are profoundly modified by human activities and thereby provide a unique “natural laboratory” to study potential ecosystem responses to anthropogenic environmental changes. Because urban environments are now affected by urban heat islands, carbon dioxide domes, and high-level nitrogen deposition, to some extent they portend the future of the global ecosystem. Urbanization in the metropolitan region of Phoenix, Arizona (USA) has resulted in pronounced changes in air temperature (Tair), atmospheric CO2 concentration, and nitrogen deposition (Ndep). In this study, we used a process-based ecosystem model to explore how the Larrea tridentata dominated Sonoran Desert ecosystem may respond to these urbanization-induced environmental changes. We found that water availability controls the magnitude and pattern of responses of the desert ecosystem to elevated CO2, air temperature, N deposition and their combinations. Urbanization effects were much stronger in wet years than normal and dry years. At the ecosystem level, aboveground net primary productivity (ANPP) and soil organic matter (SOM) both increased with increasing CO2 and Ndep individually and in combinations with changes in Tair. Soil N (Nsoil) responded positively to increased N deposition and air temperature, but negatively to elevated CO2. Correspondingly, ANPP and SOM of the Larrea ecosystem decreased along the urban–suburban–wildland gradient, whereas Nsoil peaked in the suburban area. At the plant functional type (FT) level, ANPP generally responded positively to elevated CO2 and Ndep, but negatively to increased Tair. C3 winter annuals showed a greater ANPP response to higher CO2 levels (>420 ppm) than shrubs, which could lead over the long term to changes in species composition, because competition among functional groups is strong for resources such as soil water and nutrients. Overall, the combined effects of the three environmental factors depended on rainfall variability and nonlinear interactions within and between plant functional types and environmental factors. We intend to use these simulation results as working hypotheses to guide our field experiments and observations. Experimental testing of these hypotheses through this process should improve our understanding of urban ecosystems under increasing environmental stresses.

Effects of Soil Moisture on the Temperature Sensitivity of Soil Heterotrophic Respiration: A Laboratory Incubation Study

The temperature sensitivity (Q10) of soil heterotrophic respiration (Rh) is an important ecological model parameter and may vary with temperature and moisture. While Q10 generally decreases with increasing temperature, the moisture effects on Q10 have been controversial. To address this, we conducted a 90-day laboratory incubation experiment using a subtropical forest soil with a full factorial combination of five moisture levels (20%, 40%, 60%, 80%, and 100% water holding capacity - WHC) and five temperature levels (10, 17, 24, 31, and 38°C). Under each moisture treatment, Rh was measured several times for each temperature treatment to derive Q10 based on the exponential relationships between Rh and temperature. Microbial biomass carbon (MBC), microbial community structure and soil nutrients were also measured several times to detect their potential contributions to the moisture-induced Q10 variation. We found that Q10 was significantly lower at lower moisture levels (60%, 40% and 20% WHC) than at higher moisture level (80% WHC) during the early stage of the incubation, but became significantly higher at 20%WHC than at 60% WHC and not significantly different from the other three moisture levels during the late stage of incubation. In contrast, soil Rh had the highest value at 60% WHC and the lowest at 20% WHC throughout the whole incubation period. Variations of Q10 were significantly associated with MBC during the early stages of incubation, but with the fungi-to-bacteria ratio during the later stages, suggesting that changes in microbial biomass and community structure are related to the moisture-induced Q10 changes. This study implies that global warming’s impacts on soil CO2 emission may depend upon soil moisture conditions. With the same temperature rise, wetter soils may emit more CO2 into the atmosphere via heterotrophic respiration.

Experimental landscape ecology

Experimentation in landscape ecology is widely conducted using diverse approaches to answer a broad range of questions. By assessing the response to controlled manipulations alternate hypotheses can be clearly refuted, model parameters quantified, and conditions are often ripe for unexpected insights. Results from landscape experiments complement the many well developed observational and modeling approaches more commonly used in landscape ecology. To better understand how landscape experimentation has been conducted and to identify future research directions, we reviewed and organized the diversity of experiments. We identified fifteen distinct landscape experiment types, which we categorized into four broad groups including (I) identifying landscape structure, (II) identifying how ecological processes vary within existing landscapes, (III) identifying how landscape structure influences ecological processes, and (IV) identifying landscape pattern formation factors. Experiment types vary along axes of scalable to real landscapes and generalizability, suitability for analysis through traditional experimental design and flexibility of experimental setup, and complexity of implementation and resource requirements. The next generation of experiments are benefiting from more explicit inclusion of scaling theories and tighter coupling between experiments and cyberinfrastructure. Future experimental opportunities for landscape ecologists include expanded collaborations among experiments, better representations of microbial-soil structure relationships at microscales, and direct evaluations of landscape interactions with global changes. The history, current practice, and future needs of landscape ecological research strongly support an expanded role of experimental approaches that complements the rich observational and modeling strengths of the field.

CAN Canopy Addition of Nitrogen Better Illustrate the Effect of Atmospheric Nitrogen Deposition on Forest Ecosystem

Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Here we, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. UAN and reviewed the merits and pitfalls of the two approaches. The following hypotheses will be tested: i) UAN overestimates the N addition effects on understory and soil processes but underestimates those on canopy-associated biota and processes, ii) with low-level N addition, CAN favors canopy tree species and canopy-dwelling biota and promotes the detritus food web, and iii) with high-level N addition, CAN suppresses canopy tree species and other biota and favors rhizosphere food web. As a long-term comprehensive program, this experiment will provide opportunities for multidisciplinary collaborations, including biogeochemistry, microbiology, zoology, and plant science to examine forest ecosystem responses to atmospheric N deposition.

Near Isometric Biomass Partitioning in Forest Ecosystems of China

Based on the isometric hypothesis, belowground plant biomass (MB) should scale isometrically with aboveground biomass (MA) and the scaling exponent should not vary with environmental factors. We tested this hypothesis using a large forest biomass database collected in China. Allometric scaling functions relating MB and MA were developed for the entire database and for different groups based on tree age, diameter at breast height, height, latitude, longitude or elevation. To investigate whether the scaling exponent is independent of these biotic and abiotic factors, we analyzed the relationship between the scaling exponent and these factors. Overall MB was significantly related to MA with a scaling exponent of 0.964. The scaling exponent of the allometric function did not vary with tree age, density, latitude, or longitude, but varied with diameter at breast height, height, and elevation. The mean of the scaling exponent over all groups was 0.986. Among 57 scaling relationships developed, 26 of the scaling exponents were not significantly different from 1. Our results generally support the isometric hypothesis. MB scaled near isometrically with MA and the scaling exponent did not vary with tree age, density, latitude, or longitude, but increased with tree size and elevation. While fitting a single allometric scaling relationship may be adequate, the estimation of MB from MA could be improved with size-specific scaling relationships.

Spatial distribution patterns of ammonia-oxidizing archaea abundance in subtropical forests at early and late successional stages.

Characterizing the spatial distribution patterns of soil microorganisms is helpful in understanding the biogeochemical processes they perform, but has been less studied relative to those of macroorganisms. In this study, we investigated and compared the spatially explicit distribution patterns of ammonia-oxidizing archaea (AOA) abundance and the influential factors between an early (ES) and a late successional (LS) subtropical forest stand. The average AOA abundance, vegetational attributes, and soil nutrient contents were mostly greater in the LS than the ES stand (P = 0.085 or smaller), but their spatial variations were more pronounced in the ES than the LS stand. The spatial distribution patches of AOA abundance were smaller and more irregular in the ES stand (patch size <50 m) than in the LS stand (patch size about 120 m). Edaphic and vegetational variables contributed more to the spatial variations of AOA abundance for the ES (9.3%) stand than for LS stand, whereas spatial variables (MEMs) were the main contributors (62%) for the LS stand. These results suggest that environmental filtering likely influence the spatial distribution of AOA abundance at early successional stage more than that at late successional stage, while spatial dispersal is dominant at late successional stage.

Ammonia-Oxidizing Archaea Are More Resistant Than Denitrifiers to Seasonal Precipitation Changes in an Acidic Subtropical Forest Soil

Seasonal precipitation changes are increasingly severe in subtropical areas. However, the responses of soil nitrogen (N) cycle and its associated functional microorganisms to such precipitation changes remain unclear. In this study, two projected precipitation patterns were manipulated: intensifying the dry-season drought (DD) and extending the dry-season duration (ED) but increasing the wet-season storms following the DD and ED treatment period. The effects of these two contrasting precipitation patterns on soil net N transformation rates and functional gene abundances were quantitatively assessed through a resistance index. Results showed that the resistance index of functional microbial abundance (-0.03 ± 0.08) was much lower than that of the net N transformation rate (0.55 ± 0.02) throughout the experiment, indicating that microbial abundance was more responsive to precipitation changes compared with the N transformation rate. Spring drought under the ED treatment significantly increased the abundances of both nitrifying (amoA) and denitrifying genes (nirK, nirS, and nosZ), while changes in these gene abundances overlapped largely with control treatment during droughts in the dry season. Interestingly, the resistance index of the ammonia-oxidizing archaea (AOA) amoA abundance was significantly higher than that of the denitrifying gene abundances, suggesting that AOA were more resistant to the precipitation changes. This was attributed to the stronger environmental adaptability and higher resource utilization efficiency of the AOA community, as indicated by the lack of correlations between AOA gene abundance and environmental factors [i.e., soil water content, ammonium (NH4+) and dissolved organic carbon concentrations] during the experiment.

Element Fluxes and Budets of a Plantation Embedded in an Agroforestry LandscapeANDSCAPE: Implication for Landscape ManagementANAGEMENT and Sustainability

Nutrient fluxes and cycling are key processes to sustaining the structure and functioning of patches as well as the landscapes in which they are embedded. In this chapter, we synthesized a 10- year study on the nutrient cycling of an Acacia mangiumplantation located in the upper slope of a watershed landscape consisting on a tree plantation, a fruit garden, a grassland meadow, and a fishpond. Element fluxes and budgets were analyzed to derive guidelines for managing this landscape. We found that the plantation acted as a nutrient sink during its early stage of development (15-20 years since establishment), as it accumulated a large amount of nutrients on its compartment pools (i.e., plant biomass, forest floor, and soil). Over 1/3-1/6 of total litter on the plantation floor could be moved to fertilize other landscape components (e.g., the fruit garden and the grassland). However, the ability of the plantation to retain nutrients started to decrease at a fast rate when it approached near-mature development stage (20-30 years old). During this stage the risk of N saturation, soil acidification, and nutrient depletion (e.g., P, K, and Mg) increased as the plantation ages, especially under conditions of large atmospheric deposition. In order to sustain its nutrient retention ability and other ecosystem services, we suggested that application of composite P, K and Mg fertilizers are needed after the plantation reached about 20-30 years old. This study also confirmed that positioning the A. mangium plantation on the upper slope of the watershed was an appropriate spatial arrangement for the studied landscape, as it decreases the risk of transporting excessive elements into the lowland fishpond via soil erosion and surface runoff processes. Further studies on element fluxes and budgets of the other three landscape components need to be conducted in order to obtain a comprehensive understanding of interactions between landscape pattern and biogeochemical processes, and to ultimately reach a sustainable management of the landscape.

Responses of sap flux and intrinsic water use efficiency to canopy and understory nitrogen addition in a temperate broadleaved deciduous forest

Increasing atmospheric nitrogen (N) deposition could profoundly impact structure and functioning of forest ecosystems. Therefore, we conducted a two-year (2014-2015) experiment to assess the responses of tree sap flux density (Js) and intrinsic water use efficiency (WUEi) of dominant tree species (Liquidambar formosana, Quercus acutissima and Quercus variabilis) to increased N deposition at a manipulative experiment with canopy and understory N addition in a deciduous broadleaved forest. Five treatments were administered including N addition of 25 kg ha-1 year-1 and 50 kg ha-1 year-1 onto canopy (C25 and C50) and understory (U25 and U50), and control treatment (CK, without N addition). Our results showed neither canopy nor understory N addition had an impact on leaf N content and C:N ratio (P > 0.05). Due to the distinct influencing ways, canopy and understory N addition generated different impacts on Js and WUEi of the dominant tree species. Canopy N addition increased WUEi of Q. variabilis, whereas understory addition treatment had a minimal impact on WUEi. Both N additions did not exert impacts on WUEi of L. formosana and Q. acutissima. Canopy N addition exerted negative impacts on Js and its sensitivity to micrometeorological factors of Q. acutissima and Q. variabilis in 2014, while understory addition showed no effect. Neither canopy nor understory N addition had an influence on Js of L. formosana in 2014. Probably owing to the increased soil acidification as the experiment proceeded, Js of L. formosana and Q. variabilis was decreased by understory N addition while canopy addition had a minimal effect in 2015. Thus, the traditional understory addition approach could not fully reflect the effects of increased N deposition on the canopy-associated transpiration process indicated by the different responses of Js and WUEi to canopy and understory N addition, and exaggerated its influences induced by the variation of soil chemical properties.