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Creators/Authors contains: "Parker, Geoffrey G."

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  1. Abstract

    One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure.

     
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    Free, publicly-accessible full text available December 1, 2024
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  3. Firms in advanced economies are increasingly outsourcing software and technology development as well as other knowledge work to a worldwide supply base. Standard economic and learning models predict that focal firms should outsource either all or none of a particular activity unless extra resources are required during cyclical demand peaks or access is needed to some tightly appropriable intellectual property. However, recent evidence shows that, even when these exceptions do not apply, many firms pursue a partial outsourcing strategy. We develop a dynamic optimization model to provide a rational explanation for this observation. In our model, learning from prior projects occurs at both the subsystem levelandthe overall systems level (e.g., systems integration and architecture). Learning at the two levels interacts such that integration capabilities can dynamically build and decay. The model generates conditions where partial outsourcing is rational and dominates the extreme conditions of complete insourcing or complete outsourcing. Our model also specifies the conditions for regime change between insourcing and outsourcing and cycling between insourcing and outsourcing, and overshooting or undershooting the long‐run outsourcing target. Furthermore, we show that these results are highly path‐dependent under short horizons. The model also provides explanation for interesting questions such as why the rate of outsourcing might be U‐shaped in the rate of technological change and why startups so often insource in contrast to more established counterparts in similar industries.

     
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    Abstract Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity. 
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  5. Abstract

    Tree growth is an important indicator of forest health, productivity, and demography. Knowing precisely how trees' grow within a year, instead of across years, can lead to a finer understanding of the mechanisms that drive these larger patterns. The growing use of dendrometer bands in research forests has only rarely been used to measure growth at resolutions finer than yearly, but intra‐annual growth patterns can be observed from dendrometer bands using precision digital calipers and weekly measurements. Here we present a workflow to help forest ecologists fit growth models to intra‐annual measurements using standard optimization functions provided by the R platform. We explain our protocol, test uncertainty in parameter estimates with respect to sample sizes, extend the optimization protocol to estimate robust lower and upper annual diameter bounds, and discuss potential challenges to optimal fits. We offer R code to implement this workflow. We found that starting values and initial optimization routines are critical to fitting the best functional forms. After using a bounded, broad search method, a more focused search algorithm obtained consistent results. To estimate starting and ending annual diameters, we combined the growth function with early and late estimates of beginning and ending growth. Once we fit the functions, we present extension algorithms that estimate periodic reductions in growth, total growth, and present a method of controlling for the shifting allocation to girth during the growth season. We demonstrate that with these extensions, an analysis of growth response to weather (e.g., the water available to a tree) can be derived in a way that is comparable across trees, years, and sites. Thus, this approach, when applied across broader data sets, offers a pathway to build inference about the effects of seasonal weather on growth, size‐ and light‐dependent patterns of growth, species‐specific patterns, and phenology.

     
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