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[ID: 517] Predicting tree population growth rates from plant functional traits, nutrient acquisition strategies, and climate
PI: Maria Martell
For over a century, ecologists have sought to understand how plant strategies enable populations to persist and thrive under specific environmental conditions, including water and nutrient acquisition strategies - especially those mediated through associations with mycorrhizal fungi. Plant strategies can be understood through the connection between (i) population growth, (ii) functional traits, and (iii) the environment. (Laughlin et al., 2020; Siefert & Laughlin, 2023), integrating individual- and population-level processes, and providing a clearer understanding of how traits influence growth at both scales. However, the relationship between traits and population fitness is not universal, it depends on the abiotic environmental context, including both atmospheric and soil conditions. By simultaneously considering plant functional traits, population growth across species, and abiotic environmental variation, we can integrate functional ecology and population ecology. Although nutrient acquisition strategies vary within and among mycorrhizal types, general patterns exist: arbuscular mycorrhizal fungi (AMF) typically scavenge soluble inorganic soil phosphorus and nitrogen, whereas ectomycorrhizal fungi (EcMF) rely on specialized extracellular enzymes to extract nutrients, particularly organic nitrogen and phosphorus from complex sources. However, how these contrasting mycorrhizal association types shape individual and population growth across different abiotic environmental contexts remains an open and understudied question. Recent work by Augusto et al. (2025) has challenged the traditional fast–slow plant economic spectrum. In field studies, they found that acquisitive tree species often grow slowly under natural conditions. Their results showed negative correlations between tree growth and traits such as leaf nitrogen content (Nmass), specific leaf area (SLA), specific root length (SRL), and mass-based maximum photosynthetic rate (Amax). They interpreted this mismatch as the outcome of environmental constraints: conservative species grew better in stressful conditions due to greater tolerance, while acquisitive species only grew well in environments with high soil fertility and ample moisture. 2 Here, we propose a complementary framework, beyond the classic fast-slow plant economic spectrum, to explain these findings, grounded in eco-evolutionary optimality theory (Wright et al., 2003; Wang et al., 2017; Franklin et al., 2020; Harrison et al., 2021). This approach introduces the idea of carbon cost relative to carbon gain, integrating nutrient acquisition strategies - particularly those shaped by mycorrhizal associations - with environmental constraints. By linking trait expression and population growth to resource economics and mycorrhizal type, we can better predict when and why certain functional strategies succeed across abiotic environmental gradients. Building on this foundation, our recent work demonstrated that nutrient acquisition strategies shape the trade-off between carbon costs of nutrient acquisition relative to water acquisition and soil resource availability (Cheaib et al., 2025b,a). Specifically, we found that the correlations between the carbon costs of nutrient acquisition relative to water acquisition, and soil nutrient concentrations (nitrogen and phosphorus), varies systematically with the type of mycorrhizal association and local abiotic environmental conditions. A key insight from our study is the role of soil carbon-to-nitrogen (C:N) ratios in determining the relative advantage of different strategies. Under high soil C:N ratios reflecting low soil N availability, tree species associated with ectomycorrhizal (EcMF) and ericoid mycorrhizal fungi were able to reduce carbon costs relative to species associated with arbuscular mycorrhizal fungi (AMF) by accessing organic nitrogen sources. In contrast, under low soil C:N ratios reflecting high soil N availability, species associated with AMF were able to reduce carbon costs relative to species EcMF-associated species by efficiently absorbing soluble inorganic nitrogen. These findings highlight the dynamic nature of plant-soil-atmosphere interactions and suggest that advantages or disadvantages of a given nutrient acquisition strategy are not fixed in space, but context-dependent. In this proposal, we aim to apply these insights - together with new analyses and additional datasets - to revisit and reinterpret the results of Augusto et al. (2025). By framing their observed trait–growth relationships within an eco-evolutionary optimality perspective, and explicitly incorporating nutrient acquisition strategies, we may uncover deeper mechanisms driving population-level growth rates across abiotic environmental gradients. This approach promises to bridge physiological ecology, population dynamics, and mycorrhizal symbiosis within a unified predictive framework.