Title: Self-thinning principle in miniature brachiaria swards: interspecific variations under reduced light and suboptimal nitrogen nutrition

 

Abstract: The classical self-thinning rule (slope of −3/2) describes a fundamental relationship between size and density in plant populations. However, its generality under contrasting functional strategies and varying resource availability remains insufficiently tested in forage systems. In this study, we integrated the results of two experimental approaches: (1) miniature swards of Brachiaria brizantha cv. Marandu (a resource-acquisitive species) and B. humidicola cv. Dictyoneura (a resource-conservative species), and (2) Brachiaria brizantha cv. Marandu subjected to light and nitrogen limitations, both managed under controlled canopy heights. (1) Despite morphological differences, both species converged along a line with a −3/2 slope, stabilizing biomass production (~4 t DM ha⁻¹) through compensatory demographic adjustments. (2) However, under resource limitations, self-thinning trajectories diverged: nitrogen restriction reinforced convergence towards the −3/2 slope, indicating reduced plasticity, whereas optimal conditions produced steeper slopes (approaching −5/2), reflecting greater morphological plasticity. (1 and 2) Interestingly, the orthogonal distance to the self-thinning line and the leaf area index (LAI) were decoupled from productivity within environments, questioning their utility as direct proxies for forage production, (2) although they retained comparative value across environmental gradients. Collectively, these findings refine allometric theory by demonstrating that biomass stability in tropical pastures can emerge from distinct demographic pathways and that environmental stress constrains allometric flexibility while reinforcing the classical self-thinning rule. This study highlights the critical role of size-density compensation in maintaining forage productivity and resilience, advocating for pasture management strategies that emphasize demographic plasticity over static structural metrics. These results expand the understanding of the mechanisms underlying allometric trade-offs in tropical forage systems, highlighting that forage productivity and system stability are emergent properties of morpho-demographic adjustments.

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Keywords: Adaptive self-thinning; Brachiaria; Sward structure; Morphological plasticity; Stress resilience.

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Author:  Pedro Antonio Garzón Camacho

Leader: André Fischer Sbrissia

Thesis (Doctors in Animal Science)