Abstract
Tree species that are successful in tropical lowlands have different acquisition strategies to overcome soil phosphorus (P) limitations. Some of these strategies belowground include adjustments in fine-root traits, such as morphology, architecture, association with arbuscular mycorrhizal fungi, and phosphatase activity. Trade-offs among P-acquisition strategies are expected because of their respective carbon cost. However, empirical evidence remains scarce which hinders our understanding of soil P-acquisition processes in tropical forests. Here, we measured seven fine-root functional traits related to P acquisition of five common tree species in three sites of the Luquillo Experimental Forest in Puerto Rico. We then described species-specific P-acquisition strategies and explored the changes in fine-root trait expression from 6 months before to 6 months after two consecutive hurricanes, Irma and María, passed over the island. We found that variations in root trait expression were driven mainly by the large interspecific differences across the three selected sites. In addition, we revealed a trade-off between highly colonized fine roots with high phosphatase activity and fine roots that have a high degree of branching. Furthermore, the former strategy was adopted by pioneer species (Spathodea campanulata and Cecropia schreberiana), whereas the latter was adopted by non-pioneer species (mostly Dacryodes excelsa and Prestoea montana). Additionally, we found that root trait expression did not change comparing 6 months before and after the hurricanes, with the exception of root phosphatase activity. Altogether, our results suggest a combination of structural and physiological root traits for soil P acquisition in P-poor tropical soils by common tropical tree species, and show stability on most of the root trait expression after hurricane disturbances.
Highlights
The availability of soil inorganic phosphorus strongly limits plant growth in lowland tropical forests where climate conditions lead to high soil weathering rates (Walker and Syers, 1976; Vitousek and Sanford, 1986; Reed et al, 2011; Lugli et al, 2021)
We explored relationships among fine-root traits related to P acquisition (Freschet et al, 2021), such as (i) morphological, architectural, and chemical traits (SRL, diameter, P concentration, branching intensity, and branching ratio) and (ii) physiological and microbial traits
Root phosphatase activity and branching ratio varied in only one species among sites, while root P concentration did not differ among species (Supplementary Table 1)
Summary
The availability of soil inorganic phosphorus (orthophosphate; hereafter, available P) strongly limits plant growth in lowland tropical forests where climate conditions lead to high soil weathering rates (Walker and Syers, 1976; Vitousek and Sanford, 1986; Reed et al, 2011; Lugli et al, 2021). Tree species that are successful in tropical lowlands have multiple acquisition strategies to overcome P limitation (Zalamea et al, 2016; Lugli et al, 2021). Belowground, plants optimize P acquisition by displaying multiple strategies commonly described by the expression of the traits of the narrowest, most absorptive roots (i.e., fine roots) and their mycorrhizal symbionts (Bardgett et al, 2014; Kramer-Walter et al, 2016; Weemstra et al, 2016; McCormack and Iversen, 2019; Bergmann et al, 2020; Lugli et al, 2021). Fine-root and mycorrhizal fungal trait expression is closely related to root function, such as resource foraging and uptake. Specific root length (SRL) and root branching are linked to soil space occupancy while root phosphatase activity and mycorrhizal colonization are critical determinants of P uptake (Lee, 1988; McCormack et al, 2017; Freschet et al, 2021)
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