Soilborne fungi have host affinity and host-specific effects on seed germination and survival in a lowland tropical forest

Abstract

The Janzen-Connell (JC) hypothesis provides a conceptual framework for explaining the maintenance of tree diversity in tropical forests. Its central tenet-that recruits experience high mortality near conspecifics and at high densities-assumes a degree of host specialization in interactions between plants and natural enemies. Studies confirming JC effects have focused primarily on spatial distributions of seedlings and saplings, leaving major knowledge gaps regarding the fate of seeds in soil and the specificity of the soilborne fungi that are their most important antagonists. Here we use a common garden experiment in a lowland tropical forest in Panama to show that communities of seed-infecting fungi are structured predominantly by plant species, with only minor influences of factors such as local soil type, forest characteristics, or time in soil (1-12 months). Inoculation experiments confirmed that fungi affected seed viability and germination in a host-specific manner and that effects on seed viability preceded seedling emergence. Seeds are critical components of reproduction for tropical trees, and the factors influencing their persistence, survival, and germination shape the populations of seedlings and saplings on which current perspectives regarding forest dynamics are based. Together these findings bring seed dynamics to light in the context of the JC hypothesis, implicating them directly in the processes that have emerged as critical for diversity maintenance in species-rich tropical forests.

Keywords: Janzen–Connell; diversity; pathogen; pioneer species; soil seed bank.

Fig. 1.

Hierarchical cluster analyses and pairwise similarities of seed-associated fungal communities reveal strong effects of plant species and seed viability on fungal community structure. Dendrograms represent Morisita–Horn (A) and Jaccard (B) similarities among communities of fungi. Tips of dendrograms represent fungal communities defined as the sequenced fungal strains isolated from seeds of a given plant species (AS, Annona spraguei; CV, Cochlospermum vitifolium; FI, Ficus insipida; and TB, Trema micrantha “black”), a specific burial duration (1, 3, 6, or 12 mo), a specific burial location (A, Armour; D, Drayton; H, 25 ha; P, Pearson; and Z, Zetek), and a specific viability class (viable or inviable seeds). Sequence data were obtained from 992 isolates from buried seeds (72%; mean ± SE, 110.2 ± 38.4 isolates per plant species; SI Appendix, Table S5). Together these represent 209 operational taxonomic units (OTU) based on 99% sequence similarity (Fisher’s alpha = 80.8). Overall, 107 OTU were represented by only one isolate (51.2%). Panels on the right summarize pairwise comparisons of fungal community similarity from the cluster diagrams based on abundance (C) and presence–absence data (D). Analysis of the entire dataset (nine plant species) also highlights the strong effect of plant species (SI Appendix, Fig. S6 and Tables S6 and S7). Error bars represent SE. **P < 0.001, *P < 0.05.

Fig. 2.

Nonmetric multidimensional scaling (NMDS) analysis representing similarity among fungal communities (points) isolated from seeds of the four best-sampled tree species. Panels depict the same ordination but different colors represent plant species (A), garden location (B), burial duration (C), and viability (D) (Inv, Inviable; Via, Viable); stress = 0.062; abbreviations match those in Fig. 1. Ellipses represent standard deviation of point scores relative to their centroid. Venn diagrams represent variation in fungal community composition that is explained by each variable and their shared variation, based on abundance (E) and presence–absence data (F). Redundancy analyses show that plant species (F = 14.26, P = 0.001) and viability (F = 2.26, P = 0.029) are significant when fungal abundance is considered, but plant species (F = 10.35, P = 0.001), burial duration (F = 1.42, P = 0.023), garden location (F = 1.30, P = 0.046), and viability (F = 2.63, P = 0.001) are significant when only presence–absence data are considered. In each case, the majority of the variation that can be explained is explained by plant species.

Fig. 3.

Associations among plant species and seed-associated fungi. Lines depict observed associations between fungal OTU (Top; OTU represented by ≥4 isolates, indicated by OTU number above each gray box) and plant species (Bottom; four best-sampled tree species), with line thickness proportional to the number of sequenced isolates per OTU from each plant species. Plant boxes and lines are colored to indicate viable (green) or inviable (blue) seeds. Box width is proportional to the number of sequences corresponding to each OTU (Top) or the number of sequenced isolates from each plant species (Bottom). OTU: 1, Hypocrea sp.; 2, Lasodiplodia sp.; 3, Lasodiplodia sp. 2; 4, Nectria sp.; 5, Nectria sp. 2; 6, Lasodiplodia sp. 3; 7, Fusarium sp.; 8, Fusarium sp. 2; 9, Hypocrea sp. 2; 10, Hypocrea sp. 3; 11, Nectria sp. 3; 12, Trichoderma sp.; 13, Pseudocercospora sp.; 14, Cylindrocladiella sp.; 16, Cylindrocladiella sp. 2. Plant species: AS, Annona spraguei; CV, Cochlospermum vitifolium; FI, Ficus insipida; and TB, Trema micrantha “black.”

Fig. 4.

Effects of focal fungi on seed germination and viability. Bars represent the proportion of germinated (Left axes) or viable (Right axes) seeds after inoculation with each of four fungal isolates. Viable seeds are defined as the proportion of seeds that were alive (determined via the tetrazolium test, see Methods) but failed to germinate, plus seeds that germinated. Bar heights correspond to values normalized by the maximum number of germinated or viable seeds observed in controls (noninoculated seeds). In some cases, inoculated seeds of Trema micrantha “black” germinated more than control seeds, so proportions exceed 1. Because surface sterilization after inoculation did not influence the germination or viability of seeds (SI Appendix, Table S10), bars include both treatments. Error bars represent SE. Fungal isolates were cultured from buried seeds that were viable (+) or inviable (−) from AM, Apeiba membranacea; AS, Annona spraguei; OP, Ochroma pyramidale; and TB, Trema micrantha “black.” Isolates 1 and 2 represent the same OTU at 99% sequence similarity, but different OTU at 100% sequence similarity (SI Appendix, Table S11).