Introduction

Neotropical forests are increasingly threatened by fire and clearing for pastures and croplands, but also have the potential to recover rapidly through both unmanaged and active reforestation. There is growing awareness that microbes which can form symbioses with plants—such as nitrogen-fixing bacteria and arbuscular mycorrhizae—may be critical for nutrient cycling and forest recovery. However, it is unclear how active the management of plant species and their symbionts must be for successful restoration.

Objective

What can we learn about plant-microbe symbioses from unmanaged recovery of Neotropical secondary forests to inform how critical they are for active restoration? More specifically, do symbiotic nitrogen fixation and mycorrhizal colonization consistently increase early in succession to aid in forest recovery?

Methods

First, we quantified changes in plant-microbe symbioses during forest recovery in two tropical forests: 1) In the southeastern Amazon, we studied the dynamics of nitrogen-fixing trees and symbiotic nitrogen fixation in a forest that was recovering from prescribed fires; 2) In the Panama, we examined how mycorrhizal colonization, used to represent plant investment in mycorrhizae, changed across secondary succession from clearcutting.

Results

In the southeastern Amazon, the basal area of putative nitrogen-fixing trees increased from about 1% to about 10% seven years after the last fire disturbance. However, symbiotic nitrogen fixation rates did not increase to the same extent, from negligible rates to ~0.5 kg N ha-1 yr-1, much lower than other Neotropical secondary forests. In Panama, total arbuscular mycorrhizal colonization did not increase or decrease across forest age, although components of mycorrhizal colonization did (such as the presence of arbuscules alone).

Conclusions

The low rates of symbiotic nitrogen fixation in the southeastern Amazon are likely due to a combination of high nitrogen availability and the species composition of nitrogen-fixing trees. The lack of a strong change in mycorrhizal colonization across forest age in Panama may be due to a high investment in mycorrhizae, the methodology of measuring plant investment in mycorrhizae, or because mycorrhizae themselves can range from symbiotic to parasitic. Although symbioses can change dynamically during unmanaged forest succession, they likely vary depending on baseline soil nutrient availability and other factors that are still unknown, implying a lack of a “one-size-fits-all” approach to restoration. Ultimately, our findings suggest that the need for active management of plant-microbe symbioses may instead depend on how disturbed the seed bank and microbial communities are after forest clearing events.