Soil impacts on microbes in forests exposed to degradation

A Spanish study reveals a little-known dynamic in the soil-microbe system. Soil, the authors explain, affects microorganism communities more than plant life cycles do

by Matteo Cavallito

 

Seasonal dynamics and microbial community composition – with a variable presence of fungi and bacteria – in declining forests depend mainly on the interaction between major functional groups and soil conditions. The life cycle of trees, on the other hand, plays a minor role at the same time. This is the main conclusion of a study published in the journal Science of The Total Environment.

The research, which involved scholars from the Instituto Pirenaico de Ecología (IPE-CSIC) in Zaragoza and the Instituto de Recursos Naturales y Agrobiología in Salamanca, focused on forests in the Mediterranean area, one of the European regions most affected by the effects of climate change.

The role of microbes in declining forests

“In a context of rising temperatures and more frequent and severe droughts, the occurrence of drought- and heat-induced forest mortality episodes is expected to increase leading to shifts in forest composition,” the study explains. In the Mediterranean region, the scientists continue, “rising temperatures are leading to changes in species composition towards more drought-adapted species.” Anyway, the soil microbiome may therefore contribute to the mortality of trees most vulnerable to this phenomenon.

However, “The close relationship between trees and soil microbes makes it challenging to determine what is the cause and what is the effect in forest decline.”

The microbiome, in short, plays an important role in forest functioning, but the impact of drought-induced decay and tree death on microbes is still unclear. The study therefore set out to unravel these dynamics.

The study

The researchers used phospholipid fatty acid analysis to quantify soil microbial biomass and study its seasonal changes in three different Mediterranean forests undergoing defoliation. In the study, the authors also measured the microclimatic parameters and the physical and chemical parameters of the soil under trees with different vigor. Tree vigor was assessed by the degree of crown defoliation. These factors were then related to seasonal changes in the microbial community.

Overall, the structure, biomass and relative presence of the major functional groups of the soil microbiome vary considerably with forest characteristics and seasons in contexts of continuous decay with high plant mortality.

However, the authors explain, “while the seasonal dynamics show predictable patterns, which should be accounted for in future studies, the within-site variation is highly variable and mainly depends on soil physical and chemical parameters.”

Crucial information for Mediterranean forests

The findings, therefore, “iindicate that the structure of the soil microbial communities, their biomass and the relative abundance of key functional groups, exhibit significant seasonal dynamics in declining Mediterranean pine forests.” Moreover, “these dynamics are not consistent between forests dominated by different pine species, fueling the complexity of soil microbial community variations.”

The soil microbiome, the scientists recall, can promote water and nutrient uptake by trees through its interaction with roots. Therefore, deterioration of the plant-microbiome interaction can weaken the trees’ resistance to drought.

Therefore, information about the change in the composition of the soil microbial community under trees with and without symptoms of drought-induced decline is really important. Indeed, knowing what is happening in the soil, the research concludes, “might be particularly relevant for Mediterranean pine forests”. These environments, in fact, “show evident signs of vigor loss (crown defoliation and needle browning) since it can guide potential management activities to improve forests and their soil microbiomes and to make them more tolerant to drought stress.”