Interventions in nature last longer than expected

Lüneburg/Kiel. Human interventions in nature have unexpected long-term consequences, in some cases they may even be irreversible. This is the conclusion drawn by a new study conducted by Kiel University, Leuphana University Lüneburg, the Helmholtz-Centre for Environmental Research in Halle, and the University of Minnesota. Researchers compared forests replanted on previously cultivated fields on the Lüneburg Heath with historical old-growth forests that have survived undisturbed for several hundred years. They came to the conclusion that the composition of microorganisms in the soil differed significantly between the historical and the recent forest sites - even if the reforestation had occurred over 110 years ago. This has direct consequences on the functioning of the ecosystem: reforested trees grow more quickly but they are more vulnerable to droughts. The research was published in the journal Soil Biology and Biochemistry.

In cultivating land, humans intervene in the ecosystem in diverse ways: they replace the original vegetation, improve the supply of nutrients through periodical fertilization, plow back harvested plants, and break up the soil in the process. This all has grave consequences for the life growing under the topsoil: fungi, which develop a symbiotic relationship with the new plants, quickly establish themselves.  Some soil bacteria profit from the new circumstances and spread rapidly, whereas other varieties disappear.    

These changes have far-reaching consequences. “Fungi and bacteria control the essential redevelopment processes in the soil,” said Dr. Andreas Fichtner of the Christian Albrecht University in Kiel. “Some fixate nutrients from the air, such as nitrogen. Others break down dead organic substances. They, therefore, influence the nutrient cycle immensely.”
What happens when trees are replanted on previously cultivated fields? Does the microbial composition below ground return to its original state?  “It was previously assumed,” said Professor Dr. Goddert von Oheimb of Leuphana University Lüneburg, “ given how quickly new bacteria and fungi spread, that the original species would in principle reestablish themselves again. After 20 or 30 years everything was supposed to be like it had been earlier - that is at least what we thought.”

This seems to have been fallacy, for, at least in the research sites in the Lüneburg Heath, the conditions still raise serious concerns, even after 110 years of reforestation. The soil in the original oak forests contains significantly fewer microorganisms when compared to recently reforested areas: the overall mass of isolated bacteria and fungi per cubic meter of soil was 44 percent lower.  Furthermore, the microbial degradation activity in historical oak populations is substantially lower.

Even the composition of microbial biomass varies depending on previous usage - presumably, because microorganisms respond strongly to the chemical properties of the soil: the soil on formerly cultivated land contains two to three times as much nitrogen and phosphorous compared to ancient forests because of previous fertilization.
Reforested areas more vulnerable to droughts

At first glance this data seems like good news for repopulated forests: nitrogen and phosphorous stimulate plant growth, and the high volume and greater activity of microorganisms in the soil improves the availability of these important nutrients for the plants. These conclusions were also confirmed when the researchers examined tree rings. On average, oaks on previously cultivated land showed significantly wider rings than trees in legacy forests. In other words, they grew more quickly. This was, however, not always the case: some annual rings were very thin - giving the impression that the trees had hardly grown at all during these periods. Such sharp variations were not found in historical oak populations.
 “Trees on reforested areas seemed to have more sensitive reactions to unfavorable environmental conditions,” Goddert von Oheimb said, interpreting the data. Paradoxically, one explanation could be the favorable nutrient supply which in good times leads to turbo-growth: the more nutrients are available, the more leaves form on the tree. Under ideal conditions, the tree can thus produce more energy-rich matter through photosynthesis and thereby grow more quickly. Over a larger surface area of foliage, the tree then looses more water to its surroundings.  This can become a problem during droughts.

Additionally, oaks on previously cultivated land have shallower roots: because the soil was plowed regularly, the soil layers close to the surface were comparatively looser and contained more nutrients. The advantage for the trees was that they could extend their roots just under the ground’s surface. However, during periods of low rainfall, these particular layers dry out much more quickly than the lower layers. “Though still just a theory,” Andreas Fichtner concedes, “it is nevertheless supported by the current research.”

No return to the original condition?

In the north German lowlands almost two thirds of forests exist on previously cultivated fields, meadows or heaths. “Every forest has its own history,” Fichtner said. “Therefore, we are examining whether our results from the Lüneburg Heath can be applied to other sites.” The first indications are that this is indeed the case.

It still remains unclear whether microbial biomass will return to its original condition again in the next 50 or 100 years. Goddert von Oheimb: “It is perfectly plausible that alterations humans create are irreversible.”
Effects of anthropogenic disturbances on soil microbial communities in oak forests persist for more than 100 years; A. Fichtner, G. von Oheimb, W. Härdtle, C. Wilken, J.L.M. Gutknecht; Soil Biology and Biochemistry, volume 70, March 2014, Pages 79–87

Professor Dr. Goddert von Oheimb
Institute for Ecology, Leuphana University Lüneburg
Telephone: 04131/677-2845

Dr. Andreas Fichtner
Institute for Natural Resource Conservation,
 Christian-Albrechts-University in Kiel
Telephone: 0431/880-1198