By reconstructing the history of Australia's ash tree forests, researchers revealed that human disturbances from logging and post-fire salvage have more negative effects on the health of the ecosystem than natural disturbances from increasingly common wildfires.
The study, published April 8 in Ecology Letters, also found that forest disturbances influence the ecosystem indirectly by interfering with plant-microbe interactions. The findings suggest that the mechanisms by which disturbances in the forest harm the ecosystem are more complicated than previously considered.
"When we look at disturbances in forests, we usually focus on one specific element of biota — be it trees, soils, animals, microbiota," said first author Elle Bowd, a postdoctoral fellow at Australian National University. "However, in doing this we often don't consider their relative interactions, which drive key ecological functions in forests."
Bowd went on to explain that disturbances in one element can trigger a "domino effect," whereby the entire system is thrown out of balance, making it much more difficult to predict how individual changes will impact the big picture.
This domino effect goes on to influence humans as well. About 31% of the Earth's surface is covered by forests, and in addition to their value to the lumber industry, forests provide valuable ecosystem services such as carbon storage, watershed protection and oxygen production.
Human disturbances including habitat encroachment and artificial light are known to interfere with important ecological processes. However, the researchers here were interested in disturbances that occur specifically in forests, namely logging and post-fire salvage, "a controversial practice whereby forests are logged immediately following a high-severity natural wildfire," as explained by Bowd.
While post-fire salvage may not appear as immediately destructive as regular logging because it occurs after the forest has already been damaged extensively by fire, salvaging removes structural components from the environment and disrupts animals' habitats. Salvaging has also been shown to reduce the overall level of biodiversity in forests.
And while wildfires are part of the natural life cycle of the forest, helping to clear away dead material, enrich the soil and even help certain seeds germinate, they are occurring more frequently due to climate change, leading to adverse effects on ecosystems.
As wildfires increase in frequency, the need to clarify the effects of post-fire salvage and compare them to those of wildfires becomes more pressing. To tackle this question, Bowd and her colleagues took to the ash forests of Australia to collect their data.
"My research was underpinned by extensive fieldwork, where I collected soils and plant record data from forests that had experienced different disturbance histories of wildfire, clear-cut logging and salvage logging," Bowd said. "In the lab, I then used environmental DNA from the soils to identify and describe microbial communities. Soil nutrient data was also extracted."
Using all this data, the researchers were able to reconstruct a 150-year chronosequence, a model of the forest changing through the years. This allowed them to examine the impact of different disturbances over time.
They found 68 direct and indirect disturbance effects, with the majority resulting in ecosystem-wide adverse effects. These effects included changes to species richness for fungi, plants and microbes, as well as changes in the concentrations of important soil nutrients such as nitrogen, potassium and phosphorous.
About 43% of these effects were indirect, meaning that they didn't occur as an immediate consequence of the disturbance, instead being triggered by the domino effect of interactions between plants and soil microbes. The team also found that the type of disturbance affected the severity of the impacts.
"Disturbance impacts were most complex, extensive and adverse in forests that were salvage-logged," according to Bowd. The team also found that in general, human disturbances accounted for far more adverse ecosystem consequences than wildfires, which aligns with the role natural wildfires play in rejuvenating the ecosystem.
The researchers are next interested in looking at the relative effects of forest disturbances more deeply to discover the precise chain of events that leads to these ecosystem impacts. They also hope future researchers will consider the indirect effects of forest disturbances more carefully.
"Our research emphasizes the importance of looking at both direct and indirect disturbance impacts in forests (above- and below-ground) to quantify the full burden of relative disturbances and make informed management decisions," Bowd said.
More broadly, the research suggests a change in forest management strategy.
As Bowd put it, the research "[calls] on managers to consider the interactive nature of disturbance impacts and their relative effects on key functions in forests above- and below-ground, especially in the face of a rapidly changing climate and environment."
The study, "Direct and indirect disturbance impacts in forests," published April 8 in Ecology Letters, was authored by Elle J. Bowd, Sam C. Banks and David B. Lindenmayer, Australian National University; Andrew Bissett, The Commonwealth Scientific and Industrial Research Organization; and Tom W. May, Royal Botanic Gardens Victoria.