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 Prescribed fires do more than that, though. New University of
Illinois research shows frequent fires can help keep invasive
plants in check by reducing nitrogen availability in soils.
“There's a lot of research that shows the opposite: Invasive
species respond positively to fire. That's one of the ways –
through disturbance – that many invasive species are thought to
establish. And I think that's still true. But our research shows
that repeated fire at a relatively high frequency can set in
motion ecosystem changes that can suppress invasive species over
time,” says Jennifer Fraterrigo, associate professor in the
Department of Natural Resources and Environmental Sciences at
Illinois and co-author on the new study.
When low-intensity fires move through a forest, they burn off
dead leaves, branches, and other organic matter in the top layer
of soil. Those materials typically contain a lot of carbon, but
there are other nutrients important for plant growth, like
nitrogen and phosphorus, tied up in that organic matter. Under
normal circumstances, it can take a long time for those
nutrients to break down and become available to plants.
When fires come through, the nutrients bound up in organic
matter are released quickly. That can help plants, but only
temporarily. With frequent low-intensity fires, the total
nutrient pool is diminished over time. But it’s not always clear
why that’s happening.
“It could be that nitrogen is volatilized, but based on our
soils work, we think it might be related to the formation of
pyrogenic organic matter, similar to charcoal, which is more
difficult for microbes to break down. And once microbes become
nitrogen-limited, they're more likely to use the nitrogen for
their own growth, rather than make it available to plants.
“That's how the effect on invasive plants comes around, because
many of these plants require high levels of nutrients to
maintain their high rates of growth,” Fraterrigo says. “And so
now, when you get into a situation where they're nutrient
limited, invasive plants can no longer produce as much biomass.”
Fraterrigo and co-author Mara Rembelski made the connection
between fire frequency, invasive plant growth, and microbial
nitrogen cycling by comparing forest stands with different fire
histories in southern Illinois. In some stands, managers had
used prescribed fire treatments every 3-4 years between 1990 and
2010. The other stands, part of the same forest, had not burned
for at least 50 years before the study.
To level the playing field, the researchers burned plots in both
types of stands once in spring 2017, and then set to work
measuring biomass and tissue composition of the invasive grass
Microstegium vimineum and soil microbes during the following
growing season. They compared unburned control plots with burned
plots, but that one fire in 2017 didn’t make a major difference.
“The purpose of that burn in 2017 was to account
for any confounding effects of time since the forest last
burned. There's a lot of research that shows the effects of fire
really change over time. We didn't want to have artifacts like
that,” Fraterrigo says.
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What really mattered was the legacy of fire frequency
in each stand. For example, the researchers found aboveground
biomass of Microstegium was 87% lower in stands subjected to
frequent fire.
And, as expected, they found evidence that nitrogen limitation was
responsible.
Not only was there 22% less dissolved nitrogen in the soil in the
stands with frequent fire, plant and microbial tissues also showed a
higher carbon-to-nitrogen ratio, indicating nitrogen limitation.
Soil microbial enzymes targeting nitrogen uptake became more active
in stands with frequent fire, as well.
“These enzymes are a really good indicator of how microbes are
seeing resources. The enzymes are metabolically expensive to make,
so you'd only want to invest in them to acquire resources you really
need. So if you're energy or carbon starved, you're going to produce
one set of enzymes, but if you're nitrogen starved, you're going to
activate other enzymes,” Fraterrigo says. “Those nitrogen-starvation
enzymes were what we saw in the frequent fire stands.”
Not surprisingly, pyrogenic organic matter, charcoal-like material
resulting from incomplete combustion of biomass, was higher in
stands with frequent fire. The researchers think this material,
which keeps nutrients bound up and harder to access by microbes,
explains the nitrogen limitation in those stands. They’re planning
future studies to investigate the formation of pyrogenic organic
matter more closely.
In the meantime, Fraterrigo says her results could help managers
look at fire in a new way.
“Fire is a great tool, but managers have concerns about using it to
control invasive plants because of this long-standing belief that
invaders are promoted by fire,” she says. “But we’re providing new
evidence that suggests if we burn frequently enough we could
actually have the opposite effect. Is it license to go out and burn
a lot? Not necessarily, because we still don’t really know how
native species are going to respond. But it’s something to consider
for invasive plants that fit the same profile as Microstegium.”
The article, “Frequent fire reduces the magnitude of positive
interactions between an invasive grass and soil microbes in
temperate forests,” is published in Ecosystems [DOI:
10.1007/s10021-021-00615-x]. The research was supported by USDA’s
National Institute of Food and Agriculture.
The Department of Natural Resources and Environmental Sciences is in
the College of Agricultural, Consumer and Environmental Sciences at
the University of Illinois.
[Source: Jennifer Fraterrigo
News writer: Lauren Quinn] |
