||Forests are vital ecosystems because they capture and store carbon from our atmosphere;
this is essential for maintaining the balance between carbon uptake and release in
the global carbon cycle. However, human activities—mainly burning fossil fuels—are
releasing carbon dioxide (CO2) into our atmosphere at a rate we are not confident
forests can keep pace with. As we face climate change, it is critical to understand
exactly how forests respond to elevated CO2 to inform best forest management practices.
Since trees utilize CO2 to photosynthesize, it is generally thought that increased
amounts of CO2 will increase biomass, which will create a positive feedback loop for
carbon sequestration. However, there are likely limitations—namely of nutrients—to
production that prevent this from being the case. This study aims to better understand
such tree nutrient limitations and responses to climate change conditions through
the most realistically simulated means possible.
This project uses data from the Duke Forest Free-Air CO2 Enrichment (FACE) experiment
to investigate the effect of CO2-fumigation and nitrogen (N)-fertilization on nutrient
concentrations in trees in their natural ecosystems. The experiment began in 1993
in the Blackwood Division of the Duke Forest in Durham, North Carolina on a 90-ha
loblolly pine plantation with relatively acidic soils of low fertility. The split-plot
randomized block experimental design had two main levels: first, four of the eight
30mdiameter plots were fumigated with elevated CO2; next all eight plots were divided
in half and only one half was fertilized with N. This created four distinct treatment
groups: Ambient Control, Ambient Fertilized, Elevated Control, and Elevated Fertilized—each
represented by four “half-plots.” Starting in 2010, loblolly pine and broadleaved
foliage, wood, and roots were harvested and analyzed for the six main plant nutrients:
carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium
(Mg). We hypothesized that low concentrations of any other main plant nutrients—not
just N—would mute trees’ response to elevated CO2. That is, fertilizing with only
N would not be sufficient to alleviate nutrient limitations.
Data from the Duke FACE experiment were analyzed for a signature of any nutrient limitations
under elevated CO2. Linear mixed models with random effects were created for each
main plant element (C, N, P, K, Ca, and Mg) in each tree component. These components
included foliage, branch wood, stem wood, and roots for loblolly pine, sweetgum, a
collection of other broadleaved species, small understory specimens, and vines. The
statistical models were used to assess whether nutrient concentrations were significantly
affected by elevated CO2 and N-fertilization.
Overall, elevated CO2 and N-fertilization did not significantly affect any nutrient
concentrations across all species in all aboveground components. We then compared
Duke FACE loblolly pine foliar nutrient ratios to reference values—or values that
are considered adequate for normal tree growth—and found that
the system was P- and K-limited (P:N and K:N were 24% and 26% below adequate). In
fact, adding N not only failed to alleviate P- and K-limitations, but significantly
Interestingly, we found a strong response to elevated CO2 and N-fertilization in the
roots, especially in loblolly pine. Under N-fertilization, [P] was almost 16% higher
and [K] was 29% higher under elevated CO2 than under ambient CO2 in pine roots, suggesting
increased root production and exploration for the elements that limited tree production.
The additional C supplied by elevated CO2 and exacerbated nutrient limitation by N-fertilization
created a high demand for P and K to support increased biomass production. In response,
root biomass increased, and P and K were locked belowground and used locally.
The Duke FACE experiment is special because it allows us to understand how trees will
respond to future climate change conditions before they happen. With this vital information,
we can implement preemptive forest management practices that support continued production
and carbon sequestration under elevated CO2. The results of this study underscore
that elevated CO2 will only increase tree production when all main plant nutrients
are adequately available. Thus, balanced fertilizers, rather than only N-fertilizers,
should be applied to forests to ensure that our global carbon cycle is maintained
during a most imperative time.