Century-scale wood nitrogen isotope trajectories from an oak savanna with variable fire frequencies
<p>Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. Individual fires often increase short-term nitrogen (N) availability to plants, but repeated burning causes ecosystem N losses and can ultimately decrease soil organic matter and N availa...
Main Authors: | , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2020-09-01
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Series: | Biogeosciences |
Online Access: | https://bg.copernicus.org/articles/17/4509/2020/bg-17-4509-2020.pdf |
Summary: | <p>Fire frequency exerts a fundamental control on productivity and nutrient
cycling in savanna ecosystems. Individual fires often increase
short-term nitrogen (N) availability to plants, but repeated burning
causes ecosystem N losses and can ultimately decrease soil organic
matter and N availability. However, these effects remain poorly
understood due to limited long-term biogeochemical data. Here, we
evaluate how fire frequency and changing vegetation composition
influenced wood stable N isotopes (<span class="inline-formula"><i>δ</i><sup>15</sup>N</span>) across space and
time at one of the longest running prescribed burn experiments
in the world (established in 1964). We developed multiple <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> records
across a burn frequency gradient from precisely dated <i>Quercus macrocarpa</i> tree rings in an oak savanna at Cedar Creek Ecosystem
Science Reserve, Minnesota, USA. Sixteen trees were sampled across four
treatment stands that varied with respect to the temporal onset of burning and burn
frequency but were consistent in overstory species representation, soil
characteristics, and topography. Burn frequency ranged from an unburned
control stand to a high-fire-frequency stand that had burned in 4 of
every 5 years during the past 55 years. Because N stocks and net N
mineralization rates are currently lowest in frequently burned stands,
we hypothesized that wood <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> trajectories would decline
through time in all burned stands, but at a rate proportional to the fire
frequency. We found that wood <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> records within each stand
were remarkably coherent in their mean state and trend through time. A
gradual decline in wood <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> occurred in the mid-20th
century in the no-, low-, and medium-fire stands, whereas there was no
trend in the high-fire stand. The decline in the three stands did not
systematically coincide with the onset of prescribed burning. Thus, we
found limited evidence for variation in wood <span class="inline-formula"><i>δ</i><sup>15</sup>N</span> that could
be attributed directly to long-term fire frequency in this prescribed
burn experiment in temperate oak savanna. Our wood <span class="inline-formula"><i>δ</i><sup>15</sup>N</span>
results may instead reflect decadal-scale changes in vegetation
composition and abundance due to early- to mid-20th-century fire
suppression.</p> |
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ISSN: | 1726-4170 1726-4189 |