Browsing by Subject "savanna"

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    Data supporting: Century-scale wood nitrogen isotope trajectories from an oak savanna with variable fire frequencies
    (2020-09-09) Trumper, Matthew L; Griffin, Daniel; Hobbie, Sarah E; Howard, Ian M; Nelson, David M; Reich, Peter B; McLauchlan, Kendra K; trump022@umn.edu; Trumper, Matthew L; University of Minnesota Griffin Research Lab
    Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. A single fire event often increases short-term nitrogen (N) availability to individual plants, but repeated burning causes ecosystem carbon and 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 leveraged one of the longest running prescribed burn experiments (established in 1964) to evaluate how fire frequency and changing vegetation composition influenced wood stable N isotopes (δ15N) across space and time. We developed multiple δ15N records across a burn frequency gradient from precisely dated Quercus macrocarpa tree-rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Sixteen trees were sampled across four treatment stands that varied in 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 burned in four of every five 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 δ15N trajectories would have declined over time in all burned stands, but at a rate proportional to fire frequency. We found that wood δ15N records within each stand were remarkably coherent in their mean state and trend through time. A gradual, temporally synchronous decline in wood δ15N 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 δ15N that could be attributed directly to long-term fire frequency in this prescribed burn experiment in temperate oak savanna. Our wood δ15N results may instead reflect decadal-scale changes in vegetation composition and abundance due to early to mid 20th century fire suppression.
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    Fire and vegetation effects on productivity and nitrogen cycling across a forest-grassland continuum
    (Ecological Society of America, 2001) Reich, Peter B; Peterson, David W; Wedin, David A; Wrage, Keith
    Mixed tree–grass vegetation is important globally at ecotones between grasslands and forests. To address uncertainties vis-à-vis productivity and nitrogen (N) cycling in such systems we studied 20 mature oak savanna stands, ranging from 90% woody dominated to 80% herbaceous dominated, growing on comparable soils in a 32-yr-old fire frequency experiment in Minnesota, USA. Fire frequencies ranged from almost annual burning to complete fire protection. Across all stands, aboveground net primary productivity (ANPP) ranged from 2 to 12 Mg·ha−1·yr−1, decreased with fire frequency (r2 = 0.59), increased with woody canopy dominance (r2 = 0.83), and increased with soil net N mineralization rates (r2 = 0.79), which varied from 25 to 150 kg·ha−1·yr−1. ANPP was positively related to total biomass (r2 = 0.95), total canopy leaf N content (r2 = 0.88), leaf area index (LAI; r2 = 0.87), annual litterfall N cycling (r2 = 0.70), foliage N concentration (r2 = 0.62), and fine root N concentration (r2 = 0.35), all of which also increased with increasing tree canopy cover. ANPP, soil N mineralization, and estimated root turnover rates increased with woody canopy cover even for stands with similar fire frequency. ANPP and N mineralization both decreased with fire frequency for stands having a comparable percentage of woody canopy cover. Fine root standing biomass increased with increasing grass dominance. However, fine root turnover rate estimated with a nitrogen budget technique decreased proportionally more with increasing grass dominance, and hence fine root productivity decreased along the same gradient. Via several direct and indirect and mutually reinforcing (feedback) effects, the combination of low fire frequency and high tree dominance leads to high rates of N cycling, LAI, and productivity; while the opposite, high fire frequency and high grass dominance, leads to low rates of N cycling, LAI, and productivity. Carbon and N cycling were tightly coupled across the fire frequency and vegetation type gradients.
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    Pathways in old-field succession to white pine: Seed rain, shade, and climate effects
    (2005) Dovčiak, Martin; Frelich, Lee E; Reich, Peter B
    Trees slowly colonize old fields on sandy outwash in the prairie–forest ecotone of the north-central United States, and in the absence of fire, succession is expected to proceed toward oak woodland. We analyzed whether a case of unusually rapid and spatially extensive invasion by white pine (Pinus strobus) could be explained by the presence of specific temporal or spatial opportunity windows suitable for such invasion. We tested whether the invasion was temporally restricted to the period immediately after abandonment or to periods of favorable climate, and whether it was spatially restricted to areas of high seed rain or high forest-edge shade. White pine invasion into the field occurred in two waves separated from each other by a 1987–1989 drought period. The first wave (1980– 1985) occurred during a period of average climate and led to the establishment of dense sapling patches in shade near forest edges. The second wave (1991–1994) occurred during a period of high precipitation and cooler than normal temperature, and resulted in colonization of the unshaded field center. In addition to the two temporal windows, white pine invasion occurred within two spatial windows: in areas highly sheltered by forest edge and in areas receiving high white pine seed rain. Overall these windows produced three different successional pathways: (1) a slow, creeping white pine invasion into highly shaded areas with low seed rain near forest edges; (2) a rapid, discrete-step invasion in areas where seed rain was abundant enough to overcome mortality in lower shade and where early arrivals facilitate filling in by later arrivals; and (3) a deferred invasion in the field center where low seed rain and lack of shade allowed the persistence of a grassland stage until favorable climate resulted in a white pine recruitment pulse. Temporal variation in climate can accelerate or decelerate any of the three successional pathways.