DOI: 10.5061/dryad.r4xgxd2k7
title: Kruger National Park EBP carbon

description: Woody plant encroachment (WPE) is a global trend occurring in many biomes
 including savannas and accelerates with fire suppression. Since WPE can
 result in increased storage of soil organic carbon, fire management which
 may include fire suppression can enhance ecosystem carbon sequestration in
 savannas. At our study site in Kruger National Park, South Africa, we used
 a long-term (~70-year) fire experiment to study the drivers and
 consequences of changes in woody (trees and shrubs) cover and soil organic
 carbon (C) sequestration. We surveyed four fire manipulation treatments
 replicated at eight locations within the park, annual high-intensity burn,
 triennial high (dry season) and low-intensity (wet season) burns and fire
 exclusion, to capture the range of fire management scenarios under
 consideration. Changes in woody cover were calculated over a period
 similar to the experiment’s duration (~80 years) using aerial photographs
 (1944–2018). Soils were analysed for soil organic C (SOC) and d13C to 30
 cm, under and away from tree canopies to isolate local- and
 landscape-level effects of WPE on SOC. The largest increases in woody
 cover occurred with fire exclusion. We found that plots with greater
 increases in woody cover also had higher SOC. Yet, trees were not the only
 contributor to SOC gains, such that sustained high inputs of C4-derived
 carbon (from grasses), even under canopies in fire suppression plots,
 contributed significantly to SOC. We observed little difference in SOC
 sequestration between the cooler triennial (wet season) burns and fire
 suppression. Synthesis: Grass inputs to SOC remained high across the full
 range of woody cover created by varying burning regimes. Total SOC stocks
 stored from tree inputs only matched grass-derived SOC stocks after almost
 70 years of fire exclusion. Our results point to C4 grasses as a resilient
 contributor to SOC under altered fire regimes and further challenge the
 assumption that increasing tree cover, either through afforestation
 schemes or fire suppression, will result in large gains in C sequestration
 in savanna soils, even after 70 years.
description: Soil samples were obtained under
 and away from the canopies of five large (&gt; 6 m)
 <em>Sclerocarya birrea</em> trees on each plot in 2016.
 Under-canopy samples were taken halfway between the tree trunk and the
 edge of the canopy. Away from canopy samples were taken away (&gt; 5
 m) from tree canopies. After litter was scraped away, a trowel was used to
 sample to 5 cm and, below this depth, a 4 cm-diameter soil auger was used
 to sample from 5-30 cm (at increments of 5–10 cm, 10–20 cm, and 20–30 cm)
 and soils kept separate for each depth. The soil was dry sieved through a
 2 mm sieve to remove all roots after which a bulk sample was taken to make
 one sample per canopy and one away from canopy per plot per depth (two
 vegetation types ´ 4 replicates
 (strings) ´
 4 fire treatments ´ canopy/away from
 canopy ´ 4
 depths = 256 samples). After soil sampling, soil texture was analysed at
 Elsenburg Laboratory (Western Cape, South Africa) following the hydrometer
 method for soil particle analysis (Committee, 1990). Soil carbon and
 <sup>13</sup>C/<sup>12</sup>C ratios of the soil
 carbon were determined using a Thermo Finnigan Delta plus XP mass
 spectrometer coupled with a conflo III device to a Thermo Finnigan Flash
 EA1112 Elemental Analyser with automatic sampler (Thermo Electron, Bremen,
 Germany). Although we did not expect to find carbonates in our study soils
 based on the pH values of the soils, we treated a subset (about 25%) of
 the soil samples with HCl to remove carbonates and reran these for
 variation in the C content, which confirmed the absence of inorganic
 carbon. These results were calibrated relative to Pee-Dee Belemnite as
 well as to correct for drift in the reference gas. The results are
 expressed as parts per thousand (‰) and relative to the Pee-Dee Belemnite
 standard are denoted by the term δ, with precision of duplicate analysis
 0.1‰ (February et al.,
 2011). Based on the δ<sup>13</sup>C values of the
 soil and end member (mean) δ<sup>13</sup>C values of the
 grasses (-13.17‰) and trees (-27.61‰) at our study site (February & Higgins, 2010),
 a standard end-member mixing model was used to determine the relative
 proportion of C<sub>3</sub> (trees)- and
 C<sub>4</sub> (grass)-derived carbon in the soil. This mixing
 model was only applied to the 0–30 cm soil horizon (surface soils) because
 of unrelated fractionation processes at deeper depths causing enrichment
 of soil δ<sup>13</sup>C unrelated to the inputs from
 C<sub>3</sub> or C<sub>4</sub> derived carbon
 (Balesdent & Mariotti, 1996;
 Nel et al., 2018). We
 calculated bulk density of the soil<em> </em>(Wigley et al., 2013) for each
 depth category and used this value to convert C concentrations to total C
 per volume of soil (i.e., stocks). Stocks of C per
 m<sup>2</sup> for each depth was calculated before summing
 these values and converting to total soil C per ha for soils from 0-30 cm
 deep (Mg ha<sup>-1</sup>). We then incorporated the impact of
 localised enrichment of tree canopies on SOC by weighting our calculation
 of C stocks by the relative tree cover using the following
 equation: plot total C (Mg
 ha<sup>-1</sup>) = ((proportion woody cover ´ soil
 C<sub>tree</sub>) + (proportion grass cover ´ soil
 C<sub>grass</sub>)), where grass cover equals 1 - woody
 cover.
description: # Kruger National Park EBP carbon *** Soil carbon for four treatments
 (annual Aug, triennial Aug, triennial Feb and no burn) at Pretoriuskop and
 Skukuza experimental burn plots in the Kruger National Park. ##
 Description of the data and file structure * EBP location (Pretoriuskop or
 Skukuza) * String (this is the repeat within the EBP location) * Treatment
 is fire treatment applied (aAug\_1 is the annual burn in August, bAug\_3
 is triennial burn in August (which is the end of the dry season), dnb
 stands for no burn and * cFeb\_3 is a triennial burn every February, which
 is height of growing season) * Tag is a unique qualifier that combined
 EBP, string and treatment * D<sup>13</sup>C: is the soil delta
 <sup>13</sup>C value * Fire: is the fire received per period
 (so a measure of how often fire returns, e.g., 0.1 will be once in ten
 years) * Fire\_int: I used the above Fire received to arbitrarily divided
 the interval into categories of return, either low or high, high was 0.8
 or above * Heights: should be read with columns J to N, it refers to the
 heights of the woody vegetation, so if lower than 5 m, then you can see
 how much biomass for instance in that height class, vs total *
 Grass\_biomass: was measured with a disk pasture meter before each burn,
 therefore "no burn" had no values The next four columns, kg\_ha,
 stems\_ha, kg\_stem and ba\_m2\_ha were computed from actual tree surveys
 conducted on the ground * kg\_ha: biomass worked out per ha using
 allometric relationships to convert stem diameter * stems\_ha: computed
 average stem per ha from survey data * kg\_stem: divided biomass by number
 of stems to get weight per stem in kg * ba\_m2\_ha: basal diameter was
 measured for each stem, so this is summing basal diameters to get basal
 area (m<sup>2</sup>) per ha * mg\_ha: Kg\_ha was transformed
 to mg\_ha, which is equivalent to ton per ha * Veg: the amounts under
 trees vs grasses, with total=the sum of these two * TC\_ha: used bulk
 density to work out total soil C per ha, ie converts percentage to per ha.
 * Soil C change: gives the difference between the carbon for Aug3 and the
 other treatments, we assumed that Aug3 was closest to the natural
 background fire return * tN\_ha, th same as for C but total nitrogen per
 ha * Sum\_grass and sum\_tree is how much of this is derived from grass vs
 trees. Can be read with column O (Veg), then you can see how much carbon
 under trees come from trees vs grass * Under sum\_grass, total is the
 total of the tree + grass values * Tree\_cover and grass\_cover (%) were
 obtained via aerial photos, and adds to a 100%. * Tree\_change gives the
 change in tree cover over 80 years, as obtained from aerial photos. *
 Enrich\_C: the differences between the Aug3 and the other treatments in
 delta 13C. * %N: percentage N in soil * P: percentage P in soil * K:
 percentage K in soil * Ca: percentage Ca in soil * Sand: percentage sand
 present in soil * Clay\_sand: clay:sand ratio * Claysilt\_sand:
 clay+silt:sand ratio ## Sharing/Access information This is a section for
 linking to other ways to access the data, and for linking to sources the
 data is derived from, if any. Links to other publicly accessible locations
 of the data: None Data was derived from the following sources: None