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Data from: Repeated clearing as a mechanism for savanna recovery following bush encroachment

Wedel, Emily1; Nippert, Jesse1; O'Connor, Rory2; Nkuna, Peace3; Swemmer, Anthony3

Published Apr 16, 2024 on Dryad. https://doi.org/10.5061/dryad.tht76hf6j

Cite this dataset

Wedel, Emily et al. (2024). Data from: Repeated clearing as a mechanism for savanna recovery following bush encroachment [Dataset]. Dryad. https://doi.org/10.5061/dryad.tht76hf6j

Abstract

Many savannas are experiencing increased cover of trees and shrubs, resulting in reduced herbaceous productivity, shifts in savanna functional structure, and potential reductions in ecotourism. Clearing woody plants has been suggested as an effective management strategy to mitigate these effects and restore these systems to an open state with higher rates of grass production and herbivory. This study investigated the effectiveness of repeated shrub clearing as a tool to mitigate bush encroachment in a semi-arid savanna in Southern Africa.

We present data from a 7-year experiment in the Mthimkhulu Game Reserve bordering Kruger National Park, South Africa. Colophospermum mopane stems and re-sprouting shoots were basally cut 2-3 times per year (2015-2022) in 3 pairs of treatment and control plots of 60 m x 60 m. We monitored changes in soil moisture, grass biomass, and herbivore activity via dung counts. We assessed C. mopane physiological responses to repeated cutting using non-structural carbohydrates and stable water isotopes to infer changes to energy storage and functional rooting depth, respectively.

The cleared treatment had higher soil moisture and grass biomass than the control treatment. Dung counts showed impala and buffalo visited the cleared treatment more frequently than the control treatment.

Repeated cutting had limited effects on C. mopane survival in the first 2-3 years after initial clearing, but 80% of individuals were dead after 7 years. Repeatedly cut C. mopane had lower belowground starch concentrations and used water from shallower soil depths than C. mopane in control plots.

Synthesis and applications: Repeated cutting increased soil moisture availability and grass biomass, and attracted charismatic grazing herbivores. While more costly than once-off clearing methods, this practice created more employment opportunities for a neighbouring rural community. Transforming portions of the ecosystem to a grass-dominated state may increase ecotourism potential through improved game viewing in open systems.

README: Repeated clearing as a mechanism for savanna recovery following bush encroachment

https://doi.org/10.5061/dryad.tht76hf6j

We established 3 pairs of treatment and control plots of 60 m x 60 m at Mthimkhulu Game Reserve in Limpopo Province, South Africa. Colophospermum mopane stems and resprouting shoots were basally cut in the treatment plots 2-3 times per year from 2015-2022. We monitored changes in soil moisture, grass cover and biomass, and herbivore activity via dung counts. We assessed changes in C. mopane physiology in response to repeated clearing using non-structural carbohydrates and stable water isotopes to infer changes to energy storage and functional rooting depth, respectively.

Description of the data and file structure:

Data for analyses provided in Wedel et al. (2024) - Repeated clearing as a mechanism for savanna recovery following bush encroachment. There are 8 data files that correspond to the response variables mentioned above. Below are the descriptions for each column within each file. Columns for Date, Year, Treatment, Block, and Plot are the same across files.

A. soil_moisture

Average daily soil moisture at 10, 30, and 80 cm soil depths. There was one probe per depth in each plot. Growing season is from November through April.

  • Date = date data were collected. Formatted as year-month-day.
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • Depth = soil depth (10, 30, or 80 cm)
  • VWC = average daily volumetric water content

B. mopane_mortality

In each cleared plot, we recorded whether the nearest neighbor of a randomly selected C. mopane shrub was dead or alive until 100-250 shrubs were recorded.

  • Date = date data were collected. Formatted as year-month-day.
  • Treatment = Cleared only
  • Block = block ID (1-3)
  • Plot = plot ID (1b, 2b, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • n_Dead = number of dead C. mopane individuals.
  • n_Sampled = total number of C. mopane individuals counted.

C. mopane_frequency

Frequency of C. mopane size classes. We established two 50 m long transects in each plot. At each 1 m point along each transect we measured the presence of C. mopane cover directly overhead. The height of each C. mopane shrub was measured and categorized into one of four height classes. The number of C. mopane in each height class was summed across the two transects in each plot. Frequency of each height class was calculated as the percentage of points with shrub cover directly overhead for each size class.

  • Year = year of sampling. Sampling took place in March or April each year.
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • Size_class = C. mopane height size class. small = < 0.5 m. med = 0.5 - 2 m. large = 2 - 5 m. xlarge = > 0.5 m.
  • Frequency = % of points along two 50 m transects with C. mopane cover directly overhead for each size class.

D. grass_cover

Average distance (cm) to nearest perennial tuft and width of tuft (cm) in each plot. Averages were derived from two 50 m transects in each plot. At each 1 m point along each transect we measured the distance to the nearest perennial grass tuft and the width of the tuft.

  • Year = year of sampling
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • Avg_dist = distance to the nearest perennial grass tuft (cm) averaged across the two transects in each plot.
  • Avg_width = width of the nearest perennial grass tuft (cm) averaged across the two transects in each plot.

E. grass_biomass

Grass biomass (g m^-2) measured at the end of each growing season. Grass was clipped in four - six 0.5 m x 0.5 m quadrats randomly located in each plot.

  • Year = year of sampling
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • Quadrat = quadrat number (1-6)
  • Grass_biomass = biomass of grass in each quadrat (g m^-2)

F. herbivore_dung

Total standardized dung counts and dung deposition per day for each herbivore species and sampling date in each plot. Totals were derived from two belt transects in each plot. The number of dung piles for each herbivore species was summed across the two transects. Transect lengths varied for a few sampling dates and dung counts were standardized to a 50 m length.

  • Date = date data were collected. Formatted as year-month-day.
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • Species = herbivore species ID
  • Standardized_dung = standardized dung counts for each sampling date summed across the two transects in each plot.
  • Days_between = number of days between sampling dates.
  • Deposition_per_day = dung deposition per date calculated as Standardized_dung divided by Days_between

G. stable_water_isotopes

Stable hydrogen and oxygen isotopes (per mille) of soil water and xylem water for grasses and C. mopane shrubs in cleared and control plots.

  • Year = year of sampling.
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • Type = sample type (grass, C. mopane, or soil)
  • Depth = soil depth sampled (cm). Plant samples are labelled "xylem" here.
  • dD = hydrogen stable isotopic signature (per mille)
  • d18O = oxygen stable isotopic signature (per mille)
  • PC1 = Value of PC1 axis. PCA analysis collapsed dD and d18O into a single metric.

H. NSC

Glucose, sucrose, and starch concentrations (mg g^-1) of C. mopane belowground stems/boles in technical replicates (duplicate or triplicate).

  • Year = year of sampling.
  • Treatment = Cleared or Control
  • Block = block ID (1-3)
  • Plot = plot ID (1a, 1b, 2a, 2b, 3a, 3b). Number refers to the block ID and letter refers to treatment. a = control. b = cleared.
  • NSC_type = nonstructural carbohydrate type (glucose, sucrose, or starch)
  • Concentration = concentration of nonstructural carbohydrate on a per mass dry weight basis (mg g^-1)

Funding

National Science Foundation, Award: 1928875

South African Department of Forestry, Fisheries, and Environment, Environmental Monitors Program

National Research Foundation, South African Environmental Observation Network

Kansas State University, Division of Biology