This study was conducted in a large conservation area, that is, a natural ecosystem excluding most of the anthropogenic threats that are present outside of the park. The study illustrated that in the duneveld the population studied was self-sustaining, with recruitment occurring and large individuals presumably dying of old age. Although fire caused a few individuals to coppice, no fire-related deaths were reported. In the Nossob riverbed, surveys started in a stand of predominantly young trees and the size class distribution at that stage already showed a lack of recruitment. This stand is ageing and will likely disappear at this site; however, new young stands are appearing at other sites in the Nossob riverbed. Under the current conditions with negligible anthropogenic influences, it therefore appears that some

Ecological patterns generated from a single snapshot in time should be used carefully when trying to identify the underlying processes driving the system (Jeltsch, Moloney & Milton

In arid savannas, the tree component may seem to be constituted by even-aged cohorts (Midgley & Bond

In the southern Kalahari,

From 1978, surveys were initiated in the Kalahari Gemsbok National Park (KGNP) to gather information on the growth rate, lifespan and population dynamics of

The KGNP is situated in the southern Kalahari (24°15’S–26°30’S and 20°00’E–20°45’E) and covers approximately 9600 km² (

Location of the two

The mean annual rainfall recorded from 1976 to 2015 at Nossob Rest Camp in the park is 194 mm (

Total annual rainfall (January to December) at Nossob Rest Camp from 1976 until 2016. The orange line represents the mean annual rainfall of 194 mm.

Sampling was conducted at two sites (

Two study sites (Dankbaar site and Grootkolk site) were surveyed on numerous occasions; however, survey methods used at these two sites differed. In 1978, all

At the 1 hectare (ha) Grootkolk site, height and stem circumference of all

Because of the different survey methods used at the two sites; the data collected could not be analysed in the same manner for both the sites. For example, only the 18 marked trees at Dankbaar were surveyed throughout the 38-year period; thus an analysis of the population structure is inappropriate as the entire population in the 1 ha plot was not surveyed.

An estimate of the growth rate per year over the entire study period (38 years) was calculated for each marked tree at the Dankbaar site by subtracting the first measured stem diameter (1978 survey) from the last live measured stem diameter and dividing by the number of years between these two measurements. Similarly, this method was used to calculate changes in height of a tree. A linear regression using GraphPad 7.02 (GraphPad Software,

The mean stem diameter in 1978 and 2016 of all marked trees was used to estimate the average age using the growth rate determined in the current study. The average age of all the marked trees in 1978 and in 2016 were calculated by dividing the mean stem diameter for the particular year by the growth rate determined in the current study. For comparative purposes the mean age of the trees was also calculated by the same method using the mean diameter growth rate for only the interior duneveld (3.2 mm/year) and for the interior duneveld and riverbed combined (3.6 mm/year) reported by Steenkamp et al. (

The stem circumferences for trees at the Grootkolk 1 ha plot were classified into 12 equal stem circumference classes (0–20, > 20–40, > 40–60, > 60–80, > 80–100, > 100–120, > 120–140, > 140–160, > 160–180, > 180–200, > 200–220, > 220–240 starting with class 1 and ending with class 12) for each of the survey years. The size class distribution was examined visually to classify the population into one of four population structure types (Gaugris & Van Rooyen

Idealised shapes of the four different population curve types.

For the 1 ha Grootkolk site the number of individuals recorded per stem circumference size class was calculated in order to obtain the density of trees per size class. Regressions were performed on the density (Di) of individuals against size class (Mi) (Gaugris & Van Rooyen

Two additional mean values were calculated for the stem diameter distribution for each survey year: (1) the ‘centroid’ was calculated as the arithmetic mean of all stem diameters recorded; and (2) the ‘midpoint of stem diameter range’ was calculated as the mean of the upper and lower limits of the diameter range included in the size class distribution. Niklas et al. (

Over the period of 38 years, 6 of the 18 originally marked trees at the Dankbaar site died (33%) and three showed coppice regrowth following substantial dieback because of a fire (

Age calculations for

Method | Mean age (years) | ||
---|---|---|---|

Eighteen trees (1978) | Nine live trees (2016) | Six dead trees | |

Mean diameter growth rate of 2.5 mm/year (current study) | 94 | 113 | 142 |

Mean diameter growth rate of 3.6 mm/year: interior dunes and riverbed combined (Steenkamp et al. |
66 | 78 | 99 |

Mean diameter growth rate of 3.2 mm/year: only interior dunes (derived from Steenkamp et al. |
74 | 88 | 111 |

Relationship between stem circumference, converted to diameter, and radiocarbon age: interior duneveld and riverbed combined (Steenkamp et al. |
47 | 56 | 71 |

A linear regression of growth rate against stem diameter (

Linear regression of stem diameter versus annual stem diameter growth rate for the 18 marked

At the Grootkolk site, 61 live and 14 dead individuals were recorded in the initial survey in 1979. The number of live individuals at Grootkolk in the Nossob riverbed gradually decreased over the survey period (

Size class distribution curves of

Using calculations from Steenkamp et al. (

Age calculations, derived by different methods, for

Method | Mean age (years) | |||
---|---|---|---|---|

Largest live tree class (> 220 cm – 240 cm) | Dead tree class (> 40 cm – 60 cm) | Dead tree class (> 60 cm – 80 cm) | Dead tree class (> 80 cm – 100 cm) | |

Mean diameter growth rate of 2.5 mm/year (current study) | 280–307 | 51–76 | 76–102 | 102–127 |

Mean diameter growth rate of 3.6 mm/year: interior dunes and riverbed combined (Steenkamp et al. |
195–212 | 35–53 | 53–71 | 71–89 |

Mean diameter growth rate of 5.2 mm/year: only the riverbed (Steenkamp et al. |
134–147 | 25–37 | 37–49 | 49–61 |

Relationship between stem circumference, converted to diameter, and radiocarbon age: interior duneveld and riverbed combined (Steenkamp et al. |
136–148 | 25–37 | 37–49 | 49–62 |

In all survey years the position of the centroid (arithmetic mean of all stem diameters recorded per site) was to the left of the midpoint of the size class distribution. However, the difference between these two values decreased over time and in 2016 the two values were nearly the same (

Marked

Depending on the equation used to determine the age of the dead trees there could be a twofold or greater difference in calculated age at both the Dankbaar (

On the basis of the position of the centroid in relation to the midpoint of the stem diameter class range, the Grootkolk population could be considered as a population gradually changing from young and growing to mature to old. This is supported by a visual assessment of the size class distribution, which showed a gradual transition from a Type IIIb to a Type IV curve shape over the 38 years. The large peak of young individuals in size class 4 that was evident at the first survey gradually became smaller and shifted to size class 5, and a prominent second peak emerged at size class 8 at the last survey.

At Grootkolk, few seedlings were recorded and few individuals appeared to be recruiting into the population. The low survival of size class 2, 3 and 4 individuals over the monitored period could possibly be ascribed to individuals progressing to larger size classes over the monitored period without being replenished. This is supported by the fact that size classes 5 and 8 had more individuals at the end of the monitoring period than when monitoring commenced. Overall, the survival of individuals in the larger size classes (e.g. size classes 6, 7 and 9) exceeded that in the smaller size classes (e.g. size classes 1, 2 and 3). In spite of the fact that the Grootkolk population was almost halved over the 38-year period, the slopes and intercepts of the size class distribution regressions showed no differences between years (

In their study, Steenkamp et al. (

Seed germination of

There is growing concern that mortality rates in

The continued monitoring of the population at the two sites, where baseline data are already available, is highly recommended. However, monitoring sites should also be established at additional sites outside of park borders to incorporate sites in which various threats to this species can be investigated. It would also be imperative to determine growth rates in the riverbeds. Rainfall stations should be erected at all sites to enable the investigation of the influence of significant rainfall events and drought on seed germination, seedling survival and adult mortality.

The authors gratefully acknowledge the financial assistance of the University of Pretoria, Centre for Wildlife Management and the National Research Foundation (GUN 2053522, grant numbers 61277, 81089 and 91434). The numerous honours students in Wildlife Management and Botany are thanked for their assistance in the field. South African National Parks (SANParks) are thanked for the opportunity to conduct the research in the Kalahari Gemsbok National Park. The field surveys of 2016 were successfully conducted with the assistance of SANParks and South African Environmental Observation Network employees. The anonymous reviewers and editor are also thanked for their valuable contributions to this manuscript.

The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.

N.v.R. donated the data set to South African Environmental Observation Network. The authors provided financial and/or institutional support and conducted numerous field surveys over the years. H.v.d.M. and M.W.v.R. compiled the manuscript with valuable input from the remaining co-authors.

Funding for this study was provided by the University of Pretoria, Centre for Wildlife Management and the National Research Foundation.

Changes in stem diameter (mm) from 1978 to 2016 of 18 marked

Number | 1978 | 1980 | 1983 | 1985 | 1988 | 1989 | 1991 | 1994 | 1998 | 1999 | 2008 | 2016 | Growth rate (mm/year) |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|

1 | 299.36 | 308.92 | 309.87 | 318.47 | 337.58 | 339.17 | 315.29 | 340.76 | 331.21 | 340.76 | 334.39 | 340.76 | 1.090 |

2 | 127.39 | 149.68 | 175.16 | 162.42 | 178.34 | 184.71 | 178.34 | 210.19 | 175.16 | 200.64 | 232.48 | 178.34 | 1.341 |

3 | 181.53 | 197.45 | 200.64 | 186.31 | 229.30 | 232.48 | 226.11 | 254.78 | 254.78 | 257.96 | Dead | Dead | 3.640 |

4 | 136.94 | 140.13 | 138.54 | 141.72 | 146.50 | 175.16 | 159.24 | 200.64 | 159.24 | 175.16 | 197.45 | 229.30 | 2.430 |

5 | 111.46 | 117.83 | 118.79 | 125.80 | 124.20 | 151.27 | 149.68 | 178.34 | 159.24 | 168.79 | 191.08 | 296.18 | 4.861 |

6 | 404.46 | 426.75 | 429.94 | 426.75 | 445.86 | 436.31 | 439.49 | 442.68 | 445.86 | 452.23 | Dead | Dead | 2.227 |

7 | 292.99 | 324.84 | 328.03 | 369.43 | 308.92 | 334.39 | 328.03 | 340.76 | 337.58 | 334.39 | 331.21 | 76.43 |
- |

8 | 92.36 | 92.36 | 105.10 | 98.726 | 117.83 | 117.83 | 124.20 | 136.94 | 130.57 | 133.76 | 143.31 | 181.53 | 2.347 |

9 | 156.05 | 187.90 | 187.90 | 200.64 | 200.64 | 221.34 | 222.93 | 238.85 | 242.04 | 254.78 | 273.89 | 324.84 | 4.442 |

10 | 101.91 | 117.83 | 117.83 | 111.46 | 111.46 | 125.80 | 111.46 | 124.20 | 121.02 | 121.02 | Dead | Dead | 0.910 |

11 | 181.53 | 203.82 | 211.78 | 213.38 | 213.38 | 213.38 | 203.82 | 222.93 | 219.75 | 226.11 | 229.30 | 251.60 |
- |

12 | 117.83 | 140.13 | 136.94 | 149.68 | 146.50 | 160.83 | 146.50 | 159.24 | 162.42 | 159.24 | 203.82 | 235.67 | 3.101 |

13 | 350.32 | 372.61 | 369.43 | 369.43 | 394.90 | 396.50 | 394.90 | 404.46 | 407.64 | 407.64 | Dead | Dead | 2.730 |

14 | 184.71 | 197.45 | 203.82 | 210.19 | 207.01 | 234.08 | 213.38 | 248.41 | 222.93 | 222.93 | 261.15 | 299.36 | 3.017 |

15 | 407.64 | 452.23 | 429.94 | 417.20 | 439.49 | 426.75 | 417.20 | 436.31 | 420.38 | 423.57 | 477.71 | 458.60 | 1.341 |

16 | 436.31 | 487.26 | 512.74 | 452.23 | 445.86 | 444.27 | 410.83 | Dead | Dead | Dead | Dead | Dead | 0.723 |

17 | 445.86 | 468.15 | 484.08 | 468.15 | 471.34 | 480.89 | 442.68 | 474.52 | 452.23 | 474.52 | 477.71 | Dead | 1.062 |

18 | 216.56 | 229.30 | 245.22 | 257.96 | 264.33 | 269.11 | 254.78 | 283.44 | 267.52 | 273.89 | 296.18 | 254.78 |
- |

, Coppice.

Statistical comparison of slope values for linear regressions fitted on the density of individuals against the diameter class midpoint of live trees at Grootkolk in the Kalahari Gemsbok National Park using an analysis of covariance.

Survey year | 1979 | 1984 | 1986 | 1989 | 1994 | 1996 | 1999 | 2008 | 2016 |
---|---|---|---|---|---|---|---|---|---|

Equation | |||||||||

1979 | - | ||||||||

1984 | - | - | |||||||

1986 | - | - | - | ||||||

1989 | - | - | - | - | |||||

1994 | - | - | - | - | - | ||||

1996 | - | - | - | - | - | - | |||

1999 | - | - | - | - | - | - | - | ||

2008 | - | - | - | - | - | - | - | - | |

2016 | - | - | - | - | - | - | - | - | - |

Statistical comparison of intercept values for linear regressions fitted on the density of individuals against the diameter class midpoint of live trees at Grootkolk in the Kalahari Gemsbok National Park using an analysis of covariance.

Survey year | 1979 | 1984 | 1986 | 1989 | 1994 | 1996 | 1999 | 2008 | 2016 |
---|---|---|---|---|---|---|---|---|---|

Equation | |||||||||

1979 | - | ||||||||

1984 | - | - | |||||||

1986 | - | - | - | ||||||

1989 | - | - | - | - | |||||

1994 | - | - | - | - | - | ||||

1996 | - | - | - | - | - | - | |||

1999 | - | - | - | - | - | - | - | ||

2008 | - | - | - | - | - | - | - | - | |

2016 | - | - | - | - | - | - | - | - | - |

Calculated centroid (arithmetic mean of all stem diameters recorded per site in mm) and the midpoint (mm) of the size class distribution for each year surveyed at the Grootkolk site in the Nossob riverbed.

Year | 1979 | 1984 | 1986 | 1989 | 1994 | 1996 | 1999 | 2008 | 2016 |
---|---|---|---|---|---|---|---|---|---|

Centroid (mm) | 2.85 | 3.10 | 2.98 | 2.89 | 3.27 | 3.16 | 3.34 | 3.32 | 3.48 |

Median (mm) | 3.82 | 3.82 | 3.50 | 3.18 | 4.46 | 4.46 | 4.46 | 4.46 | 3.50 |

Linear regressions fitted on the ln (Mi) against ln (Di + 1) of live trees at Grootkolk in the Kalahari Gemsbok National Park in (a) 1979, (b) 1984, (c) 1986, (d) 1989, (e) 1994, (f) 1996, (g) 1999, (h) 2008 and (i) 2016, where Mi, midpoint of the size class and Di, density of individuals.