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Large tree mortality census KNP (2006-2018)

Citation

Thaker, Maria; Vanak, Abi Tamim; Slotow, Rob (2021), Large tree mortality census KNP (2006-2018), Dryad, Dataset, https://doi.org/10.5061/dryad.2280gb5sh

Abstract

Fire and elephant herbivory are major drivers of large tree mortality in savanna ecosystems. While the spatial variation of these agents is well-studied, less attention has been paid to how disturbance history influences mortality risk for trees over time. In a long-term cohort study, we examined how the sequence of fire- and elephant-induced damage influences mortality of trees, and determined whether risk of mortality is compounded with time. Data on over 2500 large trees were collected from 22 transects in Kruger National Park, South Africa, in 2006, and trees were re-sampled in 2008, 2011, 2015, and 2018. Over the twelve year period, we recorded a cumulative death toll of 47.6% with an estimated annual mortality rate of 3-5% between 2006 and 2015, and a sharp increase to 8.8% in 2018. The main attributed agent of tree mortality was elephant damage, occurring either once or across multiple census periods. A classification tree (CT) analysis partitioned over different census periods showed that the probability of mortality for the ten most common species depended not only on the type and intensity of fire and elephant-induced damage, but also on the historical sequence of damage by these agents. In fact, elephant damage to the main stem incurred even up to 12 years earlier increases risk of mortality for large trees, especially in combination with fire damage. As expected, vulnerability to damage and risk of mortality varied between species, resulting in the potential for long-term changes in species composition at the landscape scale. Overall, this study highlights how multiple interacting agents cause emergent and lagging patterns of mortality risk for large trees in savanna ecosystems; a result that only becomes apparent through fine-scale long-term tracking of cohorts.

Methods

This dataset was used in the analysis of the causes of large tree mortality in Kruger National Park, South Africa, as presented in the manuscript titled, "The Importance of History in Understanding Large Tree Mortality in African Savannas".

A cohort of large trees (> 5m in height) was tracked across 22 transects in KNP between 2006 and 2018. The survival status of the trees and the type and severity of elephant and fire damage to individual trees was measured for the following census years: 2006, 2008, 2011, 2015 and 2018.

The excel workbook contains metadata information, which is presented in the first tab of the workbook and explains each data field. The actual data is presented in the 'alldata' tab of the excel workbook and the codes for type and severity of elephant and fire damage are presented in the 'key' tab of the excel workbook.

The study area, transect locations and sampling strategy are described in detail in Druce et al. (2008), Shannon et al. (2008) and Vanak et al. (2012).

In the 2006 survey, the following were recorded:

Tree characteristics: 1) species, 2) total height, 3) lower crown height, 4) crown diameter, 5) diameter of main stems,

Landscape features: 1) landscape type (Gertenbach classification), 2) geology, 3) soil type, 4) slope position, 5) distance to nearest water body, 6) distance to nearest river, 7) distance to nearest road, 8) elevation, 9) aspect

In all surveys:

Damage status: 1) Type and intensity of elephant damage, 2) Type and intensity of fire damage, 3) Incidences of mortality

Funding

National Research Foundation South Africa, Award: FA2006032300024 (to RS)

Amarula Elephant Research Programme

Inyuvesi Yakwazulu-Natali

National Research Foundation South Africa, Award: 103659 (to ATV)