DOI: 10.5061/dryad.v41ns1rw1
title: Woody encroachment happens via intensification, not extensification, of species ranges in an African savanna

description: Widespread woody encroachment is a prominent concern for savanna systems
 as it is often accompanied by losses in productivity and biodiversity.
 Extensive ecosystem-level work has advanced our understanding of its
 causes and consequences. However, there is still debate over whether local
 management can override regional and global drivers of woody encroachment,
 and it remains largely unknown how encroachment influences woody community
 assemblages. Here, we examined species-level changes in woody plant
 distributions and size structure from the late 1980s to the late 2000s
 based on spatially intensive ground-based surveys across Kruger National
 Park, South Africa. This study region spans broad gradients in rainfall,
 soil texture, fire frequency, elephant density, and other topographic
 variables. Species-level changes in frequency of occurrence and size class
 proportion reflected widespread woody encroachment primarily by
 Dichrostachys cinerea and Combretum apiculatum, and a loss of large trees
 mostly of Sclerocarya birrea and Acacia nigrescens. Environmental
 variables determining woody species distributions across Kruger varied
 among species but did not change substantially between two sampling times,
 indicating that woody encroachers were thickening within their existing
 ranges. Overall, more areas across Kruger were found to have an increased
 number of common woody species through time, which indicated an increase
 in stem density. These areas were generally associated with decreasing
 fire frequency and rainfall but increasing elephant density. Our results
 suggest that woody encroachment is a widespread but highly variable trend
 across landscapes in Kruger National Park and potentially reflects an
 erosion of local heterogeneity in woody community assemblages. Many
 savanna managers, including in Kruger, aim to manage for heterogeneity in
 order to promote biodiversity, where homogenization of vegetation
 structure counters this specific goal. Increasing fire frequency has some
 potential as a local intervention. However, many common species increased
 in commonness even under near-constant disturbance conditions, which
 likely limits the potential for managing woody encroachment in the face of
 drivers beyond the scope of local control. Regular field sampling coupled
 with targeted fire management will enable more accurate monitoring of the
 rate of encroachment intensification.
description: This data was collected
 from Kruger National Park (22° 20' to 25° 30' S; 31° 10' to
 32° 00' E). Woody species distribution data for 1989 was collected
 using a rapid Braun-Blanquet method. Briefly, 1794 sites were selected
 across the park. At each site, woody vegetation cover and species were
 examined in a 20 m × 20 m plot according to three height classes: small
 (0.75–2.5 m), medium (2.5–5 m), and large (&gt; 5 m). These records
 were stored as presence and absence for each woody species in each size
 class at each site. Woody species distribution data for 2008 were
 collected from 342 sites across the park. At each site, a 50 m × 60 m plot
 was established and four 50-m transects running at 0, 20, 40, and 60 m
 along the length of the plot were laid out. Woody plant data was collected
 from eight sampling points located along the transects: 30 m and 50 m
 along the first and third transects, and 20 m and 40 m along the second
 and fourth transects. At each sampling point, woody plants were sampled
 according to their height classes (≤ 1 m, &gt; 1 and ≤ 3 m, and
 &gt; 3 m) within three corresponding circles with a radius of 1 m, 2
 m, and 5 m, respectively.  In other words, woody plants with a height of 1
 m or less that were found within a 1-m radius of each sampling point were
 counted and identified to species; woody plants within a 2-m radius with a
 height of &gt; 1 m up to 3 m were measured in height and identified to
 species; and woody plants within a 5-m radius and taller than 3 m were
 measured in height and identified to species.
description: This data is comprised of four components: (1) data and code for
 species occurrence and size distribution analysis; (2) data and code for
 species distribution modeling; (3) data and code for linear regression
 modeling; and (4) predicted maps of species distribution. Please refer to
 the paper for more details.