@article {102564, title = {The importance of history in understanding large tree mortality in African savannas}, journal = {ECOGRAPHY }, year = {2021}, month = {11/2021}, abstract = {

Fire and elephant herbivory are major drivers of large tree mortality in savanna ecosystems. Although the spatial variation of these agents is well studied, less attention has been paid to how disturbance history influences tree mortality 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 over time. Data on over 2500 large trees were collected from 22 transects in Kruger National Park, South Africa, in 2006, and trees were resampled in 2008, 2011, 2015 and 2018. Over the 12-year period, we recorded a cumulative mortality of 47.6\% with an estimated annual mortality of 3-5\% from 2006 through 2015 and an 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 analysis partitioned over different census periods showed that the probability of mortality of the 10 most common tree species depended not only on the type and intensity of elephant-induced damage and fire but also on the historical sequence of damage by these agents. Elephant damage to the main stem incurred up to 12 years earlier increased risk of mortality for large trees, especially in combination with fire damage. As expected, vulnerability to damage and risk of mortality varied among species, resulting in the potential for long-term changes in species composition over large areas. 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 resolution and long-term tracking of cohorts.

}, doi = {10.1111/ecog.06012}, author = {Das, Arundhati and Thaker, Maria and Coetsee, Corli and Slotow, Rob and Vanak, Abi T.} } @article {73041, title = {Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semi-arid African savanna.}, journal = {Ecology}, year = {2020}, month = {2020 Feb 06}, pages = {e03008}, abstract = {

While studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remains unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semi-arid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C vs. C ) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22\%), woody basal area (~8 m ha ) and woody canopy cover (31\%), translating to a ~8.5 t ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54\% increase (20.5 t ha ) in total soil carbon to 30 cm depth, with ~71\% of this derived from C grasses (vs. ~76\% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g. Acacia spp.) on grass cover and productivity in this semi-arid system.

}, issn = {1939-9170}, doi = {10.1002/ecy.3008}, author = {Wigley, Benjamin J and Augustine, David J and Coetsee, Corli and Ratnam, Jayashree and Sankaran, Mahesh} } @article {35150, title = {Defence strategies in African savanna trees.}, journal = {Oecologia}, year = {2018}, month = {2018 May 12}, abstract = {

Southern African savannas are commonly polarised into two broad types based on plant functional types and defences; infertile savannas dominated by broad-leaved trees typically defended by nitrogen-free secondary compounds and fertile savannas dominated by fine-leaved trees defended by structural defences. In this study, we use trait and other data from 15 wooded savanna sites in Southern Africa and ask if broad-leaved and fine-leaved species dominate on nutrient-poor and nutrient-rich soils, respectively. We then test if there is there any evidence for trade-offs in chemical (i.e., condensed tannins and total polyphenols) vs. structural defences on different soil types. We did not find strong evidence for a general divide in fine- vs. broad-leaved savannas according to soil fertility, nor for a simple trade-off between chemical and structural defences. Instead, we found savanna species to cluster into three broad defence strategies: species were high in leaf N and either (A) highly defended by spines and chemicals or (B) only structurally defended, or (C) low in leaf N and chemically defended. Finally, we tested for differences in browser utilisation between soil types and among plant defence strategies and found that browsing by meso-herbivores was higher on nutrient-rich soils and targeted species from groups A and B and avoided C, while browsing by elephants was mostly not affected by soil type or defence strategy. We propose a framework that can be used as a basis for asking strategic questions that will help improve our understanding of plant defences in savannas.

}, issn = {1432-1939}, doi = {10.1007/s00442-018-4165-8}, author = {Wigley, Benjamin J and Fritz, Herv{\'e} and Coetsee, Corli} }