Large tree mortality census KNP (2006-2018)

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.