Abstract:
Climate variability and resilience remain gaps in tree research, challenged
by the interacting factors in climate change, long-term resilience and the
influence of evolutionary legacy effects. In a multidisciplinary approach using
90-year (1930–2020) climate-growth data, we investigated the dynamics of
climate variability on growth and resilience of the tropical African baobab
(Adansonia digitata) at the range edge in climate-variable, southeast Africa.
The main driver of climate variability, ENSO (El Niño-Southern Oscillation),
triggered 83% of droughts exacerbated by positive Indian Ocean Dipole
(pIOD) events. Growth over 90 years was positively correlated with maximum
temperature and increased after the 1976–1977 Global Warming Shift. The
influence of warming was compromised by climate variability and extreme
events. Although growth is a measure of adaptive capacity, accelerated
growth over the past 20 years contrasted with dehydration, canopy dieback
and a novel Didymella pathogen. Resilience was contingent on high genetic
diversity (polyploidy and heterozygosity) and Neotropical legacy effects of
stem water storage and longevity trade-offs of low growth and recruitment,
and also reproduction in the region. The evolution of resprouting in disturbed,
fire-prone ecosystems and bark regeneration increased recovery from
disturbance. As resource opportunists, baobabs adopted a fast-slow survival
strategy. Rainfall and warming enhanced growth while low and variable rainfall
favoured a conservative, low growth-higher survival strategy. Low rainfall,
climate extremes and topography increased mortality risk. Mortality was
higher at lower elevations on site and regionally. Low growth may conserve the baobab in climate warming but the southern hemisphere tropics is one
of two identified global hotspots with amplified hot years. The heightened disturbance predicted from increased climate variability, hot droughts and
landfalling tropical cyclones magnifies mortality risk for “Africa’s favourite
tree.”
