Resistant or Quietly Declining? Quantifying Physiological Thresholds to Predict the Climate Sensitivity of African Tropical Forest (PHYTOCLIM-AFRICA)

It was during his doctoral fieldwork in Rwanda, amid an intense dry season, that Manzi encountered a scene that would shape his scientific career. Walking through a plantation where trees had been planted at uniform 1.5m spacing, in similar soil and under the same shade conditions, he noticed an extraordinary contrast.

“Entire individuals of certain species had completely lost their leaves,” 
Manzi recalls.

“Some were already dying back, their branches bare and brittle.”
What made it striking was how close they were to other species that looked perfectly healthy. One species stood leafless and stressed, while its neighbour, just an arm’s length away, remained fully green throughout the entire dry period.

“Same soil. Same microclimate. Same drought. Same planting design,” he says. “But completely different outcomes.”

That observation led Manzi to a deeper question: what is happening inside trees before the stress becomes visible?

Manzi compares it to people living under similar conditions. “You can have people living in the same place, eating similar food and experiencing the same environment, yet they do not all develop in the same way. Some may grow taller, some remain shorter, some gain weight more easily, others stay thinner, and they can respond very differently to the same stress,” he says.

“Trees are similar. They can grow in the same environment under the same
conditions, but they won’t behave the same way because their internal physiology is different,” he explains. “In African forests, people think that green means healthy. But there are subtle, invisible changes occurring underneath. You see the forest is green, but species are being lost – you only see the ones that are still standing.

Under the green canopy

For Manzi, the key lies in the tree’s internal physiology – and in subtle differences in the conditions immediately surrounding its roots and leaves. That is why he is pioneering tree ecophysiology across central Africa: to look beneath the green canopy and identify the hidden vulnerabilities that could determine whether forests withstand climate extremes 20 to 50 years from now.

To uncover those vulnerabilities, Manzi measures things that conventional forest inventories cannot. He studies individual trees’ hydraulic and thermal limits: how much stress their water-transport systems can withstand before they fail, and how much heat their leaves can tolerate before their photosynthetic machinery begins to break down.

He then compares these physiological thresholds with the heat and drought conditions trees experience in the field. The result is a measure he calls a “safety margin” — the gap between the stress a tree can withstand and the stress it is likely to encounter.

A narrow safety margin means that relatively small increases in temperature or drought duration could push a species towards physiological failure, potentially triggering dieback before conventional monitoring registers an obvious change in the forest.

The predictive value of Manzi’s work has an immediate practical application: helping restoration projects choose trees that can withstand the climate conditions they are likely to face in the future.

“Many restoration projects fail after a few years because people ‘plant for planting’ without testing heat and drought tolerance,” Manzi observes. “You ask local communities which species grew there previously, bring seedlings, and plant them. But climate patterns have shifted. In Rwanda, for example, dry seasons that used to end in early September now stretch into late September.”

“Because you plant a species that is growing now, but you don’t know if it will grow in 10 years, 20 years – then we are investing in things that will disappear,” Manzi adds.

His research aims to provide a way of measuring that vulnerability before trees are planted. “By measuring the tolerance threshold and the stress the given tree is under, getting this safety margin gives you an idea of how much stress they can tolerate.”

For Manzi, the implication is straightforward: safety margins could help make restoration a long-term investment rather than planting trees that may disappear within a few years.

While currently based at the University of Leeds and working with the AfriTRON network of more than 200 long-term forest monitoring plots, Manzi’s $150,000 JWO

Research Grant will be administered directly through the University of Rwanda. By anchoring scientific leadership firmly on African soil, he plans to use the funding to build something that extends far beyond his own research: a growing body of open- access African data, local expertise and scientists capable of revealing how the continent’s forests will respond to a changing climate.