Tree growth in Amazonian floodplain forests

Tree growth of Nectandra amazonum (Lauraceae) in the Central Amazonian floodplains does not respond to the annual long-term flooding but to variation of minimum temperature and evapotranspiration.

Amazonian Floodplain forests

The hydrological cycle in the Amazon rainforest is changing. The three main factors contributing to this are land-use change and deforestation, the implementation of hydropower plants, and climate change. Combined, this causes both more severe and frequent droughts in some regions and floods in others. It is no surprise that trees are affected by these changes, and floodplain forests may be especially vulnerable.

But thus far, scientists know very little about the concrete effects of climate and hydrological changes on tree growth. To close this gap, Master student Janaína Quixabeira Gonçalves and her team studied the evergreen and flood-adapted tree species Nectandra amazonum. It is the dominant tree species in the so-called várzea. Várzea is one type of Amazonian floodplain forest that grows on nutrient-rich soils along white-water rivers. (The other is the Igapó forest that grows on nutrient-poor soils along black-water rivers, such as the Uatuma near ATTO.)

A Varzea environment with flooded trees along a white-water river. © Flavia Durgante / KIT
A Varzea environment with flooded trees along a white-water river. © Flavia Durgante / KIT

Tree growth variability

The scientists thus sampled trees in one such várzea at Catalão Lake close to the city of Manaus. They used a combination of different methods, including tree ring dendrochronology to determine tree growth. In correlation with local climate and hydrological data, they analyzed how tree growth changes in the period from 2001 to 2017. Surprisingly, they found no correlation between growth and the hydrological regime, although previous studies had shown such a relationship. This indicates that the Nectandra amazonum might be more resilient to disturbances of the hydrological cycle than other species.

However, the team did find variations in the tree growth and tree ring wood density, and their results suggest that temperature and evapotranspiration play an important role. Specifically, trees grow more slowly during periods when night-time temperatures, generally the minimum temperatures of the day, are higher. This relationship is even more pronounced during the months when the forest is flooded. During this time, nighttime temperatures in floodplain forests are higher because the water acts as a buffer for the temperature. During those warmer nights, the stem respiration increases, and as a result, growth is reduced.

One of the Nectandra trees that the scientists analyzed tree rings from. © Flavia Durgante / KIT
One of the Nectandra trees that the scientists analyzed tree rings from. © Flavia Durgante / KIT

In conclusion, while Nectandra amazonum seems to be more resilient than other species to changes in the hydrological cycle, it might still be vulnerable to climate change, especially to rising minimum temperatures.

Gonçalves et al. published the study “Minimum temperature and evapotranspiration in Central Amazonian floodplains limit tree growth of Nectandra amazonum (Lauraceae)” in the journal Trees.

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