Conservation news

Western Amazonian and Andean forests don’t follow the ecosystem “rules” — here’s why that’s important

  • To understand the tradeoffs between growth strategies, ecologists have developed the leaf economics spectrum (LES) theory, which posits a fairly straightforward relationship between resource acquisition and storage strategies in plants.
  • However, LES has never been studied on large geographically continuous scales — until now.
  • According to the PNAS study, leaf economics of forests are not nearly as straightforward as scientists believed them to be.

Forest ecology researchers have long focused on how environmental factors like climate, elevation, hydrology, and soil conditions affect tree growth.

They came to understand that trees have evolved to take advantage of these resources through a variety of strategies: some optimized for quick growth, relying on highly efficient photosynthesis, abundant nutrients like nitrogen and phosphorous, and optimal warm-wet climates; others for slow growth, meaning they grow leaves with strong internal structures designed to live for a long time in nutrient-poor areas where the climate is colder and drier.

For more than a decade, ecologists have worked with an established model for measuring and quantifying the growth strategies employed by trees given the environment they’re growing in. But a new study published in the Proceedings of the National Academy of Sciences (PNAS) found that forests in the Andean and western Amazonian regions of South America break these long-understood rules.

To understand the tradeoffs between these strategies, ecologists have developed the leaf economics spectrum (LES) theory, which posits a fairly straightforward relationship between resource acquisition and storage strategies in plants. This allows scientists to better understand forests’ responses to different environmental factors by plotting leaf nutrients in relation to leaf mass per area, an approach that has proven useful for predicting how forests will respond to changes in environmental and climatic conditions.

However, LES has never been studied on large geographically continuous scales — until now. According to the PNAS study, leaf economics of forests are not nearly as straightforward as scientists believed them to be.

Using a high-fidelity imaging spectrometer onboard the Carnegie Airborne Observatory, Greg Asner and his team at the Carnegie Institution for Science Department of Plant Biology at Stanford University made forest canopy maps of leaf nitrogen, phosphorous, and mass covering nearly 76 million hectare (about 188 million acres) of Peru.

Tropical rainforests of the Andes and western Amazon basin are often viewed as being similar in their growth properties, but new airborne spectroscopic measurements reveal a different pattern. Colored portions of this map of Peru indicate differences in tropical forest canopy chemicals that control tree growth. Each color reveals a different chemical makeup among Andean and Amazonian trees that are tightly related to the underlying geology, elevation and climate. Orange and red colors indicate relatively higher nitrogen concentrations in tree canopies, while green colors indicate higher phosphorus, and blues indicate thicker and tougher leaves. All other colors indicate the continuous nature of these trade-offs. These differences have a big role to play in predicting forest growth under changing climate conditions. Image provided courtesy of Greg Asner.

“We tested a traditional ecological principle at a completely new scale — in this case, the vast and under-explored Andes and Amazon region,” Asner said in a statement.

“We found that Andean and Amazonian forests have evolved into diverse communities that break simple ecological ‘rules’ previously developed through field-based studies. These forests are actually much more interesting and functionally diverse than previously thought, and have sorted themselves out across a variety of environmental templates like geology, elevation and temperature.”

In other words, forests aren’t neatly split between terrains with fast-growers and slow-growers. Asner and team found that forest canopy nitrogen, phosphorous, and leaf mass relationships are sensitive to the enormous range of geophysical conditions found throughout the region.

“Elevation and substrate were codominant drivers of leaf trait distributions,” the team write in the study. “Multiple additional climatic and geophysical factors were secondary determinants of plant traits.” The relationship between nitrogen and leaf mass per area generally followed LES theory, they added, but various topographical and soil conditions strongly mediated and at times even eliminated this relationship.

Asner and team believe that mapping the continuum of factors influencing tree growth can play a key role in determining how forests will respond to climate change. Asner said that his team’s findings also point up the need for taking airborne science into orbit as a satellite mission.

“This is the only way to create new global-scale maps,” he said, “which are needed to better understand these ecological processes and to predict the roles they will play in Earth’s future.”

Upper Amazon cloud forest and the Andes. Photo by Rhett Butler.

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