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Global distribution and climate sensitivity of the tropical montane forest nitrogen cycle

We identify a large and potentially vulnerable pool of soil nitrogen in these tropical systems as the global climate continues to warm

Alexander von Humboldt's illustration of tropical (Chimborazo, Ecuador), temperate (Mont Blanc, Alps), and subarctic (Sultielma, Lapland) mountains.

In the early 17th century, Alexander Von Humboldt noticed that mountains in the tropics maintain striking ecological zones across altitudinal gradients, mimicking how distinct ecological boundaries form latitudinally as one moves poleward away from the equator. Since then, tropical montane environments have continued to provide scientists with a natural setting for exploring environmental controls of ecosystem function and effects of global change across relatively small spatial extents.

One long-standing paradigm rooted in biogeochemical theory is that tropical montane forests are nitrogen-limited while lowland tropical forests tend to be rich in nitrogen. However, scientific evidence often reveals mixed results in this regard. Similarly, it is currently unclear how climate warming will affect soil nutrient cycles that constrain productivity and carbon storage in these ecosystems. As such, our first goal was exploring the spatial and topographic variability in nitrogen pools across these tropical mountain forests and compare that to those of the tropical lowlands. Second, we evaluate the topographic signature of the ecosystem-level tropical nitrogen cycle by examining climatic and geophysical controls of surface soil nitrogen from elevational gradients distributed across tropical mountains globally.

To even begin to probe these questions, we first needed to define the portion of the globe that is classified as montane. Because we have relatively accurate global elevation data, at first glance, this seemed like an easy task. However, diving into it, it was clear that we would need to account for topographic relief in order to remove high-elevation plateaus from our analyses. Ultimately, we used a system similar to that of Kapos et al., 2000, which takes topographic relief and slope into account for elevations below 2500m.

Next, we scraped the literature for nitrogen data across tropical elevation gradients as well as contributed novel data of our own. In total, we compiled a database from 324 humid forest sites distributed across sixteen elevational gradients and from lowland tropical forests across the major tropical land areas (Neotropics, Africa, Asia-Pacific). We obtained both nitrogen concentration data as well as data on nitrogen’s natural abundance isotope, 15N. The relative abundance of 15N compare with 14N (known as δ15N) in soils records the long-term imprint of microbial processing and nitrogen bioavailability. In more active soils, higher δ15N values indicate increased availability of nitrogen to plants and microbes and larger losses of nitrogen (i.e., via gaseous losses). In general, higher δ15N indicates isotopically enriched soils with more active nitrogen.

So, what did we find?

Of the total tropical terrestrial extent, we find that about 9% is mountainous. Furthermore, while 40% of the tropics are forested, only about half of tropical montane areas are. This means that of the ~50 million km2 of tropical land extent, only about 2 million are montane forests (or ~4%). In other words, montane forests make up only 11% of forested tropical land.

Yet, we find that, despite their small spatial contribution, we estimate that tropical montane forests account for almost 20% of global tropical forest soil nitrogen. These estimates are nearly two times higher than previous studies, indicating there is a larger nitrogen pool in these ecosystems than previously thought. Furthermore, we note steep increases in soil nitrogen and declining δ15N with increasing elevation.

Figure 3 of the paper showing the relationship between nitrogen and mean annual temperature on the X-axis in degrees C

Our models also reveal significant geographic variation across the globe with respect to the contribution of montane forests to tropical soil nitrogen pools not captured in previous studies. For instance, the Asia-Pacific region comprises only 20% of global tropical land area but holds over half of tropical montane forests and 46% of global tropical montane forest soil nitrogen. In contrast, our analysis shows higher soil nitrogen in the Neotropics and more active soil processing in Africa, but it is unclear how lower sample representation in Africa and the Asia-Pacific influence this result.

Finally, we also examine this relationship across collocated temperature data and find a positive relationship between δ15N and temperature and a negative temperature-nitrogen relationship across the global diversity of tropical mountain environments.

This information indicates that there is 1) a disproportionately large pool of nitrogen in montane tropical regions; and 2) less nitrogen cycling in higher elevations across the global tropics. Furthermore, 3) we find that this pattern is primarily driven by temperature, where cooler montane environments constrain microbial activity and gaseous losses. Taken together, these results identify a large and potentially vulnerable pool of soil nitrogen in these tropical systems as the global climate continues to warm.

Topical montane forest in Hawai'i diving into the ocean. This image shows the challenges with classifying montane environments!

Authors: Justin D. Gay, Bryce Currey, E.N. Jack Brookshire



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