The Science

Tropical forests are expected to green up with increasing atmospheric carbon dioxide (CO₂) concentrations, but primary productivity may be limited by soil nutrient availability. However, canopy-scale measurements have rarely been assessed against soil measurements in the tropics. In this study, researchers sought to assess remotely sensed canopy greenness against steep soil nutrient gradients across 50 1-ha mature forest plots in Panama. Contrary to expectations, increases in in situ extractable soil phosphorus (P) and base cations corresponded to declines in remotely sensed mean annual canopy greenness, controlling for precipitation.

The Impact

Overall, these data point to the potential utility of a remote sensing product for assessing belowground properties in tropical ecosystems.

Summary

In this study, researchers sought to assess remotely sensed canopy greenness against steep soil nutrient gradients across 50 1-ha mature forest plots in Panama. Contrary to expectations, increases in in situ extractable soil P and base cations (K, Mg) corresponded to declines in remotely sensed mean annual canopy greenness (r² = 0.77–0.85; p < 0.1), controlling for precipitation. This inverse relationship appears to be because litterfall also increased with increasing soil P and cation availability (r² = 0.88–0.98; p < 0.1), resulting in a decline in greenness with increasing annual litterfall (r² = 0.94; p < 0.1). As such, greater soil nutrient availability corresponded to greater leaf turnover, resulting in decreased greenness. However, these decreases in greenness with increasing soil P and cations were countered by increases in greenness with increasing soil nitrogen (N) (r² = 0.14; p < 0.1), which had no significant relationship with litterfall, likely reflecting a direct effect of soil N on leaf chlorophyll content but not on litterfall rates. In addition, greenness increased with extractable soil aluminum (Al) (r² = 0.97; p < 0.1), but Al had no significant relationship with litterfall, suggesting a physiological adaptation of plants to high levels of toxic metals. Thus, spatial gradients in canopy greenness are not necessarily positive indicators of soil nutrient scarcity. Using a novel remote sensing index of canopy greenness limitation, researchers assessed how observed greenness compares with potential greenness. A strong relationship with only soil N was found (r² = 0.65; p < 0.1), suggesting that tropical canopy greenness in Panama is predominantly limited by soil N, even if plant productivity (e.g., litterfall) responds to rock-derived nutrients. Moreover, greenness limitation was also significantly correlated with fine root biomass and soil carbon stocks (r² = 0.62–0.71; p < 0.1), suggesting a feedback from soil N to canopy greenness to soil carbon storage.

Principal Investigator

Daniela Cusack
Colorado State University & STRI
daniela.cusack@colostate.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

The remote sensing analysis was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Funding was provided by the National Science Foundation (NSF) Geography & Spatial Science (GSS) program (BCS-1437591), U.S. Department of Energy (DOE) Office of Science Biological and Environmental Research (BER) Program (DE-SC0016188; Early Career Award DE-SC0015898), and NASA Interdisciplinary Science (IDS) and Terrestrial Ecology (TE) programs.

Related Links

• PARCHED Facebook

References