A Tropical Rainforest Ecosystem Response to Drought

Introduction

Rainforests are one of the most unique terrestrial ecosystems in the world, producing 40% of the Earth’s oxygen and housing between 40-75% of biotic species on the planet. High rainfall, almost constantly warm temperatures and high transpiration means that rainforests are perpetually humid ecosystems. While the lush appearance of rainforests might suggest high soil nutrient contents, constantly wet conditions cause extensive nutrient leaching, leaving shallow, nutrient poor soils. What rainforests may lack in soil nutrients they make up for in their contribution to the global carbon budget. Emissions of carbon dioxide (CO2) from rainforest soils are among the highest measured globally, and despite covering less than 3% of the Earth’s surface, rainforest methane (CH4) emissions are an important contributor to global CH4 budgets.

While there are fluctuations of rainfall in an aseasonal tropical rainforest, periods of drought caused by low rainfall can significantly affect the ecosystem. Changes in global climate mean that the frequency of extreme droughts may be increasing, but the study of their impacts on the biogeochemistry of rainforests remains poorly understood.

In eastern Puerto Rico, one of the strongest droughts in recent history was experienced in 2015. Researchers from University of California Berkeley used this opportunity to study the impacts of drought on the biogeochemistry of the Luquillo Experimental Forest (LEF) rainforest.

eosAC’s deployed in Luquillo Experimental Forest

The Study & Site

The study took place in the LEF, a tropical montane rainforest in northern Puerto Rico. Typical rainfall amounts in this area average 3,500 mm to 5,000 mm per year – but during the drought year rainfall decreased by 2,035 mm/yr. The steep topography of the forest influences where this rainfall accumulates, with distinctly different soil moisture conditions in ridges and valleys of the rainforest, creating spatially and temporally variable soil redox conditions.

Researchers, Christine O’Connell, Leilei Ruan and Whendee Silver from the University of California Berkeley monitored greenhouse gas (GHG) emissions – in addition to many other biogeochemical parameters – during and after the drought event in LEF. They also studied the response and recovery of the forests ecosystem in order to capture patterns that may exist.

Their hypothesis was that during drought there would be an increase in soil O₂ concentration and CH4 uptake with the valleys having the slowest response time to drought conditions, but also the quickest recovery after drought.

They also hypothesized that the available phosphorus (P) would decline after drought.

Site Setup & Measurements

In total, 35 measurement stations were set up in the LEF in April 2015, prior to drought onset – distributed between ridge, valley, and slope sites. Each measurement station consisted of a O2 sensor and a moisture sensor. To monitor soil GHG flux, 9 eosAC automated soil gas flux chambers were set up with an eosMX multiplexer, connected to a Picarro G2508 analyzer; 3 in ridge locations, 3 in slope locations and 3 in valley locations. The continuous, real-time estimates of CO2 and CH4 fluxes were calculated using eosAnalyze software.

Using the Eosense-Picarro system, a total of 150 days of soil gas flux measurements were captured, with each individual chamber measuring on average 12 times per day. After data cleaning and accounting for days in which data could not be collected, 6,479 CO2 flux observations and 6,379 CH4 flux observations were recorded.

The Results

As expected, the drought affected the soil moisture significantly. Opening of the soil pore spaces allowed for a large increase in soil O2 concentrations, particularly in the normally saturated valley sites.

Figure 1: Soil carbon dioxide emissions across topographic zones and drought time periods.

All topographic areas showed a higher CO2 flux during drought conditions (Figure 1) with the slope and valley sites showing the largest overall increases caused by significant changes in O2 availability. During the drought recovery and post-drought periods, CO2 emissions remained significantly higher than pre-drought conditions at all topographic locations, likely caused by a slow rewetting of the soil matrix. Increases in CO2 flux during the drought, drought-recovery and post-drought periods led to an additional emission of 12.05 Mg CO2e per hectare compared to the pre-drought condition.

Figure 2: Soil methane emissions across topographic zones and drought time periods.

Drought led to a dramatic decline in the CH₄ emissions from the valley sites (Figure 2) and slightly increased the methane sink in the Ridge and Slope sites. There was little difference between the drought and drought-recovery periods for methane fluxes across all sites and there was also a marked drop in hot-moments of methane flux during these periods. The valley site quickly recovered to pre-drought methane flux levels during the post-drought period, however for the ridge and slope sites the post-drought methane fluxes are significantly higher than pre-drought conditions. This substantial increase in CH4 fluxes post drought offset 99% of the methane sink that was observed during drought conditions.

Conclusions

Real-time, eosAC automated soil gas flux chamber measurements were essential for capturing the impacts of the drought on greenhouse gas dynamics at LEF. The quick and straightforward setup – including seamless integration with the Picarro G2508 analyzer –  provided immediate data for the Berkeley research team.

These results demonstrate the vulnerability of the tropical rainforest ecosystem to extreme weather events, such as drought. Drought has important implications for biogeochemistry at ecosystem and global scales, via both direct effects (e.g., soil drying, and changes in trace gas emissions) and indirect effects (e.g., declines in inorganic P availability, and increases in organic P concentrations). Data from this study will be  critical for reducing uncertainties surrounding how terrestrial C and nutrient cycles will be modified by climate change.

“The high temporal resolution data set that we have been able to acquire [with the eosAC/MX system] is flexible, robust and allows us to track changes in the system efficiently. It’s a very exciting tool to be able to work with” – Christine O’Connel (Post Doctoral Researcher, Silver Lab)

Acknowledgements

Thanks to Christine O’Connell, Leilei Ruan and Whendee Silver from the University of California Berkeley for performing the measurements and analyzing the data associated with this study.

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