CASE STUDY: A Tropical Rainforest Ecosystem Response to Drought

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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)

ACKNOWLEDGMENTS
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.

CASE STUDY: Experimental drought reveals soil CO2 and CH4 flux dynamics under climate change

Download the complete article (PDF).


UNDERSTANDING CARBON DYNAMICS
Globally, soils are the largest terrestrial contributor of carbon (C) flux to the atmosphere. Carbon is released from soils mainly in the form of carbon dioxide (CO2) emitted as a result of microbial and plant processes, but also as methane (CH4 ), a potent greenhouse gas with 28 times the warming potential of CO2.  The production of CO2 and CH4 in soils is closely connected to physical drivers like temperature and soil water content, which may change in response to climate change. Ecosystems like heathlands and grasslands cover large areas of the Northern hemisphere and also store large amounts of C in the soil.  At the CLIMAITE climate change research facility in Brandbjerg, Denmark, researchers from the University of Copenhagen are trying to understand C dynamics under present and manipulated climate conditions.

 

eosAC automated chambers beneath a permanent rain shelter (50% rain removal) at Brandbjerg.

THE BRANDBJERG SITE
Brandbjerg is located 60 km west of Copenhagen on a former glacial outwash plain overlain by wind deposited sand. The soil is very well-drained and nutrient poor. The site is characterized as a heath- and grassland co-dominated by heather (Calluna vulgaris) and grass (Deschampsia flexuosa). The climate at the site is temperate with a mean annual temperature of 8°C and annual precipitation close to 800 mm. From 2005-2013 a climate change experiment involving increased atmospheric CO2 concentration, temperature and drought took place at the site. Starting in 2016, a new experimental set-up was introduced with a series of permanent rainout shelters removing 40%, 50% and 66% of the annual precipitation. The Brandbjerg site is part of the AnaEE Denmark consortium (www.anaee.dk).

eosAC chambers monitoring the control plot (Oct. 30 – Dec. 12)

WHERE EOSENSE COMES IN
In the current study eosAC chambers were used to gain a better understanding of the CO2 and CH4 fluxes from the soil in late autumn and winter. As part of the study, CO2 and CH4 fluxes were observed in drought (50% precipitation removal) and control plots. The eosAC has been especially helpful with measuring in detail the temporal variability of CO2 and CH4 during a period of the year where good estimates of soil respiration are often lacking. These data can help disentangle the soil temperature and moisture control on soil-atmosphere exchange of carbon in cold periods. These data are furthermore hard to come by, and although fluxes may be low, the eosAC connected to a Los Gatos Research UGGA provides the necessary precision for high quality data.

FINDINGS THUS FAR
The CO2 fluxes  displayed a clear decrease across the measurement period. All chambers start with CO2 fluxes at or above 1 𝝻mol CO2 m-2 s-1 and subsequently plateau at values between 0.4-0.8 𝝻mol CO2 m-2 s-1 from the beginning of December onward. Comparison of the CO2 fluxes in the drought and control plots indicates a consistent decrease in CO2 efflux under drought conditions. This is in line with previous manual chamber measurements in the drought plots, where a decrease in soil respiration rates has been observed during spring measurements (March to April – data not published). The temporal variability of CO2 effluxes in the drought treatment is also smaller than the variability observed in the control plots. However, Chambers 3 and 4 also show different fluxes, which can be attributed to spatial variation at the site. Therefore, it is not entirely conclusive from this data that the drought decreases the C  turnover in the soil as the difference we observe may still be due to natural spatial variability. CH4 fluxes were consistently negative throughout the measurement period, showing that the soil continued to act as a CH4 sink in the cold period of the year. Over time CH4 fluxes in control plots decreased, indicating a similar temperature effect as was observed for CO2 efflux.

Dr. Jesper Christiansen in his rain gear checking on the eosAC chambers located in the control plot.

CONCLUSIONS
Drought conditions reduced both CO2 efflux and CH4 uptake in the soil compared to the control; this indicates that soil carbon cycling rates are slowed down under lower soil moisture content. Drought also seems to reduce the response of CO2 and CH4 fluxes to environmental variability. Overall, the eosAC chambers coupled to the Los Gatos Research UGGA were capable of capturing the high-resolution temporal variability that is required to start disentangling the relationship between the environmental drivers of CH4 and CO2 fluxes. Additionally, the high accuracy of the system allowed for the observation of small net CH4 uptakes into these soils and clearly demonstrated the difference in CH4 uptake between drought and control plots. The eosAC chambers have thus provided a rare glimpse of the cold-season fluxes for a Danish heath/grassland ecosystem and provided a robust dataset for understanding the impact of drought on soil carbon cycling.

ACKNOWLEDGMENTS
Thanks to Dr. Jesper Christiansen and Dr. Klaus Steenberg Larsen for performing the measurements and analyzing the data associated with this study.

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