The effects of climate extremes on greenhouse gas emissions from Bald cypress “knees”

Climate change is leading to shifts in the duration and frequency of extremes such as droughts and floods. Because wetland and adjacent stream functions are tightly linked to their hydrologic regimes, they may act as good models for studying the effects of these extreme events on carbon cycling. The objective of this study is to investigate the effect of climate extremes on greenhouse gas (GHG) fluxes from the “knees\u27\u27 of Bald cypress (Taxodium distichum) trees within a bottomland hardwood wetland of Western Kentucky. Knees are aboveground woody root structures, which can emit or take up methane (CH4) and carbon dioxide (CO2), contributing to GHG emissions. We measured CH4 and CO2 fluxes from knees and soils within a slough in Clarks River National Wildlife Refuge. During a moderate to extreme drought in the Fall of 2022, soils took up CH4, while knees emitted CH4. Overall, both soils and knees (with a few exceptions of uptake) emitted quantifiable CO2. CH4 and CO2 emissions from knees increased with soil temperatures. Interestingly, soil CO2 emissions increased in areas with higher knee density, potentially due to increased root respiration. In the Spring and Summer of 2023, moderate levels of rain (~ 3.2 cm, averaged across a May and July event) during continued drought conditions did not have a discernible effect on CH4 emissions from knees when comparing before and after precipitation. However, a historic rain event, which dropped 17.7 cm of rain in 24 hours, resulted in significantly higher CH4 emissions after flooding. These increases were presumably due to increased methanogenesis in soils with more CH4 available for transport through knees. The results from this study can be used to more accurately quantify how GHG fluxes from exposed woody root structures are impacted by extreme climatic conditions

Assessing the effect of Taxodium distichum “knee” density on CH4 and CO2 emissions during drought conditions

With rising greenhouse gas emissions, it is crucial to include natural sources of carbon dioxide (CO2) and methane (CH4) in the global carbon budget. Wetland soils and vegetation can play a significant role in these source-sink dynamics. However, regional carbon budgets often do not include emissions from woody root structures, such as the “knees” of Taxodium distichum (bald cypress) trees in bottomland hardwood wetlands, because little has been done to study their effects. We are examining how the density of knees (e.g., surface area for gas movement) affects the balance between CO2 and CH4 uptake and emissions. Knee density was surveyed in 120 1m2 plots within a 10 m stretch of Dunn Slough in Clarks River Wildlife Refuge, which contained a source population of Taxodium distichum. Densities ranged from no knees to 24 knees per m2 (mean +/- s.d. = 3.3 +/- 3.6 knees per m2). Twenty-four plots with varying knee densities were randomly selected to measure CO2 and CH4 fluxes over six hour time periods using large-framed chambers. Additionally, twelve of these plots were randomly selected to measure fluxes from the soil alone, adjacent to knees. During drought conditions in the Fall of 2022, soils within higher density plots had higher CO2 emissions and slightly higher CH4 uptake compared to lower density knee plots. When knees were included in flux measurements, they negated differences in CH4 uptake within the soils alone. We hypothesize, this is due to the knee’s ability to transport CH4 from deep soils pools, which we have found in a complementary study. Upscaling the results of this study could provide more accurate flux rates to regional carbon budgets of areas with varying cypress knee densities

The role of beaver dams in carbon sequestration

Rising greenhouse gas emissions have called for a need in exploring carbon sequestration options, including natural solutions. Wetlands can act as carbon sinks through increased anoxic conditions influencing biogeochemical cycles and sediment accumulation rates. Once extirpated in much of their original home range, beavers are ecosystem engineers with the ability to drastically change an ecosystem through the construction of dams, often resulting in the formation of diverse wetland systems. Thus, beaver-created wetlands have the potential to similarly store carbon. Through an analysis of the literature, this study aims to investigate whether beavers promote carbon storage and how positions relative to beaver dam (i.e., upstream, in-pond, downstream) may vary carbon sequestration capacity. Results from this study could highlight just one of many ecosystem services beavers provide and the contribution of biotic factors to efforts in mitigating greenhouse gases

Measuring Organic Matter Storage in a Bottomland Hardwood Wetland of Western Kentucky

With rising greenhouse gas concentrations in the atmosphere accelerating climate change, it is important to identify and protect natural ecosystems that have the potential to sequester carbon. Wetlands are one such ecosystem, storing an estimated 20 to 30 % of soil carbon across the globe. This is partially due to their anoxic soil conditions, which slow down decomposition of organic matter. In a survey of 967 wetlands in 2011 across the contiguous United States, the National Wetland Condition Assessment highlighted that previous estimates of carbon storage in wetlands have missed deep carbon pools, especially in mineral soil wetlands. Additionally, ecosystem engineers, such as beavers, can modify freshwater ecosystems, including their biogeochemical processes. Carbon sequestration can be affected by increased flooding and, thus, slower decomposition. The goal of my study was to capture deep soil organic matter of a bottomland hardwood wetland that has been affected by beaver activity. Taking soil cores is one method of measuring organic matter in wetlands. A soil core with a depth of 102cm was taken at Travis Tract of Obion Wildlife Management Area in Western Kentucky, a known beaver impoundment. The core was divided into 2 cm increments. Organic matter was then measured through the process of loss of ignition and compared throughout the depths. Results and implications of this study will be discussed

Does Taxodium distichum “knee” density affect CO2 and CH4 emissions in bottomland hardwood forests?

Wetland soils and vegetation are significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. However, little has been done to examine the woody root structures (“knees”) of Taxodium distichum (bald cypress) trees and the effect their densities (e.g., increased surface area for gas movement) have on greenhouse gas fluxes. We are examining how the density of knees affects CO2 and CH4 emissions. Knee density was surveyed in 120 1m2 plots within a 10 m stretch of Dunn Slough in Clarks River Wildlife Refuge, which contained a source population of Taxodium distichum. Densities ranged from no knees to 24 knees per m2 (mean +/- s.d. = 3.3 +/- 3.6 knees per m2). We selected twenty-four plots with varying knee densities to collect gas fluxes over three-hour periods using large framed chambers. Interestingly, an initial test of a seven knee per m2 plot revealed a net uptake of CH4 and net release of CO2; we suggest this result is related to low water table conditions and highlights the dynamic nature of these systems. Results from this study can be upscaled to provide more accurate flux rates in areas with various densities of cypress knees

Does Taxodium distichum “knee” density affect greenhouse gas fluxes in bottomland hardwood wetlands?

With rising greenhouse gas emissions, it is crucial to include natural sources of carbon dioxide (CO2) and methane (CH4) in the global carbon budget. Wetland soils and vegetation can play a significant role in these source-sink dynamics. However, regional carbon budgets often do not include emissions from woody root structures, such as the “knees” of Taxodium distichum (bald cypress) trees in bottomland hardwood wetlands, because little has been done to study their effects. We are examining how the density of knees (e.g., surface area for gas movement) affects the balance between CO2 and CH4 uptake and emissions. Knee density was surveyed in 120 1m2 plots within a 10 m stretch of Dunn Slough in Clarks River Wildlife Refuge, which contained a source population of Taxodium distichum. Densities ranged from no knees to 24 knees per m2 (mean +/- s.d. = 3.3 +/- 3.6 knees per m2). Twenty-four plots with varying knee densities were randomly selected to measure CO2 and CH4 fluxes over six hour time periods using large-framed chambers. Additionally, twelve of these plots were randomly selected to measure fluxes from the soil alone, adjacent to knees. During drought conditions in the Fall of 2022, soils within higher density plots had higher CO2 emissions and slightly higher CH4 uptake compared to lower density knee plots. When knees were included in flux measurements, they negated differences in CH4 uptake within the soils alone. We hypothesize, this is due to the knee’s ability to transport CH4 from deep soils pools, which we have found in a complementary study. Upscaling the results of this study could provide more accurate flux rates to regional carbon budgets of areas with varying cypress knee densities

The Relationship Between Hydrogeomorphic Settings and Greenhouse Gas Emissions from Soils and Stems in Western Kentucky’s Bottomland Hardwood Forests

Bottomland hardwood (BLH) ecosystems sequester substantial amounts of carbon, yet recent studies focus on the ability of their soils and vegetation to also emit carbon as greenhouse gases (GHGs) to the atmosphere. Studies have observed differences in stem GHG exchange between extremes in hydrogeomorphic (HGM) setting, such as between dry uplands and inundated wetlands. We are investigating how GHG emissions from tree stems and soils vary across a narrower scope of HGM settings within BLHs, specifically: pond edge, lake edge, and channel. In each setting we are measuring gas flux rates, ~ monthly (starting in June 2023), from six soil chambers and six stems of Bald cypress (Taxodium distichum), a common species found across all HGMs. Additionally, we are investigating how gas diffusion rates vary from stems at two heights (40 cm and 120 cm above the ground). We hypothesize that GHG emissions vary as a function of height, with emissions decreasing with increasing stem height. We also hypothesize that GHG emissions vary as a function of HGM setting. We predict higher stem CH4 emissions in HGM settings that have higher organic matter content, a longer duration of inundation and higher temperatures compared to HGM settings with colder and shorter inundation periods. Understanding these relationships will enable accurate carbon cycle modeling in a warming world, as well as provide optimal T. distichum planting recommendations for future land managers