Hedgerow Management Strategies as Viable GHG Mitigation: Does it work?
Hedgerows have been important parts of managed farmlands for centuries in Europe, traditionally used to mark land boundaries, preventing soil loss from wind erosion, and used for fuels if managed properly. Today they still offer these same benefits, however their ecological importance is highlighted as a sanctuary for wildlife and biodiversity among farmlands, and are the last remaining semi-natural vegetation of lowland habitat.
Hedgerows and GHGs as investigated by Thiel et al. (2016)
Climate mitigation tactics are a hot topic, particularly in 2016, the hottest year on record. The practice of maintaining or creating hedgerows is being advocated for as a means to maintain the traditional benefits, as well as to serve as a way to mitigate greenhouse gas (GHG) emissions via carbon sequestration. Sequestration rates for hedgerows are not well understood just yet, which is why Thiel et al. (2016) decided to undertake their recent study. They wanted to compare the emissions (CO2, CH4, N2O) between two types of hedgerows (remnant and planted) and adjacent agricultural fields. Additionally, Thiel et al. (2016) were interested in better understanding what environmental conditions affected hedgerow emissions. The authors hypothesized that the higher species diversity in the planted hedgerows would result in a significant difference in total CO2e (carbon dioxide equivalent) emissions from the three gas species than compared to the remnant hedgerows.
Thiel et al. (2016) Methods
Their study took place in the Simon Fraser Delta, BC, where there was a mix of remnant hedgerows and newly planted ones (mix of native tree and shrub species) by the Delta Farmland and Wildlife Trust. Using opaque closed-static chambers, measurements were taken from February 2013 through to January 2014 on a bi-weekly basis. Sampling took place between the hours of 09:00 and 14:00, over 4.5 hour sampling periods on eight different farms (sites). Carbon dioxide measurements were taken over short (<10 minute) intervals, methane and nitrous oxide were taken over longer time periods (20-30 minutes) in order to detect concentration changes.
The only gas measured that had a significant difference between hedgerows and the field was carbon dioxide (111%-118% greater; growing season and non-growing season, respectively). Differences between planted and remnant hedgerows were only observed when looking at relative emissions; planted had significantly higher bi-weekly CO2 emissions than remnant (10.46 Mg CO2-C ha-1 yr-1 and 6.58 Mg CO2-C ha-1 yr-1, respectively). This was also the finding in the literature.
Methane and nitrous oxide did not yield any significant results when comparing between hedgerow management types or when compared with crop fields. However, adjacent ditches did influence methane emissions (72.1 kg CH4-C ha-1 yr-1 to 1638.1 kg CH4-C ha-1 yr-1) when compared with sites where there were no ditches present on either side of the hedgerow (-0.7 kg CH4-C ha-1 yr-1 to 65.5 kg CH4-C ha-1 yr-1). Maximum emissions seen from the planted hedgerows topped out at 295.5 kg CH4-C ha-1 yr-1 during the summer, which is comparable to saturated peatlands.
Seasonal differences were observed for N2O, with planted hedgerow N2O emissions higher between October to December, and remnant hedgerows higher during February to May. These differences could be due to timing and/or plant litter types. High Soil Organic Carbon (SOC) was correlated with increased N2O emissions.
Overall, the authors found that no significant differences between relative CO2e emissions were found between plant and remnant hedgerows, although CO2 emissions were 59% higher in planted hedgerows.
Future Improvements for Further Study
While Thiel et al. (2016) do note that there was a difference found between the two hedgerow types in CO2 emissions, they also note Wotherspoon et al. (2014) finding that CO2 emissions decrease as sampling distance from hedgerows increases. They go on to mention that the conclusion from Wotherspoon and colleagues was that CO2 sampling between 0 and 2 m from hedgerows was likely influenced by root respiration. Thiel et al. (2016) conclude that their correlations and CO2 emissions from this study are likely being masked by this root respiration. They recommend including landscape-level biomass and SOC accumulation rates in similar future studies.
While we recognize that Theil et al. (2016) have their own thoughts on future work on hedgerow GHG emissions, we’d like to add a few things we’ve come across and have been working on for the last few months.
First, Christiansen et al. (2015) found that using laser-based spectroscopy for CH4 measurements, yielded 4x higher methane flux compared to the vial-and-syringe method of gas chromatography. Their paper highlights the Minimum Detectable Flux (MDF) metric, which is used a guideline for experimental design and data quality for a closed chamber. Given that CH4 and N2O are highly variable over space and time, this metric is a handy tool. It’s possible that Theil et al., might have missed some interesting CH4 and N2O results because of the MDF.
Creelman (2015) discusses the potential biases that can be incorporated into measurement scheduling and points out the different types of emission patterns. Often times sampling takes place at a regimented time and place out of convenience, which can greatly alter results seen. For N2O, which can be an event-driven type of emission, certain sampling regimes can completely miss large episodes of N2O emissions (see graph below). In the case of Thiel et al. (2016), sampling was done bi-weekly (and monthly) between the specific hours of 09:00 and 14:00 in order to minimize soil temperature effects, however they may have greatly biased their results from routine, bi-weekly, discontinuous sampling.
Christiansen, J. R., Outhwaite, J., and Smukler, S. (2015) Comparison of CO2, CH4 and N2O soil-atmosphere exchange measured in static chambers with cavity ring-down spectroscopy and gas chromatography. Agri Forest Meteorol 211-212: 48-57
Creelman, C. (2015) Biases of sparse data and measurement scheduling on flux estimates. White paper: Eosense, Inc. Dartmouth, NS, Canada.
Thiel, B., Krzic, M., Gergel, S. E., Terpsma, C., Black, A., Jassal, R., and Smukler, S. (2016) Soil CO2, CH4 and N2O emissions from production fields with planted and remnant hedgerows in the Fraser River Delta of British Columbia. Agroforestry Systems DOI: 10.1007/s10457-016-9990-3