Beavers affect carbon biogeochemistry: both short‐term and long‐term processes are involved

Abstract

With the recent population increase in beavers (Castor spp.), a considerable amount of new riparian habitat has been created in the Holarctic. We evaluated how beaver‐induced floods affect carbon (C) dynamics in the beaver ponds and in the water‐atmosphere and riparian zone interfaces. Beaver disturbance affects soil organic C storage by decreasing or increasing it, resulting in a redistribution of C. Upon flooding, the concentration of dissolved organic carbon (DOC) increases in the water. This C can be released into the atmosphere, it can settle down to the bottom sediments, it can be sequestered by vegetation, or it can be transported downstream. The carbon dioxide (CO2) emissions vary between 0.14 and 11.2 g CO2 m−2 day−1, averaging 4.9 CO2 g m−2 day−1. The methane (CH4) emissions vary too, from 27 mg m−2 day−1 to 919 mg m−2 day−1, averaging 222 mg CH4 m−2 day−1. Globally, C emission from beaver ponds in the form of CH4 and CO2 may be 3.33–4.62 Tg (teragram, 1012 g) year−1. The yearly short‐term sedimentation rates in beaver ponds vary between 0.4 and 47 cm year−1, and individual ponds contain 9–6355 m3 of sediment. The approximate global estimate for yearly C sedimentation is 3.8 Tg C; beaver ponds globally contain 380 Tg sedimented C. After being formed, beaver pond deposits can remain for millennia. Both C sequestration and CO2 and CH4 emissions in ponds of various ages should be taken into account when considering the net effect of beavers on the C dynamics. With present estimates, beaver ponds globally range from a sink (−0.47 Tg year−1) to a source (0.82 Tg year−1) of C. More research is needed with continuous flux measurements and from ponds of different ages. Likewise, there is a need for more studies in Eurasia to understand the effect of beaver on C biogeochemistry.

Dam busy: beavers and their influence on the structure and function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems

“…CH4 fluxes from beaver ponds are also elevated (Ford and Naiman, 1988;Lazar et al, 2015;Roulet et al, 1997;Yavitt et al, 1990), especially relative to the fluxes that would likely occur from the river system in their absence (Ford and Naiman, 1988), or even relative to other regional wetlands, particuarly in boreal regions (Bubier et al, 1993;Roulet et al, 1997). However, measured CH4 fluxes from beaver systems to date are almost exclusively from the higher latitude regions of North America (Nummi et al, 2018), and are highly variable regionally (Nummi et al, 2018;Whitfield et al, 2015), locally (Bubier et al, 1993;Lazar et al, 2015), and even within a single pond (Weyhenmeyer, 1999;Yavitt et al, 1992). These increased CH4 fluxes, and to some extent CO2 fluxes, along with their high spatial and temporal variability, are a result of the expanded benthic anaerobic conditions following beaver impacts promoting metabolic pathways that include methanogenesis.…”

“…This is also a potential mechanism that can explain the increase in DOC concentrations following beaver related water level increases in Finnish lakes (Vehkaoja et al, 2015). However, Nummi et al (2018) suggest the initial DOC sources following damming are from the decay of existing organic matter stocks rather than new interactions with riparian and littoral zone organic matter. This mechanism is in contrast to most other studies examining DOC source and export dynamics that emphasize the importance of hydrological feedbacks with the riparian zone, however it does highlight the need to better understand the unique DOC source-sink dynamics that may occur in beaver systems.…”

“…The amount of woody biomass harvested by beavers remains highly uncertain, Francis et al (1985) report ~1 t per year per adult of woody biomass harvested, and Nummi et al (2018) report on average ~8.8 t per year is harvested in the browsing zone surrounding ponds per colony. However, the vast majority of this wood is used for dam construction (Nummi et al, 2018), which (Johnston and Naiman, 1990b) found on average contained ~7.7 t of wood per dam. In any case, it would be difficult to justify extrapolating these estimates beyond their local settings without further knowledge on how dependent such woody biomass harvesting may be on wood availability, type, food availability, and landscape controls on the damming activity.…”

“…The full mass balance of changes to the storage and fluxes of carbon that can occur as result of beaver modifications, especially across the spectrum of terrestrial and aquatic carbon sources and sinks, remains poorly understood (Nummi et al, 2018;Wohl, 2013). This is partly because the mass balance strongly depends on the spatial and temporal frames of reference considered, and the availability of suitable controls for context.…”

“…Increase in organic carbon storage; increase in atmospheric fluxes (CO2, CH4), and dissolved organic and inorganic carbon concentrations downstream of beaver systems. Lazar et al, 2015;Naiman et al, 1986;Nummi et al, 2018;Weyhenmeyer, 1999;Wohl et al, 2012 5.2…”