Canadian Forest Service Publications

Historical effects of dissolved organic carbon export and land management decisions on the watershed-scale forest carbon budget of a coastal British Columbia Douglas-fir-dominated landscape. 2017. Smiley, B.P., Trofymow, J.A. Carbon Balance Manage (2017) 12:15

Year: 2017

Issued by: Pacific Forestry Centre

Catalog ID: 38877

Language: English

Availability: PDF (download)

Available from the Journal's Web site.
DOI: 10.1186/s13021-017-0083-z

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Background: To address how natural disturbance, forest harvest, and deforestation from reservoir creation affect landscape-level carbon (C) budgets, a retrospective C budget for the 8500 ha Sooke Lake Watershed (SLW) from 1911 to 2012 was developed using historical spatial inventory and disturbance data. To simulate forest C dynamics, data was input into a spatially-explicit version of the Carbon Budget Model-Canadian Forest Sector (CBM-CFS3). Transfers of terrestrial C to inland aquatic environments need to be considered to better capture the watershed scale C balance. Using dissolved organic C (DOC) and stream flow measurements from three SLW catchments, DOC load into the reservoir was derived for a 17-year period. C stocks and stock changes between a baseline and two alternative management scenarios were compared to understand the relative impact of successive reservoir expansions and sustained harvest activity over the 100-year period. Results: Dissolved organic C flux for the three catchments ranged from 0.017 to 0.057 Mg C ha−1 year−1. Constraining CBM-CFS3 to observed DOC loads required parameterization of humified soil C losses of 2.5, 5.5, and 6.5%. Scaled to the watershed and assuming none of the exported terrestrial DOC was respired to CO2, we hypothesize that over 100 years up to 30,657 Mg C may have been available for sequestration in sediment. By 2012, deforestation due to reservoir creation/expansion resulted in the watershed forest lands sequestering 14 Mg C ha−1 less than without reservoir expansion. Sustained harvest activity had a substantially greater impact, reducing forest C stores by 93 Mg C ha−1 by 2012. However approximately half of the C exported as merchantable wood during logging (~176,000 Mg C) may remain in harvested wood products, reducing the cumulative impact of forestry activity from 93 to 71 Mg C ha−1. Conclusions: Dissolved organic C flux from temperate forest ecosystems is a small but persistent C flux which may have long term implications for C storage in inland aquatic systems. This is a first step integrating fluvial transport of C into a forest carbon model by parameterizing DOC flux from soil C pools. While deforestation related to successive reservoir expansions did impact the watershed-scale C budget, over multi-decadal time periods, sustained harvest activity was more influential.

Plain Language Summary

A spatially-explicit version of the Carbon Budget Model- Canadian Forest Sector3 was used to derive a baseline carbon (C) budget for the Sooke Lake watershed on Vancouver Island from 1910-2012. Streamflow and Dissolved Organic Carbon (DOC) data for three catchments were used to estimate DOC load and calibrate the model’s DOC fraction parameter, (the annual fraction of decaying humified soil C transferred to streams). The model was then used to determine forest DOC export for the entire watershed. Modelling scenarios were used to assess impacts of deforestation for reservoir expansion or sustainable forestry activities on historical C budgets.The DOC fraction parameter varied from 2-6%. Over 100 years, 30,640 Mg C was estimated to have been transferred from forest to reservoir. By 2012, reservoir expansion had resulted in 14 Mg C/ha less sequestration in the watershed while forestry alone resulted in 93 Mg C/ha less sequestration. However if C remaining in stored in wood products is accounted for, forestry would have resulted in 71 Mg C/ha less sequestration. Although often overlooked in modelling, DOC export from forests may represent a small but significant C sink. In this study, forestry had a greater impact on landscape C budgets than reservoir creation.