Mathew Brown

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Evidence for a nonmonotonic relationship between ecosystem‐scale peatland methane emissions and water table depth

Although temporal and spatial variations in peatland methane (CH4) emissions at broad scales are often related to water table (WT) using a linear relationship, a potentially complex relationship exists between these variables locally and over shorter time scales. To explore this issue, CH4 fluxes were measured using eddy covariance at the Mer Bleue bog over two summer seasons. Peak CH4 emissions (30 to 50 mg CH4-C m−2 d−1) occurred not when the WT was closest to the surface but instead, when it dropped to 40 to 55 cm below the surface. When the WT was below or above this zone, average fluxes were ~14 mg CH4-C m−2 d−1. We speculate this critical zone coincides with the necessary redox potentials and sources of fresh organic material that lead to maximum production of CH4 and/or with conditions that lead to degassing of stored CH4. However, as expected, total summer CH4 emissions were 47% lower during the drier year. This occurred in part because the WT was within the critical zone for fewer days in the drier year but also because after an extended midsummer dry period there was little recovery of CH4 emissions, even a month after rewetting.

Two Bogs in the Canadian Hudson Bay Lowlands and a Temperate Bog Reveal Similar Annual Net Ecosystem Exchange of CO2

Abstract Two ombrotrophic bogs in Canadas Hudson Bay Lowlands (HBL), an area storing an estimated 33 Gt of soil carbon, are contrasted with the Mer Bleue temperate ombrotrophic bog approximately 1000 km to the southeast to assess the net carbon dioxide (CO2) exchange between these ecosystems and the atmosphere. Peatlands in the HBL region may be impacted by not only climate change but also resource extraction practices that may cause drying of surrounding areas. Two years of eddy covariance CO2 flux measurements show the two HBL bogs to be annual sinks for CO2. Given random error and gap-filling uncertainties of 6 to 13 g C m-2 yr-1, the annual budgets of 45 to 55 g C m-2 yr-1 for the HBL bogs did not differ significantly from the temperate bogs budget of 55 g C m-2 yr-1 (in the first year) despite differences in climate and vegetation composition and abundance. The temperate bog did have significantly greater net uptake of CO2 (78 g C m-2 yr-1) in the second study year. Component fluxes of photosynthesis and respiration were much smaller at the HBL bogs and speculated to be a result of less vascular vegetation. Less growing season CO2 uptake at the HBL bogs was offset by less winter loss when compared to the temperate bog. The influence of mid-summer drying and lowered water tables was similar among all three bogs. Decreasing mid-summer net ecosystem productivity (NEP) appeared to be a result of reduced photosynthetic uptake rather than increased respiration. In the short-term, drying of the HBL peatlands might result in a decrease of their C sink strength.

The carbon, water and energy balances of two lodgepole pine stands recovering from mountain pine beetle attack in British Columbia

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