Document: JAC-3-69-22

When is net ecosystem productivity positive?

JANISCH, J.E.* and M.E.HARMON

Oregon State University, Corvallis, Oregon 97331, USA ¹

Abstract:
Pacific Northwest evergreen forests may persist for centuries and store in excess of 500 Mg C/ha. Because of this potential to store carbon, the impacts of disturbance on carbon flux in these forests are highly relevant to regional carbon dynamics, carbon credits politics, and forest management. To assess the impacts of disturbance it is essential to understand how net ecosystem productivity (NEP) changes over succession. We developed a simple model to investigate this system based on the following assumptions: 1) Carbon pools that change most in terms of absolute amounts over succession are live and dead wood, 2) stand destruction resulting from either natural causes or harvest leaves woody debris in the form of logs, stumps, and roots decomposing on the site, 3) as trees colonize and grow the stand sequesters carbon as live wood. Models including these dynamics and literature based parameterizations indicate that CO₂ emitted by decomposition of woody detritus following a stand-destroying disturbance exceeds CO₂ sequestered as live wood in the re-generating stand for 30-40 years. To provide estimates of when NEP of the replacement stand switched from negative (CO₂ source to atmosphere) to positive (CO₂ sink) we tested the prediction against a chronosequence of primary forest stands that had been harvested. Results indicate the transition from negative to positive NEP for the tree component occurred between 20 and 30 years post-harvest, earlier than model predictions. It is also estimated that regenerating 50-70 year old forest stored approximately 1/3rd as much carbon in woody biomass as 400-600 year old primary forest. These results imply that carbon pulses released by Douglas fir / western hemlock stands in the Pacific Northwest must be included in carbon budget models because stands act as CO₂ sources for two or more decades after harvest based on NEP.

Keywords: disturbance NEP CWD chronosequence

This abstract is being presented at: 11:30 AM in session:
Oral Session #25: Organic Matter Dynamics in Ecosystems.