Document: JAM-3-59-18

Contrasting soil respiration in young and old ponderosa pine stands.

IRVINE, J.* and B.ELAW

Oregon State University, Corvallis, OR 97331, USA ¹

Abstract:
Soil surface CO₂ fluxes (Fs) were estimated periodically with manual chamber measurements, and continuously with an automated multiplexing chamber system in both an old growth (O site) and 15-year-old (Y site) stand of ponderosa pine in the Oregon (AmeriFlux sites). The dependence of Fs on soil temperature at 15 cm depth (manual measurements) varied between Y and O sites (Q10 = 3.0 and 2.0, respectively), as did Fs predicted at 10 C (1.7 and 2.3 mol m^-2 s^-1). The maximum diurnal summer soil temperature at 15cm depth at the Y site was on average 3.7 C higher than at the O site in response to having a more open canopy (LAI = 1.1 Y site vs. 2.3 at O site). As a consequence, annual estimates of Fs were similar for the Y and O sites (714 and 702 g C m^-2 y^-1). Automated Fs data, which were only available during the drier summer months, provided insights into the dependence of Fs on soil moisture, soil temperature and rain events with high temporal resolution. Soil moisture and temperature were measured continuously at several depths next to each chamber. Data collected at the Y site encompassed a gradual soil dry down over the top 30 cm of soil (11 to 7.5 % VMC) with few rain events (1.7 mm). A statistical correlation model incorporating a linear soil temperature response (at 2cm depth) and a hyperbolic soil moisture response (at 30cm depth), combined in a multiplicative manner, could account for 63% of the variance of Fs. Soil temperature alone could only account for 36% of the variance of Fs. Automated data collected at the O site covered a period with frequent wetting and drying of the upper soil profile (15 to 8.2 % VMC) in response to several rain events (22.9 mm total). Under these conditions soil temperature alone could account for 70% of the variance of Fs. Soil moisture measured at either 10 cm or 30 cm depth did not significantly improve the model fit. Apparent in the O site data are immediate responses of Fs to light rain events (<1mm) that follow a period of several dry days. Such events result in more significant increases in Fs than observed following heavier rainfall several days later.

Keywords: soil respiration, ponderosa pine, automated chambers

This abstract is being presented at: 10:30 AM in session:
Poster Session #13: Decomposition and Soil Respiration.