Soil respiration and net ecosystem productivity in a chronosequence of hybrid poplar plantations

Abstract

Forest stand age can affect ecosystem carbon (C) cycling and net ecosystem productivity (NEP). In Canada, establishment of short-rotation plantations on previously agricultural lands has been ongoing, but the effect of stand development on soil respiration (Rs) and NEP in such plantations is poorly understood. These types of data are essential for constraining ecosystem models that simulate C dynamics over the rotation of a plantation. We studied Rs (including autotrophic, Ra, and heterotrophic, Rh) and NEP in 2008 and 2009 in a chronosequence of 5-, 8-, 14-, and 16-yr-old (ages in 2009) hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantations in northern Alberta. The highest Rs and NEP were generally found in the 14-yr-old stand. Seasonal variations in Rs were similar among the plantations, with most of the variation explained by soil temperature at the 10 cm depth in 2008 with far less explained in 2009, a much drier year. In diurnal measurements, hysteresis was found between soil respiration and soil temperature, with the patterns of hysteresis different among stand ages. Soil respiration in the 14-yr-old plantation had the greatest sensitivity to temperature changes. Stand age did not affect the Rh:Rs ratio, whereas the NEP exhibited strong inter-annual variability. We conclude that stand age was a major factor affecting Rs and NEP, and such effects should be considered in empirical models used to simulate ecosystem C dynamics to evaluate potentials for C sequestration and the C source–sink relationship in short-rotation woody crop systems.