Temporal changes of forest net primary production and net ecosystem production in west central Canada associated with natural and anthropogenic disturbances Zhong Li, Michael J. Apps, Werner A. Kurz, and Ed Banfield Abstract: Temporal variations of net primary production (NPP) and net ecosystem production (NEP) in west central Canadian forests over the period of 1920–1995 and their responses to natural and anthropogenic disturbances were sim- ulated using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS2). The results show that forest NPP in the region was 215 g C·year –1 ·m –2 in 1920, varied between 105 and 317 g·C year –1 ·m –2 depending on ecoclimatic province, but gradually increased to 330 (158 to 395) g C·year –1 ·m –2 in the early 1980s before declining to 290 (148 to 395) g C·year –1 ·m –2 by 1995. Forest NEP was estimated to be 53 (–13 to 88) g C·year –1 ·m –2 in 1920–1924, increased to 75 (5 to 98) g C·year –1 ·m –2 in 1960, and then declined to 26 (–14 to 53) g C·year –1 ·m –2 in 1991–1995. Natural dis- turbances played a greater role than harvest in determining the temporal pattern of forest NPP and NEP during the pe- riod because of the larger area affected by natural disturbances. This study also indicated that ignoring disturbances would lead to an overestimation of forest NPP and NEP in ecosystem modeling. Résumé : Les variations temporelles de la productivité primaire nette (PPN) et de la productivité nette de l’écosystème (PNE) dans les forêts canadiennes du centre ouest pendant la période entre 1920 et 1995 ainsi que leurs réponses aux perturbations naturelles et anthropogéniques ont été simulées à l’aide du modèle de bilan de carbone du secteur fores- tier canadien (CBM-CFS2). Les résultats montrent que la PPN forestière dans la région était de 215 g C·an –1 ·m –2 en 1920 et variait de 105 à 317 g C·an –1 ·m –2 selon la province écoclimatique mais a augmenté progressivement à 330 (158 à 395) g C·an –1 ·m –2 au début des années 1980 avant de diminuer à 290 (148 à 395) g C·an –1 ·m –2 en 1995. La PNE forestière a été estimée à 53 (–13 à 88) g C·an –1 ·m –2 en 1920–1924, a augmenté jusqu’à 75 (5 à 98) g C·an –1 ·m –2 en 1960 et a ensuite décliné à 26 (–14 à 53) g C·an –1 ·m –2 en 1991–1995. Les perturbations naturelles ont joué un plus grand rôle que la récolte pour déterminer le patron temporel des PPN et PNE forestières pendant cette période à cause de la superficie plus importante affectée par les perturbations naturelles. Cette étude a aussi montré que le fait d’ignorer les perturbations amènerait une surestimation des PPN et PNE forestières en modélisant les écosystèmes. [Traduit par la Rédaction] Li et al. 2351 Introduction Net primary production (NPP) and net ecosystem produc- tion (NEP) are two important indicator variables for terres- trial ecosystems (IGBP Terrestrial Carbon Working Group 1998; Scurlock et al. 1999). They are subject to changes in solar radiation, climate (e.g., temperature and precipitation), atmospheric chemistry (e.g., CO 2 concentration), plant at- tributes (e.g., species), edaphic conditions (e.g., soil water, texture and nutrients), as well as natural and anthropogenic disturbances (Barford et al. 2001; Churkina and Running 1998; Knapp and Smith 2001; Kurz and Apps 1999; Pastor and Post 1986; Runyon et al. 1994). Unfortunately, direct field measurement for both NPP and NEP in forest ecosystems has proven to be difficult, primar- ily because of the difficulty of measuring belowground NPP (especially fine-root biomass and turnover) and of separating the simultaneously occurring autotrophic and heterotrophic respiration (see Gower et al. 1999). The lack of species- and site-specific allometric equations also introduces uncertain- ties in measurements of aboveground NPP (see Gower et al. 1999). In general, direct NPP measurements are limited to single plants, small plots, or small patches of vegetation, oc- casionally 0.1 ha but generally much smaller (Scurlock et al. 1999). The eddy covariance techniques provide us with the best available estimate of NEP at the stand scale (~1 km 2 ), but the measurement requires heavily instrumented towers and provides no means of estimating NPP (Reich et al. 1999; Sellers et al. 1997). Currently, credible experimental data on forest NPP and NEP in the world’s major climatic zones are sparse, and their direct measurements at large scales remain problematic. Can. J. For. Res. 33: 2340–2351 (2003) doi: 10.1139/X03-168 © 2003 NRC Canada 2340 Received 5 January 2003. Accepted 18 July 2003. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 25 November 2003. Z. Li 1,2 and E. Banfield. Natural Resources Canada, Canadian Forest Service, 5320-122 Street, Edmonton, AB T6H 3S5, Canada. M.J. Apps and W.A. Kurz. Natural Resources Canada, Canadian Forest Service, 506 West Burnside Road, Victoria, BC V8Z 1M5, Canada. 1 Corresponding author (e-mail: zhong.li@ubc.ca). 2 Present address: Faculty of Agricultural Sciences, The University of British Columbia, 2357 Main Mall, Vancouver, BC V6T 1Z4, Canada.