Ž . Forest Ecology and Management 107 1998 213–229 Modeling tree growth in fertilized midrotation loblolly pine plantations Jari Hynynen a, ) , Harold E. Burkhart b , H. Lee Allen c a Finnish Forest Research Institute, Vantaa, Finland b Virginia Polytechnic Institute and State UniÕersity, Blacksburg, VA, USA c North Carolina State UniÕersity, Raleigh, NC, USA Received 7 April 1997; accepted 11 November 1997 Abstract Diameter and height growth models for fertilized loblolly pine stands were developed using data from midrotation loblolly pine plantations across the southeastern United States. Tree growth in fertilized stands was predicted with a reference growth model multiplied by an equation predicting the relative growth response following fertilization. The temporal distribution of the growth response was modeled by the Weibull function. These equations for fertilizer growth response were developed to be compatible with individual-tree simulation models. Information about dose, nutrient elements, and time elapsed since fertilization are needed to predict the relative growth response following fertilization. q 1998 Elsevier Science B.V. Keywords: Pinus taeda L.; Nitrogen; Phosphorus; Yield; Simulation; Growth response; Weibull function 1. Introduction Forest fertilization is an important silvicultural practice to improve stand productivity in loblolly pine plantations in the southeastern United States. Results from studies based on field trials demon- strate that nitrogen andror phosphorus fertilization will produce significant growth response in midrota- tion loblolly pine plantations. In 1996, approximately 280,000 ha of loblolly pine were fertilized. As of the ) Corresponding author. Finnish Forest Research Institute, P.O. Box 18, FIN-01301 Vantaa, Finland. Tel.: q358-9-8570-5324; fax: q358-9-8570-5361; e-mail: jari.hynynen@metla.fi. end of 1996, 1.52 million ha of loblolly pine stands has been fertilized in the United States since 1969. In forest management planning, reliable growth and yield models are needed to assess the growth effects of silvicultural treatments. Fertilization exper- iments have been established for numerous tree species in order to provide information about the growth effects of fertilization. Methods used to ana- lyze these fertilization data have included analysis of Ž variance or covariance Paavilainen and Simpanen, 1975; Miller and Tarrant, 1983; Opalach and Heath, . 1988; Hynynen and Kukkola, 1989 and regression Ž analysis Gustavsen and Lipas, 1975; Wells et al., 1976; Rosvall, 1980; Kukkola and Saramaki, 1983; ¨ Miller et al., 1988; Shafii et al., 1990; Hynynen, . 1993 . 0378-1127r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0378-1127 97 00334-4