A method of making a foamed cellulosic fiber-thermoplastic composite article comprising: A) providing a copolymer composition that includes: i) at least 20 wt. % of a copolymer formed by polymerizing a mixture comprising: a) 51% to 99.9% by weight of one or more primary monomers, wherein the primary monomers are selected from the group consisting of styrenic monomers and olefinic monomers and combinations thereof, b) 0.1% to 49% by weight of one or more anhydride containing monomers, and c) optionally 1% to 25% by weight of one or more other polymerizable monomers; and ii) optionally 0.1% to 30% by weight of one or more elastomeric polymers based on the weight of the copolymer; B) combining 30% to 99.9% by weight of the copolymer composition with 0.01% to 70% by weight of one or more cellulosic fibers based on the weight of the combination; C) applying an amount of heat, mixing energy and pressure sufficient to provide: i) mixing of the copolymer composition and the cellulosic fibers; ii) reaction of anhydride groups in the copolymer with hydroxyl groups in the cellulosic fibers; and iii) production of a blowing agent as a byproduct of the reaction in ii) to provide a foamable mixture of the copolymer composition and the cellulosic fibers; and D) forming the foamed article by placing the foamable mixture in a molding or extruding operation; wherein the foamed article has a structured foam central portion and micro foamed outer portion that surrounds the structured foam portion and has a higher density than the central portion; wherein voids, which are greater than 50 μm and which are separated by walls from 50 μm to 10 mm thick, are located throughout the structured foam central portion, wherein voids, which are less than 50 μm, are dispersed throughout the micro foamed outer portion, and wherein the micro foamed outer portion comprises from 5 to 22 percent of the overall thickness of the article; characterized in that at least 10% of the cellulosic fibers have been thermally modified by heat treatment at a temperature from 160 to 250°C, such as from 200 to 230°C at atmospheric pressure or at a temperature of from 120°C to 230°C at a pressure higher than atmospheric pressure prior to being combined with the copolymer composition. A) providing a copolymer composition that includes: i) at least 20 wt. % of a copolymer formed by polymerizing a mixture comprising: a) 51% to 99.9% by weight of one or more primary monomers, wherein the primary monomers are selected from the group consisting of styrenic monomers and olefinic monomers and combinations thereof, b) 0.1% to 49% by weight of one or more anhydride containing monomers, and c) optionally 1% to 25% by weight of one or more other polymerizable monomers; and ii) optionally 0.1% to 30% by weight of one or more elastomeric polymers based on the weight of the copolymer; i) at least 20 wt. % of a copolymer formed by polymerizing a mixture comprising: a) 51% to 99.9% by weight of one or more primary monomers, wherein the primary monomers are selected from the group consisting of styrenic monomers and olefinic monomers and combinations thereof, b) 0.1% to 49% by weight of one or more anhydride containing monomers, and c) optionally 1% to 25% by weight of one or more other polymerizable monomers; and a) 51% to 99.9% by weight of one or more primary monomers, wherein the primary monomers are selected from the group consisting of styrenic monomers and olefinic monomers and combinations thereof, b) 0.1% to 49% by weight of one or more anhydride containing monomers, and c) optionally 1% to 25% by weight of one or more other polymerizable monomers; and ii) optionally 0.1% to 30% by weight of one or more elastomeric polymers based on the weight of the copolymer; B) combining 30% to 99.9% by weight of the copolymer composition with 0.01% to 70% by weight of one or more cellulosic fibers based on the weight of the combination; C) applying an amount of heat, mixing energy and pressure sufficient to provide: i) mixing of the copolymer composition and the cellulosic fibers; ii) reaction of anhydride groups in the copolymer with hydroxyl groups in the cellulosic fibers; and iii) production of a blowing agent as a byproduct of the reaction in ii) to provide a foamable mixture of the copolymer composition and the cellulosic fibers; and i) mixing of the copolymer composition and the cellulosic fibers; ii) reaction of anhydride groups in the copolymer with hydroxyl groups in the cellulosic fibers; and iii) production of a blowing agent as a byproduct of the reaction in ii) to provide a foamable mixture of the copolymer composition and the cellulosic fibers; and D) forming the foamed article by placing the foamable mixture in a molding or extruding operation; wherein the foamed article has a structured foam central portion and micro foamed outer portion that surrounds the structured foam portion and has a higher density than the central portion; wherein voids, which are greater than 50 μm and which are separated by walls from 50 μm to 10 mm thick, are located throughout the structured foam central portion, wherein voids, which are less than 50 μm, are dispersed throughout the micro foamed outer portion, and wherein the micro foamed outer portion comprises from 5 to 22 percent of the overall thickness of the article; characterized in that at least 10% of the cellulosic fibers have been thermally modified by heat treatment at a temperature from 160 to 250°C, such as from 200 to 230°C at atmospheric pressure or at a temperature of from 120°C to 230°C at a pressure higher than atmospheric pressure prior to being combined with the copolymer composition.