Trees represent the largest terrestrial carbon sink and a renewable source of ligno-cellulose. There is significant scope for yield and quality improvement in these largely undomesticated species, however, efforts to engineer new, elite varieties are constrained by the lack of a comprehensive understanding of the transcriptional network underlying cambial growth and wood formation. We generated RNA Sequencing transcriptome data for four mature, wild-growing aspens (Populus tremula) from high-spatial-resolution tangential cryosection series spanning the secondary phloem, vascular cambium, expanding and secondary cell wall forming xylem cells, cell death zone and the previous years annual ring. The transcriptome comprised 28,294 expressed, previously annotated protein-coding genes, 78 novel protein-coding genes and 567 long intergenic non-coding RNAs. Most paralogs originating from the Salicaceae whole genome duplication had diverged expression, with the notable exception of those with high expression during secondary cell wall deposition. We performed co-expression network analysis to identify central transcriptional modules and associated several of these with known biological processes. This revealed previously uncharacterized complexity underlying the regulation of cambial growth and wood formation, with modules forming a continuum of activated processes across the tissues. The high spatial resolution suggested novel roles for known genes involved in xylan and cellulose biosynthesis, regulators of xylem vessel and fiber differentiation and components of lignification. The associated web resource (AspWood, http://aspwood.popgenie.org) integrates the data within a set of interactive tools for exploring the co-expression network of cambial growth and wood formation.