Allometry data on walnut (Juglans regia) in agroforestry-like systems (7-10 meters spacing) with different age (7-29 years old) was collected in the summer of 2024 across 5 locations (sandy and clayey soils) in the Netherlands. In total for 263 trees the diameter at breast height (DBH), the merchantable stem height, and total tree height were measured. For a selection of 42 out of the 263 trees, the canopy diameter in two directions, the stem diameter at the stem base, and the stem diameter at the stem top were measured. The data was collected in order to provide training and validation data for artificial intelligence (AI) models that use field-data combined with satellite data (hyperspectral, LiDAR) with the aim to predict carbon storage in the aboveground biomass (AGB) of walnut trees. The overarching goal was to contribute to cost-efficient yet reliable methods for monitoring, reporting, and verification (MRV) for agroforestry carbon farming certification schemes. The data was collected as part of the project: “Verdienmodellen voor koolstofvastlegging; gebiedsgerichte pilot in Zuid-Nederland”. The project was requested by BoerenNatuur Brabant West, executed by the Zuidelijke Land- en Tuinbouworganisatie (ZLTO) and Louis Bolk Instituut (LBI), with contributions from Space4Good on the AI model development hosted by a web-based platform "CarboCatch". The project was funded under the subsidy “Samenwerken aan groen-economisch herstel” and was granted by the Rijksdienst van Ondernemend Nederland (RVO). The data was collected by Waas Thissen (LBI) and Zarah Tomesen. Contact person for requesting the dataset is Waas Thissen.