The development of radiative transfer (RT) models requires comprehensive ground reference data on both the structural and spectral characteristics of forests. However, such kind of data are currently lacking. The dataset introduced in this paper consists detailed structural data from forest inventory measurements, terrestrial and airborne laser scanning, and digital hemispherical photography, and optical properties of tree leaves and needles, forest floor and entire stands. Such data would be valuable for RT modeling and biophysical validation studies. The paper is finely prepared and I recommend its publication in ESSD.

It would be nice if some field measurement pictures can be provided.

Fig. 7. The legend may be put in panel (a).

The study presents a new dataset based on forest inventory measurements with structural and spectral properties of 58 stands in temperate, hemiboreal and boreal European forests. The dataset is specifically designed to develop and validate radiative transfer models for forests but can also be utilized in other remote sensing studies. It includes detailed information on forest structure and spectral properties at multiple scales. The paper is well-written and structured and provides a valuable and comprehensive dataset for advancing RT models and remote sensing of forests, assuring FAIR data principles. However, I would like to ask the author for a few minor corrections:

• L. 22: Specify these new satellite missions.
• Chapter 3: Does the data set contain information of measurement uncertainty? Please describe it in the manuscript, or give indications of how it can be derived.
• L. 448 /Chapter 4: Data availability: Sharing scientific data effectively within the community is crucial, and I particularly appreciate your adherence to FAIR data principles, which greatly facilitates this process. However, FAIR principles are only mentioned and one reference is cited. Could you please give more details on how your data addresses FAIR principles, see also Box2 in Wilkinson et al., 2016 (https://www.nature.com/articles/sdata201618#Sec6)

A spectral-structural characterization of European temperate, hemiboreal and boreal forests

Summary:

This manuscript presents a ground reference dataset of structural and spectral properties of 58 stands in temperate, hemiboreal and boreal European forests with objective to benchmark 3D radiative transfer models. As a radiative transfer model developer, I realize the importance of field measurements that helps the improvement of radiative transfer models and the space mission Cal/Val activities. I appreciate the presented work. Since I am not expert in field measurement, my comments and questions are mainly from a point of view of a modeler.

Comments and questions:

(1) Section 2.3: It is hard for readers outside of the domain to understand all the details of measurement. I suggest to add several schemata to present the ideas about the measured quantities and the geometry

(2) Section 2.3.6: The reflection and transmission behaviour of leaves and trunks are usually anisotropic. Is it possible to measure BRDF (bi-directional reflectance distribution factor) and BTDF (bi-directional transmittance distribution factor) instead of DHRF and DHTF ? Also, some leaves have strong specular reflection, is the specular reflection included in DHRF ?

(3) Section 2.3.6: Our radiative transfer model is able to simulate the uncertainty propagation (given the uncertainty of leaf optical properties, our model can compute the uncertainty of the simulated image/BRF), is it possible to get simultaneously the measurement and its uncertainty ? I downloaded the spectral transmittance through the link given by the authors, but I did not find information about uncertainty.

(4) Section 2.4.1: For the atmospheric correction, how do you get the local atmospheric profile ?

The validation of BRDF models and inversion methods are in urgent need of comprehensive ground reference data of forests, as data measurement is extremely difficult. This manuscript proposes a whole dataset on the structural and spectral properties of 58 stands in temperate, hemi-boreal and boreal European forests. The dataset contains both forest structure data and spectral properties at leaves and needles, tree, forest floor and entire stands scales base on in situ and airborne measurements.  This dataset is valuable. But the manuscript can be further improved. I suggest the acceptance after revisions with clarification of minor issues.

1) Line 120, It’s better to describe the exact location of "six fixed locations".

2) Line 127, It’s look like there’s a writing mistake as "it would be 8cm if h > 16m", or maybe can explain why the thresholds were determined as this. This is in conflict with the previous definition of mature stands (Line 122).

3) Line 264, What are the criteria for determining whether a needle is current year or one-year-old needle, according to color?

4) Formula (3), What is the measurement method of S_ref,T ？According to the formula, S_ref,T in the formula seems to have the same numerical value as S_ref,R?Please clarify it.

5) Line 419 (Fig. 5.) What is the meaning of n in the picture (Number of trees?) (n=34. n=22)? Same problem also can be found in Line 124 and Fig.6.

6) Fig.7. Why only (c) has a legend?