Biomass data for young, planted Norway spruce (Picea abies (L.) Karst.) trees in Eastern Carpathians of Romania

Tree biomass data are essential for developing the biomass allometric models that are necessary for estimating carbon stock and for monitoring changes in forest biomass. In this ‘data article’ biomass records are presented for 240 Norway spruce trees (Picea abies (L.) Karst.). Trees were between 4 and 15 years of age and were sampled from 24 pure plantations located in Eastern Carpathians of Romania. Ten trees were sampled from each plantation using a cluster sampling method. For each tree, diameter at root collar height (D) and tree height (H) are provided as potential predictors for biomass. Oven-dried biomass is also recorded for the following partitions: stem (ST); branches (BR); needles (ND); roots (RT); as well as their combinations representing total aboveground biomass (AGB) and overall tree biomass (TB). Sampled trees were between 0.6 and 10.0 cm in diameter and between 53.0 and 552.0 cm in height. Total tree biomass ranged between 0.019 and 15.53 kg/tree. This dataset is related to the research article entitled “Site-effects on biomass allometric models for early growth plantations of Norway spruce (Picea abies (L.) Karst.)” (Dutcă et al., 2018) [1].


Subject area
Forestry More specific subject area Estimation of tree biomass Type of data Table  How data was acquired Data was acquired through measurement. The biomass of entire tree (including roots) was oven dried and then weighed. Precision of measurement is 1 mm for diameter, 1 cm for height and 0.1 g for biomass.

Data format
Raw Experimental features The data were collected from 24 plantations, and 10 trees sampled from each plantation. Data is available with this article.

Related research article
Site-effects on biomass allometric models for early growth plantations of Norway spruce (Picea abies (L.) Karst.) [1] Value of the data The data can be used to develop biomass allometric models, biomass expansion factors and root to shoot ratios, in order to predict tree biomass.
The data can be used to assess site-effects on biomass allometric models, and variance partitioning within and between forest stands.
The data can be used to validate other biomass allometric models. The data on partitioning of biomass (stem, branch, needle, root) can be used to assess allocation to different tree components and in calibration of growth models.
Combined with other datasets from a different region, the data can be used to assess the effect of the region on allometric models.
The data can be used with other datasets to develop generic biomass allometric models.

Data
This 'data article' presents biomass records for 240 Norway spruce (Picea abies (L.) Karst.) trees sampled from 24 plantations. Ten trees were sampled from each plantation. The sampled trees were young (up to 15 years of age), therefore, their size was generally small. The diameter at root collar height (D) ranged from 0.6 to 10.0 cm and the tree height (H) from 53.0 to 552.0 cm. The total biomass of sampled trees ranged from 0.019 to 15.53 kg/tree (Table 1 and Supplementary material).
The clustered structure of this dataset permitted the investigation of site-effects in allometric models, by separating the model variance produced by the variability of trees within plantations from the variance produced by variability between plantations [1]. Furthermore, part of this dataset was used to develop biomass conversion and expansion factors [2] and to assess the influence of age, latitude, altitude and soil properties on biomass allometric models [3].

Experimental design, materials, and methods
The trees were sampled from 24 plantations of pure Norway spruce in Eastern Carpathians of Romania, between 45.44°N and 47.76°N in latitude and between 24.96°E and 26.36°E in longitude ( Table 2). The age of trees at the moment of sampling varied between 4 and 15 years (in each plantation, the trees had similar age). Age of trees (see Table 1, in Ref. [1]) was calculated as the Table 1 Biomass of stem (ST), branches (BR), needles (ND), roots (RT), aboveground biomass (AGB, calculated as the sum of ST, BR, ND) and total tree biomass (TB, calculated as the sum of AGB and RT), for each tree in each forest stand, expressed in grams (g). Diameter at collar height (D) and tree height (H) are expressed in cm. The 'Plantation' defines the association of sampled trees in plantations.  difference between year of sampling and year of planting (provided by forest management plan), plus the number of years spent in forest nurseries (we adopted a standard number of 3 years for all sampled trees, as this is the usual practice for the species). The calculated age was then validated by counting the number of tree rings on sampled trees. Mean annual temperature (MAT) and mean annual precipitation (MAP) were derived from WorldClim, global climate data [4]. MAT varied between 2.6 and 7.3°C and MAP ranged between 643.0 and 932.6 mm. Soil pH, base saturation and texture were based on sample soil taken from each plantation.

Sampling design
The tree selection in each plantation was meant to capture the entire variability of biomass-predictor, in order to assess the site-effects on biomass allometric models [1]. Biomass allometric models, which are regression models that use tree diameter and/or height to predict biomass, are influenced by tree architecture and wood density variation. These two characteristics (i.e. tree architecture and wood density) are modulated by genotype [5], tree competition [6] and environmental conditions [7]. Since wood density is inherently species specific and because the current dataset involves only one tree species, the sampling strategy has been focused on tree architecture only. Nevertheless, an important vector of tree architecture is the H-D ratio. Therefore, to capture the variability of H-D ratio in each plantation, H was set constant while D was let to vary. Diameter was let to vary, since D has greater effect on biomass compared to H [1]. Setting H constant was also helpful to facilitate the identification of candidate trees within plantation (as H is better visually approximated compared to D, especially in young stands).

Biomass measurements
In each plantation a sample plot of 200 m 2 was delimited and all trees were measured for D and H. The 'mean height' was calculated, as the height corresponding to the mean 'collar area' (similar to basal area but calculated based on root collar diameter). Ten trees in each plantation were selected and destructively sampled. The selection criteria were (i) H of candidate trees to be close to 'mean height' (calculated for each plantation) and (ii) D was let to vary (Fig. 1). Trees showing signs of affected structural integrity or disease were avoided.
A total of 240 trees were sampled, with D ranging from 0.6 to 10.0 cm, and H between 53.0 and 552.0 cm (Table 1). For each sampled tree, D and H were measured in situ. Fresh biomass of each tree was separated in 3 fractions: (i) stem, (ii) branches with attached needles and (iii) roots. As trees were generally small, biomass of entire tree was oven-dried in the laboratory. The branches with attached needles were pre-dried for 24 hours to ease separation of needles from wooden branch fraction. After separation, all tree biomass components (i.e. stem, branches, needles and roots) were dried until constant weight (for approximately 7 days) at 80°C. All biomass fractions were weighed with an electronic scale (with a precision of 70.1 g). Tree biomass components, as used in this article, are defined as: (i) stemthe aboveground axis of the tree, from the ground (including the stump) to the highest bud and excluding therefore the branches and needles; (ii) branchesthe aboveground woody structures that were not included in 'stem' category; (iii) needlesthe non-woody aboveground tree component, including the buds; (iv) roots (coarse roots)the belowground biomass, defined by root diameter of at least 2.0 mm.