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Simple height and volume equations for on-the-fly estimation of productivity in hybrid poplar (Populus x euroamericana) plantations in the Duero Basin (northwestern Spain)
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1  Universidad Pública de Navarra


Hybrid poplar plantations are becoming increasingly important as a source of income for farmers in the Duero Basin (northwestern Spain), as rural depopulation and farmers aging prevent them from planting other labourr-intensive crops. However, forest owners, usually elderly and without formal forestry backgrounds, lack of simple tools to estimate the size and volume of their plantations by themselves. Therefore, farmers are usually forced to rely on the estimates made by the timber companies that are buying their trees. With the objective of provide a simple but empowering tool for these forest owners, simple equations based only on diameter to estimate individual tree height, and volume were developed for the region. To do so, growth in height, diameter and volume were measured for 10 years (2009-2019) in 404 trees planted in three poplar plantations in Leon province. An average growth per tree of 1.66 cm/year in diameter, 1.52 m/year in height, and 0.03 m3/year in volume was estimated, which translated into annual volume growth of 13.02 m3/ha/year. However, annual volume growth was different among plots due to their fertility, with two plots reaching maximum growth around 13 years of tree age and another at 15 years, encompassing the typical productivity range in plantations in this region. Such data allowed developing simple lineal and exponential equations to estimate height and volume explaining 84% to 97% of the observed variability. Such equations can be easily implemented in any cellphone with a calculator, allowing forest owners to accurately estimate their timber existences by using only a regular measuring tape to measure tree diameter.

Keywords: Tree plantations; growth equations; rotation length; growth rates; poplar productivity
Comments on this paper
Arthur Novikov
Effectiveness of actions
Dear researchers, I hope you have a little time to discuss the following:

1. How effective are the equations when changing climatic conditions in the study area?
2. is it Possible to apply the equations in locations other than the one under study?
3. How effective is the rejection of automatic taxation (for example, using a UAV), because you will have to measure manually at chest height a certain number of trees. By the way, how many trunks do you average per poplar plantation?
Juan Blanco
Dear Dr. Novikov,
Thank you for your interest in our work. Indeed, yours are very interesting questions:

1. Equations in changing climate conditions: As we used three different plots that encompass a range of site qualities, we are confident that these equations are reliable for the region in similar planting sites (flat, stony, riparian soils). As the regional authorities have generated several similar equations along the region, and our equations provide similar results, we think we can use our equations in the Duero basin, which has a cold Mediterranean climate. However, outside of this type of climate, the equations should not be used. On the other hand, as in the future climate change will become more noticeable, we think the direct climate impact on tree growth would be small, as the most important change will be elongated summer drought season, which is already been treated with 2-3 irrigation in the June-August session. Indirect effects (i.e. pests, etc.), we just don´t know.

2. Using this equations in other locations should be possible, as the I-214 poplar is a clonal variety selected not to be the fastest growing but more like the most sturdy, so it can reach similar growth rates under different conditions. We have compared our results with other equations in the Duero basin using 2 or 3 variables, and the results are similar. Again, the problem would be applying these equations in regions that are not cold Mediterranean climate or have different soil conditions.

3. I am not sure how to answer the question on the UAV use. We did not use drones for this work, al the work was done manually by measuring with a diametric tape for diameters and a Vertex for heights. We measured all the trees in the three plots (about 420 trees) every year. It is hard to say an average number of tree for this type of plantations in northern Spain, as they are directly linked to the property size, which in this region is very fragmented. It can be anything between just a dozen of trees to several thousands. However, the planting distance is quite uniform, 5 x 5 m, which brings a plantations density of 400 trees/ha.

Hope these answers are useful.
Best regards,
Juan A. Blanco