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)

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.