Wood decomposition and nutrient dynamics in burnt Mediterranean pine reforestations across an elevational gradient: a 15-year study

The rate of wood decomposition after a forest fire was analyzed over a period of 15 years and six sampling periods (initial, and after 2, 4, 8, 10 and 15 years). 75 cm long pieces of logs were experimentally arranged along an elevation gradient (ca. 1400, 1700, 2000 and 2200 m a.s.l.) with contrasting climatic conditions. The concentration of C, N and P was also determined for four of the sampling periods (including the beginning and after 15 years) as well as the concentration of K, Ca, Mg, Na, Fe, Cu, Mn and Zn at the beginning and after 15 years. Nutrient analyses were carried out in 2006, 2008, 2010 and 2021. In 2006, the P content was determined by the molybdovanadate method and the rest of the nutrients (Ca, Mg, K, Na, Fe, Mn, Zn and Cu) by atomic absorption of the vegetal ash solution with a Perkin Elmer 5100 spectrometer (Waltham, MA, USA) (Marañón-Jiménez et al. 2013). In 2008 and 2010, the P content was determined by the molybdovanadate method with a Perkin Elmer 2400 spectrophotometer (Waltham, MA, USA) (Marañón-Jiménez and Castro 2013). In 2021, nutrient content (P, K, Na, Ca, Mg, Fe, Cu, Mn and Zn) was determined by inductively coupled plasma mass spectrometry (ICP-MS) with a Perkin Elmer Avio 500 spectrometer. For all sampling years, total C and N were determined with a CN Gas Analyzer by using an induction furnace and thermal conductivity (LECO TruSpec CN). Log volume was estimated by the conic-parabolic formula proposed by Fraver et al. (2007). Wood density was calculated by dividing wood dry weight by its volume. To check the accuracy of the 2021 data, nutrient analyses were repeated for 53 samples. The repeated samples showed consistency for all the nutrients except for Fe and Na, that gave random values without any correlation between the two trials of analysis. Therefore the values for these nutrients for 2021 were eliminated from the data set.