Post-fire regeneration thinning, cone production, serotiny and regeneration age in Pinus halepensis

Abstract

Many pines growing in fire-prone ecosystems exhibit an abundant production of cones and the retention of part of these cones as an aerial seed bank in the canopy (serotiny). The protection of seeds and their prolific release after fire facilitate the re-colonization of the burned area. However, over-stocking in this fire regenerated stands may constraint tree growth, as well as increase the risk of new wildfires. Therefore, thinning is strongly recommended both to reduce fire occurrence and to reduce intraspecific competition. Although the benefits of thinning for tree growth in fire regenerated forests have been thoroughly documented, less attention has been paid to the effects on reproductive traits such as the timing of reproduction, the size of the cone crop and the degree of serotiny. In this study, the effects of thinning on the reproductive traits of fire regenerated Pinus halepensis forests with different regeneration age (22-, 18- and 10-yr-old) were investigated. In the thinned plots, there was a greater increase in the percentage of pines that started producing cones for the first time (thinning = 50.4 ± 11.4%, control = 13.3 ± 4.0%), as well as an increase in the number of new cones produced per pine (between two- and six-fold compared to the control stands). Our results show that thinning may decrease serotiny depending on age. Thus, the number of pines bearing open cones did not change during the study in the thinned or control plots of the younger stands (10 yrs) or in the control plots of 22 and 18 yrs, while it increased in the thinned stands of 22 and 18 yrs. Not a single cone was observed to open during the study in the thinned or control 10-yr-old plots, while in the 22- and 18-yr-stands the number of old serotinous cones per pine opened was higher in the thinned plots than in the control ones. Therefore, we conclude that thinning in fire regenerated P. halepensis forests should be conducted in younger stands (10-yr-old, in our study), because thinning at this age shortens the juvenile (non-reproductive) period and greatly increases the number of new cones produced per pine in completely serotinous individuals.