In the current study, a novel approach combining quantile regression with nonlinear mixed-effects (QR-NLME) modeling was applied to predict the maximum crown width (cw_max) of three economically important forest species—the European beech (Fagus sylvatica L.), the black pine (Pinus nigra Arn.), and the Calabrian pine (Pinus brutia Ten.) at tree level. A power QR-NLME model was fitted first to a dataset including 1414 European beech trees obtained from 29 randomly distributed sample plots, 770 black pine trees from 25 sample plots, and 1880 Calabrian pine trees from 41 sample plots in Greece, to predict the cw_max at tree level. Additionally, a nonlinear mixed-effects model (NLME) was fitted to the same dataset to predict the average crown width at tree level for all species. In the second stage, the crown competition factor (CCF) was estimated based on the population average response of the cw_max predictions. The proposed approach presented sound results when compared with the outcomes of relevant models from other regions fitted to open-grown tree data, and therefore, it can be well implemented on clustered data structures, in cases of absence of open-grown tree data.

Pinus halepensis is a Mediterranean tree species occupying areas of high tourist interest, where it forms aesthetic and recreational forests. However, intense human pressure, adverse climatic conditions and overgrazing degrade Aleppo pine forest ecosystems and render the natural regeneration of this species difficult. The ecological, landscape, recreational and soil conservation uses of P. halepensis along with its aesthetic value, make this species important for landscape planning and multi-purpose forestry. For these reasons, artificial regeneration may be required in order to render ecosystem restoration faster. Although P. halepensis is characterized by a high germination capacity and a constant temperature of 20 °C is considered optimal for germination, no research has dealt with the germination behaviour and early growth of seedlings under alternative temperature conditions similar to those dominating outdoors. Moreover, little research was conducted on seed quality characteristics of this species. Thus, in this study seed quality of P. halepensis was estimated by measuring purity, number of seeds per kg, weight of 1000 seeds, average seed weight, seed moisture content and percentage of empty seeds. Also, seed germination capacity, germination rate, percentage of infected and not germinated viable seeds, abnormal seedlings as well as the total seedling length were studied under laboratory (alternative temperature) and chamber (constant temperature) conditions with the same photoperiod. Results showed that the percentage of empty seeds and abnormal seedlings was extremely low and the total germination percentage was very high (87–90%) in both environments. Germination capacity, germination rate and the total length of seedlings did not show any differences among the two growth environments.

We estimated differences, five years after a wildfire, in soil and vegetation between Pinus halepensis stands that were once burnt (1998) and stands that were burnt twice in a short time interval (1995 and 1998), in the area of Penteli, central Greece. The parameters monitored were the physical and chemical attributes of upper soil layer and the vegetation composition, density and height. The results showed that five years after the wildfire, soil pH did not differ between areas burnt once and twice, while the organic matter was higher in the once-burnt areas. The vegetation composition was similar in the two areas and the dominant species were those pre-existing the fire. On the contrary, vegetation density was considerably lower in the twice-burnt areas. The height of woody species oscillated in the same levels in the two areas. The plant community was composed mainly by resprouting species, like Quercus coccifera, Pistacia lentiscus, or Phillyrea latifolia, and less individuals from seed-regenerated species, like Pinus halepensis, Cistus monspeliensis, and C. creticus. It is suggested that the recurrence of wildfire affected negatively the ecosystem attributes, and contributed to the increased risk of degradation.