Short communication. Recruitment and early growth of Pinus pinaster seedlings over five years after a wildfire in NW Spain

Aim of study: The main aim of this study was to analyse the post-fire recruitment and growth of Pinus pinaster seedlings during the first five years after wildfire and also to analyse the effects of climatic conditions on the survival of P. pinaster seedlings Area of study: The study area was located in a P. pinaster stand in León province (NW Spain) burned in 1998. Material and methods: Three sites in the burned area were selected. In each site three permanent transects of 20 m × 1 m were placed. In each transect, twenty 1 m sampling units were marked and the number and height of each pine seedlings was recorded at 7, 8, 9, 10, 11, 12 months and 2, 3, 4 and 5 years after the wildfire. The soil of study area is Cambisol. Mean results: Mean Pinus regeneration densities varied between 33.2 seedlings/m after 7 months and 6 seedlings/m five years after wildfire. In this P. pinaster stand, maximum mortality appeared during the summer months in the first year of regeneration. There was a significant increase in seedling height associated with a decrease in density. Research highlights: The post-fire recruitment is considered enough to ensure good natural Pinus pinaster forest regeneration. In the short term post-fire management strategy in this type of forest could be the remaining branches with cones of burned trees that allow the dissemination of the seeds during the first few years after fire and ensure natural regeneration.


Introduction
The population of Pinus pinaster Ait. in the Sierra de Teleno (NW Spain) is characterised by high frequency lightning fires (>1 fire 100 km -2 year -1 ) (Gil et al., 1991, Tapias et al., 2004. Thus fire recurrence, in addition to factors such as climate and site quality, has affected the characteristics of the population in the Sierra de Teleno, such as the fact that 97% of the Pinus trees are serotinous and produce a large number of mostly serotinous cones (Gil et al., 1991).
Since P. pinaster is a seeder species and has fixed serotinity in this population (Tapias et al., 2004), its regeneration relies on the recruitment of a post-fire cohort of seedlings. It produces an abundant yearly seed crop, part of which is held in serotinous cones, thus constituting a cumulative canopy seed bank, as occurs in other types of Mediterranean pine species (de las Heras et al., 2002). Post-fire regeneration of P. pinaster depends mainly on the canopy seed bank (Calvo et al., 2008). Seed germination and seedling recruitment occur in the resource-rich environment generated by fire (Luis-Calabuig et al., 2002;Rego et al., 1991). In general, post-fire seed germination takes place almost exclusively during the first postfire stages, as in the case of P. halepensis (Arianoutsou and Ne'eman, 2000). Seedling mortality has been reported in the first post-fire year (Thanos et al., 1996) or at some locations during the second year (Ne'eman, 1997).
Seedling establishment, growth and mortality have been described widely for P. halepensis over much of its range (De la Heras et al., 2002;Herranz et al., 1997;Thanos et al., 1996); however, studies of P. pinaster recruitment are still limited (Calvo et al., 2003;Gallegos et al., 2003;Rego et al., 1991). Therefore, it is necessary to extend knowledge about the post-fire Short communication. Recruitment and early growth of Pinus pinaster seedlings over five years after a wildfire in NW Spain regeneration and recruitment of Pinus seedlings, as well as the possible influence of climatic conditions on the regeneration process, since these factors will provide an ecological basis for def ining a suitable management strategy.
The main goal of this study is to analyse the postfire recruitment and growth of P. pinaster seedlings during the first five years after fire in a population with adaptations to recurrent f ires. Similarly, we try to def ine the influence of climatic conditions on the survival of P. pinaster seedlings.

Study area
The study area is located in a P. pinaster stand in the Sierra del Teleno, León province (NW Spain), at an elevation of 1,100 m. In this area, 3,000 ha were burned by a wildf ire in September 1998. The vegetation present in the area before the fire was dominated by 60-85 year old Pinus pinaster trees, with a density of 500-900 trees/ha.
The climate is Mediterranean with 2-3 months dryness in summer and annual precipitation of 650 to 900 mm (Ministerio de Agricultura, 1980). Mean annual temperature is 10.3°C and the soil is Cambisol (Junta de Castilla y León, 1987).

Sampling data
In each study site three permanent transects of 20 m × 1 m were placed perpendicular to the slope. The burned pines remained in the sites during the study period. In each transect, twenty 1 m 2 sampling units were marked and the number and height of each pine seedlings was recorded at 7, 8, 9, 10, 11, 12 months and 2, 3, 4 and 5 years after the wildfire.

Statistical analysis
A two-way repeated measures analysis of variance was carried out to compare changes in the density and height of Pinus seedlings in different sites (S1, S2 and S3) over time, using time as the repeated measure. The Tukey test was used to assess whether differences were significant (p = 0.05). The David et al. (1954) test was used to check normality and the Cochran (1941) test was used to check homoscedasticity. The Kolmogorov-Smirnov test was used to check the normality.
Correlations between the densities of P. pinaster, precipitation (mm) and aridity index of Martonne were tested. We also evaluated the correlations between the densities and the mean seedling height. Correlation was calculated by Pearson coefficients.

Results
Pinus pinaster highest seedlings densities ( Fig. 1) were recorded during 7 th month at sites S1 and S3 and during the 8 th month at S2. There was great heterogeneity in seedling densities among the three sites with significant differences (F 2.78 = 33.10, p < 0.01) among them during the initial years of regeneration. In general, mean regeneration densities varied between 33.2 seedlings/m 2 after 7 months and 6 seedlings/m 2 five years after wildfire.
Analysis of seedling dynamics in each site through time (Fig. 1) showed a signif icant decrease (F 9.552 = 220.0; p < 0.01) in S1 from the 7 th month (76.3 seedling/m 2 ) to the 10 th month (17.4 seedling/m 2 ). However, from this month onward, there were no significant temporal differences, reaching 9 seedlings/m 2 during the 5 th year. The site S2, with the lowest seedling density, presented a less marked decrease during the initial months of regeneration, reaching 4.68 seedlings/m 2 at 5 th year. In S3, changes in seedling density behaviour were similar to S1 and a significant decrease (F 9,552 = 37.97; p < 0.01) was observed from the 7 th month (17.2 seedlings /m 2 ) until the 9 th (8 seedlings/m 2 ). No significant changes were subsequently observed at this site. In general, at sites with the highest Pinus seedling density (S1 and S3) two important decrease points were observed, one between the 7 th and 8 th months and a second during the summer months: July, August and September (months 10, 11 and 12, respectively) in the first postfire year.

Discussion
The high density of seedlings after fire shows that there was good natural regeneration of P. pinaster in the study area. The density of seedlings is higher than those reported by other authors (Gallegos et al., 2003) in studies carried out in a P. pinaster forest in the south of Spain. The large proportion of serotinous cones in Sierra del Teleno's population (Tapias et al., 2004) that allowed good seeds protection against f ire could explain these differences.
First post-fire establishment of Pinus pinaster in the study sites occurred 7 months after fire in the richresource environment generated by the fire passage. This early seedling establishment may be related to the great dissemination power of pines and its heliophily (Thanos et al., 1996). In general, the great seedling recruitment occurs during the f irst post-f ire years, although we observed sporadic emergence of seedlings until the f ifth year, similar to the results found by Herranz et al. (1997). These new seedlings may come mainly from adhering serotinous cones on the burned trees that were present in the study zone.
The results obtained showed great heterogeneity in the regeneration process among study sites (S1 and S3 greater than S2) regardless of being located at the same elevation and slope. These differences could be explained by the different exposure of sites S1 (North), S3 (North-West) and S2 (South-West). Some authors (Gracia et al., 2002) explain that the number of Pinus in north-or northwest-facing areas was higher than in west-facing areas.
These differences among sites were also found in the analysis of the seedlings density dynamics after fire. A significant decrease was observed in zones S1 and S3 (sites with a higher seedling density) from the 7 th to the 10 th regeneration month. The main cause of pine seedling mortality was the harsh meteorological conditions, particularly summer drought as has been found by other authors (Madrigal et al., 2005;Thanos et al., 1996;Trabaud et al., 1997). In this sense, in our study sites the precipitation in the summer (64.7 mm) and also the average annual precipitation in the first post-fire year (354.7 mm) were extremely low ( an approximate reduction of 75%) in comparison with the mean annual precipitation (718.48 mm) for the area in the following years.
Seedling height increments were more pronounced from the 2nd year after the fire and showed inter-site variability. So five years after fire the S3 site showed the tallest saplings with significant negative correlation between density and height. Some authors have related the enhanced growth of pine seedlings to soil characteristics (Ne'eman, 1997;Pausas et al., 2003). In our study the soils at site S3 showed the highest quantity of nitrogen and organic matter (Calvo et al., 2008).
In conclusion, in the study area seedling density was high enough to ensure natural regeneration of P. pinaster forest. The sites with higher densities (S1 and S3) showed greater seedling mortality during the first post-f ire summer. However, site S2, with lower densities, showed less pronounced changes. During the first two years great recruitment of new seedlings was observed from cones which were opened but not killed by heat, and which remained on trees and were able to shed seeds during this time. No significant correlations were found between Pinus seedling densities and environmental factors (precipitation, and aridity index). The site with low Pinus density (S3) during the fifth year after fire showed the tallest saplings.
In the short term post-fire management strategy in this type of Pinus pinaster forest could be the remaining branches with cones of burned trees that allow the dissemination of the seeds during the first few years after fire, and ensure natural regeneration. In medium term, probably thinning of this type of forest would be a good management treatment to improve seedlings growth and to reduce fuel accumulation.