Post fire change in structure and composition of the tree stratum and fuel load in a Pinus douglasiana forest in Mexico

In pine forests with an historical fire regime of frequent low severity surface fires, under extreme weather conditions or a consequence of fire suppression, the severity of fires may increase. Response of vegetation to severe fire events was studied in a chronosequence of 8 years, 28 years and 60 years in a forest dominated by Pinus douglasiana in Mexico. Tree density (> 2.5 cm diameter) reached values of 1095 stems ha(-1) +/- 609 stems ha(-1) and a basal area of 10.0 m(2) ha(-1) +/- 2.2 m(2) ha(-1) in the 8 years stands; 28 years after stand-replacement fires, basal area (44.6 m(2) ha(-1) +/- 3.0 m(2) ha(-1)) was close to that of 60-year-old stands (49.0 m(2) ha(-1) +/- 6.4 m(2) ha(-1)). Pines showed the highest importance value across the chronosequence; with post-fire time, a formation of a sub-canopy of shade-tolerant broad-leaved species and an increase in species richness and diversity was observed. Long periods without fire increased surface fuel loading (66.0 Mg ha(-1) +/- 5.5 Mg ha(-1) for 28 years and 61.5 Mg ha(-1) +/- 4.6 Mg ha(-1) for 60 years-old stands) and changed fire behavior (spread velocity increased from 1.3 m s(-1) to 4.1 m s(- 1) and flame length from 0.7 m to 1.7 m between the 8 years stands and 28 years - 60 years stands combined). Results suggest that the fire suppression policy in pine forests should be evaluated considering the balance between biological conservation goals and mitigation of high severity fire danger.