Recurrent wildfires constrain long-term reproduction ability in Pinus halepensis Mill.

Increasing fire recurrence is a major problem threatening Mediterranean-type ecosystems. Moreover, this pattern is predicted to increase owing to global change. Although a reduction in the density and growth of post-fire regeneration is usually observed in recurrently burnt areas, the potential effects on reproductive ability have seldom been explored. The aim of the present study is to investigate whether structural changes induced by fire recurrence may constrain reproduction ability of Pinus halepensis forests. We conducted the current study in Catalonia (NE Spain) in 12 study sites, consisting of two adjacent areas differing in the number of fire events suffered throughout the last 16 years (one v. two fires). Twice-burnt areas showed a lower density of pines, lower pine height and a lower reproductive ability, namely (i) a 3-year delay in the onset of pine reproduction; (ii) a reduction of 52% in the number of reproductive pines; and (iii) a 36% lower mean cone crop per tree. The lower mean cone production per tree coupled with a lower density resulted in an ~80% lower canopy seed bank in twice-burnt areas. These results suggest that the occurrence of a third fire event in twice-burnt areas would severely constrain natural regeneration.

Regeneration by three species of Banksia on the south coast of Western Australia in relation to fire interval

The regeneration strategies of three Banksia species in relation to fire were studied over 20 years in a mediterranean heathland-shrubland on the south coast of Western Australia. Banksia baueri and B. nutans are both bushes 1–2 m high, while B. baxteri is a shrub 4 m high. All three species regenerated only from seed released from the canopy seed bank after fire. They did not start to flower until 6 years after fire and seed set took even longer. Differences between the species in age-related intensity of flowering were related to the rate at which each species accumulated seed in the canopy. Even plants over 40 years old were still increasing their overall canopy seed bank or replacing seeds that had been released or were no longer viable. The vegetation studied appeared to be little affected by humans historically and to have burnt only at intervals of 30–60 years or more. Consequently, although all three species needed fire to regenerate, management of fire regimes needs to allow adequate intervals between fires for the replacement of their canopy seed banks. Indeed, all three Banksia species studied were extinguished from one area burnt twice at an interval of 9 years. Models developed with Banksia species from the northern sand plains of Western Australia, where fires appear more frequent, may need modification to be applicable to all south-coastal species.

Aleppo Pine (Pinus Halepensis) Postfire Regeneration: the Role of Canopy and Soil Seed Banks

Pinus halepensis (Aleppo pine), is the dominant tree of a large fraction (26%) of the Greek coniferous forests; this species is an endemic pine of the Mediterranean Rim and well adapted to fire. Its regeneration is accomplished exclusively through seeds, thus its soil and canopy seed banks are of paramount importance for postfire resilience. Cone opening and seed dispersal were investigated in unburned forests of Attica (Greece) and it was found that Pinus halepensis trees maintain a significant percentage of the yearly cone crop (40-80%) closed, thus creating a persistent, canopy seed bank. Full viability of enclosed seeds was maintained for at least three years in canopy storage; moreover, preliminary results concerning the viability of seeds enclosed within the cones for four to more than 50 years showed a gradual reduction of both final percentage and rate of germination. Nevertheless, cones of up to 20 years of age contained a considerable fraction of germinable seeds. On the other hand, Aleppo pine forms only a short-lived (transient) soil seed bank; this bank was particularly abundant after a fire, as a result of the fire-induced cone opening. The germinable seed portion, although quite important prior to the start of the rainy season, was rapidly depleted, and at the end of the rainy season it was virtually absent in both burned and unburned forests. It is therefore concluded that postfire Aleppo pine seedling recruitment takes place almost exclusively during the first year after the fire and depends upon the germination of seeds in a transient soil bank which is produced by the postfire dispersal of pine seeds stored in the canopy seed bank.

Simulation of Crown Fire Effects on Canopy Seed Bank in Lodgepole Pine

Analysis of video footage taken of crown fires during the 1988 fire season in Yellowstone National Park indicated that the most frequent length of time required to completely burn tree crowns was 15-20 seconds. Lodge-pole pine (Pinus contorta Laws.) seeds were tested for ability to germinate after exposing both serotinous and nonserotinous cones for 10 to 60 seconds in a flame front designed to simulate a crown fire. Heating enhanced germination of seeds from serotinous cones but not those from nonserotinous cones. Maximum germination rates for serotinous cone seeds ranged from 37 to 64 percent and occurred after 10-20 seconds exposure of cones to flames. Germination after 60 seconds exposure averaged 0.3 to 14 percent. Maximum germination of seeds from nonserotinous cones (about 80%) occurred with no exposure and after 10 seconds in the flames.