- Top of page
Harvey & Holzman, in this issue of the Journal of Vegetation Science, describe divergent successional pathways after fire in Pinus muricata forest. Fourteen years after fire, the shrub Ceanothus thyrsiflorus had occupied flat parts of the area. They predicted it would continue to prevent succession to P. muricata forest, as in the initial floristic composition concepts of Clements and Egler.
It seems that theories of succession are to some ecologists a past-era topic. Clements' climax theory has been ‘widely discredited’ (McCook 1994), ‘found wanting’ (Goldsmith 2002), and so has been ‘replaced’ (Barbour et al. 1999), although what it has been replaced with is not clear. The primary issue is whether there is convergence to one species composition, be it called ‘climax’ or not, and whether there are alternative successional pathways to get there.
Harvey & Holzman (2014) addressed these issues for secondary succession after a 1995 fire in a Pinus muricata (bishop pine) forest near San Francisco, USA. The area had previously been burned 68 yr before that fire, which is about the maximum interval between fires in such forests. The authors must be commended on their careful work. For example, species' cover was estimated using point quadrats (genuine point quadrats with a pin less than 1-mm diameter), rather than ‘estimated by eye’ (i.e. an informed guess), as in too much work.
Species richness and diversity (H′) decreased from a maximum 1–2 yr after the fire. Variation in tree density within and among plots increased over succession, in contrast to the finding of del Moral et al. (2012) of decreased variation in species composition through time on Mount St Helens. We should expect different trends from different criteria and at different spatial grain and extent. However, the trend in this P. muricata site is opposite to the trend of random variation in the first stages, followed by increased uniformity before final sorting into micro-habitats, as suggested by Greig-Smith (1952). Any resolution will require a more complex theory.
The major finding of Harvey & Holzman (2014) was of two alternative pathways. The year after the fire, P. muricata and the shrub Ceanothus thyrsiflorus (blueblossom) comprised close to equal cover. However, by 14 yr after the fire, there was divergence. On steeper sites, with thinner soils, P. muricata became dominant. Harvey & Holzman expected that dominance to remain because of the greater height of P. muricata. On flat sites with deeper soils, on the other hand, C. thyrsiflorus was able to grow faster than P. muricata and was apparently suppressing it.
The authors compared the situation they found with Egler's (1954) ‘Initial Floristic Composition’ concept. Confusingly, there are two interpretations of that concept, both in the literature and from Egler himself (Wilson et al. 1992). One is that species from all seral stages, mid-seral and climax included, will be found in the initial vegetation (Donato et al. 2012). This was, of course, no surprise to Clements, who travelled widely and observed carefully. He commented that in secondary areas such as burns, ‘in some cases it seems that the seeds and fruits for the dominant of all stages, including the climax, are present at the time of initiation’ Clements (1916, p. 61), which could result in ‘the appearance of a species before its usual place in the sequence’ (p. 103). Harvey & Holzman see this as an explanation for the high species richness and diversity 1 and 2 yr after the fire.
The other interpretation of Initial Floristic Composition is that ‘sometimes one dominant occupies an area so completely as to exclude the others simply because it invaded first’. That quote expresses Egler's (1954) concept perfectly, although it is actually from Weaver & Clements (1929, p. 48), for Clements well understood this phenomenon 25 yr before Egler published it as new. Egler called it ‘the case of missing end-stages’. Egler's prime example, perhaps the inspiration for his concept, was a stand of the shrub/small tree Viburnum lentago, stable for 25 yr and repelling invasion by tree species (Niering & Egler 1955). This seems to parallel the stands of C. thyrsiflorus shrubs described by Harvey & Holzman (2014). C. thyrsiflorus had grown vigorously on flat areas and had suppressed, perhaps by overtopping, saplings of P. muricata. Since the latter is shade intolerant, it is unable to re-invade. Egler suggested that in such a situation, succession from shrubland to forest could be up to 1000 times slower than it had been from bare soil to shrubland. Similarly, Harvey & Holman expected that the C. thyrsiflorus stands would remain with few P. muricata until the next stand-replacing fire. Actually, this is puzzling. Fires in these P. muricata forests occur at 40- to 70-yr intervals, and they reported that before the 1995 fire the area had borne relatively uniform P. muricata forest. If P. muricata forest had been able to re-establish from the previous fire by the time of the 1995 fire, why will it not be able to re-establish in the 40–70 yr before the next one? Or is the ability of C. thyrsiflorus to get ahead dependent on weather conditions at the time: i.e. environmentally-contingent vegetation dynamics? Volume 76 of the Journal of Vegetation Science will be published in AD 2065, 70 yr after the 1995 fire. It may tell us what happened, and give us the answer to whether the Initial Floristic Composition model really works.