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The massive take over of flowers

A paper that absolutely deserves your time, even if you are not a specialist in the field (also because it is free for anyone online), is the one by Frank Berendse and Marten Scheffer (Ecology letters 12: 865-872).

I report here some of their observations and conclusions.

Charles darwin, the author of the On the origin of species, great contributor to the explanation of biological diversification on Earth, was puzzled by an abominable mistery: the angiosperm radiation in the Cretaceous period.

Angiosperms (or flowering plants) were 5-20% of the vascular plant species about 105 millions of years ago (Ma), when gimnosperms dominated the landscape and ferns where much more abundant. Nevertheless, they reached 80-100% about 65 Ma.

In this period flowering plants became more abundant and more diverse at a very impressive rate.

Before this radiation early angiosperms were limited to disturbed sites, while tall conifer forests with a dense fern understory were   definitely dominant.

At some time during the Cretaceous flowering plants entered the fern dominated understory and later angiosperms trees started to play an prominent role. This explains why the decline of ferns was more marked, while gimnosperms’ decline occured later and more slowly.

Several explanations have been proposed for this rapid and massive process, on the linked paper you can read a synthesis of several hypothesis and their limits. On the other hand, the authors relate the great take over of flowering plants to their differences in ecological requirements and litter quality. In fact the authors propose that the massive scale of shift in Earth vegetation composition may be explained as a result of a positive feedback between angiosperms and their environment.

Flowering plants have higher growth rates and need higher nutrient levels than the gimnosperms; at the same time they promote soil nutrient levels by producing litter that is more readily decomposed. In contrast gimnosperms do relatively well under low nutrient conditions and mantain low nutrient levels in the soil due to the nature of their litter.

In fact, gimnosperms generally have longer leaf life span , higher lignin concentration and low nutrients (N and P) concentrations. Their strategy aims at increasing the carbon assimilation per unit of adsorbed nutrient and reduce nutrient losses. Angiosperms require and create a higher soil nutrient levels.

Once flowering plants are present in sufficient densities they enhance soil fertility through litter implying a positive feedback that might produce a runaway process once angiosperms have reached a certain critical abundance.

Evidence for this hypothesis comes from recent vegetation shifts monitored by long-term field observations: the change from ericaceous dwarf shrubs (with leaf traits similar to gimnosperms) dominated communities (heathlands) to monospecific stands of perennial grasses (Molinia caerulea) in Western Europe; and the expansion of dwarf shrubs and graminoid in bog ecosystemsoriginally dominated by conifer trees.

Molinia caerulea dominated landscape in WalesConifer trees in a bog, Vermont - U.S.A.

In both cases an increase in nutrient supply (due to the abandonment by farmers in heathlands and by N-pollution in bogs) led to a rapid increase of flowering plants to the detriment of conifers and species with gimnosperms-like strategies.

I think that the integration of data coming from fossils and recent vegetation shifts and, above all, an ecological perspective make this hypothesis likely and definitely fascinating.

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