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The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence

The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence Gymnosperms, and conifers in particular, are sometimes very productive trees yet angiosperms dominate most temperate and tropical vegetation. Current explanations for angiosperm success emphasize the advantages of insect pollination and seed dispersal by animals for the colonization of isolated habitats. Differences between gymnosperm and angiosperm reproductive and vegetative growth rates have been largely ignored. Gymnosperms are all woody, perennial and usually have long reproductive cycles. Their leaves are not as fully vascularized as those of angiosperms and are more stereotyped in shape and size. Gymnosperm tracheids are generally more resistant to solute flow than angiosperm vessels. A consequence of the less efficient transport system is that maximum growth rates of gymnosperms are lower than maximum growth rates of angiosperms in well lit, well watered habitats. Gymnosperm seedlings may be particularly uncompetitive since their growth depends on a single cohort of relatively inefficient leaves. Later, some gymnosperms attain a higher productivity than co‐occurring angiosperm trees by accumulating several cohorts of leaves with a higher total leaf area. These functional constraints on gymnosperm growth rates suggest that gymnosperms will be restricted to areas where growth of angiosperm competitors is reduced, for example, by cold or nutrient shortages. Biogeographic evidence supports this prediction since conifers are largely confined to high latitudes and elevations or nutrient‐poor soils. Experimental studies show that competition in the regeneration niche (between conifer seedlings and angiosperm herbs and shrubs) is common and significantly affects conifer growth and survival, Fast‐growing angiosperms, especially herbs and shrubs, may also change the frequency of disturbance regimes thereby excluding slower‐growing gymnosperms. Shade‐tolerant and early successional conifers share similar characteristics of slow initial growth and low plasticity to a change in resources. Shade‐tolerant gymnosperms would be expected to occur only where forest openings are small or otherwise unsuitable for rapid filling by fast‐growing angiosperm trees, lianas or shrubs. The limited evidence available suggests that shade‐tolerant conifers are confined to forests with small gap sizes where large disturbances are very rare. The regeneration hypothesis for gymnosperm exclusion by angiosperms is consistent with several aspects of the fossil record such as the early disappearance of gymnosperms from early successional environments where competition with angiosperms would have been most severe. However there are unresolved difficulties in interpreting process from paleoecological pattern which prevent the testing of alternative hypotheses. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Biological Journal of the Linnean Society Oxford University Press

The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence

Biological Journal of the Linnean Society , Volume 36 (3) – Mar 1, 1989

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References (122)

Publisher
Oxford University Press
Copyright
Copyright © 1989 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0024-4066
eISSN
1095-8312
DOI
10.1111/j.1095-8312.1989.tb00492.x
Publisher site
See Article on Publisher Site

Abstract

Gymnosperms, and conifers in particular, are sometimes very productive trees yet angiosperms dominate most temperate and tropical vegetation. Current explanations for angiosperm success emphasize the advantages of insect pollination and seed dispersal by animals for the colonization of isolated habitats. Differences between gymnosperm and angiosperm reproductive and vegetative growth rates have been largely ignored. Gymnosperms are all woody, perennial and usually have long reproductive cycles. Their leaves are not as fully vascularized as those of angiosperms and are more stereotyped in shape and size. Gymnosperm tracheids are generally more resistant to solute flow than angiosperm vessels. A consequence of the less efficient transport system is that maximum growth rates of gymnosperms are lower than maximum growth rates of angiosperms in well lit, well watered habitats. Gymnosperm seedlings may be particularly uncompetitive since their growth depends on a single cohort of relatively inefficient leaves. Later, some gymnosperms attain a higher productivity than co‐occurring angiosperm trees by accumulating several cohorts of leaves with a higher total leaf area. These functional constraints on gymnosperm growth rates suggest that gymnosperms will be restricted to areas where growth of angiosperm competitors is reduced, for example, by cold or nutrient shortages. Biogeographic evidence supports this prediction since conifers are largely confined to high latitudes and elevations or nutrient‐poor soils. Experimental studies show that competition in the regeneration niche (between conifer seedlings and angiosperm herbs and shrubs) is common and significantly affects conifer growth and survival, Fast‐growing angiosperms, especially herbs and shrubs, may also change the frequency of disturbance regimes thereby excluding slower‐growing gymnosperms. Shade‐tolerant and early successional conifers share similar characteristics of slow initial growth and low plasticity to a change in resources. Shade‐tolerant gymnosperms would be expected to occur only where forest openings are small or otherwise unsuitable for rapid filling by fast‐growing angiosperm trees, lianas or shrubs. The limited evidence available suggests that shade‐tolerant conifers are confined to forests with small gap sizes where large disturbances are very rare. The regeneration hypothesis for gymnosperm exclusion by angiosperms is consistent with several aspects of the fossil record such as the early disappearance of gymnosperms from early successional environments where competition with angiosperms would have been most severe. However there are unresolved difficulties in interpreting process from paleoecological pattern which prevent the testing of alternative hypotheses.

Journal

Biological Journal of the Linnean SocietyOxford University Press

Published: Mar 1, 1989

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