Gap-dependence in mangrove life-history strategies: a consideration of the entire life cycle and patch dynamics

Authors


Author to whom correspondence should be addressed: L. López-Hoffman. E-mail: lauralh@u.arizona.edu.

Summary

  • 1Tropical rain forest trees have been classified according to the importance of canopy disturbances for their life histories. To assess gap-dependence vs. shade tolerance, we propose using demographic analysis of the degree to which gaps are critical for population growth.
  • 2Population growth is the composite of the successes and failures of individuals experiencing different life-cycle pathways (i.e. gap, building, and mature forest patches). To assess the contribution of each pathway, it is necessary to study patch dynamics.
  • 3We use stage and patch structured matrix models and loop analysis to assess the life-history strategies of two Neotropical mangrove trees, Avicennia germinans and Rhizophora mangle. We use data from field studies in Lake Maracaibo, Venezuela, that were specifically designed to parameterize these models.
  • 4In R. mangle, seedling to juvenile progression was limited to high light patches (> 20% canopy openness), even though they comprised < 10% of the area, and population growth depended entirely on gaps. A. germinans seedlings progressed to juveniles from all light patches, and progression rates increased with light level. However, low and medium light patches (< 20% openness) were most important to population growth because they were most frequent. We suggest that R. mangle is gap-dependent while A. germinans is shade-promoted (i.e. shaded patches are demographically more important to A. germinans population growth even though seedling progression rates increase with canopy openness).
  • 5Previous studies considered R. mangle shade-tolerant because its propagules can establish and initially survive in the shade, but our classification was based on its entire life cycle. For A. germinans, the important measure was not the growth response to light, but the frequency and demographic importance of different light patches.
  • 6Synthesis: These examples show that observations of a plant's germination biology or photosynthetic/growth response to light do not necessarily translate into demographic behaviour because physiology can be decoupled from demography by patch dynamics. Thus, it is critical to consider the entire life cycle in the context of forest patch dynamics when assessing the life-history strategies of tropical trees.

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