Secondary succession occurs when the severity of disturbance is insufficient to remove all the existing vegetation and soil from a site. Many different kinds of disturbances, such as fire, flooding, windstorms, and human activities (e.g., logging of forests) can initiate secondary succession. Pioneers of secondary successions face quite different conditions from those that accompany primary succession. Secondary successions often start with resource-rich conditions associated with high light availability and reduced competition for nutrients and moisture. Disturbances may also be short-lived; for example, gaps created in forest canopies close as the crowns of surrounding trees expand and as seedlings and saplings in the understory grow up in response to increased light. Pioneers rely on recruitment from propagules present in the soil, or that disperse into the site after disturbance occurs. Pioneers are able to outcompete established vegetation that survived the disturbance by maintaining high juvenile growth rates. Some of the fastest growing trees are pioneers in tropical rain forests. Individuals of the balsa tree Ochroma pyramidale, for example, can grow from seedlings to adults with >30 cm trunk diameter in <10 years.
The difference between pioneer and nonpioneer species is difficult to delineate (Table 1). Attempts to define distinct life-history strategies (implying coordinated evolution of life-history traits) are confounded because key traits such as propagule size and juvenile growth rate can vary over several orders of magnitude within a community and show broad overlap among species with contrasting habitat requirements. Nonetheless, interactions among traits can be used to describe some life-history tradeoffs that largely constrain the habitat requirements of pioneers. For vascular plants, paramount among these is a tradeoff between growth in the sun and survival in the shade (Figure 3). The high growth rates of pioneers are maintained by allocating a large fraction of the plant’s resources to new leaf area production, and by investing in nutrient-rich leaf tissue that can attain high-maximum photosynthetic rates. A consequence of preferential allocation to leaf production is that few resources remain that can be used to defend the plant against herbivores and pathogens, or to recover from physical damage. This results in high mortality, particularly in the shade, where resources needed for tissue replacement are most limiting.
Table 1. Characteristics of pioneer tree species in tropical forests that distinguish them from nonpioneer species
Pioneer speciesNonpioneer species1. Juveniles recruit from seed following disturbance; seedlings are unable to survive beneath a forest canopySeedlings and saplings persist in the shade of a forest canopy2. Seeds germinate in response to cues provided by changes in light, temperature, or soil nitrate concentrations indicating disturbance to canopy vegetationSeeds germinate immediately after dispersal or seasonally during periods favorable for establishment3. Seeds generally small; frequently dispersed by windSeeds may be large; frequently dispersed by vertebrates4. Seeds often persist in the soil (weeks to decades after dispersal)Seeds lack dormancy or remain in the soil for less than a year5. High height growth rate and juvenile mortality rateLower height growth, crowns often show lateral spread in the shade6. High maximal photosynthetic rate, light compensation point, and foliar nutrient concentrationsLow maximal photosynthetic rate, light compensation point, and foliar nutrient concentrations7. Short-lived leaves with high leaf area per unit leaf massLeaves of juvenile plants may persist for several years with low leaf area per unit leaf mass8. Open canopies with sparse branchingClosed canopies9. Low wood densityMedium–high wood density10. Low investment in chemical anti-herbivore defenseHigh investment in chemical and structural defenses11. Often form defensive mutualisms with antsDefensive mutualisms uncommon12. Adult lifespan typically <100 yearsAdult lifespan up to 500 years13. Wide geographic and ecological rangeOften restricted geographic range
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