Testing the Janzen-Connell mechanism: pathogens cause overcompensating density dependence in a tropical tree

Area of Science:

• Ecology
• Botany
• Forest Science

Background:

• The Janzen-Connell hypothesis is a key theory for tropical plant diversity.
• It posits that natural enemies reduce seedling survival near parent trees, favoring rarer species.

Purpose of the Study:

• To test the assumption of overcompensating density dependence in the Janzen-Connell hypothesis.
• Investigate seedling mortality dynamics of the tropical tree Pleradenophora longicuspis.

Main Methods:

• Conducted parallel shadehouse and field density-series experiments.
• Monitored seedling survival and density over time.
• Utilized fungicide treatments to differentiate pathogen impacts.

Main Results:

• Observed strong, overcompensating mortality in P. longicuspis seedlings, reaching 90-100% within 4 weeks.
• Fungal pathogens were identified as the primary drivers of this rapid mortality.
• Fungicide application significantly reduced mortality, indicating density-independent effects without pathogens.

Conclusions:

• The study confirms overcompensating mortality driven by natural enemies, supporting the Janzen-Connell hypothesis.
• Rapid, density-dependent mortality can prevent conspecific replacement, promoting species coexistence in tropical forests.