Beyond the Classical Janzen–Connell Hypothesis: The Role of the Area Under the Parent Tree Crown of Manilkara zapota

Abstract

The effect of the parent tree on seedling recruitment has been studied in various research studies. The Janzen–Connell (JC) hypothesis states that the closer the seedlings are to the source tree, the greater the risk of mortality and/or impact from pathogens and herbivores. Despite the extensive existing literature, there are not many studies that evaluate the influence of crown area, as well as the effects on leaf asymmetry, an important measure of biotic and abiotic stress. (1) This study evaluates the effect of distance from the parent tree and the crown’s area of influence on mortality, growth, and leaf asymmetry of Manilkara zapota seedlings, as well as insect herbivory and damage from leaf pathogens in a Mexican neotropical forest. (2) We selected 10 reproductive adult trees (Diameter at breast height, DBH ~ 10–25 cm) and established four 10 m × 1 m transects around each tree in four directions (north, south, east, and west). Each transect produced 10 quadrants of 1 m², and the quadrant where the shadow of the parent tree extended was marked as either under crown or crown-free. All M. zapota seedlings were counted in each quadrant. For one seedling in each quadrant, we recorded height, leaf asymmetry (LA), insect herbivory, and damage from leaf pathogens. Herbivory by insects, damage from leaf pathogens, and LA were only measured on the newest leaves. Mortality was determined after 9 months per quadrant, as well as light availability (photosynthetic photon flux density), temperature, and relative humidity. (3) We found that mortality and relative growth rate (RGRHeight) increased near and under the parent tree. Furthermore, LA decreased at greater distances from the parent tree and only outside the crown’s influence. Additionally, LA had a strong positive influence on damage caused by insect herbivory and leaf pathogens, impacting both more strongly under the crown. A high dependency of leaf pathogens on damage from insect herbivory was also recorded. Finally, the most frequent type of herbivory was that caused by chewing insects. (4) To our knowledge, we present one of the few studies that has addressed the JC hypothesis, considering not only the distance from the parent tree and seedling density but also the influence of the crown on the performance of M. zapota seedlings. Studies that consider the influence of the microenvironment are of fundamental importance for a comprehensive understanding of the JC hypothesis.