Fruit production is essential for forest regeneration, and variation in fecundity among species may drive changes in community composition. In wet tropical rainforests, palm species account for approximately one quarter of individual trees and are among the only species to fruit multiple times in a year, making palms an important plant community component and food source for animals. The amount of fruit produced by an individual palm may be influenced by many factors, including tree size and soil nutrient availability. However, the influence of specific nutrients may vary between species, and is of special interest because of increasing atmospheric deposition of nitrogen in the tropics and associated declines in base cations. To investigate the importance of different nutrients across species, fruit production was estimated for approximately 700 trees of six subcanopy palm species at La Selva Biological Station, Costa Rica at three time points. Individuals were sampled from five locations which represent a range of soil types present at La Selva. Fine scale soil measurements were taken at each location, and soil nutrient availability was imputed to each tree. Influences of tree size and nutrient availability on fruit production were tested by fitting individual-based models for each species.
Results/Conclusions
In Euterpe precatoria, Iriartea deltoidea, Socratea exorrhiza, and Welfia regia, fruit production was positively correlated with diameter. Also, in W. regia, fruit production was negatively correlated with magnesium. When analyses were restricted to reproducing trees, fruit production was negatively correlated with magnesium in S. exorrhiza, calcium in E. precatoria, and potassium in Prestoea decurrens. Contrastingly, potassium was a positive predictor for Cryosophila warscewiczii. The positive effect of potassium in C. warscewiczii suggests that it is limiting to reproduction in this species, but base cation toxicity in other species seems unlikely. Because soil cation concentrations are often positively correlated with each other it is difficult to determine which, if any, are driving these relationships. However, a related study suggests that base cations limit growth of dicot, non-legume tree species, but not the growth of these subcanopy palms. Thus, under high base cation availability, the faster growth and higher resource demands of dicot competitors may indirectly constrain fruit production in palms. Although this possible competition effect requires further investigation, soil base cations were the strongest correlate of fruit production in tropical palms.