Interannual variability of growth and reproduction in Bursera simaruba: The role of allometry and resource variability

Background/Question/Methods

Plants are expected to differentially allocate resources to reproduction, growth and survival in order to maximize overall fitness. Life history theory predicts that the allocation of resources to reproduction should occur at the expense of vegetative growth. Although it is known that both organism size and resource availability can influence life history traits, few studies have addressed how size-dependencies of growth and reproduction and variation in resource supply jointly affect the coupling between growth and reproduction. In order to understand the relationship between growth and reproduction in the context of resource variability, we utilize a long-term observational dataset consisting of 670 individual trees over a 10-year period within a local population of the tropical tree Bursera simaruba (L.) Sarg in the 15-hectare San Emilio Long Term Forest Dynamics Plot (SE-LFDP) located in Sector Santa Rosa of Área de Conservación Guanacaste in northwestern Costa Rica. We: (i) quantify the functional form and variability in the growth reproduction relationship at the population and individual-tree level and (ii) develop a theoretical framework to understand the allometric dependence of growth and reproduction.

Results/Conclusions

First, we show that growth and reproduction are not isometric and thus do not scale similarly with size. Second, the relationship between RA and RGR is ultimately governed by the underlying allometries of growth and reproduction. Third, observed variation in growth and reproduction is consistent with resource variation and tradeoff mechanisms each playing important roles in the observed patterns. Our findings suggest that the differential responses of allometric growth and reproduction to resource availability both between years and between microsites underlie the apparent relationship between growth and reproduction. Thus, a plot of relative growth rate against reproductive allocation cannot evaluate the nature of the tradeoff since growth and reproduction are not isometric with tree size. Finally, we offer an alternative approach for quantifying the relationship between growth and reproduction that accounts for variation in allometries.