Biomass allocation and phosphorus economics of rain-forest seedlings: effects of fertilization and radiation on soil specialists and soil generalists

Concurrent nutrient and radiation limitation in forests may engender trade-offs between P-use and radiation-use efficiency in tree species. To quantify these trade-offs, structural and physiological traits were examined among five rain-forest species subjected to four levels of fertilization and two levels of radiation in a glasshouse experiment. Schist specialists. Cryptocarya lividula and Ceratopetalum virchowii, occur only on P-poor schist soils, whereas soil generalists, Cryptocarya mackinnoniana, Franciscodendron laurifolium and Myristica insipida, occur on both P-poor schist and P-rich basalt soils. Wild seedlings less than 20 cm tall and 1 y old were collected from field sites, treated with fungicide. sorted into treatments (48 plants per species), and grown for 11 mo. We hypothesized that soil specialists would possess mainly non-plastic traits conferring high P-use efficiency, whereas soil generalists would possess markedly plastic traits conferring high radiation capture and use, enabling them to outcompete specialists on P-rich soils. Only generalist C. mackinnoniana and specialist C. virchowii supported these hypotheses. Cryptocarya mackinnoniana had more plastic root mass fraction, leaf area ratio, P uptake, and higher C assimilation than C. virchowii. which resulted in greater relative growth rates in high P treatments, but lower P-use efficiency in low P treatments. In contrast, specialist C. lividula demonstrated similar trait plasticity as C. mackinnoniana, suggesting that plasticity in these traits may be poor indicators of fitness on P-poor soils.