Understanding Biology Of Guam's Native Fadang Tree

Green leaves must pay back their own construction costs, but each leaf is also required to supply the energy needed to maintain its live tissues throughout its lifespan. Additionally, large plants like Guam's Cycas have an abundance of non-green tissue that requires energy to maintain. Therefore, large plants with this leaf construction strategy must also possess a payback strategy that allows recovery of these costs coupled with an excess in food synthesis to support plant growth.

"These phenomena are understood using a financial analogy where borrowing a modest amount of money for one investment strategy is much easier to pay back and less risky than borrowing a substantial sum of money for a different investment strategy," said Marler. "The manufacture of food by the leaf is analogous to the income generated from the investments."

Results of the research indicate these plants approach the substantial leaf construction investments by delaying the decline in food synthesis capacity that accompanies the leaf aging process. Many fast-growing species, for example, constantly replace their leaves. For these species, food synthesis may be reduced to negligible levels in less than one month after leaf construction. In contrast, the decline in food synthesis for leaves of Guam's Cycas is minimal, with rates of two-year-old leaves sustained at 80% of the initial maximum. Marler indicates this sluggish pace of decline is 65-80 times slower than other species for which this trait has been determined.

The financial risk analogy illuminates a critical environmental issue because Guam's Cycas population is being threatened by alien species that feed on its leaves. Guam's alien deer and several alien insects halt a leaf's payback process such that construction costs are not recovered before the leaves are eaten. As the process is repeated the plant effectively starves to death.