Fertility and Effective Population Size in Seedling Seed Orchards of Casuarina equisetifolia and C. junghuhniana

Two seedling seed orchards each of C. equisetifolia and C. junghuhniana established by thinning provenance trials in coastal and inland locations in South India were evaluated for sex expression and fertility variation at four years. More than 80% of the trees in C. equisetifolia orchards were fertile in both sites with a similar pattern of more (almost 2 times) female trees and equal proportion of monoecious and non-flowering trees. In C. junghuhniana, the coastal orchard had twice the proportion of fertile trees as that of the inland. Orchards established in coastal environment had less fertility variation and hence maintained lower coancestry values in both species. Coastal site has more trees contributing effectively to seed production than inland locations and the orchards maintain higher (almost two times) effective population sizes. Genetic drift is also 3 times higher in inland locations in both species. Male and female trees in inland orchards of both species however had greater reproductive output than coastal trees. Monoecious Casuarina equisetifolia trees showed a different trend of greater male fertility in coastal site, but seed output was the same in both locations. Gene diversity values of all orchards are high though it is marginally higher in coastal sites. Measures like constrained seed collection from large number of trees and promoting representation of superior provenances with low fertility would be useful in checking diversity loss during domestication.