Sex ratios and clonal growth in dioecious Populus euphratica Oliv., Xinjiang Prov., Western China

Using a microsatellite assay, we investigated sex ratios at three levels (apparent, intrinsic, genet) for Populus euphratica stands in Xinjiang, China and possible consequences of sex-specific costs of reproduction in terms of clonal growth and individual growth or mortality. Sex ratios at all levels tended to be male biased (60 % of 3,295 flowering trees were male), although male excess was least pronounced at the genet level (52 % of 850 genets were male). Male clones comprised significantly more (708 vs. 572) trees than female clones. Reproductive investment was measured in terms of carbon (C) and nitrogen (N) contents of male and female reproductive organs: single flowers or fruit capsules, whole inflorescences or infructescences, and whole branches of ca. 2 cm diameter. Male flowers and catkins require less N than female fruits and catkins, but on average only 16 % of female catkins develop into fruits. This changes the measured investment for reproduction at branch level: now male branches spent 3.3 times more N than their female counterparts. This coincides with the annual increment of branches, measured as a possible trade-off for costs of reproduction: female branches needed 2 years less to reach a diameter of 2 cm. We conclude that full fruit set of females would give males a heavy comparative advantage, but frequent abortion of whole infructescences by females seems to be a powerful mechanism to compensate a higher reproductive effort, thus avoiding a pronounced runaway effect by more vigorous clonal growth of male trees over a long time.