In the plant kingdom, parthenogenesis and sex are far from mutually exclusive. Clones may be the way to go when nutrients and water are plentiful, but when the going gets tough, plants often switch to sex, which lets them try out different, possibly advantageous arrangements of genes.
Aspens are a good example of how sex can help an organism that usually clones itself to survive. Going by the name Pando (Latin for "I spread"), one particular colony of male aspen trees in Utah is not only the largest organism on earth (weighing in at over 6,000 tons), it is also the oldest, estimated to be 80,000 years old. The organism is actually an entire forest of an estimated 47,000 trees that are all genetically identical, and that are all feeding off a shared root system that covers over 10 acres. The whole thing grew from clones arising from a single tree.
Aspens do still have sexes, though. And recently researchers discovered that aspens can't actually clonally reproduce forever--at some point, to avoid petering out, they do need to have sex. It seems as they age, they become infertile, and stop being able to reproduce sexually, because genetic mutations build up over time with each new clone produced.
Researchers were able to date the age of each new clone back to the parent tree by comparing the everyday mutations that happen with known regularity during the process of cell division. By counting the number of these mutations they could tell which of the clones were newer and which were older, building a family tree of cloned trees. They found that the trees produced less pollen as they aged--and could be completely infertile by the tender young age of 20,000 years.