Cottonwood and the River of Time: On Trees, Evolution, and Society

17. The Farmer’s Trees

17 / the faRmeR’S tReeS i f annual crops allowed early farmers in the near east to take advantage of nutritious plants with short life cycles, reaping the benefits of culture within months and thereafter adding successive improvements year after year, this was not as easily accomplished with the trees growing in their surroundings. as a consequence the shift from collecting nuts and fruit from dry woodlands to actually planting and tending these trees near settlements happened much later, only around 6,000 years ago. Several factors contributed to this delay. first and foremost, trees take three to eight or more years before they bear any fruit. This means that a stable sedentary existence is a precondition for starting even the mere beginnings of an orchard. another problem has to do with the genetics of trees—by now no longer a secret to the reader. in contrast to the selfing habits of crop plants, trees are predominately 161 162 The farmer’s Trees outcrossers, requiring cross-pollination to set any fruit. and as a consequence they are highly heterozygous, giving rise to highly varied progenies , most of which would be unsuitable for cultivation. in turn, that means that it takes at least two trees with desirable fruit or seed characteristics to get fruit production going, that only a small fraction of their progeny will inherit those traits, and that these characteristics will only reveal themselves once the seedlings have reached maturity. Therefore, selection gains via sexual reproduction are slow and late in coming, requiring major up-front investments with few early returns. These factors explain why what occurred almost as a free by-product with cereals and other annuals, thanks to their genetic makeup, presented a major obstacle with trees. no wonder that the earliest evidence for tree cultivation in the near east is from trees such as olives, figs, and date palms—species in which vegetative propagation allowed the grower to bypass sexual reproduction . in fact, a recent discovery in the Jordan valley suggests that fig cultivation may have been initiated as early as 11,400 years ago—even before the domestication of cereals and legumes.1 vegetative propagation worked well by taking cuttings in figs, by the rooting of basal knobs in olive, and by transplanting offshoots in date palms.2 Grapevine cultivation via the use of cuttings began about the same time. vegetative propagules were passed on from one generation of farmers to the next and migrated with them. in fact, some of these clones may still be in cultivation today, as suggested by the comparable fingerprint of certain current date pits with well-preserved ancient specimens from the same region (daniel Zohary, personal communication). only much later, during the first millennium B.C., did apple, pear, plum, and other fruit trees appear in the ranks of domesticates, and this strictly by the use of grafting—a technique possibly introduced from China that provided another method of bypassing sexual reproduction. Thus, the capture of suitable cultivars was a question of identifying rare deviants among wild types that held promise as desirable individuals, and then making more copies of them through cloning. however, since among these rare deviants even fewer were governed by a dominant mutation, subsequent breeding brought little reward. one such exception was almond , in which a single, dominant gene changed the bitter, toxic seed into a sweet one.3 almond culture could therefore be pursued through The farmer’s Trees 163 collecting non-bitter seed, planting them again, and later culling the fraction of bitter trees among their progeny.4 apart from fruit trees, other trees were clearly of use to the early farmers, among them those that we have already encountered in the earliest remains of abu hureyra, 11,500 years ago—namely poplars and willows. Both grew close to fresh water, where humans liked to settle, and both had properties that made them useful in many ways. Their low-density wood dried relatively quickly for use as fuel, while their foliage was an excellent forage for goats. Goats, the notorious browsers around the perimeter of a settlement, probably revealed to their keepers a most essential trait about poplars and willows, which is their propensity to regrow by root suckers and sprout shoots. nothing would have been more welcome than a perpetual supplier of twigs and leaves near one’s abode, especially if it exhibited a growth rate far superior to that...