First reproduction and tropical tree restoration

Background/Question/Methods

Sixty years ago LaMont Cole offered what seemed a preposterous insight. He argued that in expanding populations, "for an annual species, the absolute gain in intrinsic growth which could be achieved by changing to the perennial reproductive habit would be exactly equivalent to adding one individual to the average litter size." Charnov and Schaffer generalized the insight by showing that repeated reproduction had a clear advantage when adult-survival greatly exceeds offspring survival, the general case in tree populations.  Early reproduction at small stature in some rainforest trees shows that the logic of a powerful advantage to early reproduction is not limited to pioneers.  We use experimental restoration to offer insights into choice of species of rainforest trees for restoration of degraded land.  In 2006, 16 fenced 30 x 30 m plots were planted in eroded pasture (median soil depth 18.5 cm) at Los Tuxtlas, Mexico.  Eight include 12 individuals of 12 native tree species that are dispersed by animals, eight include 12 individuals of native tree species dispersed by wind.

Results/Conclusions

While the ultimate hypotheses address differential colonization of treatments by forest species, the leading edge of  reproduction in the first 8 planted species to flower and fruit is enlightening.   As expected, pioneer Cecropia obtusifolia (normal adult height 5-20 m) grew and matured rapidly; 20 individuals reproduced in 4 years at normal stature (6.7 +/- 1.6 sd), juvenile to adult “inflection” was reached in 5 years, and virtually all 75 survivors were reproductive at 6. Moreover, one mid-pioneer fig (Ficus yoponensis, 25-35 m) produced fruit in 4 years at 2.4 m, late pioneer Stemmadenia donnell-smithii (10-20 m) produced fruit in 3 years at 2.4 m, and late pioneer Ceiba pentandra (20-40 m) produced flowers in 5 years at 3.0 m.  Astonishingly, later successional forest endemic Amphitecna tuxtlensis (3-10 m) flowered in 4 years at 1.2 m, Vochysia guatemalensis (15-30 m) flowered in 4 years at 3.6 m, Platymiscium pinnatum (15-30 m) seeded at 4 years at < 1 m, and Tabebuia guayacan (15-30 m) first flowered in 5 years at 1.7 m.  While some of these species have light wood (0.26-0.5 g cm^-3), the latter two do not (0.71 and 0.84 g cm^-3).  For forest species able to survive in open pasture, early reproduction at very small stature is likely a fitness advantage to the trees, and a boon to restoration ecologists who wish to include structural complexity in planted forests.