Reproduction by big-leaf mahogany trees at Marajoara is a year-long process that begins when the rainy season draws to a close in May and early June. The forest begins to dry out as days and weeks pass with no rain falling, seasonal streams dry up, the water table begins its descent to several meters below the ground surface, and deciduous tree species begin to shed portions or all of their leafy crowns. By the middle of the dry season in July, most mahogany trees drop all of their leaves and, within a week of the last leaf falling, begin to flush new crowns. Tiny emerging leaves are bright pink before turning lime green, expanding within a month to full size. As new leaves grow, branching inflorescences emerge from leaf axils, with each flower stalk bearing up to 100 flower buds. The new crown requires just over one month to mature. Then flowers begin to open, initiating the next annual reproductive cycle.
Individual flowers are pale yellow, less than 1 cm in diameter, and mildly scented. A large mahogany tree in full flower can have several hundred thousand flowers open at one time on its crown. A diverse community of small diurnal butterflies and nocturnal moths visit the flowers, with most visitation occurring during the late afternoon and early evening. Many species of ants, thrips, small beetles, flies, bees, and wasps also visit flowers, many of these feeding on the basal nectary. Compared to many other tree species with large floral displays, insect activity within heavily flowering crowns was muted at Marajoara even when flowers were newly receptive – mahogany does not attract swarms of diurnal bees, nor large butterflies. Pollination occurs when insect pollinators move pollen from flowers on one tree to flowers on a neighboring tree; mahogany is largely self-incompatible, that is, it rarely self pollinates. Large emergent trees with spreading crowns present the largest floral displays and probably attract the largest pollinator crowds. Unless adult tree densities fall to extremely low levels (see Distribution Patterns), distances between flowering trees are probably not a problem for small flying insects to traverse; studies in Mexico have shown them capable of moving up to 4.5 kilometers between isolated Swietenia humilis trees. Whether population density influences fruit production rates on the pollinator end is an interesting question that only careful field research can answer.
Individual mahogany trees flower for up to a month at a time, while local populations take up to four months to flower, from July to October at Marajoara. This means that early-flowering trees are reproductively isolated from late-flowering trees in the sense that their flowering schedules do not allow for pollen exchange. By the time the rainy season sets in, small fruit have begun to develop on tree crowns after flower pollination. Woody fruit mature through the wet season, gradually becoming full-sized. We have occasionally seen scarlet macaws break into fruit on the crown and eat seeds. Seeds developing inside fruit capsules are also susceptible to attack by the mahogany shootborer (Hypsipyla grandella), a nocturnal moth whose larval caterpillars bore into the fruit and consume the seeds.
Mahogany trees 20–30 cm diameter fruited at all four field sites in southeast Pará, but this happened rarely; more commonly, trees began to fruit annually or supra-annually after they were larger than 30 cm diameter. At Marajoara, trees larger than 60 cm diameter produced significantly more fruit on an annual basis than trees 30–60 cm diameter (15 fruit yr-1 vs. 4 fruit yr-1, respectively), echoing results from field studies in Bolivia, Belize, and Fazenda Mogno II within the same region. After many years of observation we consider a ‘large’ fruit crop to be 25 or more capsules, with exceptional fruit crops on individual trees in excess of 300 occasionally observed. These fruit production rates are low compared to populations in Acre and Bolivia in southwest Amazonia, where large (130 cm diameter) trees have been observed to produce 900+ fruit in a single year. It could be that fruit production in southeast Pará is depressed by logging, which leaves few large individuals and increases distances that pollinators must travel between flowering crowns. It is also possible that nutrient-poor soils in this region limit the ability of trees to mature large numbers of woody capsules.
As the rainy season begins to wind down in late April and May, and tree crowns thin as leaves begin to fall, fruit held above the crown begin to dry out and split open. Five woody valves (pericarps) peel off of each fruit and drop to the forest floor, exposing approximately 60 winged seeds attached to the top of the fruit column by a threadlike funiculus, ready to disperse on the first wind capable of dislodging them from the parent tree. Seeds disperse through the dry season as tree crowns go completely bare and then reflush new leaves. By the time a tree has a new crown and is flowering heavily, few seeds will be left to disperse.
Thus completes the annual reproductive cycle. Most seeds land within 35 meters of parent trees, but occasional wind gusts can disperse seeds several hundred meters. There on the ground they will wait for the rainy season’s return, when repeated wetting triggers germination and, hopefully, seedling establishment. Until that happens, seeds must survive both animal and insect predators, fungal pathogens, and other random mortality agents like falling branches or smothering leaves.
Much of what we know about floral and pollinator behavior comes from many hours spent observing flowers and insect visitors at close range from tree towers built into mature mahogany crowns. We built three quite different structures for this work. The first was a wooden scaffolding enclosing a pindaiba tree (Xylopia aromatica, Annonaceae) growing beside a 90 cm diameter mahogany tree near camp where we lived. This tower, shown here, rose 25 meters into the interior of the tree’s crown. The second tower, also shown here, was built around the stem of a 40 cm diameter mahogany tree, rising 15 meters directly into the heart of its crown. A third ‘tower’ was a simple platform wedged into the crown of another 40 cm diameter tree, perched 13 meters above the ground. All three were designed by Jurandir Galvão and built by him and the field crew.
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