After dispersing from tree crowns in the mid dry season, mahogany seeds must wait 1–3 months for the rainy season’s return before they can germinate – triggered by moisture imbibition – and establish as seedlings on the forest floor. A lot can happen to seeds on the ground during this time, most of it bad. Trees, branches and large leaves can fall on them; they can get stepped on by browsing animals; and predators, including fungal pathogens, insects, and forest rodents like pacas (Cuniculis paca) and spiny rats (Proechimys spp.), can eat them. [GRAPH]
Studies at Marajoara and elsewhere have demonstrated why first-year seedlings are so ephemeral on the landscape. Only 67–72% of apparently viable seeds actually germinated two months after dispersal at Marajoara. In surveys of dispersed seeds around fruiting trees we found that about 10% of seeds on the ground suffer animal attack at some level during the interval between dispersal and germination. This effect was proportionally higher near parent trees where seed densities are highest, a possible example of the Janzen-Connell density-dependent effect. Overall, seed predators eliminated ~40% of viable seeds, while 36% germinated and the rest did not for unknown reasons. After 9 months only 14% of outplanted seeds survived as seedlings, with this number falling to 8.5% after 1 year.
Once seeds have successfully germinated and the seedling is establishing on the forest floor, growing to 15–25 cm height with two pairs of simple leaves (see DESCRIPTION), mahogany faces a different kind of predator, a small nocturnal moth named Steniscadia poliophaea (Lepidoptera: Noctuidae) which lays its eggs on the underside of new expanding leaves. Tiny yellow larval caterpillars emerge from the eggs within a few days and begin to feed on new leaf tissue. As far as we know this seedling predator preys exclusively on mahogany seedlings; its only food source apparently is meristematic leaf tissue, that is, expanding or flushing leaves and stem tissues.
Female Steniscadia poliophaea moths somehow locate recently germinated mahogany seedlings in the forest understory, as well as older seedlings and saplings that are flushing new leaves, and lay their eggs with exquisite timing so that larval caterpillars emerge in time to defoliate whole crowns. We have seen infestation rates up to 90% on new seedling crops beneath parent trees at Marajoara. Studies by Julian Norghauer and colleagues have demonstrated that Steniscadia poliophaea exerts a strong Janzen-Connell density-dependent effect on mahogany seedlings and saplings. That is, as seeds disperse further from parent trees, the moth manages to locate and infest progressively lower percentages of establishing (and established) seedlings, allowing mahogany to ‘escape’ by long-distance dispersal. Interestingly, we have seen the moth and its negative effects on mahogany seedling populations at field sites across southern Brazil as far west as Acre and Peru, but it has not been reported in Bolivia or Central America.
Seedlings that escape Steniscadia poliophaea need 10–14 days to reach their initial full height and set two pairs of simple leaves; apical growth then pauses while new green tissues lignify (become woody). Underground, the seed radicle forms a taproot 10–15 cm deep, soil conditions permitting, with short lateral roots growing longest and densest near the soil surface. The vigor of the seedling’s first flush of new leaves, two to four weeks after lignification, depends on light conditions, rooting success, soil nutrient status, and attacks by herbivorous insects. When conditions are ideal – especially where plenty of light is available – the first flush may add up to eight simple leaves and 5–10 cm of stem growth, with new leaves much larger and more elongate than the first leaf pairs. Vigorous seedlings may form compound leaves during the second flush four to six weeks later, setting bi- or trifoliolate leaves and then, in successive flushes as height extension accelerates, four- to eight-foliolate leaves up to 18-foliolate as saplings grow taller than 1.5 m. New leaves are set in spiraling formation along the expanding apical leader, widely spaced at first and then bunching tightly as apical growth slows during each flush, the last (highest) leaves tending to be much smaller than the first. A robust seedling growing in high light and nutrient-rich soil can flush new leaves three to four times annually; bunched leaf scars on stems of saplings and poles indicate where successive apical flushes slowed and stopped. Vigorously growing saplings and poles form a distinct cambial ring with each flush, indicating that rings on adult stems correspond to annual dry season crown replacement.
Certain parent trees at Marajoara produce albino seedlings at low rates (< 5%) each fruiting year. These are normal-sized but pale pink, and cannot photosynthesize in full or partial sunlight, never surviving longer than two to three months.
Mahogany is strongly heliotropic, capable of growing at steep angles towards sunlight in open space as overhead canopies close tree- or branchfall gaps. Fast-growing saplings can add up to one meter’s height in a single flush, setting 20–30 large compound leaves whose combined weight may cause the new crown and even the entire stem to flop groundwards. Flopping crowns are most frequently seen at Marajoara in open conditions where soil nutrient status is especially high – for example, where seedlings establish beside decaying stumps in full sunlight, or where drainage conditions foster the formation of nutrient-rich translocated soil horizons. As saplings grow taller than 2 m height, apical dominance weakens and lateral sprouting may occur along the upper stem. This tendency is more pronounced on plants in open growing conditions compared to plants growing in dense secondary regrowth (for example, in treefall gaps).
Mahogany seedlings and saplings remain evergreen through the dry season, losing their crowns completely only where soil moisture availability is extremely limiting. Seedlings and saplings normally retain leaves from two to three flushes at a time; those retaining deep crowns through the dry season indicate growing sites with persistent soil moisture availability. Though not universal, evidence that nutrient translocation occurs from leaves to the seedling/sapling stem or roots is common, with leaves often turning crimson before falling. Saplings and poles typically shed many or all leaves during extended cloudy periods in the mid to late rainy season, flushing new crowns before the dry season begins. Most remain dormant through the dry season, flushing again in early September, often days before the first sporadic rains. Crowns flatten umbrella-like, with leaves tightly bunched and chlorotic, where overhead canopies close off incoming light, where soil nutrient status is limiting, or where soils waterlog during the rainy season.
The transition from juvenile to adult growth patterns – from an evergreen habit with multiple annual apical flushes to complete crown loss and reflush once annually during the dry season – occurs during the early pole phase when the stem is between 5–10 cm diameter. Leaf size and leaflet number also diminish once adult growth patterns are set. Whether this transition is prompted by environmental factors or by genetic ones is unknown. At Marajoara, no pole-sized trees > 10 cm diameter flush repeatedly through the year, barring rare small adults that suffered basal injury during logging.
At this point we must consider a second and highly problematic insect predator of mahogany, the shootboring moth Hypsipyla grandella (Lepidoptera: Pyralidae) which lays its eggs on the expanding apical meristem of flushing saplings. Larval caterpillars hatch and burrow into the growing apical tip, feeding on new plant tissues and eventually disabling the leader. This in turn causes sprouting, which can ruin tree form and contribute to hollow stems on larger trees. Because the shootboring moth (and its larval caterpillars) is much larger than Steniscadia poliophaea described above, it cannot attack seedlings until they reach ~1 m tall, with enough apical meristem during new flushes to support larval caterpillars through several instar phases. Thus Hypsipyla grandella is a stem predator of saplings and pole-sized mahogany, while Steniscadia poliophaea is a leaf predator on the crowns of seedlings and small saplings.
The shootborer is resident in many mahogany adult trees from year to year, though its impact on healthy individuals is negligible; during the first three years of growth censuses we saw evidence of the shootborer in 3–24% of tree crowns at Marajoara and in 1–21% of tree crowns at Agua Azul. Trees of all sizes were attacked, but heaviest infestation was in trees weakened by basal injuries incurred during logging, or in clusters of trees exposed to full sunlight after logging. These infestation rates probably underestimate actual rates because many trees were censused before or after the optimal period for noting shootborer presence (after crown flush but before leaf maturation). We have also noticed a tendency by late-flushing saplings, poles and trees to suffer higher rates of shootborer attack than early-flushing individuals, possibly a result of the shootborer’s cyclical population dynamics within dry seasons. During the wet season when few flushing apical meristems are available as a food source, the shootborer can infest maturing mahogany fruit on tree crowns.
Little is known about the shootborer's role in regulating mahogany population structures in natural forests. At our field sites, mahogany saplings and poles growing isolated in natural forest or logging gaps rarely escape attack – the moth is very good at locating mahogany plants as hosts for its eggs and larvae. Unlike Steniscadia poliophaea the shootborer also attacks other trees in the Meliaceae family such as Cedrela odorata and Guarea spp., providing alternate host species when mahogany densities are very low. Plantation managers have long been aware that planting mahogany seedlings at low densities in secondary vegetation reduces shootborer attack rates. Even so, Hypsipyla grandella remains the principal obstacle to successful plantation management across mahogany's natural range, and a major obstacle to sustainable management of mahogany in natural forests.
It definitely is not easy growing up to be a mahogany tree.
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