When does a mahogany tree become an ‘adult’? Is it a question of age, of stem (bole) size or canopy height, of sexual activity or sexual maturity? Pole-sized mahogany trees smaller than 20 cm diameter flower and fruit regularly in plantations and occasionally in natural forests, depending on how tall they are and on the amount of competition for sunlight they face from adjacent trees. These trees we consider precocious teenagers. In our view, mahogany adulthood in southeast Pará sets in when trees in natural forests begin to flower and fruit on an annual basis. This generally begins to happen when trees reach 30 cm diameter and larger, with crowns attaining co-dominant or dominant canopy position. Of course, exceptions are common: trees both smaller and larger than this size may or may not flower and fruit annually depending on their specific growing conditions. But in the absence of detailed long-term reproductive data for every mahogany tree in the forest, we find it useful to define trees 30 cm diameter and larger as adults, based on observations of hundreds of trees in this region since 1995.
In western Amazonia, where forest canopy heights tend to be taller and more closed than in southeast Pará, reproductive onset may occur at larger stem sizes because 30 cm diameter mahogany trees may still be competing on the vertical axis for canopy position. There’s no time (or rather, no resources) available for flowering and fruiting until a tree’s crown is well established in the forest canopy. See Reproduction for more information on flowering and fruiting.
While images we see most frequently of mahogany in natural forests are of huge, emergent trees (see Description), in fact few individuals grow fast enough and survive long enough to attain these large sizes. As demonstrated in Populations, in southeast Pará most adult trees are smaller than 100 cm diameter.
Relatively little is known about environmental and physiological processes governing mahogany's growth once it achieves adult size. As a deciduous species exchanging its crown each dry season, mahogany both avoids and tolerates seasonal water stress. Since flowering and leaf flushing occur before the end of the dry season, carbohydrates and water either must be accumulated and stored within the tree, most likely in the roots, during the rainy season, or water uptake must continue through the dry season through roots tapping deep-soil water. Wood color (orange-red to red) and the astringent properties of mahogany sap and seeds indicate a high rate of investment in secondary compounds for anti-fungal and anti-herbivory purposes, a trait common in the Meliaceae.
Diameter growth rates of pole-sized and adult trees in natural forests have not been widely documented because mahogany populations are difficult to locate and protect long enough to study – scientists usually arrive after loggers, not vice versa. What published data we can find in the literature is largely consistent across mahogany’s natural range. An early study (1930s) in British Honduras (Belize) reported mean diameter increment (growth) rates between 0.36–0.91 cm yr-1 over eight years for all size classes of mahogany trees. In the early 1990s in Mexico, juvenile and young adult trees grew 0.38–1.09 cm yr-1, with the fastest-growing trees between 15–30 cm diameter. At these rates mahogany trees would need 122 years, on average, to achieve the minimum commercial diameter of 55 cm in Mexico. A two-year study in Bolivia reported annual diameter growth rates ranging from 0.26–0.90 cm yr-1 across size classes, with maximum growth rates for individual trees approaching 2 cm yr-1. The fastest growing trees were 20–80 cm diameter, with rates declining as trees grew larger than 80 cm diameter. At these rates mahogany would need 52–148 years to grow to merchantable diameter, or 80 cm in Bolivia, averaging 105 years. More recently a four-year study in Belize found much higher growth rates than previously reported, with averages approaching or exceeding 1 cm yr-1 for all size classes larger than 20 cm diameter (and increasing as tree stem size increased). As well, faster growth rates were observed during years with higher total rainfall. Differences in soil type within regions have been shown to influence growth rates, with faster growth on more nutrient-rich soil types.
At Marajoara in southeast Pará we have documented annual diameter growth rates for large numbers of trees since 1997. Median growth rates hover at or slightly higher than 0.5 cm yr-1 for trees 20–60 cm diameter, and begin to decline after this size (note that ‘sub-adult’ trees 20–30 cm diameter are included in this discussion). But 75th percentile growth rates, that is, the rate at which the 75th-ranked tree in each size class is growing (on a scale with 100 being the fastest), approach or exceed 1 cm yr-1 for trees up to 70 cm diameter, and 90th percentile trees approach 2 cm yr-1 (see ‘whiskers’ in the accompanying chart, part (a) at the top). This means that the healthiest, most robust individuals are capable of quite rapid diameter growth. Similar growth rates were observed in a two-year study at Fazenda Mogno II near Agua Azul (see part (b) in the chart and Populations).
What factors influence growth rates? At the level of all trees in a population we have found at Marajoara that the amount of annual rainfall, especially rainfall totals during the first half of the rainy season, is positively correlated with mean annual growth rates. The accompanying chart (see ‘A’) shows this: population-level growth rates at Marajoara generally rise and fall as annual rainfall totals are high or low relative to the long-term average. This corroborates findings from Belize described above.
If we measure changes in stem diameter at four-week intervals for several years we find that mahogany trees expand during rainy periods and do not during the annual three- to four-month dry season. This is illustrated in the accompanying chart (‘Figure 3’).
At the level of individual trees, we have found that the single most important factor predicting how fast or slow a given tree will grow in a given year is how fast it grew last year, and, with gradually diminishing predictive ability, during up to 10 years prior to the year of measurement (see accompanying chart (a)). While this phenomenon, called growth autocorrelation, may seem self-evident – why wouldn’t we expect a healthy 75th percentile tree to continue rapid growth next year and the next? – the implications for understanding both individual tree history and population dynamics are quite profound. Growth autocorrelation means that trees within a population sub-divide into different growth trajectories that may persist for years, decades, or even lifetimes. Of course, today’s fast-growing trees do not always maintain this position within a given population – they may experience injury, or be overtopped by competing trees, slowing growth rates relative to other individuals. But generally speaking, growth autocorrelation means that a population’s largest individuals have, on average, grown faster over longer periods of time than smaller trees. Further, this means that the largest trees in a population aren’t necessarily the oldest; the stronger the growth autocorrelation ‘signal’ is, the more this is likely to be true for a given large tree. This helps explain why size class frequency distributions (see Populations) may represent poor approximations of age distributions within a population.
The second most important tree-level factor influencing diameter growth rates at Marajoara is the degree of crown vine coverage, that is, what proportion of the tree crown is covered in vines? Vines compete with trees for sunlight without paying the cost of building woody support for leaves held tens of meters above the forest floor. Some forests in southeast Pará, including Marajoara, have extremely high density and diversity of woody vines, often to such an extent that moving through these forests on the ground can be quite difficult (at least for a human being). Comparing growth rates by mahogany trees with progressively higher vine ‘loads’, we see a steady decline in growth rate and vigor as the proportion of vine coverage increases (see chart (e)). Other factors influencing diameter growth rates include stem diameter – as trees get larger, growth rates gradually decline; crown illumination – as tree crowns become more exposed to overhead sunlight, growth rates increase; and the number of fruit produced in a given year – more fruit mean slower growth.
Trees grow, but they also die. In fact, it turns out that mahogany trees of all sizes consistently die at a rate slightly higher than 1% per year. At Marajoara, two main mortality agents account for most of deaths we have observed since 1995: vines and wind. About 50% of trees die standing, and most of these deaths occur because vine coverage is so high that trees literally starve to death, unable to photosynthesize the carbohydrates necessary for respiration and growth because their own crowns – their leaves – have no access to sunlight beneath dense vines. Crown vine loads can be so heavy that trees gradually lean and tip over, their roots wrenching slowly out of the ground on one side. The remaining 50% of trees die ‘on the ground’, either uprooted or snapped somewhere along the stem by violent wet season windstorms. Or they may be killed beneath larger neighboring windthrown trees. Again, vines contribute to many of these deaths, pulling trees over during the wet season when rain exaggerates their combined weight.
When we look at mortality from a statistical point of view similar to growth, we find weaker relationships at the level of individual trees. More robust, faster-growing trees are at lower risk of mortality than slow-growing trees. Mortality risk rises also as the degree of crown vine coverage increases. Other factors, like tree (stem) size and the degree of crown illumination, have a perceptible but non-significant influence on mortality rates.
For forest managers, some mortality risk can be identified and avoided by cutting vines from trees with heavy vine loads. But mortality risk associated with storms is largely random and cannot be actively ‘managed’. This means that when forest managers attempt to predict how populations will recover after planned harvests, they must accept that approximately 1% of surviving trees will die, on average, every year during cutting cycles between harvests. This has important implications for modeling efforts examining diameter growth and timber yields from management sites for mahogany. More on this can be found under Dynamics and The Model.
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