By ‘population’ we mean the sum of all mahogany individuals – adults, juveniles, seedlings, seeds – present on the landscape. In a spatial sense, population size depends on how much forest area we are considering. Boundaries defining a given population may be set by how much forest we can afford to inventory, or by land use change around the perimeter of a management or research site (for example, forest clearing for agriculture or ranching). In a temporal sense, seeds and seedlings are relatively ephemeral life stages, appearing in annual bursts from the late dry season to the early rainy season, their numbers diminishing rapidly as seeds germinate and new seedlings experience high mortality on the ground. And on a practical level, seed, seedling, sapling and even adult tree numbers are extremely difficult to quantify in dense forest where hundreds of other tree species and other plant forms are competing for growing space and resources (light, soil nutrients, etc.). So population ecologists often must settle for ‘snapshot’ views of tree populations, quantifying as best possible the ephemeral early life stages while describing adult and sub-adult life stages using size class frequency distributions to show the range and number of tree stem sizes growing within a given forest area.
This chart labeled (D) shows the size class frequency distribution of mahogany trees ≥ 20 cm diameter in an area of 204 hectares at Marajoara, in a rectangular plot 1200 meters wide by 1700 m long. (‘(D)’ indicates which field site this was in the published article.) The bottom (X) axis shows the range of size classes by 10-cm intervals up to 180 cm diameter; ‘30’ represents trees 20–30 cm diameter. On the left (Y) axis we can see that six trees in the first (20–30 cm) size class were found within this area. The size class with the most trees was 60–70 cm diameter, with 11 trees per 100 hectares. The largest tree was between 140–150 cm diameter. The gray fill indicates live trees that survived logging at this site in the early 1990s, while white fill indicates logged trees. That is, most of the trees in this population are logged stumps. In the accompanying map we can see these trees on the landscape, clustered along the banks of seasonal streams running south (down) and north (up) off the map. The contour lines show gradual topography following streams downhill.
We describe this ‘population’ in 204 hectares as having a roughly flat frequency distribution larger than 20 cm diameter, with a frequency peak between 60–70 cm diameter and relatively few trees larger than 100 cm diameter. From transect surveys we know that saplings and pole-sized trees from 2–20 cm diameter are uncommon within this area, while germinating seeds and seedlings appear in short-lived bursts each year. While we assume that large trees are likely to be older than small trees, we cannot describe this population in purely demographic terms, that is, as a distribution of tree ages rather than of tree size classes. Because tree diameter growth rates can vary dramatically among trees, it is possible, for example, that a tree 30 cm diameter is older than a tree 60 cm diameter. This population occurs at a density of 0.65 trees ≥ 20 cm diameter per hectare, or 65 trees per 100 hectares.
Tree population ecologists are interested in how populations change over time, both in terms of size and age distributions. Is the population structure we observe today stable, meaning that in one or more tree generations from today we would see size and age class distributions roughly equivalent to today’s? Or was the pre-logging population at Marajoara atypical compared to historical populations – more or less dense, perhaps, or with fewer or more large trees? For management purposes the main question is what will happen to the trees that survive logging: can the post-logging population recover through growth and recruitment of new individuals to pre-logging densities, and if so, how long will this take? To answer these questions we need detailed, long-term observational data that quantifies growth, mortality, and reproductive rates for all adult and juvenile size classes, as well as information about seed dispersal, germination, seedling establishment and growth across a variety of environmental conditions, and forest canopy disturbances necessary for seedling survival and growth to adult size (also known as ‘recruitment’). The spatially explicit Big-Leaf Mahogany Growth & Yield Model available on this website synthesizes these types of data into an interactive application that simulates mahogany population dynamics at Marajoara under natural conditions and after logging.
Contrast the Marajoara population frequency distribution with one from a site called Fazenda Mogno II located approximately 100 km north near the small community of Agua Azul. In chart (C) we see 100% of trees ≥ 20 cm diameter in an area of 859 hectares, over four times larger than the area inventoried at Marajoara. At this site the distribution is weighted more towards smaller trees, with a peak between 40–50 cm diameter and few trees larger than 60 cm diameter. Again the grey fill indicates live trees after logging. Note that densities per 100 hectares of forest (see the left (Y) axis) are much higher than at Marajoara; this population occurs at a density of 1.18 trees ≥ 20 cm diameter per hectare, or 118 trees per 100 hectares. A key basic question is, why does this population appear so different from the one at Marajoara? A key applied question is, which population will recover more quickly after logging? It turns out that, even though this population yields much lower merchantable timber during the first harvest, it is likely to recover faster than the Marajoara population during the years between harvests because small, sub-commercial trees represent a higher proportion of the population.
Mahogany population frequency distributions in southwestern Amazonia tend to look different from those southeast Pará. In chart (G) we see trees ≥ 20 cm diameter in 685 hectares at a forest management and research site called Fazenda São Jorge, located ~40 km south of Sena Madureira near the Rio Iaco. This population has many more quite large trees than the two previous examples, and more irregular frequencies across the range of size classes (see the ‘valleys’ at 50–60 cm and 110–120 cm diameter). Also note that density is much lower, which is characteristic of mahogany populations in southwest Amazonia, including Peru and Bolivia; this population occurs at a density of 0.12 trees ≥ 20 cm diameter per hectare, or 12 trees per 100 hectares.
Relatively few mahogany population structures have been described in the scientific literature. The main obstacle to doing so is the cost of intensive inventory efforts in dense tropical forests across large areas, considering mahogany’s typical low-density occurrence pattern. Population structures from Mexico and Bolivia (see Snook & Gullison et al. references below) have been described as unimodal (‘single-peaked’) and interpreted as representing single-aged cohorts of trees that established after large-scale disturbance events. After establishment differences in growth rates spread the trees through the observed range of size classes. We can see an example of this in the final chart showing population structure in the Chimanes region of the state of Beni, Bolivia. However, at field sites in southeast Pará and Acre several lines of evidence indicate that mahogany populations are multi-aged, with regeneration occurring at irregular intervals rather than all at once as would be the case in single-aged cohorts.
Grogan J, Jennings SB, Landis RM, Schulze M, Baima AMV, Lopes JCA, Norghauer JM, Oliveira LR, Pantoja F, Pinto D, Silva JNM, Vidal E & Zimmerman BL (2008) What loggers leave behind: impacts on big-leaf mahogany (Swietenia macrophylla) commercial populations and potential for post-logging recovery in the Brazilian Amazon. Forest Ecology and Management 255: 269-281.
Gullison RE, Panfil SN, Strouse JJ & Hubbell SP (1996) Ecology and management of mahogany (Swietenia macrophylla King) in the Chimanes Forest, Beni, Bolivia. Botanical Journal of the Linnean Society 122: 9-34.
Snook LK (1993) Stand dynamics of mahogany (Swietenia macrophylla King) and associated species after fire and hurricane in the tropical forests of the Yucatan Peninsula, Mexico. PhD dissertation, Yale University School of Forestry & Environmental Studies, New Haven, CT, USA.