‘Regeneration’ refers to mahogany seedlings and saplings growing in the forest understory and in treefall gaps near adult trees. Felling large emergent and overstory trees opens growing space at ground level where seedlings live, growing space that seedlings can fill if they are there to fill it, and if they are capable of responding to suddenly available light in logging gaps. Some management systems rely on regeneration in place at the time of logging – the technical term is ‘advance regeneration’ – to replace logged trees during the intervals between harvests. So two key questions are: do mahogany seedlings occur at sufficient densities in natural forests to manage, and if so, are they capable of responding to growing space created by logging or other canopy interventions?
Unfortunately the answer to the first question is almost always no. As discussed elsewhere (see Ecology > Juveniles), mahogany is a light demanding species that fares poorly in understory shade. Paradoxically, its wind-dispersed seeds rarely fly further than 100 meters from parent trees, ensuring that most seeds land and germinate in the forest understory. These shady conditions are good for seed germination (moist, hidden) but bad for seedling growth (too dark); if some kind of disturbance opening overhead canopy space does not occur within a year or so after germination, these seedlings will likely die for lack of light. Under natural conditions, most seedlings that manage to establish in the forest understory die within the first two years.
For these reasons advance regeneration densities of mahogany are almost always low to non-existent in natural forests, except in certain transitional environments in Mexico and Central America as reported by Lamb (1966). At forest sites where mahogany occurs across the southern Amazon in Brazil and Bolivia, we have never seen abundant seedling regeneration except near the occasional parent tree during the wet season following a heavy fruit set. This finding extends to logged forests where we might expect to find mahogany seedlings in treefall gaps, but in fact rarely do. This is because felling operations often occur before seeds are dispersed by the middle of the dry season; because seasonal winds disperse most seeds from east to west across southeast Amazonia, meaning there will be few seedlings available for growth where trees are felled to the east side (roughly 50% of all cases); and because individual trees alternate years between heavy and light fruit production, meaning that logging events often do not correlate with abundant seed availability.
Where advance mahogany regeneration exists, can it be ‘released’ for rapid growth by opening overhead canopy space? The answer to this question is provisionally yes, depending on a number of factors. First, the sooner the better: first-year seedlings in the forest understory will respond better to canopy release than second-year seedlings and so forth, that is, post-release survival and growth rates will increase as the period of suppression decreases. Second, larger suppressed seedlings and saplings will respond more vigorously to release than smaller individuals of the same age. Put another way, relative seedling size can inform us about initial growing conditions, and the better those initial conditions were – that is, the larger the seedling or sapling – then the better the chances are that a given seedling will survive and thrive following canopy release. This means that most seedlings in the forest understory cannot be managed for future timber production because they had a poor start in life and have been suppressed for too long to be able to respond to canopy release.
If forest managers decide to include seedling regeneration as part of their management plan, then a couple of approaches are possible. The earliest attempts to manage mahogany regeneration in natural forests were made by British foresters in Belize (then British Honduras) back in the 1920s. They used shelterwood systems to encourage seedling establishment during 1 to 3 years before felling adult trees. This involved opening the forest canopy by selectively felling non-timber trees in the vicinity of mahogany ‘seed trees’, increasing light levels at ground level which in turn encouraged early seedling survival and growth. Adult mahogany trees would then be harvested after the next generation was established at ground level. These experiments offered promising preliminary results but were abandoned due to budgetary issues. Shelterwood systems for mahogany regeneration could be used in the Amazon; in fact a crude shelterwood system had been implemented at the Agua Azul field site in the year before we first visited in 1995. However, a shelterwood system requires capital investment and delaying harvests until seedlings are established. There is also the risk that regeneration might fail, either for lack of seed production or due to germination failure or seedling predation by specialized herbivores.
An alternative to shelterwood is to manage natural regeneration at the time of timber harvest. Harvests could be restricted to years of heavy mahogany seed production, which occur every 2nd or 3rd year in southeast Amazonia. The timing of tree felling could be delayed until after seed dispersal, ensuring the maximum number of seeds on the ground before removing adult trees. Site preparation could include clearing understory vegetation to some degree to increase understory light levels, which would encourage seedling establishment and early growth. Where possible, trees could be felled in the direction of prevailing winds – to the west in southeast Pará where dry season winds blow out of the east, for example – to ensure that logging disturbance is centered over the highest possible concentration of germinating seedlings. Long-term success will depend on maintaining open canopies above natural regeneration during the years following harvest and seedling establishment by periodically return to the site to remove competing secondary vegetation that might suppress mahogany seedlings by closing the overhead canopy. Again, any system targeting natural regeneration for future timber production will require capital investment.
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