Topic 10.4: Regeneration Requirements

A source of sexually or vegetatively produced propagules or perenniating structures is a pre-requisite for regeneration, but the environmental conditions also have to be appropriate. For example, retention of seed trees may result in a heavy seed rain, but most of the seeds will fail to establish seedlings if they fall on dense leaf litter in deep shade.  To take this example a bit further, regeneration will also fail if the pollinators or seed dispersal agents are absent, or seed predators are exceedingly abundant.

Regeneration from seed can fail at any phase of the process of flower production, pollination, seed set, seed dispersal, and seedling establishment.  For species that are not represented by abundant seedlings or dormant seeds prior to harvesting, timing of harvesting operations can be critical.  For example, felling of Swietenia macrophylla trees during the early dry season before they have produced mature fruits is unfortunate but understandable given the seasonal cycle of agricultural activities in the region.  Setting a minimum diameter limit for harvesting that is close to or less than the minimum size at which trees start to reproduce is another obvious way to assure regeneration failures.  Unfortunately, diameter limits are all-too-often set without regard to the biology of the species being harvested.

Seed set of most tropical plants requires pollination by animals.  Although many species of canopy trees have flowers that can be pollinated by a diversity of unspecialized insects, there are plant taxa that require the services of a narrow set of specialized pollinators.  If populations of these pollinators are reduced due to hunting, creation of conditions unsuitable for nesting, or other environmental changes, seed production will be jeopardized and regeneration rendered unlikely.

Seeds may be produced in abundance but regeneration nevertheless fail if seed dispersers are absent or limited in abundance due to over-hunting.  Although many of the best known timber trees in the tropics have wind-dispersed seeds, many other timber-producing species, as well as most understory trees and virtually all palms, shrubs, and herbs produce seeds that require the services of mammals, birds, reptiles, or even fish for their dispersal.  Seeds that are not dispersed mostly fall under the parent plant where they suffer great competition, seed and seedling predation, and the impacts of pathogens.

Pre- and post-dispersal seed predation can greatly reduce the numbers of seeds available for germination.  In some cases, mammals and birds (e.g., parrots and doves) eat large numbers of seeds before they are mature.  Similarly, many insects (e.g., some beetles and flies) lay their eggs on flowers or young fruits; the larvae hatch and bore inside where they are nourished at the expense of developing seeds.  Many  mature seeds are sacrificed to animals that serve as both dispersers and seed predators. Squirrels and other rodents that scatter-hoard seeds for future consumption are a familiar example of this dual function; the seeds they disperse but fail to recover are the most likely to survive and contribute to the next generation.

Dispersed seeds that escape predation may nevertheless fail to germinate or establish as seedlings if the environmental conditions of the place to which they were dispersed are not suitable.  For example, seeds that are stimulated to germinate by high red:far red ratios of light will fail to germinate if they land in the forest understory.  More commonly, seedling establishment fails because the seedling root fails to find a reliable source of water.  Seedlings from small seeds that germinate on top of leaf litter are particularly prone to desiccation.  When the reserves of carbohydrates stored in seeds are exhausted, seedlings will die if they are not able to photosynthesize enough to balance their respiratory carbon losses. Herbivory and damage from fallen branches and trampling also result in the death of many seedlings, as do nutrient deficiencies, but desiccation and carbon imbalances (often associated with fungal infection) apparently kill the majority of seedlings.

It perhaps goes without saying that most seeds and seedlings fail to survive to maturity, but detailed and long-term studies of population biology are often required to determine whether apparent “bottlenecks” at the seed or seedling phases actually threaten population maintenance.  Nevertheless, silviculturalists need to be careful to avoid inadvertently jeopardizing sustainability by creating conditions favorable to weeds, seed predators, herbivores, and pathogens, or that are unfavorable to pollinators or seed dispersal agents.  In some cases, seedling establishment enhanced by removing surface litter and near-ground competition with controlled burns, or exposing mineral soil by mechanical scarification with a tractor-drawn plow. Such intensive site preparation treatments are more commonly used in plantations than in managed natural forests, but they should not be disregarded as silvicultural options.