Issue:
118
Page: 44-49
Sustainable Harvest of Wild Plant Populations
by Charles M. Peters, PhD
HerbalGram.
2018; American Botanical Council
Harvesting
plant resources from wild populations affords a number of benefits. For many
medicinal and aromatic species, wild material is thought to be qualitatively
superior to cultivated stock because plants that grow in natural, more adverse
environments typically produce increased amounts of secondary metabolites.1
Wild-harvested plants frequently provide an important source of income for
local communities and can also offer a powerful incentive for conserving
natural habitats.2,3 Finally, harvesting plants from the wild, as
opposed to cultivating them, allows their populations to keep growing,
regenerating, and evolving in response to an ever-changing array of selective
pressures. Wild plant populations typically exhibit a high degree of genetic
diversity; cultivated plants, as a rule, do not.
The risk of harvesting wild populations
is that they can be overexploited and degraded easily. Unfortunately, this
appears to be happening with increasing frequency all over the world. It is
estimated that about 20% of the wild-harvested sources of medicinal plants
worldwide are currently exhausted or threatened by overexploitation.4
For example, several of the most valuable wild rattan species (from various
genera in the Arecaceae family) in the Greater Mekong region of Southeast Asia
are seriously depleted,5 numerous native fruit trees in Amazonia are
disappearing,6 and the gaharu (Aquilaria malaccensis,
Thymelaeaceae) trees of Indonesia and Malaysia — whose fungal-infected
heartwood is a valuable incense, perfume, and medicine — have been in Appendix
II of the Convention on International Trade in Endangered Species of Wild Fauna
and Flora (CITES) since 1995.7 The interaction between wild plant
populations, uncontrolled exploitation, and demanding market economies has
been, to say the least, rather dismal.
There is a finite quantity of leaves,
stems, roots, fruits, seeds, or exudates that can be harvested each year from a
wild population of plants. Once this limit is passed, the regeneration dynamics
of the population are affected and the number of individuals that compose it
will decrease. If exploitation continues at the same intensity, the population
will eventually disappear. To avoid the dangers of overexploitation, several
assessment tools have been developed with criteria for ranking a species’
resilience to harvesting based on life history characteristics, habitat
specificity, and demand.8,9 For species that receive a low score
(i.e., “high-risk” species), cultivation rather than wild harvesting is
recommended. While these efforts are undeniably useful, it is important to note
that even the most “low-risk” species can be overexploited and depleted, and
wild populations of even the most vulnerable species can be harvested
sustainably. What is needed to avoid the former and achieve the latter is a
clearer understanding of the productive capacity of the resource, a
conscientious management effort, and a modicum of control over both the people
and the plants.
Over the past 35 years, I have been
involved with numerous projects in collaboration with local communities focused
on the management and sustainable harvest of wild populations of valuable plant
resources.10,11 This work was conducted in tropical regions around
the world with different ethnic groups and different types of botanical
resources. Most of the resources I studied were trees (harvested for timber,
carving wood, fruit, oleoresin, or useful bark) and vines (such as rattan), but
several species of woody perennials or herbaceous annual plants were included
in the management initiatives of some communities. Similar management protocols
were used in every case. Building on this research, the purpose of this article
is to review the conceptual foundation that underlies the sustainable harvest
of wild plant populations and outline the basic data requirements for
developing a management plan. The benefits of harvesting botanicals from the
wild are numerous, and doing so in a sustainable manner is not as complicated
or difficult as it might seem.
Basic Population Management Concepts
From a management perspective, a wild
resource is most usefully described in terms of two parameters. The stock
of a resource is the number of stems or individual plants of the
resource-producing species (whether tree, shrub, or herb) that is found in the
forest or field at one point in time. The yield is the rate at which a
particular resource grows, multiplies, or increases in quantity each year. The
amount of new timber (cubic meters/hectare), rattan cane (m/ha), bark (kg/ha),
or latex (liter/ha), or the number of fruits, leaves, or stump sprouts that a
species produces each year is the yield of that resource.
There is a close relationship between
the current stock and the yield of a wild resource. Abundant species with dense
populations have a large stock and produce a large amount of harvestable
resource each year, while sparse, low-density populations exhibit a much lower
yield. As the stock of the resource increases within a given area, so does the
annual yield. The converse is also true.
The relationship between stock and yield
can have profound consequences for the sustainable exploitation of a wild plant
resource. In order to exploit the same species year after year in the same
place, it is important to harvest no more than its annual growth each year.
Harvesting more than the annual growth in a single year will diminish the
current stock of the resource, and the resource will be depleted over time. This
functions in much the same way as the relationship between the principal and
the interest of an endowment or a savings account. As long as annual
withdrawals are less than or equal to the interest generated by the principal
each year — and the principal is left intact — such withdrawals can continue
(theoretically) for perpetuity. However, if withdrawals are larger than this
each year, the principal gets smaller and the account is eventually overdrawn.
Resource
Depletion Scenario
A graphic example of this process is
shown in Figure 1. The initial stock of rattan in the forest is assumed to be
1,000 commercial canes (at least four meters long), with each of these canes
exhibiting an annual growth rate of two meters per year. Based on the stock and
the growth rate, the annual yield from this rattan population is estimated to
be 500 canes. By the end of Year 1, the initial stock of 1,000 rattan canes has
produced 500 new canes (i.e., the existing stock is now 1,500 canes). During
the first harvest, an order for 700 canes is fulfilled, reducing the stock to
800 canes. In Year 2, the reduced stock yields less new material (i.e., 400
canes), but harvest rates are held constant at 700 canes to satisfy the
demanding buyer. By Year 3, the population now exhibits a stock of 500 canes
and barely grows enough to produce 250 new canes. The final harvest of 700
canes reduces the stock to 50 canes, which will produce only 25 new canes and
certainly not support another commercial harvest. In this example, the rattan
population is severely overexploited in only a few years. To repeat, it is of
utmost importance that no more than the annual growth of a wild resource be
harvested each year. Defining this critical harvest limit will inevitably
require the collection of baseline data.
Data Requirements
The ability to exploit a wild plant
population with minimal ecological impact improves dramatically when more is
known about the species.12,13 Regardless of the species, habitat, or
plant part harvested, the most important ingredient required to achieve a truly
sustainable form of resource use is information,14 such as
quantitative data on the stock and annual yield of the plant.
The stock of a population is assessed
through a forest inventory. Foresters and ecologists have developed a variety
of plot sizes and shapes and methodologies to survey wild plants, and
trade-offs of time, cost, and statistical precision are inherent in each one.15,16
Inventories conducted at the village level are strongly recommended, but they
require a sampling methodology that is easy to understand and implement and
that does not involve the use of specialized or expensive field equipment.
Based on these considerations, and after many years of experimentation with
different inventory methods in collaboration with villagers, a systematic
sample composed of parallel, 10-meter-wide transects appears to work best for
counting and measuring forest resources.
Transects should run straight along a
pre-determined compass bearing. The bearing should be chosen so that the
transects run across topographical features (i.e., up and down slopes and
across rivers, rather than parallel to them). Orienting the transects in this
way will maximize the number of different habitats encountered in the inventory
and provide a more representative sample of local habitats. The distance
between transects determines the sample intensity (i.e., the percentage of the
total area that is included in the inventory). The closer the transects are
together, the higher the sample intensity. For example, separating each
10-meter-wide transect by 100 meters would give a sample intensity of 10%,
while separating the transects by 200 meters would yield a sample intensity of
5%. Given the paucity of density data that exists for even the most
economically important forest resources, a 5% sample would be sufficient to
estimate the existing stock of a wild resource.
Finally, in addition to counting the
plants of a particular species in the inventory, individual specimens should
also be measured or visually estimated into size classes. Diameter at breast
height (DBH) is the most convenient measurement for trees, and basal diameter
is an appropriate size-class parameter for shrubs. Height classes can be used
for vines, palms, and smaller woody perennials, and life stages (i.e.,
seedling, sapling, juvenile, adult) can also be used for species such as agaves
(Agave spp., Asparagaceae) that are difficult to measure. The reason for
assigning individuals to size classes or life stages is to divide the
population into groups that are likely to be experiencing the same growth
conditions.
The yield of different plant resources
is quantified through a growth or yield study. Growth studies are used when the
resource of interest is stem or root tissue (e.g., timber, rattan, ginseng [Panax
spp., Araliaceae] roots); yield studies are used to measure fruit production,
latex yield, or bark growth. The objective is the same: to quantify the
size-specific annual production of the resource of interest in different
habitats. How much rattan, timber, bark, latex, or floral nectar, or how many
native fruits or leaves, are produced by a given species within a particular
habitat? This is an important consideration, because it will ultimately
determine how much of a given resource can be harvested from the wild.
In selecting the sample individuals to
measure for yield studies, care should be taken to choose individuals that
represent a range of different sizes (or ages), canopy covers, and habitats.
Plants typically grow faster (i.e., produce more wood, leaves, or fruit) when
there is more available light and/or nutrients; taller plants with a better
canopy position also usually grow faster than suppressed individuals. Both
fast-growing and slow-growing specimens should be included in the selection of
sample plants for the yield study. If only fast-growing individuals are
sampled, the annual yield of the population will likely be overestimated, and
too much of the resource will be harvested from the area. Conversely, if the
selection of sample plants contains too many slow-growing individuals, yield
will be underestimated, and resources that could have been harvested
sustainably will be left in the field. While this may seem obvious, it is often
tempting to bias the selection of sample individuals toward the smaller-size
classes, which are often more numerous, accessible, and easier to measure.
These individuals also usually grow more slowly.
Defining a Sustainable Harvest
With data on the density and size-class
distribution of a plant population and relatively precise yield estimates, it
is possible to calculate the total quantity of the resource produced by the
species in a single year. Multiplying the size-specific growth rate by the
number of individuals in that class and then totaling the result over all
classes provides an estimate of total population productivity. For resources
like timber, rattan cane, bark, and roots, the harvest of which inevitably
kills the plant, this estimate represents the limit of how much material can be
sustainably harvested from the population in one year. If the population
produces a total of 1,000 kilograms of new bark each year, then this is all
that can be exploited. The exact number of trees that are harvested will depend
on the size and bark volume of the trees that are felled or de-barked, but the
total amount of bark extracted from the site in a single year should never
exceed 1,000 kg.
For fruits and seeds, which can be
harvested without killing the plant but impact the regeneration dynamics of the
population, defining a sustainable harvest limit is a bit more complicated.
Clearly, not all of the fruit produced by the population can be harvested year
after year. It is necessary to determine how many fruits need to be left on the
site to facilitate the recruitment of new seedlings into the population. This
can be accomplished in two main ways: through successive approximation (i.e.,
harvesting a certain percentage of the fruit crop, monitoring the impact on
regeneration, and adjusting subsequent harvests as warranted17) or
by dividing the management area into separate harvest units of comparable size
and leaving one unit untouched or fallow each year; the fallow plot should be
rotated sequentially and be different each year.
Impact Monitoring
Even when harvest
limits are respected and maintained, collecting commercial quantities of
resources from wild plants can cause changes in the population being exploited.
Shifts in biophysical parameters (e.g., rainfall, temperature, presence or
absence of pollinators or predators) can cause plant populations to produce
varying numbers of seedlings or to exhibit varying rates of mortality each
year. These demographic changes, in either direction, should not go undetected.
It is best to set up a series of permanent plots in the forest and re-inventory
them every five years or so.14
In particular,
changes in the size-class distribution of the harvest population that suggest
adverse effects on rates of regeneration should be noted. To assess this
change, the results from the initial inventory should be used to produce a
size-class histogram (i.e., a graph that shows the number of individuals in
each size class). A histogram serves as a baseline and represents the initial
structure or “pre-harvest” condition of the population.
In spite of the
variety of different reproductive and growth strategies used by plants, wild
populations exhibit a limited number of size-class distributions. Three of the
most common distributions are shown in Figure 2 with a representative example
of each type. Size classes indicated are diameter classes (cm DBH) for the tree
species (tengkawang nyamuk and uvos) and height classes
(m) for the rattan species (may sap).
The Type I
size-class distribution, illustrated by tengkawang nyamuk (Shorea
atrinervosa, Dipterocarpaceae), a valuable seed oil-producing tree in
western Borneo, displays a greater number of small individuals than large ones
and an almost constant reduction in number from one size class to the next.
This type of population structure is characteristic of shade-tolerant plants
that maintain a relatively constant rate of recruitment. It is probable that
the death of an adult tree will be supplanted by the growth of individuals from
the smaller size classes.
The Type II
size-class distribution, illustrated by uvos (Spondias mombin,
Anacardiaceae), a popular native fruit tree from the Peruvian Amazon, is
characteristic of species that show discontinuous or periodic recruitment. The
actual level of seedling establishment may be sufficient to maintain the
population, but its infrequent recruitment causes notable discontinuities in
the structure of the population as the newly established seedlings and saplings
grow into the larger size classes. This type of diameter distribution is common
among tree species that depend on canopy gaps for regeneration. Many of the
mast-fruiting Dipterocarpaceae in Southeast Asia also exhibit a Type II
distribution (although not tengkawang nyamuk, which fruits every year).18
The large number of individuals in the first size class, as shown in the uvos
histogram, suggests that gap colonization by this species was particularly
successful in the past.
The final
size-class distribution, Type III, is illustrated by may sap (Calamus
dioicus, Arecaceae), an important commercial rattan from the Greater Mekong
Region. A Type III distribution is seen in species whose regeneration is
severely limited for some reason. Population density is low, seedling numbers
are greatly reduced, and very few individuals are in the intermediate size
classes. Type III distributions are frequently encountered among
light-demanding, pioneer species that require large canopy gaps for
regeneration. In the absence of factors required for regeneration, these
species may disappear from the forest. The situation with may sap is aggravated
by intensive commercial harvesting of adult plants for their cane.
The size-class
distribution of a population is extremely sensitive to the population’s
regeneration rate. A Type I distribution can easily change into a Type II if
existing recruitment levels are diminished or interrupted. Further constraints
on regeneration may drive the population to a Type III distribution. It is
perhaps most useful to view these three distribution types as a single sequence
through which a wild species passes on its way to extinction.
Size-class
histograms should be constructed every time a harvest area is re-inventoried,
and the distribution of individuals in the current population should be
compared to the “baseline” histogram from the original inventory. Are there
notable changes in the shape of the distribution? Have the number of
individuals in the initial size class decreased (i.e., is the population
recruiting fewer new individuals)? Or, ideally, has the general shape of the
size-class distribution remained relatively unchanged over the five-year
period?
Pronounced changes
in the size-class distribution should be accompanied by immediate reductions in
the amount of material harvested from the population each year. These harvest
reductions should be maintained until the structure of the population returns
to baseline conditions. If, after five years, the number of individuals in the
smaller size classes continues to decline, the harvest intensity should be
further decreased and the response should be monitored. By conscientiously
monitoring the population response to different harvest levels, a level of
resource extraction that the population can support will eventually be
determined. This is where sustainability happens.
Recommendations
and Next Steps
The sustainable
management of wild plant populations is both a science and an art. In most
cases, the actual sustainability of an enterprise hinges more on the
willingness of the collectors and buyers to follow the prescribed harvest
limits than on the statistical rigor of the data used to estimate them. It is
important not only to count the plants and measure the growth as accurately as
possible, but also to make sure that everyone understands how and why these
data were collected and to continually affirm that things will get better if
the harvested controls are respected.
In terms of next steps:
Start small. Identify a specific plant
resource-collector-market system in which information exists about the biology
of the plant species, market conditions for the resource are favorable, and
stakeholders are enthusiastic about the prospects of using wild resources
sustainably.
Conduct a quantitative inventory of the plant population
and start a yield study, ideally involving the collectors and/or local community.
Calculate a sustainable harvest level in collaboration
with all stakeholders and initiate harvesting. Promote the sustainability
initiative as much as possible.
Periodically re-inventory the harvest area to assess the
impact of harvesting. Adjust harvest rates as necessary and carefully explain
to all stakeholders the reason(s) for the modification.
Freely share information with colleagues, local
authorities, retailers, business representatives, and the general public.
Both the harvested and the harvester ultimately benefit
from sustainable resource use.
Charles
M. Peters, PhD,
is the Kate E. Tode Curator of Botany at the Institute of Economic Botany at
the New York Botanical Garden and an adjunct professor of tropical ecology at
the Yale School of Forestry and Environmental Studies. His recent book, Managing the Wild: Stories of People and
Plants and Tropical Forests (Yale University Press and NYBG Press, 2018)
describes more than three decades of fieldwork with local communities in
tropical forests to sustainably manage wild plant resources.
References
- Schippmann U, Leaman D, Cunningham AB. A comparison of
cultivation and wild collection of medicinal and aromatic plants under
sustainability aspects. In: Bogers RJ, Craker LE, Lange, D, editors. Medicinal
and Aromatic Plants. Wageningen, the Netherlands: Springer Netherlands;
2006;75-95.
- Shackleton
C, Shackleton S. The importance of non-timber forest products in rural
livelihood security and as safety nets: A review of evidence from South Africa.
South African Journal of Science. 2004;100(11-12):658-664.
- Townson
IM. Forest Products and Household Incomes: A Review and Annotated
Bibliography. Tropical Forestry Papers No. 31. Oxford, UK and Bogor,
Indonesia: Oxford Forestry Institute and Center for International Forestry
Research; 1995.
- Hamilton
AC. Medicinal plants, conservation and livelihoods. Biodiversity and
Conservation. 2004;13(8):1477-1517.
- Evans
TD. The status of the rattan sectors in Lao People’s Democratic Republic,
Vietnam and Cambodia-with an emphasis on cane supply. In: Dransfield J, Tesoro
F, Manokaran N, eds. Rattan: Current Research Issues and Prospects for
Conservation and Sustainable Development. Rome, Italy: Food and Agriculture
Organization; 2002;115-144.
- Vazquez
R, Gentry AH. Use and mis-use of forest harvested fruits in the Iquitos area. Conservation
Biology. 1989;3(4):350-361.
- Wyn
LT, Anak NA. Wood for trees: A review of the agarwood (gaharu) trade
in Malaysia. Petaling Jaya, Malaysia: TRAFFIC Southeast Asia; 2010.
- Castle
LM, Leopold S, Craft R, Kinscher K. Ranking tool created for medicinal plants
at risk of being overharvested in the wild. Ethnobiology Letters.
2014;5:77-88.
- American
Herbal Products Association. Section 4: Wild collection assessment tool. In: Good
Agricultural and Collection Practices (GACP). Silver Spring, MD: AHPA. July
7, 2017. Available at: www.ahpa.org/Resources/GACP-GMPAssessmentTools/TabId/405/ArtMID/1249/ArticleID/825/Section-4-Wild-Collection-Assessment-Tool.aspx.
Accessed March 4, 2018.
- Peters
CM. Community forestry and sustainability res
earch at The New York Botanical
Garden. Brittonia. 2016;68(3):290-298.
- Peters
CM. Managing the Wild: Stories of People and Plants and Tropical Forests.
New Haven, CT: Yale University Press; 2018.
- Foster
S. Harvesting medicinals in the wild: The need for scientific data on
sustainable yields. HerbalGram. 1994;24:10-16.
- Cunningham
AB. Applied Ethnobotany: People Wild Plant Use and Conservation. London,
UK: Earthscan; 2001.
- Peters
CM. Sustainable Harvest of Non-timber Plant Resources in Tropical Moist
Forest: An Ecological Primer. Washington, DC: Biodiversity Support Program;
1994.
- Husch
B, Beers TW, Kershaw JA. Forest Mensuration. 4th ed. Hoboken, NJ: John
Wiley and Sons, Inc.; 2003.
- Peters
CM. Beyond nomenclature and use: A review of ecological methods for
ethnobotanists. In: Alexiades MN, ed. Selected Guidelines for Ethnobotanical
Research: A Field Manual. Bronx, NY: NYBG Press; 1996;241-276.
- Peters
CM. The Ecology and Management of Non-Timber Forest Resources. World
Bank Technical Paper No. 322. Washington, DC: The World Bank; 1996:82-85.
- Peters
CM. Illipe nuts (Shorea spp.) in West Kalimantan: Use, ecology, and
management potential of an important forest resource. In: Padoch C, Peluso NL,
eds. Borneo in Transition: People, Forests, Conservation, and Development.
Kuala Lumpur, Malaysia: Oxford University Press; 1996;230-244.
|