Forest Regeneration - University of Washingtoncourses.washington.edu/fm323/NotesNotes/Forest... · 2014-04-01 · Treatments of vegetation, slash, or soil to create and maintain conditions

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Forest Regeneration

Chapters 7 & 10 Smith, et al.

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Forest Regeneration

!  Trees live long, but not forever so the time comes when they naturally are, or, must be, replaced by new ones.

!  Regeneration plays such a prominent role in silviculture that most silvicultural systems are named for their method of regeneration

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Regeneration or Reproduction Method “ a process or procedure by which a stand is

established or renewed ” !  Two main objectives

!  Free up growing space (either partially or totally); translates into removing some of mature trees

!  Create environment for establishment of new !  Other included considerations

!  Treatments of vegetation, slash, or soil to create and maintain conditions favorable to the establishment and early growth of regeneration

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Two types of regeneration or reproduction !  Natural

!  Establishing and releasing seed or vegetative sprouts (propagules) originating from sources within or adjacent to the stand being regenerated

!  Artificial !  Regeneration is artificially seeded or

planted, and planted trees can be either from natural origin dug from elsewhere in the forest or grown in seedling nurseries or greenhouses

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Artificial Regeneration

!  Crucial decision and investment

!  Site preparation and species match

!  Planting / Direct seeding

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!  Planting stock !  bare-root seedlings

↑ light, easy transportation and cheaper ↓ planting can be done only in short periods of time

!  containerized seedlings ↑ quick root-soil contact, can be grown faster, greater

uniformity makes planting cheaper, less seed is used ↓ more costly to transport

!  vegetative propagation ↑ genetic qualities are perpetuated, easier and faster

establishment if species forms roots readily, reproduce several individuals from a single one

↓ no root system (some planting limitations), few species

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!  Size of plants !  “bigger is not necessarily better!” !   large planting stock vs. small planting stock

!  Season of planting !  Breaking of dormancy - Active root elongation

period !  Temperature and moisture

!  Site preparation for improvement of site !  Reduction of competing vegetation !  Drainage of water !  Placement of seedlings

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Methods of planting

!  Manual or hand planting !  compression methods: bar-slit (sandy soils) ,

grub-hoe-slit (stony or clay soils) !  dug-hole methods (when previous methods fail):

side-hole, center-hole, wedge !  uniform holes can be made using special devices

for containerized seedlings (uniform size & shape) !  Mechanized planting (mostly compression

method) !  flat or gently sloping sites, stump free (abandoned

agricultural fields) !  Usually more effective and cheaper !  Afforestation

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Density of plantations

!   Important decision !  Good control of initial density and spatial

arrangement !  Best to envision the kinds of trees wanted in

the stand late in the rotation and work backward from this goal - tendency to think how many trees will be needed to achieve early occupancy of the growing space

!  High density vs. low density → site and objective of production

!  Spatial arrangement: square, hexagonal, staggered, “random,” clumped

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Plantation protection

!  Plantations require a large investment → losses represent a much larger waste of money compared to natural regeneration

!  Part of the cost of artificial regeneration goes towards protection

!  Damaging agents: wild animals (rodents, browsing, ants, etc) and competing vegetation (woody plants)

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Direct seeding

!  Requires !  Control of seed-eating animals !  Favorable site and seedbed conditions !  Moisture during germination (rain) !  Seed supply !  Seed treatment (often for germination)

!  Site factors and site preparation define the feasibility of the method

!  Types: broadcast or strip and spot seeding

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Natural regeneration and disturbance !   Natural succession and development of new stands

proceeds in one of two ways !  Advance regeneration (already established survivors) !  Germination of seeds

!   Once established, woody perennials usually command growing space so tenaciously that only the death of some of them can make vacancies large enough for the establishment of new ones

!   Disturbance, particularly lethal, figures prominently in the ecology of natural regenerative processes

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Natural regenerative disturbance

!  Fire is most common natural disturbance !  Ignited by lightning, volcanoes, spontaneous !  Kills small trees most efficiently, so old stands are

killed from the bottom upwards !  Favors the return of two major categories of

species !  Those that sprout from roots or bases of fire girdled

stems !  Species germinating from small, wind-blown seeds

adapted to germinate on mineral soil surfaces bared by fires

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Adaptations to fire (some examples) !   Closed-cone pines, such as

jack pine in the Lake States, lodgepole pine and Monterey in the west – regeneration occurs after a crown fire has killed the old crop, exposed mineral soil, and opened the cones

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Adaptations to fire (more examples)

!  Species w/hard-coated seeds that survive long periods in the forest floor, springing up after fires, such as Cherries and Ribes shrubs

!  Sprouting species, mainly angiosperms (broad-leaved trees) and some conifers such as pitch pine and coast redwood

!   Light-seeded species that thrive on seedbeds of bare mineral soil exposed by fire but not outstandingly resistant to fire (e.g., alder, cottonwood, poplar)

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Adaptations to fire (ya example)

!   Species with sufficiently fire-resistant bark to withstand burning at intervals throughout an entire single generation, like the hard pines of the South, such as longleaf, loblolly, and shortleaf, and ponderosa pine in the northwest and west

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Kinds and severity of disturbances !  Geologic, erosional events are the most

severe kind of natural disturbance

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Kinds and severity of disturbances !  Geologic, erosional events are the most

severe kind of natural disturbance !  E.g., landslides, melting of glaciers, formation of

new land by water, wind, volcanism !  Ecologically, true primary succession occurs after

these events !  Ordinarily, herbaceous vegetation colonizes these

areas, though some tree species can also, such as true poplars, alders, some others

!  SELDOM wise to imitate this with silviculture, but foresters are often called upon to re-vegetate these areas as well as strip mines, or such areas caused by other events

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Kinds and severity of disturbances

!  Next most severe is a hot fire that burns large amounts of dead fuel created by blow-downs or pest outbreaks

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!  Next most severe is a hot fire that burns large amounts of dead fuel created by blow-downs or pest outbreaks !  Some fires can burn so hot as to be almost as

lethal as landslides, but after fire organic matter usually remains in the soil

!  A few silvicultural techniques artificially create similar disturbances, leaving mostly bare soil with organic matter intact, freeing all the growing space for establishment of new vegetation from seed or other methods

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Kinds and severity of disturbances !  Next most severe are “releasing

disturbances” caused by wind or pests of large trees that kill forests from the top down, sparing most plants in lower strata !  Species adapted to such disturbances have

foliage constructed and displayed in ways that make them tolerant to shade !  These species may exist as seedlings or saplings for

many years beneath old stands, retaining the capacity for rapid height growth whenever some lethal event releases them

!  Tend to be limited to sites and regions that are continuously moist enough to reduce exposure to fire or drought stress

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!  Natural disturbances are not uniform in severity, type, or extent

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!  Natural disturbances are not uniform in severity, type, or extent !  Differences are degree of exposure of

mineral soil, extent of freedom from root competition, intensity of solar radiation

!  Pioneer communities are the result of catastrophic disturbances

!  Late-successional communities result from a long series of frequent, light, “releasing” disturbances

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Environment of the microsite

!  The first few weeks in the life of a seedling are the most dangerous for its survival

!  Seedlings respond to light, water, carbon dioxide, chemical nutrients, and biotic influences (regardless of the name of any silvicultural treatments)

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Environment of the microsite - Water loss

!  Water is lost to the atmosphere by direct evaporation from the uppermost layers of soil and organic material

!  Loose materials lose water so fast they are seldom hospitable for plants

!  Direct evaporation explains the loss of water from the top layers of soil when there are no roots (capillary action)

!  Shade and mulching can greatly slow down direct evaporation losses

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Environment of the microsite - Surface Temperature !   Survival of new unshaded seedlings depends heavily

on physical processes dissipating solar energy load !   Reflection, convection, conduction, evaporation, and

outgoing re-radiation are the processes that take heat energy away from absorbing surfaces of plants

!   Shading is the most important physical process that is subject to silvicultural manipulation

!   Chlorophyll reflects heat energy (longwave infrared radiation) to some extent, but opaque objects like rocks and trees are most effective in reflecting harmful radiation away from seedlings

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Environment of the microsite - Light

!   Light regimes beneath forest canopies can be dramatically altered by changes in quality, intensity, and proportion of direct versus diffuse light

!   For deciduous canopies, moisture and temperature affect the times when leaves are present that reduce solar radiation to the forest floor

!   Seedlings of many species thrive by being out of synchrony !   Some by early emergence before older trees leaf out in

spring or before wet season !   Others by delaying leaf shedding in fall or at beginning of

dry season !   The greatest difference in light radiation quality beneath a

forest canopy occurs across forest openings – under direct or indirect manipulation of the silviculturist

!   Slope and aspect also will influence groundstory radiation 28

Regeneration Process

!   Initiation of natural regeneration from certain kinds of disturbances can be mimicked to a degree with silviculture

!  Goal is providing right amount of growing space for establishment and growth of the desired species !   Selection Systems (single-tree, group) !   Shelterwood Systems !   Seed-tree Systems !   Single-cohort Systems (coppice, clearcut)

!  Success of regeneration can be regulated through manipulation of overhead cover and the characteristics of surface materials

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Essential stages of natural regeneration

!  Overview !  Flowering !  Seed supply !  Seed dispersal !  Storage !  Germination !  Succulent stage !  Growth and establishment

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!  Flowering !  All tree species can

reproduce sexually from seed !  Monoecious species

have both functional sexes on the same plant, but in separate flowers, e.g. Red alder

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!  Flowering !  All tree species can reproduce sexually from

seed !  Dioecious species have trees with flowers of only

one functional sex, e.g. Black cottonwood

!   These strategies can contribute to creation of a viable propagule that will advance the genetic vigor of the future population 32


!  Flowering !  Some species occurring in some environments

rely heavily on vegetative (asexual) regeneration e.g. Quaking aspen

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!  Seed supply !  First prerequisite! – no tree or group of

trees is a dependable source unless it is sufficiently mature and vigorous to produce seed

!  Seed bearers should be located so that wind or other agencies ensure pollination and proper distribution of seed over the site

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!  Seed supply !  Not every year is a “good” seed year

Periodicity of cone crop in 4 w. white pine trees from same stand

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!  Seed dispersal !  Modes of seed dissemination include

almost every imaginable mechanism (i.e. animals, wind, water, gravity)

!  Distances of effective dispersal vary widely, but usually no greater than a few times the height of the seed bearers

!  Predators (insects, bats, birds, rodents, etc.) account for some losses

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!  Storage !  Seeds produced in a “good” year often

must survive at least one cold or dry period and be ready to germinate the next growing season (stratification)

!  Varying degrees of dormancy help this process

!  Forest floor conditions usually supply the appropriate conditions of a cool moist period to break dormancy

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!  Germination !  Adequate amounts of moisture (most critical and

variable), heat, and oxygen, sometimes certain wavelengths of red to far red light starts development

!  Depends heavily on the amount of rainfall and conditions right at the spot where the seed fell

!  Species such as oaks with large heavy seeds are generally the only ones that can establish themselves in thick leaf litter – on bare mineral soil, the blunt roots would simply roll them around over the surface without penetrating the soil

!  Light-weight, wind-disseminated species germinate best on or slightly beneath moist seedbeds - bare mineral soil is best 38


!  Succulent stage !  Seedlings are most vulnerable during the

first few weeks of life, while stems are still green and succulent

!  Heat injury takes a heavy toll !  Damping-off, caused by mildly parasitic

fungi in shaded conditions also takes its toll

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!  Growth and establishment !  The key to success is extending roots into the soil

quickly enough to maintain contact with portions of the soil where water is available

!  Roots that penetrate below the “capillary fringe” and tops that extend into the turbulent air strata are much less subject to drought and heat injury

!  Once a seedling is established, the ecological and silvicultural rules that govern their lives as trees are quite different

Natural Reproduction Triangle

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Summary !  Two major methods of regeneration: natural, ‘artificial’

!  Natural regeneration is affected by type and intensity of ‘disturbance’

!  Natural Regen initiated in any particular stand through methods deliberately regulating cover is best specified by explicitly stating… !  Propagule origin !  Descriptors of site treatment !  Spatial pattern of operation !  Vegetation structures remaining

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Summary (cont’d) !  Success of natural regeneration depends on

the occurrence and timing of of seven (7) essential events

!  Artificial Regeneration requires careful thinking about cost / investment, species, methods

!  Two artificial methods: direct seeding, planting !   If planting, decisions needed for

!  Stock type !  Stand density !  Planting method !  Planting season !  Plantation protection

Documents

Forest Regeneration - University of Washingtoncourses.washington.edu/fm323/NotesNotes/Forest... · 2014-04-01 · Treatments of vegetation, slash, or soil to create and maintain conditions