Doug Maguire, Oregon State University, Image 2714015, http://www.forestryimages.org/ Stand Initiation Stage: Single-Cohort stands Forest Stand Dynamics Winter 2003 The Stand Initiation phase of forest development begins after existing vegetation is killed by a disturbance. The Oliver model discusses two types of disturbance, stand replacing and minor, and concentrates on species regeneration strategies. The Franklin model focuses on remaining biological legacies. AGE RANGE and SPATIAL PATTERNS of TREE INVASION Trees do not usually invade a disturbed area in a single year or in a uniform spatial pattern excepting with artificial regeneration following harvest. The age range of the new stand depends on the length of time it takes for trees to invade and establish. Age distributions in stands result from disturbance frequency and magnitude. A single-cohort stand is only one of many possible outcomes. Trees invade until all growing space is occupied. Age range will be narrow if the invading or preexisting stems rapidly occupy the growing space, excluding later arrivals. a) on productive sites Narrow age ranges occur: b) if regeneration strategies promote rapid growth (sprouting or advance regeneration) c) if many trees invade rapidly following disturbance (lodgepolepine or larch after fire) 1
Most rapid invasion is generally by artificial regeneration, provided seedlings survive and grow. Slow invasion occurs: on low productivity sites if invading species are slow to arrive (limited regeneration strategies, poor seed years, distance to seed source). Trees invading a site very early or very late may not compete as effectively and die, narrowing the age range. Bimodal age ranges occur where some growing space is quickly filled by advance regeneration or sprouts and the rest by seedlings. On harsh sites the age distribution can be very long if only a few early invaders arrive and later invaders arrive slowly. Multi-cohort stands can develop if disturbances do not completely remove the existing overstory. Age distributions can be regular or random, depending on disturbance frequency. Numbers of trees in successive cohorts depends on disturbance severity and the ability of previous cohorts to capture released growing space and prevent a new cohort from establishing. Multi-cohort stands occur where disturbances are: a) very frequent and light but older cohorts cannot completely capture released growing space b) very infrequent (gap phase dynamics) c) where new stems invade slowly or invade rapidly, but soon die (root rot pockets) Naturally-regenerating stands can have from less than 100 to over 100,000 stems per ha at the end of the SI phase. Initial stem density is a result of regeneration strategy and substrate availability. Lodgepolepine regenerates densely from seeds stored in serotinous cones that fall on a favorable seedbed. Ponderosa pine may regenerate densely or more sparesly (related to soil moisture). 2
Stands regenerate in aggregated patches or more dispersed patterns depending on microsite availability. In clumps, density dependent mortality begins sooner than for the stand as a whole. Invasion patterns result from regeneration mechanisms of various species, growth and mortality rates, and tolerances for shade and other limiting resources. A light fire in a black spruce stand or other bog forest may kill overstory trees but not consume enough organic material to allow seedlings to establish. Some species may initially dominate a stand (red maple, black birch, western hemlock, pin cherry) but have such a high (or early) mortality rate that they soon lose their dominance to other species with lower mortality rates or longer life spans. Abandoned fields are more uniform with respect to growing space and may regenerate densely and uniformly. Most forests have very un-uniform microsites which may persist through generations of trees (pit and mound topography, colonnades). Seedling mortality is usually higher than sprout mortality. Sprout mortality among COMPETING sprouts is high until one or two sprouts dominate, then mortality is low. Although seedling mortality is usually high, sometimes it is overcome by sheer numbers of seedlings which can swamp species with other regeneration mechanisms, especially if seedling growth is also rapid (red alder). Moderately poor sites generally have the greatest amount of plant diversity. Very good sites allow rapid colonization by one or two species that outcompete all others. VERY poor sites are generally colonized by a few species with high tolerances for resource limitations. 3
Soil texture can determine which species are able to colonize and compete successfully. Glacial soils include outwash which is variable with respect to texture and moisture-holding capacity and till which has a more uniform texture but differs in depth, compaction, and oxygen availability. Alluvial soils vary from coarse sand to fine clays, depending on the speed of the water that deposited them. Soils characterized by pit and mound topography vary over small areas with respect to growing space. Advance regeneration and stump sprouts occur on undisturbed soil between pit/mound microrelief. Species with windblown seeds colonize the bare mineral soil on mounds, while only species with tolerance for saturates soils colonize pits. Trees that colonize pits are usually outcompeted by those colonizing the more favorable mound sites. Only species that can tolerate both shade and saturated soils (red maples) are able to persist in the pits. Pit and mound microrelief can perpetuate itself: trees on mound sites grow tall, are blown over and create a new pit/mound in the same location. Higher microrelief is important where snowfall is high. Tree stumps and logs are dark, so snow melts faster. These sites support hemlock regeneration. Fir seedlings are sturdier and can tolerate greater amounts of snow. Trees that begin life on organic substrates need to develop strong prop roots for support when the substrate disintegrates. Nurse logs result in a colonnade of trees with prop roots. Spatial pattern and species composition following a disturbance is based on the distribution of parent material existing BEFORE the disturbance. Existence of pre-disturbance parent material is based on what occurred before; thus there can be a selfperpetuating tendency BUT IT IS CAUSAL AND NOT CO-EVOLVED. Competition from existing plants, including allelopathy and patchy germination can influence subsequent tree distribution patterns. Western redcedar seedlings grow better near parent plants because chemicals exuded by the roots of the large redcedar change soil chemistry in such a way that redcedar seedlings are at a competitive advantage to western hemlock (both are shade tolerant). Over generations, more of the stand would, in theory, favor WRC, leading to that species dominating the stand. 4
STAND INITIATION SINGLE COHORT STANDS After death of the overstory, initiating trees must compete with herbaceous and woody plants before beginning to interact with each other. There are 5 important characteristics of the stand initiation phase of forest development. 1. More individuals and species interact than during any other phase of stand development. 2. The relative age range of trees in the initiating cohort narrows throughout this phase. 3. Small sizes of plants magnify growth effects caused by small variations in environmental conditions. Small events determine who survives to maturity. 4. The biogeochemical cycle is reacting to the initial disturbance AND to the developing stand. Environmental conditions (water, nutrients, temperature) change rapidly. 5
Species other than trees compete vigorously for growing space. Trees may be initially or totally excluded. Ferns, grasses, and woody shrubs can dominate a site for decades. 5. Growing space is not fully occupied; trees are growing essentially without competition from other trees. The Stand Initiation phase of forest development generally has the greatest amount of biodiversity, but species tend to be generalists (including weeds). The RELATIVE differences among competing trees decreases with total age. When trees are very young, a year or two in age may confer a great competitive advantage. This advantage decreases over time: there is not a great deal of difference between a 60 year old tree and a 70 year old tree. This paradigm has NOT been adequately tested, because in OG forests, soil flora, fauna, and arthropods have not been thoroughly inventoried and may account for much of that phase s diversity. Small trees are more susceptible to their microenvironments than large trees: grass competition fluxes in temperature, moisture, light animal browsing As tree grows taller, and roots expand, they become less susceptible to the microenvironment. There is, however, a LOT of difference between a 3 year old tree and a 13 year old tree. Younger trees are outcompeted by older trees. As the stand develops toward the Stem Exclusion phase, most of the 3 year old trees will have died. 6
Growth of plants modifies the surrounding environment, dampening fluctuations in temperature, humidity, soil moisture levels, windspeed etc. Red maples can invade swampy areas and make them less swampy via evapotranspiration; then other species can invade (an example of relay floristics). Raised microsites can become too dry to support germinating seedlings. Biological legacies decay, changing C/N ratios and reducing availability of soluble nitrogen. Soil structure and nutrient status changes with root penetration and litter accumulation. Rapid decomposition can cause nutrients to leach below rooting zone. Slow decomposition can build up organic matter and restrict nutrient availability. 7
During Stand Initiation, invading trees are NOT competing with one another EXCEPT where they are in close proximity. Forest invasion on a talus slope can take centuries and radiates outward from suitable microsites. Trees Versus Other Species During Stand Initiation the 6 strategies of invading trees 1. Trees invade the site before other vegetation can establish. This strategy occurs with species that sprout or produce advance regeneration. Lodgepolepine seedlings are also able to invade sites and overwhelm other kinds of vegetation quickly. 2. Trees invade suitable sites that were previously occupied by annuals or perennials that die back to the root collar. 4. Trees invade areas where a few early invaders have reduced the ability of existing herbs and shrubs to occupy the growing space (through shading or at higher elevations in the form of tree clumps). 3. Trees invade older herb or shrub communities where existing plants have lost their vigor: high elevation brush fields following fire. 8
5. Trees invade areas where small disturbances release growing space previously occupied by herbaceous species. 6. Trees invade where other trees and shrubs are growing, but are able to outcompete them and take over the site. (Ponderosa pine on site previously artificially regenerated with Douglas-fir) Once a tree is established it grows upward, outward, and downward, it s rate of expansion varying by species, site, and competition. The more it grows, the better the tree can withstand changes in its environment AND keep competitors shaded, shorter, and less competitive. Disadvantages to later arrival are more pronounced in single species stands where trees require the same set of resources. Management Implications Species composition of the stand can be managed more easily in SI than any other phase Manage to confer competitive advantage to desired species Can be accomplished through site preparation that mimics (or doesn t) a specific disturbance, thus conferring a regeneration advantage to desired species. Management Implications Shortening the SI phase can result in loss of habitat diversity or future nutrient conditions Killing red alder / ceanothus reduces N inputs Management Implications Stand structure at the end of the SI phase will determine the composition (and value) of the stand for many decades. 9
Management Implications During the SI phase, silviculturists need to determine 1. Can the species of interest survive? 2. Will the species of interest be competitive? Too many individuals of a desired species may be more of a problem than too few: 1. Diameter growth may be affected by tight spacing 2. Pre-commercial thinning (expensive!) may be required to maintain optimal growth rates. 10