Forest Stand Dynamics

1. Forest Stand Dynamics

2. Defining Forest Stand Dynamics • Forest dynamics describes the underlying physical and biological forces that shape and change a forest • Continuous state of change altering forest composition and structure • Basic elements of forest dynamics: • Disturbance • Succession

3. Disturbance and Succession • Forest disturbance is an event that causes change in forest structure and composition, resource availability, and the physical environment • Succession is the process that gradual replacement of one community of plants by another

4. Disturbance and Succession

5. Disturbance and Succession

6. Disturbance and Succession

7. Range of Forest Disturbance • Forest disturbances vary in type, frequency, spatial scale, and severity • Disturbance types: Fire, wind, insects, disease, flood, ice storms, grazing/herbivory, timber harvest, road construction, conversion to nonforest • A continuum of disturbance from individual tree-level to landscape scale

8. Major (Stand-Replacing) Disturbance

9. Major (Stand-Replacing) Disturbance

10. Phases of Stand DevelopmentFollowing Major (Stand-Replacing) Disturbance • Stand initiation (reorganization phase) • Stem exclusion (aggradation phase) • Understory reinitiation (transition phase) • Old-growth (complex phase, steady-state) • Each phase of stand development is accompanied by changes in stand structure and species composition.

11. Stand Initiation Stage • Follows major disturbance (wind, fire, clearcuts) • Regeneration from seed, sprouts, or advance reproduction • Rapid increase in the number of stems and biomass • Structure • Single cohort (“even-aged”) stand • “Brushy” stage with herbaceous, shrub, small trees • Invasion continues until all growing space is occupied • Stage ends when canopy becomes continuous and trees begin to compete with each other for light and canopy space

12. Stem Exclusion Stage • Begins at about crown closure, characterized by onset of density dependent mortality (“self-thinning”) • Canopy continues to have one cohort and canopy too density to allow new trees to grow into canopy • Crown differentiation occurs • Biggest trees tend to get bigger, the smaller ones tend to die • Mortality rates are high, especially in intermediate and overtopped crown classes • Least competitive individuals die • Crowns are small enough so that when a tree dies, others fill the vacant growing space by expanding their crowns • Phase ends when biomass peaks

13. Crown Classification Overtopped http://www.extension.umn.edu/distribution/naturalresources/images/3473-12.jpg

14. Crown Classification • Dominant: Crown is larger than average and typically above the general upper level of the canopy; receives full top light, considerable side light • Codominant: Top of crown is at upper canopy height; receives full top light, little from sides; medium-sized crown, usually somewhat crowded on its sides. Often wide range around “average canopy” tree. • Intermediate: Top of crown is below the top of the general canopy; receives some top light from directly above, none from the side; conspicuously narrower, smaller and shorter than the average crown. • Overtopped: Crown entirely below some foliage of the upper canopy; receives no direct top light; small, weak crown with low vigor

15. Understory Reinitiation Stage • Mortality of individuals cannot be closed by adjacent individuals • Crowns of trees are now large enough so that when one overstory tree dies, the surrounding trees can not fill the gap • Permanent canopy gaps form • Permanent understory forms • Tree reproduction becomes re-established beneath parent stand • Primary factors influencing species composition • Understory light availability • Species degree of shade tolerance

16. Old-Growth (or Complex) Stage • Natural mortality of large overstory trees produces irregular canopy gaps • Mortality and recruitment and are in balance and biomass is stable • Can mark transition from an even-aged to an uneven-aged stand

17. Stand Initiation Stem Exclusion Old growth (Complex Stage) Understory Reinitiation

18. Johnson, P.S., S.R. Shifley, and R. Rogers. 2002. The Ecology and Silviculture of Oaks. CABI Publishing, New York, NY. 503 p. Stand Development in the Central Hardwood Region Stand Initiation Phase: 10 – 20 years

19. Johnson, P.S., S.R. Shifley, and R. Rogers. 2002. The Ecology and Silviculture of Oaks. CABI Publishing, New York, NY. 503 p. Stand Development in the Central Hardwood Region • Stem Exclusion: • Begins after 10 years • Concludes before age 70

20. Johnson, P.S., S.R. Shifley, and R. Rogers. 2002. The Ecology and Silviculture of Oaks. CABI Publishing, New York, NY. 503 p. Stand Development in the Central Hardwood Region • Understory Reinitation: • Tree reproduction begins to develop under maturing overstory • Begins after age 50, concludes before age 120

21. Johnson, P.S., S.R. Shifley, and R. Rogers. 2002. The Ecology and Silviculture of Oaks. CABI Publishing, New York, NY. 503 p. Stand Development in the Central Hardwood Region • Complex Stage (old-growth): • Oak forest typically require 100 years or longer to reach this stage.

22. Johnson, P.S., S.R. Shifley, and R. Rogers. 2002. The Ecology and Silviculture of Oaks. CABI Publishing, New York, NY. 503 p. Stand Development in the Central Hardwood Region *Assume no significant stand-scale disturbances occur *Actual durations of stages of development vary with species composition, site productivity, and other factors

23. Tree Growth and the Environment

24. Photosynthesis • Photosynthesis: Conversion of light energy to chemical energy • Production of carbohydrates from CO2 and H2O in the presence of chlorophyll using light energy. 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O • Photosynthetic activity is a major factor in the production of biomass • Rates of photosynthesis are influenced by both plant and environmental factors light chlorophyll

26. Respiration • Respiration is the process by which energy fixed by photosynthesis is made available for metabolic processes

27. Environmental Factors Influencing Photosynthesis • Light • Temperature • CO2 concentration • Water availability • Nutrient availablity

28. Environmental Factors Influencing Photosynthesis • Light • Photosynthesis uses solar radiation in the visible spectrum (400-700 nm wavelengths) • Wavelength range known as photosynthetically active radiation (PAR) • Light directly affects tree growth by its intensity, quality, and duration

29. Environmental Factors Influencing Photosynthesis • Light • Light environment in a stand is influence by the vertical and horizontal forest structure • Density and vertical distribution of foliage alters light transmittance from sky to forest floor • Silviculturists manipulate light environment in stands by altering forest structure

30. Inverse Relationship between Canopy Openness and Light Availability

31. Light and Photosynthesis Below some light, carbon uptake is negative, as respiration exceeds photosynthesis

32. Light and Photosynthesis As light increases, a light compensation point is eventually reached where CO2 through photosynthesis is exactly balanced by losses through respiration

33. Above the light compensation point, photosynthesis increases until the amount of carboxylation enzyme or available CO2 limits photosynthesis. • Plateau in the rate of photosynthesis is know as light saturation point

34. Light and Photosynthesis • Light compensation points and light saturation levels vary: • Among species • Among individuals of the same species • Among leaves on the same tree • With changing environmental conditions

35. Environmental Factors Influencing Photosynthesis • Temperature • Temperature is a very important factor in photosynthesis but unlikely to become a limiting factor in forests of temperate regions except during the winter

36. Environmental Factors Influencing Photosynthesis • CO2 concentration • CO2 concentration in the atmosphere is low, about 0.03% by volume • Concentrations in the forest are often higher but show vertical gradients which change diurnally and seasonally • Stands whose structure permits continued circulation of air provide more favorable conditions from the standpoint of CO2 supply than those with a tight canopy or those which are multi-storied. • CO2 enrichment (i.e. atmospheric rise due to fossil fuel burning) has been shown to increase growth rates

37. Environmental Factors Influencing Photosynthesis • Water availability • Only minute quantities of water are consumed in the process of photosynthesis • The main effect of water on photosynthesis is indirect through hydration of protoplasma and stomatal closure

38. Environmental Factors Influencing Photosynthesis • Water availability • Issues with moisture availability for photosynthesis and hydration: • Water stress in temperate regions occurs on xeric to intermediate quality sites and due to seasonal drought • Water is a primary growth limiter in arid and semi-arid regions • Wider spacing is one way for the silviculturists to reduce the impact of low moisture supply in such regions arid and semi-arid regions (U.S. southwest and mountain west)

39. Environmental Factors Influencing Photosynthesis • Water availability • Moisture availability is dictated by • Soil properties • Soil texture • Soil profile and depth • Topography • Aspect • Slope position • Slope shape • Elevation

40. Environmental Factors Influencing Photosynthesis • Water availability and topography • Aspect • Solar radiation exposure strongly effects evapotranspiration • North, northeast, and east slopes more cool and moist • West, southwest, and south slopes have highest transpiration loss due to perpendicular orientation to incoming solar radiation • Slope position • Upper slopes drier • Middle to lower slope positions more moist and productive • Slope shape • Convex landforms shed water • Concave accumulate water

41. Environmental Factors Influencing Photosynthesis • Nutrients • Photosynthetic efficiency of foliage depends decisively on soil nutrient supplies • With improving nutrient status among sites photosynthetic capacity of trees also improve • The effect is both direct (i.e., quantity of CO2 fixed by gram of foliage) and indirect by increasing size of individual leaves, total size of crown and root system • Nutrient availability is dictated by a site’s soil properties • Exception is the use of fertilization

42. Plant Factors Influencing Photosynthesis • Leaf age • Position within crown • Crown class and species • Sun and shade adaptations

43. Plant Factors Influencing Photosynthesis • Leaf age • In conifers fully expanded one-year-old foliage is the most efficient of all age classes • Difference between age classes is mainly a consequence of varying rates of respiration, and by insect or disease damage

44. Plant Factors Influencing Photosynthesis • Position within crown • The most productive leaves are in the upper crown. The lowest whorls contribute little to net photosynthesis.

45. Plant Factors Influencing Photosynthesis • Crown class and species • Differences in photosynthetic efficiency between dominant, co-dominant, intermediate, and overtopped trees are relatively minor when one compares similarly exposed foliage and expresses efficiency per unit of leaf surface • The major factor causing differences in photosynthetic capacity of trees of different crown classes and of different species is the enormous difference found in leaf area.

46. Plant Factors Influencing Photosynthesis • Sun and shade adaptations • Not all tree species possess the same photosynthetic efficiency • Photosynthetic rates and efficiency also varies with species shade tolerance • Shade tolerant, intermediate tolerance, and intolerant • Photosynthetic efficiency varies between shade and sun leaves on the same tree • Shade leaves and shade-tolerant species have higher photosynthetic efficiency per unit of leaf area under low light conditions than sun leaves • Under high light conditions the reverse is true

47. The Carbon Budget of Trees Carbon budget of a tree (or any plant) can be expressed like a bank balance: Income = carbohydrates manufactured in photosynthesis Expenditures = carbohydrates used in growth and maintenance (construction and maintenance respiration) Balance = carbohydrates stored (so-called nonstructural carbohydrates and other compounds)

48. Individual Tree Growth • Amount of carbohydrates produced through photosynthesis by a given tree is influence by: • Resources available to the tree (i.e., growing space) • Ability to harvest light and the roots ability to supply the foliage • Extent to which a tree increases mechanical support (i.e., stem diameter) depends upon: • Carbohydrates remaining after supplying essential functions

49. Shade Tolerance • Shade tolerance • Definition: Having the capacity to compete for survival under shaded conditions • Understanding of shade tolerance is a cornerstone of silviculture • Critical to silviculture in the following ways: • Regeneration of desired tree species • Regulating forest growth and understanding plant succession • Influencing species composition

50. Shade Tolerance and Photosynthesis • Shade tolerant species • Species adapted to growing at reduced light intensities • Generally have lower compensation points and levels of light saturation than shade intolerant species • Shade intolerant species saturate at relatively high light levels • Yield increased carbon gain in high light environments