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THE INFLUENCE OF STAND CONDITIONS ON TREE FORM

This study explores how stand conditions influence tree growth, diameter, and height, beyond DBH and HT effects. Various methods and models are used to analyze stem form based on different treatments and vegetation management. The results highlight the significance of thinning intensity and other variables on stem taper and growth differences.

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THE INFLUENCE OF STAND CONDITIONS ON TREE FORM

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  1. THE INFLUENCE OF STAND CONDITIONS ON TREE FORM Sean M. Garber and Aaron R. Weiskittel Oregon State University June 21, 2004

  2. INTRODUCTION • A goal of silvicultural research is to detect changes in growth. • Changes in stand conditions affect diameter and sometimes height: • Density (spacing and thinning) • Social position within the stand • Control of competing vegetation • Fungal diseases

  3. INTRODUCTION • Consequently stem volume also responds to these stand conditions.

  4. INTRODUCTION • Existing volume and taper equations: • Most are functions of only DBH and HT; • Regional development; • May not include intensively managed stands or other dramatically altered stand conditions; • Averages lose subtle differences.

  5. INTRODUCTION • Questions: • Is there an affect of stand conditions above and beyond the effect on DBH and height? • Does this influence the ability to discern growth differences among treatments in silvicultural studies?

  6. INTRODUCTION HT DBH

  7. METHODS • Analytically assessed stem form on four separate studies: • Two vegetation management studies • A western larch LOGS thinning study • Two mixed-species spacing trials • Study of Swiss needle cast

  8. Represents shape METHODS • Kozak variable-exponent taper model applied in all studies: d = γ1DBHγ2XC + ε

  9. METHODS • Shape component (XC) • X varies from 0 (top), 1 (p), > 1 below p. • C = linear function of: • Z = stem position (h/HT) • DBH/HT • Treatment variables

  10. Positive term Negative term Model dynamics

  11. METHODS • Sample trees were climbed or felled: • Sampled across range of within treatment DBH; • DBH, HT, HCB, and CW measured; • Upper stem diameters (ib and ob) and height measured.

  12. METHODS • Autocorrelation accounted for in two ways: • Model fit by tree and parameters modeled using SUR. • Model fit using GNLS with CAR(1) • Multicollinearity was also a problem • Screened to reduce VIF

  13. Douglas-Fir Stem Taper Under Early Vegetation Control Study part of the Vegetation Management Research Cooperative, Oregon State University.

  14. INTRODUCTION • As trees develop under intensive management: • Form differs from a cone (/12)D2H, • Trees large enough for existing volume or taper equations.

  15. Summit Marcola

  16. Vegetation Management sites • Study description • Planted in 1993, • Completely randomized design, • 8 treatments, • 3 replicates, • Plot area = 0.112 ac, • 49 seedlings planted at 9.8 ft square spacing.

  17. METHODS • Treatments • No herbicide, • 4 ft2 full control, • 16 ft2 full control, • 36 ft2 full control, • 64 ft2 full control, • 100 ft2 full control, • 100 ft2 woody vegetation control, • 100 ft2 herbaceous vegetation control.

  18. METHODS • Diameter outside bark was collected at tree base, breast height, 8 ft, and every 4 ft above 8 ft. • Observed DOB’s were modeled using variable-exponent model using GNLS. • C was a function of site, area treated, and target vegetation.

  19. RESULTS • Modeling • Model fits were excellent (R2 > 0.95); • Impact of autocorrelation eliminated w/ CAR(1); • Multicollinearity present, albeit small. • Treatment variables • Area treated and herb control significant

  20. Stem Taper in a Western Larch Levels-of-Growing-Stock Thinning Study From Lennette A.P. 2000. Twenty-five-year responses of Larix occidentalis stem form to five stand density regimes in the Blue Mountains of eastern Oregon. M.S. Thesis, Oregon State University. 59 p.

  21. METHODS • Study description • Established in 1933, • Completely randomized design, • 5 treatments, • 2 replicates, • Plot area = 0.4 ac, • Thinned in 1966, 1975, and 1985

  22. METHODS • Treatments based on bole surface area (BSA) targets:

  23. METHODS • Data analysis: • Parameters fit by tree; • Parameters tested among treatments by MANOVA; • Parameters modeled using SUR.

  24. RESULTS • Stem form responded to thinning: • C = a1sin-1(Z) + a2Z2; • MANOVA suggested differences among the treatments; • Parameters increased w/ thinning intensity: • a1 = f (DBH/HT) • a2 = f (CR)

  25. BSA: 15,000 to 25,000 BSA 10,000 BSA 5,000

  26. RESULTS • CR improved model fit after accounting for DBH/HT • Treatment variables did not account for additional variation beyond DBH/HT and CR.

  27. Increasing thinning intensity

  28. Stem Taper in Two Mixed-Species Spacing Trials From Garber, S.M. and Maguire, D.A. 2003. Modeling stem taper of three central Oregon species using nonlinear mixed-effects models and autoregressive error structures. For. Ecol. Manage. 179: 507-522.

  29. Study sites • Two study sites located at Pringle Falls EF on the Deschutes National Forest: • Pringle Butte (34 years) • Lodgepole pine and Ponderosa pine • 4500 feet elevation, west aspect, and dry • SI100 = 100 ft (Barrett 1978) • Lookout Mountain (27 years) • Grand Fir and Ponderosa pine • 5100 feet elevation, east aspect, and “more mesic” • SI100 = 110 ft (Barrett 1978)

  30. GF MX Whole plot PP Split plot MX P F P F P F P F P Experimental Design • Completely randomized split-plot design • Whole plot factor: spacing • Split-plot factor: species composition • Replacement series setup • Only one mixture 50:50

  31. INTRODUCTION • Mixed-species stand development: • Spacing • More spacing, less stratification • Species composition • PP over GF • LP over PP From Garber, S.M. and Maguire, D.A. 2004. Stand productivity and development in two mixed-species spacing trials in the central Oregon Cascades. For. Sci. 50: 92-105.

  32. Top height growth across spacing

  33. RESULTS • Modeling • Model fits were excellent (R2 > 0.95); • Impact of autocorrelation eliminated w/ random effects and CAR(1). • Profiles • Showed differences among spacing; • Slight differences between pure and mixed stands.

  34. SPACING

  35. SPECIES COMPOSITION Mix PP (subordinate) Pure PP

  36. SPECIES COMPOSITION Pure PP Mix PP (dominant)

  37. DOUGLAS-FIRSTEM TAPER & SNC Part of the Swiss Needle Cast Cooperative, Oregon State University. From Weiskittel, A.R. 2003. Alterations in Douglas-fir crown structure, morphology, and dynamics imposed by Swiss needle cast in the Oregon Coast Range. M.S. Thesis, Oregon State University. 389 p.

  38. INTRODUCTION • SNC disrupts normal needle physiology, leading to premature loss of foliage • 10 yrs of extensive defoliation has dramatically altered Douglas-fir crown structure & morphology • modified age class structure and vertical location of foliage • reduced branching • increased crown recession rates

  39. HYPOTHESES • loss of foliage has decreased stem increment in the crown • increased crown recession rates modified stem form • stem taper equations require additional SNC covariates

  40. METHODS • 105 trees from 31 plots destructively sampled • DBH; 12 – 65 cm • Relative density; 21.5 - 73.6 • Site index; 27.7 – 47.4 m @ 50-yr • Foliage retention; 1.2 – 4.4 yrs • 3 to 4 trees/plot & 10 to 15 discs/tree

  41. ANALYSIS • utilized modified Kozak’s equation • includes Z, DBHZ, DBH/HT, FOLRET • little correlation between covariates • continuous AR1 error structure • tree volume predicted using the developed equation and Bruce & Demars (1974) equation • results compared graphically and statistically

  42. RESULTS • R2 of 0.95 • parameters all highly significant (p<0.001) • likelihood ratio test also suggested that foliage retention significantly improved model fit (p<0.0001) • for a given DBH/HT, foliage retention significantly reduced dibs throughout the stem, except below BH

  43. RESULTS

  44. DISCUSSION • Does relative stem form change w/ stand conditions? • Differences in profiles after accounting for DBH and HT. • Treatments variables significant after accounting for DBH/HT: • Vegetation treatment; • Thinning level; • Foliage retention.

  45. DISCUSSION • Does relative stem form change w/ stand conditions?

  46. DISCUSSION • Do these changes affect results of silvicultural studies? • Significant difference between volumes estimated from SNC equations and Bruce-DeMars; • Bias a function of top height, DF QMD, FOLRET, and CLSA; • Mean volume losses due to SNC were 31%.

  47. DISCUSSION • Mechanisms • Differential basal area growth rate along the stem differences among treatments. • Stem growth rate responds to increased resources; • Response increases w/ decreasing stem height. • Swiss needle cast • Reduces tree leaf area (thins crown) • Decreases upper stem growth

  48. Curves diverge

  49. DISCUSSION • Measure upper stem diameters. • Assess existing volume or taper equations. • Develop new site-specific equations. • Incorporate crown ratio into the model • Include stand or treatment variables into models.

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