The potential for uneven-aged silviculture in restoration and management of old forests

Kevin L. O’HaraUniversity of California - Berkeley

• Uneven-aged – 3 or more age classes• Multiaged – 2 or more age classes• Selection systems – uneven-aged silvicultural

systems• Stocking control – the process of controlling the

amount and distribution of growing stock• Cutting cycles – intervals between harvest entries

in multiaged stands

Uneven-aged – a.k.a.

• Continuous cover forestry• Near-natural• Close-to-nature• Nature-based• Back-to-nature

Common North American stocking control procedure

0

20

40

60

80

100

120

Diameter Classes

Num

ber o

f tre

es

• Q-factor = quotient of number of trees in a diameter class to the number in the next larger diameter class. For a q-factor of 1.5, 100 trees in one diameter class requires 67 in the next larger class.

DeLiocourt 1898

Meyer 1952

• Meyer suggested “virgin” stands were “balanced” because he observed their diameter distributions were straight lines on semi-logarithmic scales.

• The “balanced” stands were capable of yielding a constant volume while maintaining the structure.

• Meyer considered virgin stands to be balanced “because they have been maintained for long periods of time by natural processes of growth and mortality.”

• He promoted using the forests of an entire county or state to determine the appropriate balanced diameter distribution.

Meyer also developed this

diagram to represent the balance between

size classes in uneven-aged stands. The Swiss stand was

seen as more balanced because of

its position in the middle of the

diagram.Meyer 1952. Journal of Forestry

Swiss stand

US stand

From Smith et al. 1997

Another definition of the balanced uneven-aged stand is based on age/size classes occupying equal amounts of growing space.

• This form of “balance” is analogous to area control forest regulation at the stand level. Each age class occupies equal amounts of growing space and many more small than large trees are required for full site occupancy.

• The miniature even-aged forest model allowed for estimation of growth and stocking relationships using even-aged yield tables.

Summary: the “balanced” stand

• Straight line on semi-log scale• Found in “virgin forests”• Harvest = growth• Similar amounts of volume in each size class• Each age/size class occupies equal area

Uneven-aged stands managed to maintain a

negative exponential

diameter distribution.

Cutting Cycles

• Balanced stand concept implied that any stand with a different structure was out of balance.

• Cutting cycles were therefore short to maintain the balanced structure

Tree Diameter Classes

Num

ber o

f Tre

es

Mortality overestimatedDensities too highRotated sigmoid distribution may be more common

Alternative diameter distributions

4 (youngest) 3 2 1 (oldest)

Age classes

Num

ber

of tr

ees

infreq firefreq firevar. reten.

Regeneration events in complex stands are the result of combinations of appropriate conditions for germination and early tree growth, and presence of seed sources. Disturbances play a major role.

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5.00

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35.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

tree diameter (in)

num

ber o

f tre

es

ponderosa pine with 4 cohorts

Regeneration events in complex stands are the result of combinations of appropriate conditions for germination and early tree growth, and presence of seed sources. Disturbances play a major role.

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5.00

10.00

15.00

20.00

25.00

30.00

35.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

tree diameter (in)

num

ber o

f tre

es

ponderosa pine with 4 cohorts

Central European model for selection silviculture

(Schutz 2006)

Basal area – Multiaged stands

Year

1900 1920 1940 1960 1980 2000

Bas

al A

rea

(m2 /h

a)

0

10

20

30

40

50

60

70

(O’Hara et al. 2007)

Shannon H (size diversity) – Multiaged stands

Year

1900 1920 1940 1960 1980 2000

Sha

nnon

Siz

e D

iver

sity

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(O’Hara et al. 2007)

Shannon H diversity – even-aged

Year

1900 1920 1940 1960 1980 2000

Sha

nnon

Siz

e D

iver

sity

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(O’Hara et al. 2007)

Shannon species diversity (H) for basal area –Multiaged stands

Year

1900 1920 1940 1960 1980 2000

Sha

nnon

Div

ersi

ty -

Basa

l Are

a

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(O’Hara et al. 2007)

European model

• Increasing stocking• Expanding diameter range• Increasing species diversity• For these very old, long-term research plots,

Plenter system is still evolving.

Cohorts in a Multiaged Stand

(O’Hara 1996)

Increment with Constant LAI

0

0.5

1

1.5

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2.5

3

3.5

4

1 2 3 4 5 6 7 8 9 10 11 12

Stocking Scenarios

Volu

me

Inc

(m3/

ha/y

r)Gross Increment

Recoverable Increment

(O’Hara 1996)

0

10

20

30

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50

60

cohort 4 cohort 3 cohort 2 cohort 1

youngest ------ oldest

tree

s/ha

TOTAL Leaf Area Index (LAI) 6Cohort 1 Cohort 2 Cohort 3 Cohort 4 TOTAL

Number of Trees/Cohort/Hectare 50 12 25 40 127Percent of LAI/Cohort 90 7 2 1 100

Cohort 1 Cohort 2 Cohort 3 Cohort 4 TOTALLeaf Area Index/Cohort ECC 5.4 0.4 0.1 0.1 6.0Leaf Area Index/Cohort BCC 1.8 0.1 0.0 1.8Leaf Area/Tree (m̂ 2) ECC 1000.0 350.0 48.0 15.0BA/Cohort (m̂ 2/ha) ECC 26.9 2.1 0.4 0.1 29.5BA/Cohort (m̂ 2/ac) BCC 8.6 0.2 0.1 8.9Avg. Vol. Increment/Tree (m̂ 3/yr) ECC 0.13 0.02 0.00 0.00Avg. Vol. Increment/CC (m̂ 3/ha/yr) 3.9 0.2 0.1 0.0 4.2Quadratic Mean DBH/Cohort (cm) ECC 73.4 41.5 12.7 5.7Tree Vigor (cm̂ 3/m̂ 2/yr) 99.410 76.925 68.914 48.700Stand Density Index ECC 280.0 27.0 8.4 3.8 319.3Stand Density Index BCC 112.5 4.0 2.4 118.9

Ponderosa pine MASAM - MONTANAUSER-SPECIFIED VARIABLES

DIAGNOSTIC INFORMATION

(O’Hara et al. 2003)

Uneven-aged – What’s wrong

• Obsession with maintaining precise diameter distributions

• Not “natural” as practiced• Notion of the “balanced stand” is arbitrary• “Baggage” of uneven-aged• Traditional means of managing uneven-aged

stands are therefore not appropriate for old forests

Multiaged – What’s Right• Many sustainable options: single-tree,

variable retention, groups• Sustainable old forest structures can take

many forms.• This is a tremendous advantage because it

provides flexibility for land managers.• Complexity in stand structure with treatments

that are not all the same and may be simpler to design and implement

• Multiaged stands have lots to offer: continuous cover, carbon storage, more natural for some situations