early seral ecosystem creation and dynamics

Early Seral Ecosystem Creation and Dynamics

Thomas SpiesPNW Research Station

Topics• What is early seral?

• Disturbance effects

• Successional pathways

• Regional patterns and trends

His

ES

High Severity Fire

Mixed-Severity Fire

A Definition

• An stage in forest succession that is the immediate product of a disturbance that removes most of the canopy cover– Less than 20% canopy cover remaining

– Large enough patch for light-demanding vegetation to establish

– Contains biological legacies from previous forest

– Short lived, depending site productivity

Components of Early Seral

• Loss of tree canopy • And reduction or loss one

or more of following:– Standing trees

– Understory vegetation

– Forest floor litter

– Large woody debris

– Soil organic matter

– Tree roots

StructuralElement

Wind Fire Landslide Clear cut/Burn

Canopy foliage X X X XStanding Trees X x X XLWD X X/xUnderstoryvegetation X X XForest Floor X X XOM Soil x X xRoots x

Disturbances that create early seral ecosystemsvary in their effects on ecosystem structure

0 20

Low

Mod

High

Tree Canopy Closure (%)

Dea

d an

d L

ive

Bio

logi

cal L

egac

ies

Wildfire

Windthrow

Landslide

Early Seral Based on What Remains

Retention Systems

Blowdown, Bull Run Watershed

HOH Fire

Tillamook Fire

High Severity Wildfire

Yacolt Burn

Landslides in Coast Range

Photo by Rob Pabst

Long-lived early seral related to land-use history and site conditions

Early Seral on Intensively Managed Forest Land

Gap Disturbances

Gap

Height

Diameter/Ht Ratio = 1.0

Diameter/Ht Ratio = 0.5

Gap Size Effect Dense Conifer Forests

ES plant species begin to appearES plant species infrequent

Gap diameter/canopy ht ratio

Num

ber

of G

aps

Most natural gaps are too small for early successional conditions

Fire as a gap process in dry, fire-prone forests

Historical dynamics of fire and succession in Ponderosa Pine

R. Van Pelt

Time Zero

20 yrs

40 yrs

60 yrs

80 yrs

Fire Suppression

ES ES

ES

ESESSuccessional Pathways

ES

ES ES

ES

Early seral creation in the Biscuit Fire

Biscuit Fire Crown Damage byVegetation Cover Class

• Median crown damage was 74%– 96% for shrub/regen– 88% for hardwoods– 62% small conifers– 53% mixed small and large conifers– 32% large conifers (>20 inches dbh)

Thompson and Spies In Press

(ES)

Historical Range of VariabilityOregon Coast Range

% of Landscape

Early Seral: 10-25%

Old Growth: 35-60%

% of Landscape

Pro

babi

lity

3.1 % (incl BB and DL)

3.6 %

3 %9.5 %

2.3 %

0.4 %

0 %

0 %

0 %

Wildfire Conversion of Old Forest to Early Successional1993-2003

Percent Loss of Older Forest on a Decadal Basis By Province

1.4 %

0 %

Based on Moeur and Spies et al. 2005

Conclusions

• All ES Share the same element: Loss of tree canopy cover to low levels (e.g. <20%)

• Disturbances produce different kinds of ES vegetation

• A transitory stage that lasts 15 ~ 30 years depending on site productivity and vegetation type

Conclusions

• Gaps have limited capacity to create ES in wetter forests

• Small disturbance patches are more effective ES in dry forest types than in wetter types

• ES can arise at any stage in succession

Conclusions

• Rate of ES creation has varied over time

• Amount of diverse ES is declining in wetter forests across all ownerships

• ES created by intensive forest management is not the same as that created by natural disturbances