Patterns of recruitment and mortality in mixed coast live oak/ Engelmann oak

woodlands in southern California over 24 years

Dawn M. Lawson

U.S. Navy SSC Pacific

Leslie Seiger

Mesa College

We would like to thank the UC Integrated Hardwood Range Management Program

and Marine Corps Base, Camp Pendleton for funding and access for this work.

Paul H. Zedler

University of Wisconsin-Madison

Coast Live OakEngelmann Oak

Quercus agrifoliaQuercus engelmannii

Typical Oak Woodland

Distribution of Mixed Oak Woodlands on Camp Pendleton

0 10 205Kilometers

Establishment and Recruitment Lawson 1993; Lawson, Zedler and Seiger 1997

• Seedling establishment is episodic and patchy.

• Canopy location is the most important factor affecting both establishment and relative growth rate, but it does so in opposite ways.

• The seedlings and saplings of the two species do not differ in growth rate.

Seedling Bank

Growth Rate in Gaps

Establishment and Recruitment Lawson 1993; Lawson, Zedler and Seiger 1997

• Q. engelmannii has a greater ability to establish in the gaps where mortality is lower and relative growth rate higher.

• Q. agrifolia establishes more seedlings and recruits more individuals into the sapling size class.

• Seedlings and saplings may be maintained in smaller size classes in areas of high fire frequency.

• Based on the number of stems, seedlings and saplings may be most severely effected by moderately frequent fires.

Suppression and Burnout Model

High fire frequency causes juvenile oaks to be maintained in the smaller size classes.

Fire causes the continual attrition of larger trees through the “burnout” process.

The canopy slowly declines at rates that may be difficult to discern in short term studies.

Episodic Escape Model

High fire frequency causes juvenile oaks to be maintainedin the smaller size classes.

Fire causes the continual attrition of larger trees through the “burnout” process.

but occasional recruitment to the canopy offsets decline.

dead oak

ring of recruits

Episodic Escape Model

Canopy Change Analysis

Oak Canopy 1928

Canopy Change Analysis

Oak Canopy 2000

Canopy Change 1928 to 2000

• N = 9• Mean +78%

• Range 31% to 166%• stdev – 30%

• The canopy in coast live/ Engelmann oak woodlands increased increased significantly between 1928 and 2000.

• High fire frequency did not prevent this increase.

• The largest increase occurred in the sparsest woodlands.

Was this canopy increase a result of increasing size of individuals or recruitment of new individuals?

Did the increase involve one or both species?

Methods• Sampled oaks along belt transects

• saplings and adults in 15m belt • seedlings (<50 cm ht) in 1m belt

• Resamples 1997 and 2011 5 stands

• Initial sample 1987 15 stands

• Transects oriented along the primary environmental gradient.

• Recorded • stem diameter at base, dbh,

height and canopy location

Stand Characteristics

• Density ranges from 50 to 171 trees (dbh>10cm) per ha.

• Rainfall varies from 35 cm at the lower elevations to 60 cm at the higher elevations.

• Fire history ranges from 4 to 10 fires over the last 40 years.

• The stands occur in two elevational bands from 100 to 200 m and from 600 to 800 m.

• Engelmann oak composition varies from 8% to 27%.

Stand Structure Change 1987 to 2011(n=5)

Stand Structure Change 1987 to 2011low elevation (n=3) & high elevation (n=2)

mortality = 11.8% mortality=11.5% 1987 – 2011 1987 - 2011

Conclusions• The canopy in coast live/ Engelmann oak woodlands has

increased significantly in the last century.

• In the last 24 years a pulse of recruitment has moved through the stands.

• Q. agrifolia has increased in numbers but not at the expense of Q. engelmannii.

• High fire frequency did not prevent this increase.• The large seedling bank under canopies contains more

than enough oaks to compensate for mortality.

• The stand structures appear to reflect both fire history and by differences in precipitation between low elevation and high elevation sites

Conclusions cont.• Demographic differences between high and low elevation

sites may be useful in anticipating the effects of climate change.