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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.

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Presentation on theme: "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."— Presentation transcript:

1 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

2 Coast Live OakEngelmann Oak Quercus agrifoliaQuercus engelmannii

3 Typical Oak Woodland

4 Distribution of Mixed Oak Woodlands on Camp Pendleton

5 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.

6 Seedling Bank

7 Growth Rate in Gaps

8 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.

9 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.

10 Episodic Escape 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. but occasional recruitment to the canopy offsets decline.

11 dead oak ring of recruits Episodic Escape Model

12 Canopy Change Analysis Oak Canopy 1928

13 Canopy Change Analysis Oak Canopy 2000

14 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 High fire frequency did not prevent this increase. The largest increase occurred in the sparsest woodlands.

15 Was this canopy increase a result of increasing size of individuals or recruitment of new individuals? Did the increase involve one or both species?

16 Methods Sampled oaks along belt transects saplings and adults in 15m belt seedlings (<50 cm ht) in 1m belt Resamples 1997 and stands Initial sample stands Transects oriented along the primary environmental gradient. Recorded stem diameter at base, dbh, height and canopy location

17 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%.

18 Stand Structure Change 1987 to 2011 (n=5)

19 Stand Structure Change 1987 to 2011 low elevation (n=3) & high elevation (n=2) mortality = 11.8% mortality=11.5% 1987 –

20 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

21 Conclusions cont. Demographic differences between high and low elevation sites may be useful in anticipating the effects of climate change.


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