A 400 year history of canopy disturbance in pine-oak forests of the Ozark Highlands, Missouri, USA

Background/Question/Methods

It has been well established that fire historically played a primary role in the establishment and maintenance of shortleaf pine (Pinus echinata, Mill.) forests in Missouri.  Yet, other disturbances have likely contributed to the structure of these forests.  Using dendrochronology and growth release models we analyzed the canopy disturbance history of shortleaf pine-oak forests in the Missouri Ozark Highlands encompassing the years 1588-2008.  The objectives of this study were:  1) identify growth release events using living and remnant shortleaf pine and oak; 2) analyze the temporal aspects of canopy disturbances; 3) determine effects of changes in the disturbance regimes.

Increment cores and remnant wood were collected from two sites (Mill Hollow (MH):  37^o24’N, 91^o26’W; Randolph Tract (RT):  37^o25’N, 91^o38’W) in which 106 and 141 samples were collected, respectively.  Ring-widths were measured and standard cross-dating procedures were used for all samples.  Growth release models were used to identify periods of canopy disturbance in shortleaf pine and oak.

Results/Conclusions

A total of 133 release events occurred in MH (1670-2008) and 54 shortleaf pine releases occurred in RT (1765-2008).  Growth release analysis suggests periods of synchronous release events within sites (MH:  1770’s, 1800’s, 1840’s, 1980’s; RT:  1900’s, 1980’s).  The first release of trees based on mean diameter suggests trees were in an understory position (MH:  shortleaf pine = 11.1cm, SD = ±5.9cm; white oak = 8.82cm, SD = ±8.6cm; RT:  shortleaf pine = 10.08cm, SD = ±8.2cm).  There appears variation in mean release interval (MH = 45.2 years; RT = 34.5 years).  Correlations between release date and PDSI suggest drought conditions existed just prior to growth releases.  MH shortleaf pine had a confined establishment (1690-1720); and based on our sampling, there was no establishment between 1720 and 1890.  However, shortleaf pine at RT revealed a wider establishment period in which at least one pine established in 15 of 25 decades beginning in 1765.

Current management of these two pine-oak sites includes single-tree selection.  However, historical data indicates differences in the frequency of canopy disturbance.  Differences in establishment may be attributable to the frequency and magnitude of multiple disturbances (fire, canopy) contributing to the structure of the forests.  Despite the similarity between sites (south-facing slopes) and the relative proximity between sites (~10km), there appears to have been differences in the historical disturbance dynamics between these pine-oak forests.