Growth response of a deciduous oak species to fire, loss of neighbors, and climate within a juniper-dominated woodland ecosystem

Background/Question/Methods

The declining health and poor regeneration of oaks in mixed-species forests and woodlands is a widespread and growing concern.  In our study we investigate the growth response of a deciduous oak species, Quercus buckleyii (Texas Red Oak) within a juniper-dominated woodland.  Understanding which factors positively influence oaks is important for management of mixed-species communities where oaks are a keystone species. In this study, we used tree rings, fire scar, and multi-temporal aerial photographic data to assess response of oaks to disturbance type and resultant impact on woodland patches.  Three hundred and seventy tree slabs from downed and dead red oaks were collected in the Balcones Canyonlands National Wildlife Refuge outside Austin, Texas.  We analyzed tree rings from these slabs to determine recruitment date, annual ring width, and where evident, time of fire. Changes in tree ring widths associated with canopy openings were derived from neighborhood analysis of digital aerial photos from 1939, 1951, 1964, 1980, 1995, and 2004.

Results/Conclusions

Results indicated that red oaks increased radial growth following fire.  Analysis of canopy openings associated with the aerial photographs showed that the oak species did not respond to canopy openings with increased radial growth as predicted by gap-phase dynamics.  Climate impacted average radial ring growth as demonstrated by comparison with Palmer Drought Severity Index values (p = .56).  Given that radial growth is influenced by both fire and climate, we explored the possibility that dramatic climate and related disturbance events (drought and high occurrence of fire) of the 1950’s created a possible ecological regime shift.   Changes in both mean index values and disturbance frequency were noted during the 1950’s.  These results were confirmed by landscape analysis of disturbance patches identified from the historical photographs which show cutting and burning occurred with the highest frequency between 1951 and 1964 with 7.6 % of the study area affected.  With uncertainty in future climate, understanding the range of possibilities from past events is important with the predicted possibility of more extreme El Nino/La Nina cycles.