Using historic maps and citizen science to investigate the abundance and condition of survey reference ‘blaze' trees

Large old trees often possess important biodiversity and heritage values. This study investigated the occurrence and condition of survey reference trees in a typical rural landscape in New South Wales. A community survey (citizen science) method, in conjunction with a systematic examination of historic maps, was conducted to locate reference trees for subsequent field survey of their biophysical attributes and statistical analyses. In a combined study area ~5000 km2, we found 81 old reference trees, where an estimated 1.3–2.6% of the original number remains. Tree blazes ranged from 14 to 140 years in age, where most were 120–130 years old, and predominately of the Eucalyptus species. Blaze age was strongly correlated with the thickness of bark overgrowth. Types of reference trees identified included several PM (permanent marker) trees, a cemetery marker, a trig station and 15 benchmark (BM) trees. Eleven BM trees located were related to 1928–1932 irrigation surveys and were previously undocumented. Many reference trees were found in minor road reserves or in conjunction with clumps of other large trees. Being a form of living heritage, our results show that survey trees are succumbing to the ravages of time.