Fire resistance and bark properties of trees in a seasonally dry forest in eastern Bolivia

As forest fragmentation and intentional burning of grasslands increase, the frequency of fires penetrating the dry and subhumid tropical forests of Bolivia is also likely to increase. To expand our understanding of the role of fire in tropical dry forest, the physical and thermal properties of barks of tree species were studied to determine their relative resistances to cambial damage by fire. For 16 tree species found in the dry forest of the Lomerio region of eastern Bolivia, bark thickness, moisture content, and specific gravity were measured. Insulating capabilities of bark were measured by obtaining cambial and surface temperatures during experimental wick fires. Bark thickness on trees 5-100 cm dbh (diameter at 1.4 m) ranged from 2-51 mm and both thick- and thin-barked species were represented. For all species, bark thickness increased as stem diameter increased. Bark thickness explained more (63%) of the variation in peak cambial temperatures during fires than did bark moisture content (4%) or specific gravity (1%). A threshold bark thickness of 18 mm was associated with the ability to withstand lethal cambial temperatures during the experimental, low intensity fires. For 13 of the 16 species included in this study, trees ≤20 cm dbh have bark thickness below the threshold 18 mm and, therefore, are likely to experience cambial injury from low intensity fires. Our results suggest that the forest presently characteristic of the Lomerio region did not develop with frequent fires and that species composition is likely to be substantially affected by an increase in fire frequency.