Tradeoffs in fire survival traits of oaks in the Trans-Pecos Mountains

Background/Question/Methods

Trees may survive fire through persistence of above or below ground structures; investment in bark aids in the first while investment in carbohydrate storage aids in the second. Investment in defense structures comes at a cost, necessitating tradeoffs.  We hypothesize that relative investment in bark or carbohydrates changes with tree age and with fire regime: we expect delayed investment in bark and early investment in carbohydrates under low frequency, high severity fire regimes. Common oaks of the Trans-Pecos (Quercus emoryi, Q. gambelii, Q. gravesii, Q. grisea, Q. hypoleucoides, Q. muehlenbergii, Q. pungens) were sampled at three sites with historically mixed fire regimes: the Chisos Mountains, the Davis Mountains and the Guadalupe Mountains. Bark thickness was measured on 30 individuals per species per range following Adams and Jackson's (1995) contour method, involving tracing, coring and digitizing cross sections. Carbohydrate concentration in taproots was measured after initial leaf flush and before secondary shoot development. Investment in each trait was standardized by bole diameter  and analyzed using major axis regression.

Results/Conclusions

Five species showed little variation in investment through ontogeny:  Q. emoryi, Q. gambelii, Q. gravesii, Q. muehlenbergii, Q. pungens. Two species showed opposite and contrasting patterns of investment. Q. grisea invests early in bark with an estimated bark depth of 3.7mm at 5cm diameter and reduces investment through time (allometric slope, alpha=0.75). In contrast, Q. hypoleucoides has thin bark in young stems (1.4mm at 5cm diameter); it accelerates investment over time (alpha=1.25) resulting in among the thickest bark when mature. Within a pair wise comparison the rates of investment of Q. hypoleucoides and Q. grisea were significantly different (p<0.05), representing alternate strategies in fire survival. Root carbohydrate concentrations were similar across all species and sizes, suggesting that any differences in below ground storage are likely to be in total volume of storage tissue rather than carbohydrate concentration.  Investment patterns in above ground survival matched the predicted fire regime of Q. hypoleucoides; it prefers wetter  sites and therefore experiences longer fire return intervals but potentially more intense fire. Q. grisea grows in dry sites, which probably experience more frequent low severity fires. Through expanding our understanding of fire survival traits we are better equipped to determine species success under novel disturbance regimes.