Oak bark allometry and fire survival strategies in the Chihuahuan desert Sky Islands, Texas, USA.

Dylan W Schwilk,Helen M Poulos,Maria S Gaetani

doi:10.1371/journal.pone.0079285

Abstract

Trees may survive fire through persistence of above or below ground structures. Investment in bark aids in above-ground survival while investment in carbohydrate storage aids in recovery through resprouting and is especially important following above-ground tissue loss. We investigated bark allocation and carbohydrate investment in eight common oak (Quercus) species of Sky Island mountain ranges in west Texas. We hypothesized that relative investment in bark and carbohydrates changes with tree age and with fire regime: We predicted delayed investment in bark (positive allometry) and early investment in carbohydrates (negative allometry) under lower frequency, high severity fire regimes found in wetter microclimates. Common oaks of the Texas Trans-Pecos region (Quercus emoryi, Q. gambelii, Q. gravesii, Q. grisea, Q. hypoleucoides, Q. muehlenbergii, and Q. pungens) were sampled in three mountain ranges with historically mixed fire regimes: the Chisos Mountains, the Davis Mountains and the Guadalupe Mountains. Bark thickness was measured on individuals representing the full span of sizes found. Carbohydrate concentration in taproots was measured after initial leaf flush. Bark thickness was compared to bole diameter and allometries were analyzed using major axis regression on log-transformed measurements. We found that bark allocation strategies varied among species that can co-occur but have different habitat preferences. Investment patterns in bark were related to soil moisture preference and drought tolerance and, by proxy, to expected fire regime. Dry site species had shallower allometries with allometric coefficients ranging from less than one (negative allometry) to near one (isometric investment). Wet site species, on the other hand, had larger allometric coefficients, indicating delayed investment to defense. Contrary to our expectation, 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 in carbohydrate concentration.

Highlights

Fire has been a powerful disturbance on the global landscape for hundreds of thousands of years, promoting traits in plants which confer an advantage in the presence of fire [1,2]
Understanding variation in fire response strategies both across and within fire regimes is a major goal of plant fire ecology [1,3,4,5]
Thick bark and tall height that keeps leaves and young tissue away from the flames of a surface fire are the main traits associated with above ground survival [15,16], while basal or below ground resprouting can allow individual survival even when above ground tissue is killed

Summary

Introduction

Fire has been a powerful disturbance on the global landscape for hundreds of thousands of years, promoting traits in plants which confer an advantage in the presence of fire [1,2]. Many previous studies have focused on this dichotomy by contrasting non sprouting fire recruiters with basal resprouters [6,7,8] or with fire-surviving species that rely on height and/or bark to avoid tissue damage [9,10]. Plants can survive fire by protecting above ground tissue from heat damage or by relying on basal or below ground resprouting after above ground tissue is killed [3,14]. Thick bark and tall height that keeps leaves and young tissue away from the flames of a surface fire are the main traits associated with above ground survival [15,16], while basal or below ground resprouting can allow individual survival even when above ground tissue is killed. Even plants that lose all above ground tissue during a fire and resprout basally may have an advantage over the seedlings of non-sprouting post-fire recruiters because the resprout has access to stored carbon and a more extensive root system than the seedling

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