Fine root growth of Quercus pubescens seedlings after drought stress and fire disturbance

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Abstract

Post-fire resprouting is an important process in the Mediterranean climate regions of the world and involves considerable rearrangement of biomass allocation. We have investigated the morphological changes occurring in the fine root portion of Quercus pubescens seedlings growing in controlled conditions in which fire disturbance is superimposed on drought-stressed plants. We measured the absolute (length, number of apices) and relative (specific root length and root tissue density) morphometric traits of fine roots, and the biomass and water content of the main plant compartments (leaves, shoot, taproot and lateral fine roots). Initially, soil drying significantly increased the fine root standing mass and decreased the specific fine root length irrespective of the fire, but fine root biomass declined after a critical length of time. Fire significantly decreased the above-ground biomass and its water content notwithstanding the drought stress interruption. On the contrary, time, water supply and fire disturbance factors showed significant interaction effects for the plastic morphological traits, namely, length and number of apices. In fact, fire reduced and postponed the peak of root growth in terms of the thinnest fine root fraction (0.0–0.5 mm diameter) and number of apices. These findings indicate the advantages of shedding over maintaining the roots under a condition of severe drought. Indeed, shedding makes the overall reduction of the root system more functional, and induces a partial increase in water particularly in the thicker fraction of the fine roots (0.5–2.0 mm). Shoot removal by fire seems to lessen and prolong the acclimation process to drought, but the decrease in non structural carbohydrate reserves appears to impede the recovery process at least after persistent drought.

Highlights

• Drought significantly increased the fine root mass independently from fire. • In burnt plants, drought stimulated root growth without an equivalent tissue density increase. • Fire postponed the growth peak for the 0.0–0.5 mm diameter fine root fraction. • In the short term, fire may lessen and prolong the acclimation process to drought. • In the long term, the NSC reserves decrease strongly affects the recovery process.

Introduction

Wildfires are strong unpredictable disturbance events that cause the partial or complete loss of the above-ground plant biomass. They are frequent in Mediterranean climate regions of the world, and plants have evolved a range of adaptations that enable them to survive fire (Keeley, 1991). At species scale, plant responses to disturbances have typically been classified dichotomously, separating species that survive and persist vegetatively (resprouters) from those that are killed and regenerate from seeds (seeders or non-sprouters) (Wells, 1968, Midgley, 1996). At community scale, the high resilience of Mediterranean plant communities after fire can be explained by the ability of plant species to recover by means of resistant structures, namely the bud bank (Malanson and Trabaud, 1988, Hodgkinson, 1998) and germination of fire-protected seeds stored in the soil or in the canopy bank (Noble and Slatyer, 1980, Lloret, 1998).

Growth resumption of resprouters appears to depend on a supply of stored non-structural carbohydrates (NSC) present both in stem-derived organs such as lignotubers, rhizomes, stolons, and in roots (Jones and Laude, 1960, Donart and Cook, 1970, White, 1973, Kays and Canham, 1991, Bowen and Pate, 1993, Van der Heyden and Stock, 1996, Canadell and Zedler, 1995, Canadell and Lopez-Soria, 1998). Resprouters allocate more biomass and NSC in their roots than non-root-sprouting congeners (Pate et al., 1990, Bowen and Pate, 1993, Langley et al., 2002, Palacio et al., 2007). Water shortage strongly affects the Mediterranean vegetation during summer, thereby disturbing plants that are already drought stressed (Trabaud, 1987). Therefore, it is the response of fine roots to the combined effect of increasing soil dehydration and fire disturbance rather than the two conditions separately that is crucial for plant survival.

We investigated the combined effect of drought and fire at root system level in the resprouting process of Mediterranean species seedlings of Quercus pubescens, Quercus ilex and Fraxinus ornus (Chiatante et al., 2005, Chiatante et al., 2006). This study shows there is considerable re-arrangement of fine roots after the onset of soil drying in all the species studied, whereas our results concerning fine root growth rate in relation to fire disturbance are inconsistent. Stored NSC in Quercus congenera roots greatly affects damaged shoot recovery, and it was observed to decrease after resprouting (Kabeya and Sakai, 2005). Thus, assuming that root NSC availability decreases after fire disturbance, we expect root growth rate to decrease particularly during the period following the fire event.

Our general objective was to determine if, in controlled conditions, measurement of absolute morphometric traits (length and number of apices), relative traits (specific root length [SRL], root tissue density [RTD]), and biomass allocation in Q. pubescens seedlings confirms our previous findings of the bell-shaped fine root growth-response curve after the onset of soil drying, obtained under semi-natural condition (nursery) and the same water/fire treatment combinations. Our specific objective was to characterize the occurrence of different fine root growth rate, if any, between the unburnt and burnt drought-stressed plants.

Section snippets

Plant material

One-year-old dormant Q. pubescens Willd. seedlings (seeds from a local provenance, Central Apennine Ranges) of very narrow range of sizes (top bud 15–20 cm high) were transplanted in 5000 cm3 pots filled with a 2:1:1 mixture of clayey loam soil:moist peat:vermiculite respectively. The plants were grown in a growth chamber under 25/18 °C day/night temperature, 300 μmol m−2 s−1 PPFD, 16 h photoperiod and 45/70% relative humidity (RH) throughout the experiment. A typical watering regime for all potted

Results

Soil water content 17 days after the onset of soil-drying treatment was 22% lower than control (27.2%) (Table 1), but there were no differences in the traits examined between treated and control plants (Fig. 2). Thirty-seven days after the onset of soil-drying, the soil water content decreased by a further 15% in plants not exposed to fire, and by only 2% in fire-exposed plants. At the end of the experiment, day 64, the water content of the soil decreased by 61% and 50% versus initial water

Discussion

Drought stress after soil drying induced relevant changes in root system architecture. The short harvest intervals we used revealed changes in the fine root portion and SRL of fire-exposed plants that eluded detection in previous studies. Moreover, the constant atmospheric conditions in growth chamber did not directly affect shoot physiology, therefore the changes we identified in root morphology were strictly related to increasing soil drying rather than to inputs from the shoot. As general

Acknowledgements

The authors thank Solena Sciandra for technical assistance in the laboratory. We are also grateful to Jean Ann Gilder (Scientific Communication SRL) for revising and editing the text. Some of the results reported herein were obtained when working on the European project “Eco-slopes” (QLK5-2001-00289). The authors also acknowledge the Italian MIUR (Italian Ministry for University and Research) and the European project “COST Action E38”.

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