Fine root dynamics of pedunculate oak and narrow-leaved ash in a mixed-hardwood plantation in clay soils

The minirhizotron technique was used to study the temporal dynamics of fine roots (diameter ≤2 mm) over a twelve-month period in pedunculate oak (Quercus robur L.) and narrow-leaved ash (Fraxinus oxyphylla Bieb.) trees growing in clay soil. The experiment was conducted in an eight-year-old mixed hardwood plantation established in northern Italy’s Po lowlands on former farm land. Minirhizotron tubes were placed in mono-specific patches of oak and of ash and monitored from May 2000 to May 2001. Tubes were observed weekly from spring to fall and every two weeks during winter. The collected data for both species showed that while cumulative root length density of production (RLDp) and mortality (RLDm), as well as the RLDp’s vertical and diameter distribution, did not differ statistically between them (RLDp was 7.6 ± 0.89 and 7.31 ± 0.74, LDm 4.74 ± 1.10 and 2.77 ± 0.60 mm cm−2 yr−1 ± Standard Error for oak and ash, respectively), the seasonal course of root production and mortality markedly differed. The oak trees displayed a nearly steady rate of root production from early spring regrowth to mid-autumn and the ash produced most of its fine roots over a shorter period, fromMarch to late May. The rate of root mortality peaked duringwinter in oak and in ash was fairly constant throughout the experiment. The coarse fine roots (e.g., root 1.1–2.0mm in diameter) in ash were preferentially produced in June and their frequency increased with soil depth and in oak were produced simultaneously and with the same vertical distribution of the smaller roots. Root median longevity (Lm) was slightly higher for ash than for oak and for summer than for fall and winter root cohorts; that in both species Lm was negatively related with root diameter confirms that roots smaller than 1.0 mm are highly dynamic. These findings may be relevant for a better understanding of the ecology of mixed pedunculate oak and narrow-leaved ash stands in clay soil.