Influence of individual oak (Quercus robur) trees on saturated hydraulic conductivity
Introduction
It is generally accepted that the saturated hydraulic conductivity (Ks or coefficient of permeability: Bear, 1972) of forest soils is higher than that of soils supporting other vegetation types (Pritchett and Fisher, 1987, McCulloch and Robinson, 1993); however, a search of the scientific literature reveals that there are surprisingly few studies testing this hypothesis. While most studies support the idea that trees enhance Ks (Table 1), the results of some suggest that this is not universally true. For example, Jaiyeoba (2001), investigating the effect of reforestation on degraded land in Nigeria, found that a significant increase in Ks was observed under only one of the six plantations studied. Yeates and Boag (1995) reported higher soil permeability in pasture compared with the Pinus radiata agro-forestry systems in their study. Likewise, the results of a study by Giertz et al. (2005) showed that within undisturbed areas, savannah soils had higher mean Ks than those under natural forests. Furthermore, a localised reduction in Ks beneath Sitka spruce (Picea sitchensis) was reported by Chappell et al. (1996), with a factor of 5.4 reduction in Ks directly beneath individual conifers compared with soil 2 m away from each tree. Clearly, there is a need for further research to advance our understanding of how trees affect Ks, including consideration of the possible mechanisms by which individual trees influence soils.
The concept of ‘single-tree influence circles’, first introduced by Zinke (1962), is well established in studies of soil properties such as exchangeable cations and pH (e.g., Hornung, 1985, Boettcher and Kalisz, 1990, Amiotti et al., 2000, Graham et al., 2004). The influence of single trees has also been investigated for soil moisture depletion (Ziemer, 1968), rainfall interception (David et al., 2006) and earthworm abundance (Boettcher and Kalisz, 1991). The work reported here seeks to determine the influence of individual trees on the Ks of the A horizon of a Dystric Gleysol, with a focus on isolated oak trees. The benefits of studying isolated, individual trees are 3-fold. First, comparison of the Ks of a grassland topsoil beneath individual trees can be compared with that in open land in the immediate vicinity, thus reducing the likelihood of catena-related variations in soil type (Chappell and Ternan, 1992) affecting the results. Secondly, the study of isolated trees allows us to investigate the sphere of influence of individual trees on Ks. Finally, quantification of Ks at different distances from tree trunks may help indicate those factors which regulate the influence of trees on Ks, such as fine root density (see Rasse et al., 2000), soil compaction from tree weight and movement (Chappell et al., 1996), organic matter inputs (Lehmann et al., 2001) and chemical inputs from litter-fall, root decay or leaching from the canopy or roots (Rhoades, 1996).
The trees chosen for study were English oaks (Quercus robur), also known as pedunculate or common oaks, growing in a parkland setting. Oaks are considered to be the most important tree in broadleaf forestry within Great Britain, comprising 30% of the total broadleaved forest area (Hibberd, 1991). All of the individuals selected in this study were isolated trees planted in former hedgerows or as the parkland was established in 1899. Indeed, Great Britain has more trees in hedgerows and parkland (>1.8 million; Barr et al., 2001) than in woodlands or forests (Rackham, 2003). In 1998, there were 655,000 oak (Quercus spp.) trees in hedgerows in Great Britain (Barr et al., 2001).
The specific objectives of this study are to:
- 1.
Quantify the statistical and spatial variability of Ks at 0.10–0.25 m depth in a Dystric Gleysol under grassland around six isolated oak trees and at more than 20 m away from these trees.
- 2.
Review the literature pertinent to the possible effects of trees on Ks, to allow tentative explanation of the observed spatial variability.
Section snippets
Research site and methods
Field measurements were undertaken within a 10 ha area of parkland (54°00′50′′N, 2°47′22′′E), now part of the campus of Lancaster University, Lancashire, UK. Prior to 1899, this area was divided into three fields of pastureland. The six isolated, mature oak trees within this area were selected for investigation, together with nineteen sampling sites all in excess of 20 m away from any isolated trees or woodland (Fig. 1a). The girth, diameter breast height (d b h), crown radius and approximate
Statistical distribution of Ks
The asymmetry of the statistical distribution of the 119 Ks values is clear from the asymmetry of a Box-and-Whisker plot of the data (Fig. 2a; Freund and Wilson, 2002). A logarithmic (loge) transform of the data does produce a symmetrical plot (Table 2b) indicating that the centroid of the distribution of these data is better represented by the geometric rather than arithmetic mean. Many other Ks studies (e.g., Nielsen et al., 1973, Talsma and Hallam, 1980, Bonell et al., 1983, Chappell and
Review of the positive and negative impacts of trees on Ks
Consideration of why Ks may be different close to individual oak trees planted within grassland requires a new and possibly unique review of a broad range of scientific literature. This review would suggest that there are several mechanisms that might increase Ks close to isolated oak trees, and a similar number of mechanisms that could reduce it.
Conclusions
Four conclusions arise from this study, namely:
- (i)
The research is the first to quantify the local effect of individual trees on the Ks using a borehole permeameter in a radial sampling pattern. As such, it provides a sampling strategy the future more extensive sampling that may allow generalisation across different soils and tree species.
- (ii)
The local impact of isolated English oaks (Q. robur) on the Ks of an A soil horizon beneath grassland was observable for the Gleysol tested. The local effect was
Acknowledgements
The authors would like to thank Heather McCollin, Sharon Hall and Rebecca Watts for their assistance in the field and Anne Wilkinson for her support with the laboratory analysis.
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