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Measurements of root-length density (RLD) in a range of 31 apple, kiwifruit, peach, Asian pear and grape orchards were used to derive indices to describe the exploration and exploitation of rooting volumes. Orchards were of various ages and located on a range of soil types, geographic regions, management systems etc. Data were obtained from core samples of volume 1.66x 10 -4 m 3 randomly taken within a "standard volume", determined by average planting grids, of 2 m radius centred on tree stems, and 1 m depth. Root systems were described using an exploitation index, E(~b), and an exploration index, E(O). E(4) is defined as the proportion of the soil volume which contains roots at RLD greater than or equal to some specified value, q~. E(O) is defined as the proportion of the soil volume which contains roots at any RLD greater than zero. These indices are dependent on sample size, as are all volumetric or soil-coring data.Estimates of E(0) for each orchard were obtained as the proportions of cores containing any RLD>O and assessed for dependence on species. Peach trees had a significantly higher value of E(O), equal to almost 1.0, compared to the other four species where E(O) was approximately 0.8 (p<0.01) or less. There was also some variation with age. E(O) was lower for very young plants which had not fully occupied the sampled soil volume. Exploration indices for woody roots increased with rootstock age but otherwise did not explain large differences in E(~) between species for given ~b values.For example at ~b = 0.05x 104 m.m -3, E(~b) was approximately 0.45 for peach and kiwifruit, and 0.05 for apple, Asian pear and grape, whereas at ~ = 0.5× 10 4 m.m -3 the corresponding values were 0. I and almost zero. Negative exponential curves relating E(~5), scaled by dividing by E(0), to 6 were fitted for each of the 31 orchards. Exponents for these curves, k, were significantly smaller for kiwifruit and peaches than apples, grapes and Asian pears (p<0.05), and smaller for apples than grapes and Asian pears (p_<0.05). A larger k implies a rapid fall-off in E(~b) as ~ increases. Although all five species contained zero and low RLD samples, only kiwifruit and peaches contained higher RLD values and consequently have higher mean RLD. This trend was consistent across all soils, regions, sampling dates, and plant ages.The analyses demonstrate that core sampling can give useful insights into macro-scale root-system distribution, such as the proportion of a soil volume explored and how it is exploited. If positions of core samples are noted during sampling using angular direction, depth and radial distance as spatial coordinates the method can be used to describe root-system structures.

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The root systems of apple trees from five orchards ranging in age from 1.5-y to 14-y were sampled to depths of between one and two metres using soil cores. Although trees came from orchards which differed in soil-type, tree spacings and management, consistent patterns were found in root systems. In orchards of 4-y and older, roots of adjacent trees met so that soil volumes within the planting grids (i.e, tree spacings of approximately 5 m inter-row x 4 m intra-row distances) were completely explored, although not completely occupied by roots. Mean root-length densities declined with depth for these orchards. In the 1.5-y orchard, roots from adjacent trees did not meet and root-length densities declined with radial distance from the stern as well as with depth.Root-length densities in the top i m ranged from zero to about 1.0 cm.cm -3 in all orchards and were highly variable. The proportions of core samples having zero values for root-length density were used to subdivide the root zone into volumes in which all samples contained roots, and volumes in which some samples had no roots.Results suggest that roots in an 'average' tree penetrate to at least one metre depth in all but very young orchards so that soil in this volume is fully explored. Volumes filled by roots and volumes occupied at any particular root-length density appear to reach a maximum at about 4 years. Volumes of soil occupied at any particular root-length density were equal in all orchards older than 4 years. This suggests that root growth was balanced by root death. In contrast woody roots continue to accumulate with time.