Datasets for Statistical Analysis

The fine root dataset

Keywords:

root, plants

Categories:

biology;
generalized linear models

Description:

The data concerns the underground root system of eight separate apple trees. Three root stocks are considered (Mark, MM106 and M26) with two plant spacing (4x2 meters and 5x3 meters). Soil core sampling units were taken from each plant, and classified as belonging to an inner or outer zone. The response variable is the density of fine roots (or root length density, RLD). RLD have zeros as well as continuous positive values.

There are 511 observations (193 or 38% have a zero response) on five variables.

Variables:

Plant	Plant number (1 to 8)
Rstock	Root stock: one of Mark, MM106 or M26
Spacing	Plant spacing: one of 5x3 or 4x2 (in meters)
Zone	Zone relative to the plant the soil core is taken from: Inner or Outer
RLD	Root length density (in cm/cm3)

Data Quality:

There are no missing values.

Source:

Statsci.org, provided by Nihal de Silva.

Notes:

The design is not a full factorial design:

• plants 1 and 2 are tested only with the Mark root stock and at a spacing of 5x3;
• plants 3 and 4 are tested only with Mark root stock at a spacing of 4x2;
• plants 5 and 6 are tested only with root stock MM106 at a spacing of 5x3;
• plants 7 and 8 are tested only with M26 root stock at a spacing of 4x2.

In addition, the Mark root stock is tested at both plant spacings but the MM106 only at 5x3 and M26 only at 4x2. So there are four unique treatment combinations: Mark stock at 5x3 and 4x2, MM106 at 5x3, and M26 at 4x2.

References:

• de Silva, H. N., Hall, A. J., Tustin, D. S., and Gandar, P. W. (1999). Analysis of distribution of root length density of apple trees on different dwarfing rootstocks. Annals of Botany, 83, 335-345.
• Dunn, P. K., and Smyth, G. K. (2004). Series evaluation of Tweedie exponential dispersion model densities. Statistics and Computing, to appear.