Pothole Creek
Study Area

• Mensuration, Growth and Yield
• Canopy Modelling and
  Stem Analysis
• Regeneration and Seed Dispersal
• Coarse Woody Debris
• Light Relationships
• Fire History
• Tree Physiology and Moisture Dynamics
• Root Excavation

Related Work

Root competition has traditionally been ignored in stand-level growth models, and we would like to find out if this neglect is justified. We used hydraulic excavation to unearth the root systems of both young and mature trees growing at the Pothole Creek Research Site, and greenhouse techniques to study root anatomy. Our objective was to gain some insight into below-ground competition on dry interior Douglas-fir sites. It is hoped that these studies will lead to a better understanding of root system development, architecture, and anatomy.

In the summer of 1997, we concentrated mainly on developing methods for excavating, mapping and recording root systems. Using an 18 horse power fire-fighting pump (capable of pumping water at pressures in excess of 250 psi), we excavated an 8 m x 5 m plot to a depth of between 0.5 and 1.0 m. Roots were flagged with coloured tape to indicate the tree of origin. We then took photographs from atop a scaffold, and combined about 10 photographs to make composite images covering about 1.5 m x 2 m. These composites were then assembled into a map of the entire plot. A simple model predicting total root system biomass from tree diameter was developed (Richardson and zu Dohna 2003).

Greenhouse experiments were then conducted to study the impact of potting medium (sand, topsoil, a commercial mixture of sphagnum, dolomite, and a wetting agent called Pro-mix), and a drought treatment on both root anatomy and root topology. The following pictures illustrate the anatomical plasticity of Douglas-fir in response to below-ground conditions (Figure 1).

Drought, Pro-Mix	Well-watered, Pro-Mix

Fig. 1. Slide images showing morphological differences caused by drought treatment of Douglas-fir seedlings grown in Pro-Mix.

Extension Rates

To study root branching patterns and growth rates, we excavated two or three roots of varying size from each of four trees, and took sections at 1 m intervals along each root . We aged each section and used linear regression to estimate the annual extension rates of each root. The results were as follows (rate ± SE, in m / yr):

Table 1. Estimates of annual extension rates of interior Douglas-fir sample roots at the Pothole Creek Study Area.

Coarse root extension rates varied both between trees, and between roots on the same tree; the mean rate was found to be 7.4 cm per year (Richardson 2000).

Radial Growth

We also measured the width of root growth rings for each of the last 30 years on sections from 3 trees. Because of the eccentricity of root rings, ring widths from different sections were only modestly correlated. However, a general chronological pattern is discernable.

Fig. 2. Annual radial increment for three different roots, 1968-1998. 2-1a and 2-1b are different radial axes on the same section (Richardson 1999).

Root Biomass

In 1998, we continued our root excavation activities to compare root biomass under different forest vegetation types. We dug a series of 1-m x 1-m x 1-m pits (3 pits per cover type) under:
1. Grassy openings;
2. Clumps of mature timber (100± year old trees);
3. Clumps of regeneration (Douglas-fir approximately 30 years old, 2-4 m high).

Fig. 3. Total live and dead root biomass in 1 m³ plots under three cover types.

The biomass of smaller roots (< 2 cm diameter) was roughly the same under all three cover types (Figure 3). Since the smallest roots are most important for water and nutrient uptake, we can conclude that below-ground competition is as fierce under grassy openings as it is under clumps of regeneration or mature clumps of timber.

Larger roots (> 2 cm diameter) were more abundant under the mature clumps than either the regeneration clumps or the grassy openings. Dead roots were more abundant under both the grassy openings and the regeneration clumps than under the mature clumps. Presumably, these patterns can be explained by the high turnover of smaller roots and the harvest which took place approximately 30 years ago (Richardson et al. 2003).

Mapping

We are experimenting with different ways of recording and mapping the root systems of trees. The goal is to develop a method which is convenient and conveys as much information as possible. One possible method is illustrated below (Figure 4).

Fig. 4. Polar rhizogram for Douglas-fir root system.

The tree is plotted in the center of the graph. It is a polar chart using angles and radii to illustrate placement of roots. The rays emanating from the center have a length proportional to the log of the diameter of the root. Thus the viewer gets a visual impression of the size and spatial distribution of the roots of a given tree. Information for this sort of display can be collected quickly and easily, with a minimal amount of excavation.

References

Richardson, A. 1999. Excavation of interior Douglas-fir root systems at the Pothole Creek Research Site. Summer 1998 Work Term Report prepared for the Ministry of Forests Research Branch, 47 pp.

Richardson, A. 2000. Coarse root elongation rate estimates for interior Douglas-fir. Tree Physiol. 20:825-829.

Richardson, A.D., C. Bealle Statland, and T.G. Gregoire. 2003. Root biomass distribution under three cover types in a patchy Pseudotsuga menziesii forest in western Canada. Ann. For. Sci. 60:469-474.

Richardson, A.D. and H. zu Dohna. 2003. Predicting root biomass from branching patterns of Douglas-fir root systems. Oikos 100:96-104.
 

Contact: Andrew Richardson: andrew.richardson@unh.edu