What is the arrangement of a ponderosa pine root system? 



This question and the limited study of root systems of this species (Woolsey 1911; 

 Yeager 1935) prompted the writer in 1950 to investigate the root system of a 16.9-inch 

 d.b.h. , 60-year-old ponderosa pine 67 feet high. The tree was a vigorous dominant 

 specimen growing in a stand supporting a basal area of 95 square feet per acre in Boise 

 Basin Experimental Forest, Idaho City, Idaho. The exposure was easterly and had 

 about a 15 -percent slope. Availability of a good water supply and sufficienr equipment 

 enabled most of the excavating to be done by v/ashing. The coarse granitic loam soil 

 contained many large aggregates; as excavating proceeded, it became apparent that the 

 tree was beside a diorite formation. Hence, it became necessary to alternate the wash- 

 ing with hand-picking tools and to vary the pressure of the water stream to avoid injury 

 to the small roots. The excavating started in mid-July and continued intermittently 

 until September 6. 



Besides the diorite, the soil included sandy loam and gravelly loam. Most of the 

 lateral root system was growing in the sandy loam . The diorite formation was near 

 the surface at the downhill base of the tree, but also extended up the slope above it. On 

 the uphill side the coarse gravelly loam covered a gravel bed to a depth of a Little more 

 than 2 feet. 



The direction of main lateral root growth appeared to be closely related to the 

 soil type in which the roots grew. In the loose gravelly soil, which had small stones 

 throughout, the roots grew in a generally straight-line direction. Where the soil was 

 compacted and contained larger stones, the root direction changed often. Main root 

 growth in the diorite formation followed fissures and cleavage planes. The ends of 

 several main laterals were dead up to 43 percent of their length and therefore were not 

 mapped. 



All the main lateral roots were excavated, their directions determined, and their 

 diameters and depths measured. Wherever a main lateral forked, only the larger fork 

 was followed and mapped. The taproot and sinker roots were traced as far as hardness 

 of the substratum permitted- -never more than 50 inches. Secondary laterals were 

 tallied by diameter by 5-foot lengths along the main laterals. Sections of several main 

 laterals near the trunk were cut and removed after measurement to facilitate excavation 

 of roots close to or beneath them. Because of the recording and plotting system fol- 

 lowed, the delineation of the root system and the tally of the roots present a conserva- 

 tive picture. 



The author gratefully acknowledges assistance of Richard H. LeDosquet, Bureau 

 of Land Management, Fairbanks, Alaska, in the excavating. 



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