Ask Mr. Smarty Plants

The  US Forest Service describes three growth forms—single-stemmed trees, a bush or tree with multiple stems, and a prostrate low-growing bush.  The single-stemmed trees can reach 20 to 40 feet and the largest trees grow where there is a reliable water source such as a streamside or in a floodplain.  In fact, the environmental conditions (water availability, soil, crowding) affect how rapidly the plant grows and size doesn't necessarily correlate with age. The US Forest Service reported 27 year-old plants within a fenced area in north-central Texas that ranged from 0.7 feet to 4.9 feet.

Mr. Smarty Plants suspects that you are interested in mesquite as a crop and are looking for optimum conditions to produce fast growth. We can find some answers to your question in an article by Nancy L. Patch and Peter Felker, Silvicultural treatments for sapling mesquite (Prosopis glandulosa var. glandulosa) to optimize timber production and minimize seedling encroachment, Forest Ecology and Management, Volume 96, Issue 3, 1 September 1997, pages 231-240. Their study involved establishing 6 treatment plots of mesquite trees of similar size and designed to have 4 rows of 4 trees each spaced 10 meters apart.  The initial size of the mesquites in the area ranged from 3.32 cm (1.3 in) to 3.73 cm (1.47 in) diameter. They estimated that these trees were less than 10 years old.  Here are the different treatments:

1.  Control — no manipulation in the plots.

2.  Harvest — strip harvested (all woody growth removed) between the 16 trees in the plot using a biomass harvester.

3. Harvest + herbicide — strip harvested between the 16 trees (as above) followed by herbicide application. 

4.  Harvest + prune — strip harvested (as above) and followed by pruning the 16 trees to a single stem.

5.  Harvest + prune + plow — strip harvested (as above), followed by pruning the trees to a single stem, and  plowing (disking) the area between the trees to a depth of 40 cm (15.7 in).

6.  Harvest + prune + plow + rye — strip harvested (as above), followed by pruning the trees to a single stem, followed by plowing (disking) the area between the trees to a depth of 40 cm (15.7 in), plus seeding with rye grass (Secale cereale).  The rye grass was sowed to inhibit more invasive grasses and weeds from being established in the experimental plots.

Here are the results for the Control (1) and the most successful treatment, Harvest + prune + plow + rye (6).

1.  Control:  The trees in this experimental plot began at an average diameter of 3.32 cm (1.3 in) and grew in diameter at an average of 0.54 cm per year (0.21 in) so that at 9 years their diameter was approximately 8.8 cm (~3.5 in)

6.  Harvest + prune + plow + rye:  The trees in this experiment plot began at an average diameter of 3.56 cm (1.4 in) and grew in diameter at an average of 1.2 cm per year (0.47 in) so that at 9 years their diameter was approximately 14.36 cm (~5.7 in).

Here is a quote from the "Discussion" section of the Patch and Felker paper:

"The control continued to have the slowest growth, and the two treatments that received disking had the greatest diameter and basal area growth.  Other treatments besides disking that decreased competition, i.e., elimination of mesquite in interstitial areas with herbicides also stimulated growth of the crop trees.  The continued significance in treatment differences over time suggests that these management practices continue to affect stand-dynamics years after initiation.  With more frequent disking and perhaps fertilization, presumably even greater growth rates could be achieved." 

And, commenting on the results of treatment 6 (Harvest + prune + plow + rye) that if the increase in diameter of 1.2 cm per year over the first nine years could be maintained that it would be possible "to achieve a 46-cm basal-diameter tree in a 30-yr rotation."  [46 cm = 18.11 in]

This study reported by Patch and Felker was carried out using only available rainfall and no irrigation.  A previous study by A. B. Duff, J. M. Meyer, C. Pollock and Peter Felker [Biomass production and diameter growth of nine half-sib families of mesquite (Prosopis glandulosa var. glandulosa) and a fast growing Prosopis alba half-sib famiy grown in Texas, Forest Ecology and Management, Volume 67, Issues 1-3, August 1994, Pages 257-266] reports:

"In 1986, a year of average annual rainfall preceded by a year of high annual rainfall, the mean basal diameter growth rate for native P. glandulosa families was 2.83 cm per year."

This would indicate that irrigation along with proper spacing and pruning of trees would positively affect the rate of growth of mesquite.