Background
Out of sight, out of mind may apply to many aspects of our daily life; but when it comes to the daily life of trees, what none of us sees is of paramount importance. And that is why a group of scientists associated with the USDA's Agricultural Research Service (two of them now retired) looked closely at the root growth responses of a group of sour orange trees to a 300-ppm (~75%) increase in the atmosphere's CO2 concentration over a period of seventeen years, as they grew from young seedlings to fully-mature heavily-fruit-bearing trees.

What was done
Eight sour orange (Citrus aurantium L.) seedlings were planted directly into the ground (an Avondale loam) at Phoenix, Arizona (USA) in July of 1987, where they grew within four identically-vented, open-top, clear-plastic-wall chambers that were constructed around pairs of trees, as described by Idso and Kimball (1991). Through two of these chambers (from their bottoms to their tops), normal ambient air was continuously pumped both day and night, while through the other two chambers a continuous flow of air enriched with an extra 300 ppm of CO2 was similarly pumped. Throughout the 17 years of this experiment, the trees were fertilized and flood-irrigated according to standard commercial orchard practices, so as to maintain ample nutrients and soil moisture, as described by Idso and Kimball (2001). Then, at the end of the seventeen years, 105-cm-long soil cores were extracted from each chamber, using the methods described by Prior and Rogers (1992), at distances of 0.5, 1.0, 1.5 and 2.0 meters along lines emanating from each tree in the four cardinal directions. These cores were subsequently divided into 15-cm segments (0-15, 15-30, 30-45, 45-60, 60-75, 75-90, 90-105 cm), after which root dry weight and length densities were evaluated for each of these soil column intervals.

What was learned
Prior et al. discovered that, overall, elevated CO2 increased root length density by 35.6% and root dry weight density by 39.1%. In the two uppermost soil layers (0-15 cm and 15-30 cm), however, where the sour orange trees had their greatest root presence, they report that the 300-ppm increase in the air's CO2 concentration increased fine root length density by 64.5% and 57.2%, respectively, while corresponding increases in root dry weight were 80.3% and 82.8%.

What it means
In the words of the five researchers, "these large root responses, along with the large increases in fruit and wood production noted by Kimball et al. (2007), indicate that long-term citrus productivity can be enhanced as atmospheric CO2 continues to rise, particularly when trees are grown under conditions of water and nutrient supply typical of orchard conditions."

References
Idso, S.B. and Kimball, B.A. 1991. Effects of two and a half years of atmospheric CO2 enrichment on the root density distribution of three-year-old sour orange trees. Agricultural and Forest Meteorology 55: 345-349.

Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.

Prior, S.A. and Rogers, H.H. 1992. A portable soil coring system that minimizes plot disturbance. Agronomy Journal 84: 1073-1077.