Woody Plant Encroachment and Groundwater Recharge

In a review of all large FACE studies conducted over the prior 15 years, Ainsworth and Long (2005) reported that the greatest CO₂-induced benefits were accrued by trees, which experienced a mean biomass increase of 42% in response to a 300-ppm increase in the atmosphere's CO₂ concentration. In comparison, they found that C₄ sorghum posted a yield increase of only 7%, while the C₃ crops rice and wheat exhibited yield increases of 16% and 22%, respectively. Thus, it is only natural to presume that as the air's CO₂ content continues to climb ever higher, earth's woody plants will gradually encroach upon areas where herbaceous plants previously ruled the landscape; and, in fact, such is typically observed to be the case throughout the entire world. However, as noted by Wilcox and Huang, trees typically use deeper water than grasses; and, consequently, they say the "prevailing belief is that woody plant encroachment leads to declining groundwater recharge and, therefore, to lower groundwater contributions to streams," which would typically be an undesirable outcome. But is this belief correct?

In a study they designed to explore their newly-posed question, the Texas A & M University researchers analyzed the long-term (85-year) trends of both baseflow (groundwater-derived) and stormflow (precipitation-derived) streamflow components of four major rivers in the Edwards Plateau region of Texas (USA) -- the Nueces, Frio, Guadalupe and Llano Rivers -- over which time period the region experienced a significant increase in the presence of woody plants.

Results indicated that "contrary to widespread perceptions," streamflows in their study region "have not been declining." In fact, they write that "the contribution of baseflow has doubled -- even though woody cover has expanded and rainfall amounts have remained constant."

Noting that their findings "run counter to current thinking in both lay and scientific circles," the Texas researchers speculate that "baseflows are higher now than in pre-settlement times, because rooting by trees has facilitated groundwater recharge." In addition, the transpiration-reducing effect of atmospheric CO₂ enrichment may also have played a role in this regard, as has been suggested by several prior studies of river basin hydrology (Idso and Brazel, 1984; Gedney et al., 2006; Betts et al., 2007). In any event, and whatever the answer or answers may be, it would appear that good things have been happening to degraded grasslands throughout the world, as the atmosphere's CO₂ concentration has been gradually rising and woody plants have been extending their ranges and growing where they had previously been unable to survive, while at the same time helping to make more water available for many other uses ... by man and nature alike.

Additional References
Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO₂ enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO₂. New Phytologist 165: 351-372.

Betts, R.A., Boucher, O., Collins, M., Cox, P.M., Falloon, P.D., Gedney, N., Hemming, D.L., Huntingford, C., Jones, C.D., Sexton, D.M.H. and Webb, M.J. 2007. Projected increase in continental runoff due to plant responses to increasing carbon dioxide. Nature 448: 1037-1041.

Gedney, N., Cox, P.M., Betts, R.A., Boucher, O., Huntingford, C. and Stott, P.A. 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.

Idso, S.B. and Brazel, A.J. 1984. Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature 312: 51-53.