Woody Plant Encroachment Paradox: Rivers Rebound as Degraded Grasslands Convert to Woodlands

The related phenomena of degradation and woody plant encroachment have transformed huge tracts of semiarid and subhumid rangelands. This transformation may have enormous consequences for regional water supplies, but to date few assessments have been done at scales larger than that of small catchments. Woody plant encroachment in particular is assumed to reduce groundwater recharge and, hence, baseflow to streams. For the study reported on in this paper, we analyzed the long-term (85 years) streamflow trends of four major river basins in the Edwards Plateau region of Central Texas. This region, in which springs are abundant because of the karst geology, has undergone both degradation and woody plant encroachment. We found that, contrary to common and widespread perceptions, streamflows have not been declining. In fact, the contribution of baseflow (supplied by springs and groundwater) has doubled—even though woody plant cover has expanded and rainfall amounts have remained relatively constant. We attribute this increase in springflow to a general landscape recovery that has taken place concurrent with woody plant expansion—a recovery brought about by lower grazing pressure and improved land management. Our results indicate that for drylands where the geology supports springs, it is degradation and not woody plant encroachment that leads to regional-scale declines in groundwater recharge and baseflows. Further, our results indicate that when woody plant expansion follows on the heels of degradation, it may even help reverse these declines.