Woody plant encroachment modifies the ecohydrology of a sub-tropical savanna landscape: Implications for hydrological connectivity and plant community

Shishir Basant and Bradford Wilcox, Ecology and Conservation Biology, Texas A&M University, College Station, TX

Background/Question/Methods
In dryland ecohydrology, connectivity (overland flow, runoff-runon dynamics) and function (productivity, patch distribution influenced by connectivity) have served as a useful framework to understand ecohydrology of woody plant encroached landscapes. However, this framework has rarely been applied or tested in sub-tropical woodlands which are different because, a) higher precipitation than arid regions and b) grazing intensities have reduced across these landscapes allowing for herbaceous recovery in the intercanopy areas.
The objective of this research was to address two fundamental questions using a sub-tropical woodland as a model landscape: i) Has the decline in grazing pressure altered the ecohydrological connectivity (overland flow and runoff–runon dynamics) in these landscapes? and, ii) Is the plant-water use dynamics (and hence, plant productivity) influenced by runoff-runon dynamics?
We report the findings of a three-year long study, where we measured multiple ecohydrological attributes along a catena in a South Texas mesquite woodland. These included overland flow, soil moisture, and field saturated hydraulic conductivity (Kfs) from an upland savanna-parkland area to the adjacent downslope, more densely wooded areas. Along the same catena, continuous sapflux measurements were made for 14 tree individuals – 6 were Prosopis glandulosa (a pioneer overstory specie) and 8 individuals of Diospyros texana (a dominant understory specie). Overland flow was monitored using 3 large runoff plots (4m X 11m), soil moisture was measured using both neutron probes (upto 250 cm) and continuous using TDR probes. Kfs was measured using Beerkan method and disc infiltrometers. Sapflux was monitored using Granier Thermal Dissipation probes. We also report on our findings on active sapwood areas in different tree individuals and accounted for these corrections in our results. Micrometeorological data was collected from an eddy covariance tower at a neighboring shrubland site.
Results/Conclusions
We found no evidence for redistribution of water from the uplands to the drainage areas under the current ungrazed conditions. Further, Kfs at the ungrazed study site was two orders of magnitude higher than that at the grazed site. Contrary to our hypotheses, transpiration rates were higher for upland locations indicating a possible role of lateral root spreads and more soil moisture exploration. Daily water uptake and flux rates in D. texana (the understory) was higher than P. glandulosa (the overstory) which also contradicted the commonly held belief related to higher water use by the dominant mesquites. We summarize these findings to comment on the ‘modified’ hydrology of a landscape after ~200 years of WPE.