LB 37-313 - CANCELLED - Are hydrological services overlooked in our advocacy for savanna restorations? Insights from three woody encroached savannas

Over the past century, large proportions of grasslands and savannas globally have been lost to woody plant encroachment – i.e., increasing dominance of woody plants in savannas over time. The effect of woody plant encroachment on hydrology has been extensively researched and documented. While there is wide recognition of woody-encroached savannas being originally open-canopy savannas, we still know little about the hydrological state of pre-encroachment or remnant savannas. Consequently, it is unclear as to what may happen to the hydrology if woody-encroached savannas were successfully restored to open-canopy savannas or grasslands. To restore woody encroached savannas, prescribed fires, tree-thinning, and grass-seeding are widely practiced. The advocacy for most savanna restoration efforts often rests on the need for forage production and biodiversity conservation. Here we draw attention to improved hydrological services as an often-overlooked outcome during savanna restoration. Specifically, we propose that savanna restoration can improve hydrological services such as deep drainage and drought-resilience and that if recognized, it could provide a more valuable incentive for grassland restoration. To support our viewpoint, we present findings from three woody encroached savannas. Site 1 is a South Texas mesquite savanna. Site 2 is a woodland-pasture in Central Texas where, due to land-conversion and subsequent abandonment, a habitat mosaic including thicketized oak woodlands and open pastures has been created. Site 3 is an oak savanna in East Texas which offers a mosaic of vegetation ranging from 60-year-old savannas to thicketized woodlands. At each site, soil moisture was measured to a depth of 200 cm for a year and soil water storage was compared for pastures or grasslands with dense woodlands. In addition, chloride concentrations were measured in soils at Site 2 and Site 3 to a depth of 300 cm and then used for estimating deep drainage rates under the different vegetation covers. Across both sites (Site 2 and Site 3), deep drainage rates were at least an order of magnitude higher in the pastures or the grasslands. Similarly, soil water storage in the grasslands and pastures at all sites was higher across all seasons with woodlands showing lower moisture contents even in deeper soils during droughts. We suggest that these findings make a strong case that restoring woody-encroached savannas can cause a marked improvement in soil water storage and deep drainage.