Dryland tropical forests are important but little‐understood ecosystems. These landscapes are at risk globally because of accelerated deforestation and the consequent losses of hydrological functions. An example is the Caatinga shrublands in northeastern Brazil; these dry tropical forests are undergoing rapid change as the land is converted to cultivated cropland and pastureland and then often abandoned after the land is exhausted, allowing forests to regrow. In this study, we evaluated how these changes influence soil hydraulic properties and soil erosion in Caatinga landscapes, by examining four sites of different ages: a recently abandoned pasture, a young (7 years of regrowth) secondary forest, an intermediate (35 years of regrowth) secondary forest, and an older (more than 55 years) forest. Rainfall simulation was used to assess soil infiltrability and susceptibility to erosion. In addition, we determined hydrological conductivity in the field using the Beerkan method (Kfs) and collected samples for laboratory‐determined measurements (Ks). We found that infiltrability and Kfs values were progressively higher as time since land abandonment increased, whereas Ks values were not significantly different across sites. Infiltrability was approximately 2 times smaller than Ks and Kfs—possibly owing to the formation of soil crusts from raindrop impact. Soil erosion did not show a progressive improvement with forest age and was better explained by site characteristics, such as herbaceous cover and slope. In general, our results suggest that in Caatinga landscapes, more than 35 years is required for the recovery of soil hydraulic properties following land abandonment.

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Grasslands and savannas across the globe have undergone a dramatic transition over the past century. Historical overgrazing has set in motion a cascade of events ranging from desertification in arid climates to woody plant encroachment (WPE) in semiarid and subhumid climates. In recent decades, grazing pressure on many of these landscapes has declined significantly, and where rainfall is sufficient (as in most semiarid and subhumid settings), herbaceous vegetation in intercanopy areas will recover. An important question is, how has this transition altered ecohydrological connectivity (overland flow and runoff-runon dynamics)? A woody-plant-encroached, subhumid savanna site in South Texas with a history of heavy grazing (but ungrazed since 1981) was used as a model landscape to address this question. Overland flow, soil moisture, and field saturated hydraulic conductivity (K fs ) were measured along a catena extending from the upland savanna-parkland areas to the downslope, more densely wooded areas. For comparison, K fs and infiltrability were also measured at a moderately grazed upland site 14 km east of our study site, selected as a surrogate for past conditions at our site. In contrast to the prevailing hypothesis that the downslope areas ('drainage-woodlands') at our study site have continued to be supported by runoff generated from the upland areas, our measurements yielded no evidence for the redistribution of water from the uplands to the drainage areas under the current ungrazed conditions. Further, K fs at the ungrazed study site was two orders of magnitude higher than that at the grazed site and infiltrability was twice as high at the ungrazed site than the grazed site. These findings, coupled with historical information from the site, strongly suggest that historical overgrazing amplified the runoff-runon process, resulting in significant subsidies of water from the uplands to the drainage areas. Then, with the relaxation of grazing pressure and subsequent landscape recovery, redistribution of water via surface runoff was relatively rare. We believe that our results are generalizable for savannas that have recovered from overgrazing. When these savannas are heavily grazed, ecohydrological connectivity is greatly increased; but if grazing pressure relaxes, ecohydrological connectivity will collapse. These changes in ecohydrological connectivity have important, but not always well understood, ecological consequences.

Semiarid karst landscapes are often the source areas for regionally important groundwater supplies. Like savannas across the globe, these landscapes are experiencing an increase in woody plant cover—often referred to as woody plant encroachment. Although this phenomenon is commonly viewed as leading to increased transpiration and reduced groundwater recharge, this may not be true of all ecosystems. For example, in the Edwards Plateau region of central Texas—where the underlying geology is karst—dramatic increases in baseflows have occurred concurrently with the expansion of woody plants. It has been suggested that in this context woody plants, especially juniper (Juniperus spp.), are partially responsible for boosting recharge by improving soil infiltrability, but this hypothesis has not been systematically evaluated. Our study examined the effects of an important encroaching shrub (Redberry juniper) on soil infiltrability in the Edwards Plateau. We carried out a large number of infiltration tests to determine soil infiltrability and used a dye tracer followed by soil profile excavation to estimate the potential for deep percolation. Tests were performed at increasing distances under juniper shrubs of five size classes, ranging from young seedlings to mature shrubs. We found that in soils underlying shrubs, infiltrability was quintupled and percolation depth almost tripled compared with soils in intercanopy zones. Surprisingly, shrub size was not a significant factor. Even the soils beneath the smallest shrubs had much higher infiltrability than intercanopy soils, showing that these woody plants modify soil properties at very early stages. We also found that both infiltrability and percolation depth gradually increased with proximity to the trunk and showed a strong correlation with litter thickness. Our results provide support for the hypothesis that in semiarid karst landscapes, woody plant encroachment—especially the invasion of juniper—can play an important role in enhancing groundwater recharge by improving the soil infiltrability.

Grasslands and savannas in drylands have been and continue to be converted to woodlands through a phenomenon often described as woody plant encroachment. This conversion has profound implications for the ecosystem services that these landscapes provide, including water. In this paper, using examples from six case studies across drylands in the Great Plains and Chihuahuan Desert regions of the United States, we explore the ecohydrological changes that occurred following woody plant encroachment (WPE). In all cases, the increase in woody plant cover brought about modifications in connectivity, which led to profound ecohydrological changes at both the patch and landscape scales. At the wet end of the dryland spectrum (subhumid climates), increases in evapotranspiration following WPE led to reduced streamflows and groundwater recharge. In drier regions, woody plant encroachment did not alter evapotranspiration appreciably but did significantly alter hydrological connectivity because of changes to soil infiltrability. In semiarid climates where rainfall is sufficient to maintain cover in intercanopy areas concurrent with woody plant encroachment (thicketization), overall soil infiltrability was increased—translating to either decreased streamflows or increased streamflows, depending on soils and geology. In the driest landscapes, woody plant encroachment led to xerification, whereby intercanopy areas became bare and highly interconnected, resulting in higher surface runoff and, ultimately, higher groundwater recharge because of transmission losses in stream channels. On the basis of our review of the studies’ findings, we argue that the concept of ecohydrological connectivity provides a unifying framework for understanding these different outcomes.