The influence of forest regrowth on soil hydraulic properties and erosion in a semiarid region of Brazil

Abstract

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.

Previous Land Use Affects the Recovery of Soil Hydraulic Properties after Forest Restoration

“…These findings are in agreement with those reported by several other studies in the Atlantic Forest [63,65]. Additionally, the K s spatial variability observed in both remnant forests is in line with previous work by Hassler et al [7], who attributed the K s variability in Panama forest soils to overland flows that result in erosion [19]. Other factors that might have caused the K s spatial variability in remnant forest plots were the steepness of the sample plots and the soil distribution in the landscape positions (U, M, D) influenced by contrasting slope and topography.…”

“…This result can be associated with the high ρ b and a consequent increase in the soil Mic that was noted in the remnant forest plot F5D. The soil attributes and K s values in remnant forest sites could be explained by the longer time that these forests have remained undisturbed, which allows a better soil structure to develop and the storage of more soil carbon [19,66]. These findings are in agreement with those reported by several other studies in the Atlantic Forest [63,65].…”

“…where γ and β are infiltration coefficients commonly set at 0.75 and 0.6 as explained by Lassabatere et al [3,7,16,19], r (L) is the radius of the disk source, η is a shape parameter that is estimated from the capillary models [45], and θ i and θ s are the initial and final water contents, respectively. Note that θ i should not exceed 0.25 θ s ; however, Di Prima et al [43] showed that BEST-steady can be applied in initially wetter soil conditions (θ i > 0.25 θ s ) without an appreciable loss of accuracy in the predictions of K s .…”

“…In general, important differences were observed in the soil attributes and K s between land-cover classes. These differences could be related to many factors such as intensity of past land use [4,23], spatial and topographic variations in soil types along the toposequences [63,64], density and diversity of plants, root system, vegetation type, canopy cover and soil faunal activity, among others [19]. Unfortunately, the influence of these factors on soil attributes and K s after forest restoration is poorly understood and needs to be included in future studies.…”

“…Conversely, restored forest R3 presented visually a canopy structure with greater understory vegetation. Indeed, the canopy cover determines the interception rainfall, reducing raindrop impact and surface sealing, which could enhance the K s [19]. Additionally, the higher ρ b values in restored forest R4 are an indication of lower root and soil organism presence [70]; this may reduce plant seed germination, reduce root growth and decrease soil oxygen availability, becoming an ecological filter in the natural regeneration processes [71,72].…”