Selection and interpretationof soil quality indicators for forest recovery after clearing of a tropical montane cloud forest in Mexico

Abstract

Through slash-and-burn techniques, vast areas formerly occupied by tropical montane cloud forest (TMCF) in Mexico have been converted into croplands and secondary forest of different ages. Despite the dramatic changes in soil properties and processes detected during cropping and forest regeneration, no attempts have been made to develop soil quality indicators (SQI) to assist in the assessment of soil conditions during such changes. SQI are considered to be essential in evaluating plans of forest restoration or management; as such, the objectives of this study were to (i) select soil properties that can be used as SQI during forest regeneration for abandoned crop fields in a TMCF area managed under the slash-andburn method; and (ii) examine the ecological significance of stand age for function-based interpretations of the selected SQI. To this end, the soil properties of three adjacent chronosequences in El Rincón, Sierra Norte, Southern Mexico were analyzed. Each chronosequence consisted of ordered series of five stands of different age after abandonment: a cornfield and adjacent forests of 15 (incipient forest), 45 (young forest), 75 (mature forest), andP100 (old-growth forest) years after abandonment. The soil properties of undisturbed old-growth forest stands were used as a reference. After inspection of principal component analysis results and control charts, the following soil properties were chosen as SQI in TMCF areas: soil organic carbon, pH, plant-available P, O horizon thickness and exchangeable Al3+. The selected SQI displayed different rates of change during forest regeneration. Soil organic carbon had a fast recovery rate and, therefore, a greater ability to return to its original level after disturbance. In contrast, O horizon thickness, soil pH, plant-available P, and exchangeable Al3+ showed a slow rate of change during the fallow period. SQI did not always change linearly nor improve with the age of the forest. The highest exchangeable Al3+ concentration was detected in 45-year-old forests, suggesting that at this forest age, soil become an important filter against Al3+ sensitive species, potentially affecting vegetation composition. Considering the slow recovery rate of some SQI, we estimate that fallow periods of at least 100 years are required in order to reach good soil quality in TMCF ecosystems. Management practices should therefore consider the maintenance of forest of different ages spanning at least 100 years in the landscape. Doing so would achieve more sustainable management practices by allowing a relatively continuous recovery of the ecosystem without prolonged interruptions of land utilization.