Old-growthness level assessed by structural heterogeneity indices in Mediterranean Quercus pubescens forests

Abstract

Old-growth forests are of major importance for biodiversity conservation and climate change mitigation, as well as being a benchmark for the implementation of sustainable forest management. Although dedicated studies have significantly increased in the last decades, there is still limited knowledge of Mediterranean forests, especially those dominated by Quercus pubescens and related taxa. To fill this knowledge gap, we primarily studied in the field two downy oak forests possessing old-growth traits, localized in Sicily (Mediterranean, Italy). Second, we used a structural heterogeneity index (SHI) to assess their old-growthness level, in comparison with the downy oak stands surveyed in the Regional Forest Inventory (RFI) of Sicily. Third, we tested the effect of different sets of structural parameters on SHI scores, thus assessing whether their choice could affect the final score and the stand assessment. SHI was well proven to discriminate these two stands from the others, both showing, on average, a SHI score just higher than 80, whilst SHI in RFI plots was just under 50, a significantly lower value. The methodological approach used in our study highlights the need to standardize the parameters used to characterize the old-growthness level of Mediterranean forests in order to allow more reliable comparisons. Most of the structural parameters were higher in the two selected stands, except for the attributes related to standing deadwood, suggesting a still limited contribution of standing dead trees and snags in the potential old-growth stands under investigation. The application of a structural index has proven effective for the purpose it was tested for, demonstrating its usefulness in discriminating between two potential old-growth stands from ordinary stands of the same forest type. We believe that both forests deserve primary attention and tailored management measures, as well as inclusion in the recently established Italian Network of old-growth forests.