Monitoring forest soil erosion under long-term post-fire conditions and during insect outbreak in Mediterranean region (Greece)

Abstract

<p>Natural-related disasters like insect outbreaks might alter how the surface runoff is intercepted in affected forests, leading to more intense floods and soil erosion. The quantification of soil erosion and its relationship to the phenomenon of insect outbreaks are currently not well-explained or insightfully understood in the literature.</p> <p>In addition, wildfires are the most destructive phenomena that significantly affect the soil erosion rate. Most studies primarily focus on the "disturbance window" (ranging usually 3 to 10 years). There are few studies investigating the long-term post-fire consequences on soil erosion rates, particularly those examining the time period beyond 20–30 years after the wildfire.</p> <p>The present research is conducted in Thessaloniki's suburban forest (Seich Sou), North Greece. Over 50% of the woodland area was burned by a wildfire in 1997. The bark beetle <em>Tomicus piniperda</em> caused a severe insect infestation (in May 2019) . More than 300 ha of <em>Pinus brutia</em> forest were lost as a result of the infestation. To minimize and prevent the spread of the infestation, diseased trees have been cut down and removed from the forest.</p> <p>The aim of the research is to investigate the long-term effects of wildfire (1997) on erosion rate and, for the first time in the literature, to estimate the effect of insect outbreak on soil erosion.</p> <p>Soil erosion was monitored by installing silt fences on 25 field plots (each plot area: 5 x 22 m) in five representative forest areas: (1) areas naturally reforested after the wildfire of 1997, (2) areas characterized by reforestation failure, (3) areas infected by insects, (4) areas with insect outbreak and logging of dead trees, and (5) undisturbed areas as a control.</p> <p>Soil erosion was lowest in (5) undisturbed plots (control) (0.023 t/ha/year), whereas it was highest in (2) areas of reforestation failure (0.51 t/ha/year). When the four affected sites are compared, the plots with (3) "insect outbreak (no logging)" present the lowest soil erosion rate (0.027 t/ha/year). Furthermore, the difference between (3) "insect outbreak (no logging)" and (5) "control" plots was negligible, revealing that the influence of entomological infestation on soil erosion was observable and measurable, although quite low. The plots with (4) "insect outbreak and logging of dead trees" showed double amount of erosion compared with the plots of (3) "insect outbreak (no logging)" and (5) "control" plots. This fact revealed that logging, as a method to prevent infestation spread, might have positive results, but it could also have negative impacts on soil erosion rates.</p> <p>Concerning the long-term consequences of the 1997 wildfire on soil erosion, the results from plots (1) with "natural reforestation" indicated that erosion rates are triple (0.062 t/ha/year) 25 years after the wildfire compared to the (5) "control" plots (0.023 t/ha/year). It appears that the "window of disturbance" in the reforested region has not been closed, and the forest environment has not been substantially recovered. Restoration of soil erosion rates to normal levels under Mediterranean conditions, where soils are generally thin and stony, can take more than two decades, depending on site quality, geomorphology and meteorological conditions.</p>