Predicting understory maximum shrubs cover using altitude and overstory basal area in different Mediterranean forests

Abstract

In some areas of the Mediterranean basin where the understory stratum represents a critical fire hazard, managing the canopy cover to control the understory shrubby vegetation is an ecological alternative to the current mechanical management techniques. In this study, we determine the relationship between the overstory basal area and the cover of the understory shrubby vegetation for different dominant canopy species (Pinaceae and Fagaceae species) along a wide altitudinal gradient in the province of Catalonia (Spain). Analyses were conducted using data from the Spanish National Forest Inventory. At the regional scale, when all stands are analysed together, a strong negative relationship between mean shrub cover and site elevation was found. Among the Pinaceae species, we found fairly good relationships between stand basal area and the maximum development of the shrub stratum for species located at intermediate elevations (Pinus nigra, Pinus sylvestris). However, at the extremes of the elevationclimatic gradient (Pinus halepensis and Pinus uncinata stands), stand basal area explained very little of the shrub cover variation probably because microsite and topographic factors override its effect. Among the Fagaceae species, a negative relationship between basal area and the maximum development of the shrub stratum was found in Quercus humilis and Fagus sylvatica dominated stands but not in Quercus ilex. This can be due to the particular canopy structure and management history of Q. ilex stands. In conclusion, our study revealed a marked effect of the tree layer composition and the environment on the relationship between the development of the understory and overstory tree structure. More fine-grained studies are needed to provide forest managers with more detailed information about the relationship between these two forest strata.

A silvicultural stand density model to control understory in maritime pine stands

“…The "regulating" effect of overstory density on understory has been reported for other forest systems (Bailey & Tappeiner 1998, McKenzie et al 2000, Gracia et al 2007, Coll et al 2011, Botequim et al 2015. Results are also in agreement with the expected dynamics of development of these ecosystems.…”

“…2) reveals a tendency similar to that reported for Fw, with the denser stands less prone to present understory vegetation. Other authors have also reported strong negative correlations of canopy cover with understory development, quantified in terms of SDI (McKenzie et al 2000) or stand basal area (Gracia et al 2007, Coll et al 2011, Botequim et al 2015). …”

“…Gracia et al (2007) found that overstory cover and basal area in temperate forests in NE Spain negatively affected the presence of two shade-intolerant shrub species, decreasing their abundance when light decreases. A significant relationship between stand cover on understory vegetation has been reported also by Coll et al (2011), referring to some species of the Pinaceae and Fagaceae families in forests of Catalonia (Spain), and by Botequim et al (2015) in Portuguese forests of different cover types.…”

“…Nevertheless, as understory fuel is a critical component of fire hazard, it is crucial to keep a dense canopy cover to shade the understory and reduce the available fuel, as an alternative to the fuel treatments proposed by Graham et al (2004), such as the case of prescribed fire. As stated by Coll et al (2011), managing the canopy cover to control the understory shrubby vegetation is an ecological alternative to the current mechanical management techniques. The silvicultural option for pine maritime forests suggested in this study can help the adaptive management of maritime pine forests in areas prone to forest fires.…”

Patterns of shrub diversity and tree regeneration across topographic and stand-structural gradients in a Mediterranean forest

“…Although shrub recruitment might be nil in Mediterranean forests during very dry years (Mendoza et al, 2009b), it is unlikely that climatic conditions and, specifically, water availability have limited the expansion of shrub communities in the Navahondona forest for at least three reasons. First, results showed that altitude (a proxy of climate) does not influence shrub abundance, i.e.…”