Understory structure and function following release from cattle grazing and overstory thinning in Mediterranean conifer plantations

Key message: Cattle grazing and overstory cover restrict understory growth and interact in shaping the understory community structure in Mediterranean conifer plantations. Context: Understanding how silvicultural manipulations drive understory structure and function in Mediterranean pine plantations is essential for their multifunctional management. Aims: This paper aims to study the interactive effects of cattle grazing and overstory thinning on understory structure and function. Methods: Ten plots (0.25 ha) were selected in East Mediterranean mature Pinus brutia plantation (rainfall = 600 mm year⁻¹) representing thinned (≈100 trees ha⁻¹, leaf area index (LAI) ≈ 1.6) and non-thinned (≈230 trees ha⁻¹, LAI ≈ 3.5) areas. Two subplots (100 m²) within each plot were fenced in 2000 and 2006 while a third one remained grazed. Understory growth and species composition were measured in 2010. Results: Thinning and grazing exclusion both positively influenced woody growth with their combined effect during 10 years leading to 20-fold increase in vegetation volume. An increase (15-fold) in herbaceous biomass was recorded 4 years after grazing exclusion but disappeared 10 years after exclusion due to increased woody cover. Species richness was not influenced by grazing but was positively affected by thinning. Understory composition was affected by grazing × thinning interaction with herbaceous ephemerals and short woody species being more frequent in grazed, thinned areas while larger woody species were more associated with ungrazed, non-thinned areas. Conclusion: Grazing impacts on forest understories depend on overstory cover. We propose variable grazing-thinning combinations to meet multiple management objectives.