Persistent increase in understory vegetation following thinning and grazing exclusion in a semiarid pine forest

Forests function as hotspots for plant functional diversity, hosting numerous nontree plant species in the understory. In tropical, temperate, and Mediterranean forests, density reduction (thinning) treatments have been shown to enhance the abundance of understory plants, including shrubs and herbs. These plant functional types are also known to dominate semiarid steppes and savannas. Yet, it is unknown if thinning in a semiarid forest could have such effects on understory vegetation. Here, we tested the effects of thinning and grazing exclusion on shrub cover and herbaceous biomass in a field manipulation experiment in a semiarid pine forest. Understory development was measured in 21 experimental plots (0.5 ha each) in the dry seasons of the years 2009-22. Fifteen plots were thinned to 300, 200, or 100 trees ha(-1) (0%, 33%, and 67% thinning, respectively) and grazing was excluded from half of each plot. Six additional plots were left unthinned at 400-200 trees ha(-1), and served as controls. Herbaceous biomass increased significantly with thinning, from similar to 5 to similar to 10 kg ha(-1) in most years. Shrub cover tended to increase with 67% thinning from similar to 2% to similar to 4%, and both shrubs and herbs increased with time. While annual precipitation amount did not directly affect the understory, herbs responded positively to last year precipitation. Ungrazed plots showed higher shrub and herb development than grazed plots, but overall, grazing effect was not significant. Our results show a persistent increase in understory vegetation following thinning in a semiarid pine forest. The gaps formed by thinning created conditions for regrowth of the native nontree vegetation, diversifying the forest structural diversity and its ecosystem services. Yet this change might not sustain under drying conditions.