Post-harvest recovery of microclimate buffering and associated temporary xerophilization of vegetation in sub-continental oak forests

Forest microclimate buffering is a key factor influencing living conditions of forest understorey species. Shaping open-air conditions into unique microclimate, forest canopies can mitigate climate change impacts on forest vegetation. In managed forests, canopy disturbances associated with forest harvest reduce microclimate buffering, threaten forest-dwelling species, and may trigger compositional shifts of understorey towards warm-demanding plants. However, how long does it take for microclimate to recover and whether understorey changes are permanent or temporary remain sufficiently unexplored. To reveal changes in microclimate, potential compositional shifts of understorey and time for microclimate and vegetation recovery, we measured forest microclimate and recorded understorey composition along developmental gradient of even-aged managed oak forests in the Western Carpathians (Central Europe). We compared offsets of air temperature and soil moisture to open-air conditions and thermal and moisture requirements of understorey plants between developmental stages and analysed relationships of these measures to tree layer structural variables. During growing season, all developmental stages exhibited lower maximum temperatures and higher soil moisture minima than open-air conditions, with the smaller thermal and higher moisture offsets in the stages of seed cut and clearings compared to mature stand. In a vegetation response to cutting disturbances, we did not identify thermophilization, but shift towards drought tolerant plants, i.e. xerophilization. This emphasizes higher importance of water availability than temperature for vegetation of oak forest in sub-continental conditions. Importantly, observed xerophilization was linked to occurrence of species typical for non-forest vegetation or more open forests, which are typical for oak dominated forests. Microclimate typical for mature stands recovered when tree regeneration reaches the height of ca. 10–15 m. Also, xerophilization was temporary and moisture requirements of vegetation recovered along with microclimate. Our results indicate that decrease in microclimate buffering effect may support restoration of declined vegetation diversity of sub-continental oak forest and forest understories have potential to recover along with forest succession. On the other hand, period of microclimate buffering effect loss is rather long and many plants could by threatened by extreme conditions during ongoing climate change. Thus, to safeguard all forest species novel harvest techniques combining open light gaps with closed-canopy components are required.