Data for: Fine-scale interventions can reinforce the forest character of the understory vegetation – The effects of different artificial gaps in an oak-dominated forest

Description

Data of understory survey in the frame of Pilis Gap Experiment. The study was carried out in an oak-hornbeam forest in Hungary, in 2018-2023. Data were collected in every year, in four, artificially created gap types (large circular, large elongated, small circular, small elongated gaps) and in uncut control sites, in six replicates. List of understory species was established and percentage cover of each species was estimated in 2 m x 2 m quadrats. The cover of shrub layer and the height of the understory vegetation was also estimated. Species richness, sum of the species cover and cover of 5 functional groups (annual forbs, perennial forbs, graminoids, woody regeneration and cover of Rubus fruticosus agg. as an individual species group) were calculated. Relative direct and diffuse light were estimated by hemispherical photographs, and volumetric soil water content was also measured.

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Understory vegetation was sampled in the center of each plot using a 2 m × 2 m quadrat. Percentage cover for all herbaceous vascular plant species and woody species less than 0.5 m in height was estimated visually. Most of the species were recorded in June, while in April, we recorded species that can be detected optimally in spring. Some species, occurring both in spring and summer, were sampled at both occasions, and for each species, we chose the higher cover values for analysis. The average height of the understory vegetation for each quadrat, as well as the total cover of the shrub layer (woody species above 50 cm height but below 5 cm diameter at breast height) was also registered by visual estimations. The nomenclature of plants follows Király (2009). In 2018, relative diffuse light (diffuse non-interceptance, which is the percentage of diffuse light coming through the canopy) was estimated by LAI-2000 Plant Canopy Analyzer (LI-COR Inc., Lincoln, USA). We conducted measurements in the center of each plot at a height of 1.3 meters above ground level. Measurements were taken in July and in order to prevent direct light reaching the sensor, always at dusk. A 270° view restrictor masked the segment of the sky containing the sun and the operator (LI-COR Inc. 1992). Above-canopy reference measurements were taken in a nearby open field. Since 2019, hemispherical photographs were used to estimate light in the gap centers at a height of 1.3 m using a KODAK PIXPRO SP360 camera. To reduce any bias caused by capturing the sun-disk or the reflectance of direct sunlight on the leaves, all photographs were taken at dusk. The WinSCANOPY 2019 software (Regent Instruments Inc., Québec, Canada) was used to calculate diffuse and direct components of the incoming radiation (indirect and direct site factor, ISF and DSF respectively; %) relative to above-canopy light. The analysis period for the radiation calculation was between 01 May and 30 September. To ensure comparability of light data collected by different instruments during the pre- and post-treatment periods, parallel measurements were conducted in 2020 using the Plant Canopy Analyzer and hemispherical photography. The diffuse non-interceptance values measured by the Plant Canopy Analyzer were calibrated and converted by regression to the ISF values obtained from the hemispherical photographs. Soil moisture was measured in 2018 by a FieldScout TDR 350 probe equipped with 7.5 cm steel rods (Spectrum Technologies Inc., Aurora, USA). Two measurement campaigns were carried out in the vegetation season, one in June and one in August. Four measurements were taken at the midpoints of the sides of the vegetation quadrat for each plot. The volumetric soil water content (VWC; m3/m3) of each plot was then expressed as the average of the values of the four measurements. From 2019, soil moisture (VWC) data were continuously collected by TMS-4 loggers with 15-min logging intervals (TOMST s.r.o., Praha, CZ).

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