29 Increasingly mechanized timber harvesting and the repeated use of skid trail 30 networks may affect ground vegetation differently at subsequent stages in the forest 31 rotation. At a fine scale, no studies have yet compared the influence of 32 micro-environmental factors and the effects of skid trail disturbance on ground flora 33 diversity. We investigated understory diversity patterns on skid trails in 30-, 50-and 34 63-year-old oak forests in the northern half of France. Subplots were placed on skid 35 trail center, wheel track, skid trail edge plus an off-trail control. At each subplot, we 36 measured soil moisture, soil compaction (penetration resistance and bulk density) and 37 photosynthetic active radiation and recorded the abundance of all vascular plants. The 38 richness and abundance of ground flora were calculated based on the classification of 39 their life form, seed bank persistence, light preference and moisture requirements. For 40 each ecological group, we found out its best diversity indicator from subplot location, 41 micro-environmental factors (soil moisture and compaction, light) and stand attributes 42 (stand type, basal area), then assessed the magnitude and negligibility of the effect of 43 the best indicator. 1) Higher soil compaction compared to controls was detected on 44 the tracks of skid trails in the 50-and 63-year-old stands. Neither soil moisture nor 45 light varied with subplot location whatever the stand type. 2) The best diversity 46 indicator that showed non-negligible effects included subplot location, and soil 47 moisture or soil compaction. Compared to controls, skid trails in the 50-and 48 63-year-old stands were richer in tree and short-term seed bank species, while skid 49 trails in the 30-year-old plots had no effect on ground flora. The abundance of tree 50 and shade-tolerant species was also higher on skid trails. Soil moisture was positively 51 correlated with the richness of low-and high-humidity species, shade-tolerant species 52 and transient seed bank species as well as with the abundance of short-term seed bank 53 species. Bulk density positively affected heliophilous species richness, while 54 penetration resistance was positively related to shrub abundance. Skid trails and soil 55 compaction in our research area had either no impact or a positive impact on ground 56 flora diversity. Longer-term studies of skid trail effects are needed to validate these 57 main findings.58 59
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