Is plant diversity on tractor trails more influenced by disturbance than by soil characteristics?

- Soil compaction was higher on old or recent tractor trails than on controls.
- Soil bulk density had positive relationship with heliophilous woody species richness.
- Disturbance mattered more than soil characteristics for fine-scale plant diversity.
- Diversity of most herbaceous groups was higher on tractor trails than on controls.
- Impact of tractor trails was long-lasting for most ecological groups.

Despite broad recognition that forest machinery access systems impact plant diversity, previous studies have largely focused on skid trails. As a consequence, how tractor trails (dead-end single-use trails) influence plant diversity is poorly understood. In particular, knowledge is lacking on whether fine-scale understory diversity is better indicated by tractor-trail disturbance or by the environment. We investigated the relative importance of tractor-trail disturbance conditions and soil characteristics, as well as their effects on ground floral diversity in four oak forest types (a high forest and three coppice-with-standard forests) in the northern half of France. Subplots representing five types of disturbance conditions were placed on both recent (<12 years) and older (⩾12 years) tractor trails at two locations - (trail center and wheel track), and an off-trail control subplot was added. At each subplot, we measured soil moisture, penetration resistance (PR) and bulk density (BD), and the last two were used as proxies for soil compaction. We also recorded the abundance of all vascular plants below 2 m in height on each subplot. Plant diversity was calculated based on CSR plant strategy (C-competitors, S-stress tolerators and R-ruderals), seed bank persistence, and light and moisture preference. We found that compared to controls, PR and BD were significantly greater on both old and recent tractor trails; soil moisture was greater on recent tractor trails. Comparing models showed that tractor-trail disturbance mattered more than did the soil characteristics for the diversity of most ecological groups (except for woody species richness). Compared to controls, tractor trails favored the richness of the short-term seed bank, C-strategy, CSR-strategy, heliophilous and hygrophilous herbaceous species; and abundance of heliophilous, CSR-strategy and hygrophilous herbaceous species. Furthermore, compared to controls, both richness and abundance of mesophilous herbaceous species strongly increased on recent tractor trails and strongly decreased on old tractor trails. Concerning effects of soil characteristics, only one non-negligible (positive) relationship was found: between BD and the richness of heliophilous woody species. Since plant diversity is affected by tractor trails, and since plant recovery is very slow on trails, long-term studies on their impact are needed.