Promoting plant diversity and habitat heterogeneity through vineyard terracing

Questions: Viticulture on steep slopes has shaped landscape and biodiversity in many regions, but insufficient profitability has led to management cessation and shrub encroachment. A solution to maintain cultivation economically viable could be vineyard terracing. We aimed to identify the potential of terracing to enhance plant diversity and habitat heterogeneity in vineyards, analyse effects of management intensity on vineyard vegetation, and assess how plant communities change after vineyard abandonment.

Location: Wine-growing region of the Upper Middle Rhine Valley in Hesse (50.042342°N, 7.814533°E) and Rhineland-Palatinate (50.119139°N, 7.719275°E), Germany.           

Methods: We recorded vascular plant species and local vineyard parameters in vertically oriented vineyards with vegetated and tilled open inter-rows, in terraced vineyards with tilled terrace inter-rows and extensively managed embankments and in vineyard fallows in a total of 45 study sites. We used plant species richness, Ellenberg indicator values and Grime’s strategy types to describe how traits and ecological requirements respond to distinct vineyard management.  

Results: Plant species richness and composition were determined by management-derived disturbance intensities. Extensively managed embankments had a distinct plant community, the highest plant species richness, more perennial and indicator species, and lower nitrogen indicator values compared to inter-rows. In contrast, highly disturbed open and terrace inter-rows revealed plant communities associated with annuals and ruderals, but species richness did not differ between terrace inter-rows and embankments. Plant communities of fallows were completely different with lower plant diversity.       

Conclusions: Our results highlight the potential of terraced vineyards for plant diversity with nutrient-poor, extensively managed embankments providing conditions that have become rare in modern agricultural systems. A long environmental gradient from terrace inter-rows to embankments created habitat heterogeneity at a narrow space. In contrast, intensive inter-row management in vertically oriented vineyards hampers high plant diversity and abandonment fosters the spread of woody species at the expanse of plant diversity.

Methods
Methods in brief, for details, see manuscript: 

Vegetation surveying and assessment of local environmental data:

Vegetation surveys were carried out in 2019 in two sampling rounds in March and June-July.
For each sampling round, two pairs of vegetated and open inter-rows were selected in the center of each vertically oriented vineyard and two pairs of embankments and terrace inter-rows were selected in the center of each terraced vineyard. The plant community was assessed in rectangles of 1 x 6 m in each selected inter-row and embankment (vineyard compartments) respectively, resulting in four quadrats per vineyard and sampling round. As fallows were difficult to assess due to dense scrub, 
we used one plot of 4 x 6 m for vegetation surveying. Within each plot, we identified all vascular plants and estimated their respective ground cover according to Londo (Londo 1976).
We additionally estimated the percentage cover of total vegetation, bare ground, litter, shrubs, and trees. Nomenclature follows Buttler et al. (2018). 

Data processing:

For statistical analysis, Londo cover-abundance values were transformed into mean values according to Leyer and Wesche (2007), and mean values for percentage 
cover of total vegetation, bare ground, litter, shrub, and tree cover were calculated and averaged over the two sampling rounds.
To determine the total cover of herbaceous vegetation and grasses per plot, we added up all transformed cover values of the respective grass and herbaceous species.  
To describe site-specific factors, we calculated Ellenberg indicator values for nitrogen, temperature, light, moisture, and reaction for each vineyard compartment. Only presence-absence data of species abundance were used to compute mean indicator values. Species without existing Ellenberg values were excluded from the analyses.
To account for distinct management-related disturbance intensities, we implemented a disturbance gradient ranging from high disturbance (1) in open and terrace inter-rows, to intermediate disturbance in vegetated inter-rows (2) and embankments (3) to low disturbance (4) in fallows. 
To describe ecological traits of plant species, we calculated CSR strategy types according to Grime (2002) for each site. We used a numerical implementation to handle the strategy types mathematically and translated the three categories (CSR) into values ranging from 0-3 for each of the three strategy dimensions (competitiveness, stress tolerance, and ruderality). The three scores of each species sum up to three and each score corresponds to the number of the respective letter in the strategy category. We calculated mean values for each strategy type for each site based on presence-absence data. We assigned species to different life forms (annual, biennial, perennial). 
We chose (generalized) linear mixed effect models (GLMMs) and indirect ordination methods (DCA) for analysis.