The fate of arable bryophytes in the Swiss Plateau: Agricultural intensification, sustainable farming, promoting agro-biodiversity and three decades of monitoring hornworts

Irene Bisang^1,3; Luc Lienhard²; und Ariel Bergamini^3,*;

Background – what do we know?

Humans have cultivated land over thousands of years in Europe and thus created and maintained a complex landscape. Such landscape mosaics, including low-intensity agricultural habitats, sustained a rich and diverse flora and fauna. For example, numerous plant species have adapted to regular disturbance through tillage in crop fields. The intensification of agricultural management and structural changes since the Second World War, however, have led to landscape simplifications and a severe loss of biodiversity in the cultivated fields and the environments around them. The arable flora is nowadays considered one of the most threatened plant groups in Europe. Bryophytes are likewise profoundly affected by the changes in agricultural cultivation (Hodgetts et al. 2019, Bisang 1992).

Fig. 1. Anthoceros agrestis, Phaeoceros carolinianus and Riccia sorocarpa at the margin of an arable field

Arable bryophyte specialist species are short-lived, reproduce fast and often copiously, and their spores or vegetative dispersal units may build up a persistent reservoir in the soil (diaspore bank; Bisang et al. 2021a). These species are weak competitors and depend on bare soil to grow and thrive, thus the agricultural practices in crop fields are critical whether they can occur (Bisang, 1998; 2009). Many species develop best in autumn stubble-fields that are left untreated after harvest. The two hornworts Phaeoceros carolinianus and Anthoceros agrestis are typical for the specialised arable bryophyte flora in Switzerland and Central Europe (Figs 1, 2; Swissbryophytes 2004-2021). In Switzerland, P. carolinianus is redlisted and protected by national law (Schnyder et al. 2004; NHV 2015). Recent evaluations for the updated Red List of Swiss bryophytes revealed that also A. agrestis is threatened (pers. comm.). Both species are targeted in the Swiss Environmental Objectives for Agriculture (BAFU 2019; BAFU & BLW 2016).

Fig. 2. Phaeoceros carolinianus with young sporophytes

To halt the decline of farmland biodiversity, agri-environment schemes (AES) were adopted in Switzerland in 1998 (Schweizerischer Bundesrat 2021). AES require sustainable farming practices and include the mandatory establishment of Biodiversity Promoting Areas (BPAs), a key element of Swiss AES. Farmers are financially compensated for implementing AES-measures.

What did we study – Results

We asked how arable management and the introduction of AES-measures affect the specialised bryophyte flora of arable land, whether these specialist species do occur in existing BPAs, and how weather conditions affect the development of arable bryophyte populations.

Firstly, we monitored the two typical arable bryophyte specialists, the Field hornwort (A. agrestis) and the Carolina hornwort (P. carolinianus) in the intensively cultivated lowlands of Switzerland (Swiss Plateau) over a period of almost three decades. Each year in the autumn during three periods (1991–1995, 2005–2007, 2016–2018), we surveyed the occurrence, abundance and habitats of hornworts in 28 selected crop fields. Secondly, we compared hornwort occurrences with regional weather data within the three study periods. Thirdly, we examined the bryophyte flora in different BPA-types typical for arable farming both at the surface and in the soil.

We found that hornwort occurrences have strongly declined over the past 30 years, which we could attribute to a loss of suitable habitats. Stubble-fields that are left unprocessed until late autumn or over winter had almost disappeared in the Swiss Plateau (Bisang et al 2021; Fig. 3). This is a consequence of changed arable practices (decreased cereal cultivation, early harvest and tillage) and, importantly, modified AES regulations. Since 2005, the AES directives require a prompt post-harvest cultivation by an intermediate crop or early winter crop, to prevent soil erosion and nutrient leaching. Further, dry and warm conditions during summer negatively affected the development of hornwort populations. Different BPA-types on arable land harboured arable bryophytes. However, rare, endangered or highly specialized arable species occurred only rarely in the investigated BPAs, and species richness decreased in BPAs of three years or older (Studer, 2016, Valentini, 2014). Finally, we showed that spores and / or vegetative dispersal units of many arable bryophytes survive several years in the soil of cultivated fields and BPAs. From this long-lived diaspore bank, they may emerge quickly under favourable conditions (Bisang et al. 2020).

Fig. 3. Cereal stubble fields that remain untreated until at least late October are favourite but nowadays rare habitats for arable bryophyte specialists.

Based on our results, we propose tailored amendments of AES directives to safeguard the characteristic arable specialist bryophytes in the Swiss arable landscape. The measures are described in detail in the publications (link). They will also be beneficial for other organisms that depend on extensively managed open habitats, for example arable wildflowers, farmland breeding birds and specialised arthropods.

In brief, we recommend implementing the following management in selected arable fields:

• Tillage not before the end of October
• Crop rotation with adequate proportions of cereals
• No post-harvest processing of stubble fields before the end of October or spring
• Promotion and optimization of short-lived BPAs in crop fields (e.g., conservation headlands, annual flower strips, wildflower stips)

Literature: download here

Impressum January 2022
We thank the Swiss Federal Office for the Environment (FOEN) for financial support of the “Hornwort monitoring”, Maya Valentina and Lisa Studer for the possibility to use the data from their master's theses (ETH Zurich), Katja Jacot Amman (Agroscope) and other colleagues for various contributions, and the farmers for allowing us to investigate the bryophytes flora on their arable fields.

Adresses
¹Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sverige, irene.bisang[at]nrm.se;
² Natur&Geschichte, Waldrain 16, 2503 Biel, luc.lienhard[at]bluewin.ch
³ WSL Swiss Federal Research Institut, Zürcherstrasse 111, 8903 Birmensdorf, ^*ariel.bergamini[at]wsl.ch

Photos: Irene Bisang; Lars Hedenäs