Grazing exclusion unleashes competitive plant responses in Iberian Atlantic mountain grasslands

Abstract

Questions Does the absence of equalizing mechanisms after cessation of grazing unleash strong above‐ground competitors to create large patches in the community? Do these competitive intraspecific aggregations displace and exclude other species, thereby reducing species diversity? Location Atlantic grasslands, Aralar Natural Park, Basque Country, Northern Iberian Peninsula. Methods Large herbivores were experimentally excluded from three sites (50 m × 50 m exclusion fences) during 9 yr in a productive semi‐natural grassland system with a long history of grazing, using adjacent grazed plots as experimental controls. Sampling was carried out by systematically placing 100 quadrats (0.5 m × 0.5 m) in each of the six plots. Floristic composition and abundance, as well as eight hydrological and chemical soil properties, were measured in each quadrat. The spatial structures created by competitive species were analysed using RDA in conjunction with Moran's eigenvector maps, and soil variables were simultaneously included in the analyses, thus disentangling the structures likely created by niche effects. Competitive exclusion was further determined using linear regressions between species richness and abundance of competitive species. Results Grazing exclusion unleashed competitive species such as Festuca microphylla and Agrostis capillaris, which became dominant in the exclusion plots and created large spatial patches. Furthermore, a negative linear relationship, consistent across exclusion plots, was observed between species richness and abundance of competitive species, indicating that strong above‐ground competitors outcompeted other species when herbivores were excluded. However, the outcome of grazing exclusion across sites depended to some extent on local environmental conditions (niche effects). Conclusions This work confirms that the powerful equalizing mechanism of disturbance by herbivores is crucial for species co‐existence in productive grasslands. However, important differences observed in environmental effects across sites suggest that, even in highly productive grasslands, plant traits and local environmental characteristics (niche effects) do matter for species co‐existence.

Modelling aboveground net primary production (ANPP) of an Atlantic mountain grassland based on time series approach

“…Nevertheless, in the fenced series there are other influencing ecological drivers than climate, because the community is changing due to the lack of equalizing mechanisms that herbivores exert, and the field plots were not in a stable equilibrium. In this case, a bigger increment of ANPP was revealed, which was related with the sudden spread of highly productive dominant grasses (Odriozola et al, 2017), and this enhanced the positive trend. As observed in long-term grazer exclusion experiments (Patton et al, 2007), the initial extra productivity would most likely cease after a longer period of grazing abandonment, when litter accumulation inhibits plant growth.…”

“…Since, in broad terms, similar or even higher ANPP are often reported under grazing conditions because of greater renewal of leaf tissue and lower senescence rates, the greater ANPP found in our study in excluded vs. grazed plots requires additional explanation. We believe that this is a short-term effect, most likely due to the observed fact that the growth of the dominant grasses (Festuca nigrescens and Agrostis capillaris) was released when large grazers were prevented by fencing (Odriozola et al, 2017). Compared to forbs, grasses strongly determine ANPP, and grazing may cut down the response of grasses to increased temperature (Wang et al, 2012).…”

“…As a consequence, graminoid species diversity is high, with Festuca nigrescens Lam. and Agrostis capillaris L. being the dominant species (Odriozola et al, 2017). These grasslands are phytosociologically called Jasiono laevis-Danthonietum decumbentis (Loidi, 1983) and are considered a priority habitat (6230 subtype a) by the Habitat Directive (EC, 2013).…”

“…Experimental grazing exclusion has demonstrated that the decoupling of grassland ANPP and the livestock pressure has many unwanted consequences for their conservation, apart from shrub encroachment. These changes include loss of plant diversity (Odriozola et al, 2017); reduction on forage quality as reduced digestibility and nutrient content, and enhanced fibre content (Odriozola et al, 2014); and the related reduction on microbial activity and retarded nutrient cycling (Aldezabal et al, 2015).…”

Indicadores De Sustentabilidade Com Base Na Qualidade Do Solo E Acúmulo De Fitomassa Em Pastagens Degradadas

“…De acordo com Laliberté et al (2012) e Odriozola et al (2017), as interações planta-solo são de extrema importância na manutenção da composição da comunidade vegetal, além disso, as propriedades do solo exercem um papel fundamental na transição de pastagens degradadas para ecossistemas restaurados. Han et al (2008) e Jing et al (2014 ressaltaram que a exclusão de pastejo tornouse uma prática primária indispensável para restauração de pastagens degradadas, principalmente, em regiões semiáridas.…”