Heat waves and droughts strongly impact productivity and ecosystem functioning in an abandoned subalpine grassland

<p>Climate and land-use changes have major impacts on global biodiversity and carbon cycle of ecosystems. Severe heat waves and droughts, already experienced by the European Alps, e.g. in 2015 and 2018, are expected to further increase in the near future.</p><p>In the last decades, land-use changes have led to the abandonment of several mountain grasslands and pastures, so that in Europe a net conversion of grasslands to forest is currently occurring. However, the consequences of alpine grassland abandonment on the ecosystem responses to climate extremes are still largely unknown. Understanding climate change impacts and feedbacks of alpine and subalpine grasslands is essential, because they are ecologically sensitive ecosystems, and they constitute an important C sink and hotspots of biodiversity.</p><p>In this work we aim at understanding the effects of heat waves and drought on the relative productivity of grasses and forbs and consequently on ecosystem functioning in an abandoned subalpine grassland located in the Western Italian Alps (Aosta Valley) at 2100 m asl. We took advantage of a 10-years natural experiment in which we analysed biomass production, LAI and Net Ecosystem CO<sub>2</sub> Exchange. Vegetation of the study area is characterized by a dominance of the grass Nardus stricta, and by Arnica montana, Trifolium alpinum, Geum montanum and several other forb species typical of alpine and subalpine grasslands.</p><p>In the period 2009-2019, primary production as represented by biomass and leaf area index (LAI) gradually decreased with important drops in 2015 and 2018, which were characterised by extreme climatic conditions.</p><p>Considering the functional type response to extremes, the LAI peak of grasses, which appeared always the dominant portion of the total LAI, showed significantly lower values in 2015 and 2018 compared to long-term. On the other hand, LAI peak values of forbs showed higher variability among plots and years. The clear decrease of the LAI of grasses (mainly represented by Nardus stricta) contributed significantly to the decrease of the total biomass production and to the NEE reduction. The response of Nardus stricta to heat waves and drought is very clear and influences ecosystem functioning and consequently vegetation dynamics, modifying the relative productivity of grasses and forbs. As an example, in the years 2015 and 2018 an evident phenological response was observed in Arnica montana, with an exceptional number of inflorescences.</p><p>In conclusion, we found that heat waves and droughts have the potential to influence the natural vegetation dynamics following abandonment and contribute to the reduction of plant biomass production with consequences on the net ecosystem C exchange and species competition in mountain grasslands.</p>

Clearing vs. burning for restoring Pyrenean grasslands after shrub encroachment

Anthropogenic activities have modified vegetation on subalpine belts for a long time, lowering the treeline ecotone and influencing the landscape mainly through grazing and fire. The abandonment of these traditional land use practices during the last decades and global warming are contributing largely to the colonization of woody species in subalpine grasslands, causing irreversible changes in ecosystem functioning. A variety of management strategies requiring the use of fire and/or clear-cutting are carried out to prevent the expansion of highly encroaching shrubs and improve the conservation status of subhumid high-productive grasslands ecosystems. However, it is still poorly understood how different management strategies affect the recovery of subalpine grasslands, which is influenced concurrently by the vegetation community involved. In this study we used field experiments to test the impact of management treatments on soil properties and vegetation responses in subalpine Bromion erecti grassland communities colonized by the pyrophyte shrub Echinospartum horridum (Vahl) Rothm. on the southern Pyrenees. Vegetation was monitored for 5 years in E. horridum plots (two plots per treatment) where the vegetation was removed by prescribed fire (Burnt treatment) or by mechanical removal (Clear-cut treatment). Undisturbed E. horridum plots were used as a control (C-Erizón) for shrub removal treatments and a Bromion erecti grassland community regularly grazed (C-Grass) was used as a control for subalpine grassland. Clear-cut treatment of E. horridum community was more effective to control E. horridum colonization than Burnt treatment and contributed to a higher extent to recover original grassland vegetation, because E. horridum seedling performed worse (lower germination rates) and plant species were more similar to the original grassland (legumes, non-legume forbs, and grasses). Shrubs and sub-shrubs cover in Burnt areas increased faster than in Clear-cut areas during the 5 years following the treatment, although it did not reach the level of C-Erizón. Species richness and diversity increased in comparison with C-Erizón in both treatments, but they were significantly lower than those in the C-Grass. Network connectivity was larger in well preserved grasslands, i.e, C-Grass, than in any other treatments, mainly dominated by negative plant-plant association. Soils nutrients declined in Burnt sites 4 years after the fire treatment and no difference was observed between Clear-cut and C-Erizón sites, although they did not reach the soil fertility values of C-Grass in terms of organic matter and C/N ratio. This study showed that grazing favors plant diversity and community complexity in subalpine grasslands. It also demonstrated that clearing is a better strategy than burning to restore grasslands after shrub encroachment because burning entails deeper soil degradation and faster recovering of the pyrophyte shrub, E. horridum.