Neglecting non‐vascular plants leads to underestimation of grassland plant diversity loss under experimental nutrient addition

Abstract Nutrient availability and grazing are known as main drivers of grassland plant diversity, and increased nutrient availability and long‐term cessation of grazing often decrease local‐scale plant diversity. Experimental tests of mechanisms determining plant diversity focus mainly on vascular plants (VP), whereas non‐vascular plants (NVP, here bryophytes) have been ignored. It is therefore not known how the current models based on VPs predict the rates of total (NVP + VP) losses in plant diversity. Here we used plant community data, including VPs and NVPs, from nine sites in Europe and North America and belonging to the Nutrient Network experiment, to test whether neglecting NVPs leads to biased estimates of plant diversity loss rates. The plant communities were subjected to factorial addition of nitrogen (N), phosphorus (P), potassium with micronutrients (K +μ ), as well as a grazing exclusion combined with multi‐nutrient fertilization (NPK +μ ) treatment. We found that nutrient additions reduced both NVP and VP species richness, but the effects on NVP species richness were on average stronger than on VPs: NVP species richness decreased 67%, while VP species richness decreased 28%, causing their combined richness to decrease 38% in response to multi‐nutrient (NPK +μ ) fertilization. Thus, VP diversity alone underestimated total plant diversity loss by 10 percentage points. Although NVP and VP species diversities similarly declined in response to N and NPK +μ fertilizations, the evenness of NVPs increased and that of VPs remained unchanged. NP, NPK +μ fertilization and NPK +μ fertilization combined with grazing exclusion, associated with decreasing light availability at ground level, led to the strongest loss of NVP species or probability of NVP presence. However, grazing did not generally mitigate the fertilization effects. Synthesis . In nine grassland sites in Europe and North America, nutrient addition caused a larger relative decline in non‐vascular plant (NVP) than vascular plant species richness. Hence, not accounting for NVPs can lead to underestimation of losses in plant diversity in response to continued nutrient pollution of grasslands.