Humped-back shaped response of plant species richness to increasing shrub encroachment in calcareous grasslands

Abstract

In the present study, we examined the effect of shrub encroachment in temperate semi-natural grasslands on plant species diversity. We tested the hypothesis that an initial shrub encroachment leads to enhanced habitat heterogeneity and thereby to a higher diversity. In a calcareous grassland near Göttingen (Germany) we analysed the effect of shrub encroachment on the species richness in 30, 100 m 2 plots each with 10 subplots of 1m 2 size. The 30 main plots belonged to six different shrub encroach�ent classes. A descriptive correlative gradient analysis of shrub invaded grasslands and their species nu�ber of flowering plants was performed. Within the 30 plots of different shrub encroachment a total of 203 plant species were recorded. The mean �1-diversity (level of subplots), mean �2-diversity (level of plots) as well as the �3-diversity (level of shrub encroachment classes) have their highest values at �ediu� shrub invaded sites. This finding is in line with our hypothesis of a hu�p�back relation between shrub encroachment and species richness, and can be explained by the increased habitat heterogeneity. However, Detrended Correspondence Analysis (DCA) emphasized the importance of the present vegetation composition for species richness. The dominance of highly competitive, clonal-growing grass species is accompanied by low diversity swards with a lower facilitation of shrub establish�ent. �pecies accu�ulation curves highlight the benefit of shrub encroach�ent for ��(landscape-) diversity. This result emphasises the importance of habitat heterogeneity for biodiversity and, therefore, nature conservation.

High resistance of plant biodiversity to moderate native woody encroachment in loess steppe grassland fragments

“…In forest steppes, a heterogeneous habitat mosaic of grassland and forest patches in transition between closed forest and open grassland (Erdős, Ambarli, et al, ; Erdős, Kröel‐Dulay, et al, ), most dry‐grassland species can also grow near to shrubs and trees but some shade‐tolerant and forest species are also favoured (Pykäla, ; Erdős, Tölgyesi, Körmöczi, & Bátori, ). With increasing cover of woody species, the area of fringe communities at the edge or transition zone between grassland and shrub‐ and/or tree‐dominated vegetation increases, which could also be beneficial for biodiversity (Kesting et al, ; Erdős, Ambarli, et al, ; Erdős, Kröel‐Dulay, et al, ).…”

“…The establishment of scattered trees and shrubs also increases the medium to large-scale habitat heterogeneity (see e.g., Zhou et al, 2019), which has proven to be beneficial for sustaining high species richness through providing multiple types of microsites for species establishment both for plants (Kesting et al, 2015(Kesting et al, , 2009Tamme, Hiiesalu, Laanisto, Szava-Kovats, & Pärtel, 2010) and insects (Eldridge et al, 2011;Koch, Edwards, Blanckenhorn, Walter, & Hofer, 2015). In forest steppes, a heterogeneous habitat mosaic of grassland and forest patches in transition between closed forest and open grassland (Erdős, Ambarli, et al, 2018;, most dry-grassland species can also grow near to shrubs and trees but some shade-tolerant and forest species are also favoured (Pykäla, 2005;Erdős, Tölgyesi, Körmöczi, & Bátori, 2015).…”

“…In our study, negative effects of shrub invasion on grassland plant biodiversity were detected only at relatively high levels of encroachment. Other research has even found beneficial effects on local biodiversity through the creation of greater habitat heterogeneity (Kesting et al, 2015). Thus, low to moderate woody encroachment in dry loess grasslands by native shrubs and trees seems to be tolerable in the short term but requires control of mid-term successional processes, eventually followed by management interventions.…”

“…Woody encroachment often results in ecosystem degradation with related biodiversity decline, and has considerable effects on net primary production, the cycling of surface water and nutrients, and carbon balance (Lett & Knapp, ; Knapp et al, ; Kesting, Wrage, & Isselstein, ; Maestre et al, and literature cited therein). More studies have focused on woody encroachment in semi‐arid to semi‐humid open habitat types from North America, South Africa and Australia (Cabral, Miguel, Rescia, Schmitz, & Pineda, ; Maestre et al, ; D'Odorico et al, ), whereas the Palaearctic has received less attention with most recent studies focusing on shrub encroachment in the Mediterranean region (but see Kesting et al, and Kesting, Petersen, & Isselstein, or Komac et al, ).…”

Assessing species saturation: conceptual and methodological challenges

“…Several studies have used species accumulation curves (SACs) over time (Yosef & Tryjanowski, ; de de Souza, Marinoni & Marinoni, ; Kesting, Petersen & Isselstein, ) or in combination with productivity (Guo, Shaffer & Buhl, ; Ptacnik et al ., ; Di Pippo et al ., ) as tests for saturation. While this may be appropriate in specific cases, we call attention to the risk of confusing sampling completeness with saturation.…”