Shrubs exhibit competitive interactions with herbaceous plants and shape community assemblage and functional composition in the alpine western Himalaya

Prepared by Bittu Ram and Amit Chawla

Patches of evergreen alpine shrub, Rhododendron anthopogon, at Rohtang. Photo credit: Bittu Ram

In the Himalayan region, alpine shrub meadows are one of the major ecosystems that cover a large geographical area and are very vulnerable to climate warming. The shrubs are colonizing the alpine meadows in this region at a faster rate due to rapid global warming. Generally, alpine regions are characterized by stressful environmental conditions for plants, and it has been reported that shrubs act as facilitators to the herbaceous species by providing conducive environment for their growth and persistence, and increasing local plant diversity. However, a few studies have also suggested competition between shrubs and herbs due to the common sharing of limited resources. Thus, studies in the alpine region around the globe suggest shrubs as a keystone species that structure plant communities through competition-facilitation interactions. However, limited research has been conducted to assess the role of shrubs in structuring alpine plant communities in the Himalaya. Therefore, our study aims to evaluate the role of dominant shrubs in organizing herbaceous communities in the alpine region of the Himalaya. We wanted to know how shrub-herb interaction shapes herbaceous communities along elevations having varying levels of stressful environmental conditions.

Patches of deciduous shrub, Caragana versicolor, at Komic. Photo credit: Bittu Ram

We selected three shrub species, a broadleaf evergreen (Rhododendron anthopogon), a needleleaf-evergreen (Juniperus polycarpos), and a broadleaf-deciduous (Caragana versicolor). To determine the interaction between shrubs and herbs, we estimated the Interaction Indices and other statistics, and also analyzed the functional traits of herbaceous communities. We found that these dominant shrubs have competitive interactions with the associated herbaceous species in the study region. Our results also demonstrate that though soil under the canopy of these shrubs have a higher availability of nutrients, this did not contribute toward the facilitation of herbaceous species. Instead, we found a strong competition between shrubs and herbaceous species for available resources and decreased herbaceous species richness and abundance in the shrub canopies. Thus, these shrubs have the capability to inhibit the establishment of the many herbs beneath their canopies and only those herb species can co-occur under the shrub canopies that have faster resource acquisition traits such as possessing higher leaf area, specific leaf area and plant height for rapid uptake of resources. This reflects the better competitive ability of under-canopy herb species to establish and survive in the highly competitive microhabitats created by shrubs. In contrast, the surrounding open area has a higher herbaceous species richness and abundance that exhibits resource-conservative traits as they uptake resources at a slower rate as an adaptation strategy. Moreover, our study also suggested that the competitive interaction was more intense at higher elevations because of more competition for limited resources under stressful environmental conditions. Our study concludes that the encroachment of alpine shrubs in the alpine region of the Himalayas under future climate warming may pose a serious threat and could result in the loss of many rare but important herb species. Therefore, it would be important to comprehend the causal factors, current rate of shrubification and predict future shrub colonizing areas.

Fieldwork undertaken in the Rhododendron anthopogon dominant shrub patch. Photo credit: Bittu Ram

This is a plain language summary of the paper of Ram and Chawla published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13269)