Saving tropical riverine grasslands of the Brahmaputra floodplains

What are grasslands

Grasslands are highly dynamic ecosystems that include vegetation that is mainly dominated by grass or grass-like plants. These can be in the form of natural and semi-natural pastures, woodlands, scrub and steppe formations (Intermediate areas between forests and deserts made up of small grasses). The UNESCO defines grassland as “land covered with herbaceous plants with less than 10 percent tree and shrub cover” and "wooded grassland as 10-40 percent tree and shrub cover".

Grasslands occur where rainfall is usually low and/or the soil depth and quality is poor. Low rainfall prevents the growth of a large number of trees and shrubs in abundance but is sufficient to support the growth of grass cover during the monsoon. Low rainfall can also trigger droughts and fires that prevent the development of dense forests but grasses can survive fires and heat and their stems can grow again from where they have been burnt off. Many of the grasses dry up during the summer months while the grass cover grows back from the rootstock and the seeds of the previous year during the next monsoon. This change gives grasslands a seasonal appearance with periods of increased growth followed by a dormant phase.

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Tropical grasslands, undervalued and understudied

Tropical grasslands cover approximately 10 percent of Earth’s landmass, accounting for about 15 percent of the carbon stored terrestrially, and 30 percent of global net primary productivity informs this paper titled 'Impact of management practices on the tropical riverine grasslands of Brahmaputra floodplains: Implications for conservation' published in Ecological Indicators.

However, the conservation value of tropical grasslands continues to be underappreciated leading to their widespread degradation due to agricultural conversion, afforestation, sand mining, urbanisation, changes in intensity and frequency of burning, biological invasion, overgrazing, altered edaphic conditions, and woody encroachments.

Grasslands are also vulnerable to climate change induced frequent floods and droughts. Forest restoration strategies such as afforestation, suppressing fire, excluding large grazing animals or soil fertilisers are commonly misapplied to tropical grasslands. Lack of evidence-based studies on tropical grasslands make it difficult to determine which management regime works best for their restoration and rewilding.

Management strategies to conserve riverine grasslands

Protected Areas of India and Nepal are the last remaining highly productive subtropical grassland ecosystems, which are understudied with respect to impact of management interventions on grassland integrity and the animals they support.

Although, these riverine grasslands are legally protected through the Wild Life (Protection) Act, 1972 of India, and National Parks and Wildlife Conservation Act, 1973 of Nepal, they are vulnerable to flooding and subjected to excessive grazing and poor management practices such as burning and invasive uprooting. The conservation of these vulnerable tropical grasslands requires scientifically validated effective management strategies. 

This study on the riverine alluvial grasslands of Kaziranga Tiger Reserve (KTR) in the Brahmaputra floodplains aimed at generating baseline information to aid the management authorities in formulating and upgrading management practices in the grasslands. 

The study aimed at answering the following questions: 

• Does the duration of inundation influence short grassland (SG) and tall grassland (TG) extent in KTR
• What are the vegetation characteristics and soil nutrient content (C, N, P and K) of SG and TG
• Is there any similarity between species composition of SG and TG in the selected sites
• How do environmental variables such as rainfall (RF), maximum temperature (MAXT), minimum temperature (MINT) and soil nutrient content (C, N, P and K) influence plant species distribution in SG and TG.

Kaziranga was first declared as a Reserve Forest (1908) and later redesignated to Game Sanctuary (1916), Wildlife Sanctuary (1950), National Park (1974), UNESCO World Heritage Site (1985) and Tiger Reserve (2007). Floods are an annual event in Kaziranga.

The study found that:

• Areas inundated for a longer duration were dominated by SG and have higher species diversity, while areas inundated for the shorter duration are dominated by TG and have had low species diversity
• Not only did the duration of flood impact SG and TG extent, flood-induced moist conditions in SG also could have retarded the development of invasives, while these were found to proliferate in TG and multiply more in dry season. 
• The absence of grazing animals increased plant species diversity in the grasslands due to high litter accumulation, including post-flood senescent vegetation inside the control enclosures,  undisturbed plant growth, root turnover and microbial activities as well as woody species growth, invasive species occurrence as well as soil nutrient content.
• Large and small grazing animals consumed both woody and invasive species, and can possibly reduce species diversity and growth of woody and invasive species. 
• In TG, burning decreased species diversity and woody species and enhanced soil nutrient content and proliferation of herbaceous invasives, while invasive uprooting promoted species diversity and soil nutrient content, and decreased woody and herbaceous species. 

The study suggests that:

• Short-term rotational grazing needs to be incorporated into the management regime for rewilding SG, and long-term rotational grazing for rewilding TG. 
• Controlled burning must be carried out only in completely dry grassland patches, and in invaded patches post manual uprooting of fire-adapted invasives. Additionally, manual uprooting of invasives must be carried out seasonally, during both dry and wet cycles. 
• Compared to TG, less scientific and management focus has been given to SG, especially with respect to grazing and invasive management. This study shows that if managed properly, the SG have the potential to provide more foraging ground and habitat to grazing animals , but also other animals.  
• Alteration of the flooding regime due to reduced water availability may affect the ratio of SG and TG extent in the long run, which may alter the vegetation composition and the productivity of these grasslands.