Forbs is a catch-all term for all herbaceous plants in grasslands that are not graminoids. That is, these are the species that are not in the plant families Poaceae, Cyperaceae, and Juncaceae.
The focus of this website is obviously on the graminoid family Poaceae, and grasses rightly have been the main focus of studies on current grasslands. But there has been a growing call by some researchers that forbs have been overlooked during this seeming lovefest for the Poaceae and other graminoids. They argue that forbs are actually extremely important due to the following reasons:
- In terms of species richness, forbs usually dominate grasslands. That is, there are more forb species in a grassland than there are grass (or other graminoid) species.
- Forbs also are more phylogenetically rich than graminoids in grasslands. There are more genera, family, and other higher level groupings of forbs than there are graminoids.
- Forbs also contribute substantially to the functional diversity of grasslands. The range and distribution of plant traits such a leaf size, root depth, and growth rate is grater in the forbs community in grasslands.
Given all these advantages, they wonder why forbs don’t dominate grasslands in terms of numbers and cover today. They even coined a name for this: the “Forbs Paradox” or “Paradox of Forbs”, and surmised that the answer to this paradox is that the grasslands today are missing some factor that gave an advantage to forbs in the past (Bråthen et al, 2021).
This missing factor may have been the megafauna that existed several thousand years ago, and a recent paper that used ancient eDNA to determine the proportion of grasses vs forbs in an ancient Mammoth Steppe landscape may lend some support their contention (Willerslev et al, 2014).
eDNA or Environmental DNA is genetic material from the environment, including from the soil, water, and air. It is DNA that has been shed by organisms into their surroundings and includes cellular DNA or extra-organismal DNA such as epidermal cells, pollen, spores and other traces, as well as naked DNA (extracellular DNA). The sampling of such eDNA has in many ways revolutionized the ability of researchers to detect species, assess biodiversity, and monitor ecosystems.
The two broad types of sampling for monitoring biodiversity are DNA barcoding (focusing on single species) and metabarcoding (barcoding coupled with high-throughput sequencing methods to detect multiple species or whole communities). Various studies have used eDNA sampling to detect rare, endangered, cryptic and invasive species; understand community composition, and plant–animal interactions; and reconstruct past flora (Banerjee et al, 2022).
Using large-scale ancient DNA metabarcoding, researchers looked at 50k years of arctic floral history and found interesting changes in the vegetation over that long time span, as that landscape switched from being a cold and dry Mammoth steppe to a moister tundra across the LGM (Last Glacial Maximum).
They found out that over much of the earlier period (pre-LGM and LGM, between 50k and 15k years ago), forbs dominated the landscape, especially taxa from the plant family Asteraceae, including from the genera Artemisia, Achillea, Chrysanthemum, and Tanacetum. C3 graminoids such as Bromus sp made up only around 20% of the samples at this time. Bolstering this result, a look at the diet of the megafauna using eDNA analysis of stomach contents showed that up to 63% consisted of forbs, with graminoids making up only 27% of the contents. However, after 10k years ago (post-LGM), the situation changed dramatically, with graminoids and trees becoming more dominant in the moister landscape.
Other studies have been done that seem to show similar results, wherein Mammoth steppes had forbs and graminoids dominating an environment before giving way to shrubs and trees post-LGM (Murchie et al, 2021).
The researchers noted that the change in floral composition and decrease in diversity might have been caused by the glacial change in the environment, but also proposed that the extinction of many megafauna post-LGM may have been a major cause of the decline in forbs. The trampling made by large megafauna may have favored forbs, and the higher nitrogen available from these megafauna may have also favored them.
Saying all that, there are possible caveats to the use of eDNA for determining populations in ancient environments. First of all, is the eDNA recovered actually from the time period being studied? Secondly, are the methods of recovering the eDNA and the informatics used accurate in terms representing the data, or are false positives and negatives non-negligible? Third, does the abundance of eDNA from a taxon correlate accurately with the amount of biomass it claims to represent? Finally, differential degradation of eDNA from various taxon could bias the results significantly.
Nevertheless, such recent studies offer an alternative and exciting window into exploring the ancient landscapes of the near past, and I look forward to more research being done that utilize eDNA.
References and Literature Cited
Bråthen, Pugnaire, Bardgett. The paradox of forbs in grasslands and the legacy of the mammoth steppe. Frontiers in Ecology and the Environment. 2021
Murchie, T.J., Monteath, A.J., Mahony, M.E. et al. Collapse of the mammoth-steppe in central Yukon as revealed by ancient environmental DNA. Nat Commun 12, 7120 (2021). https://doi.org/10.1038/s41467-021-27439-6
Pritam Banerjee, Kathryn A Stewart, Gobinda Dey, Caterina M Antognazza, Raju Kumar Sharma, Jyoti Prakash Maity, Santanu Saha, Hideyuki Doi, Natasha de Vere, Michael W Y Chan, Pin-Yun Lin, Hung-Chun Chao, Chien-Yen Chen, Environmental DNA analysis as an emerging non-destructive method for plant biodiversity monitoring: a review, AoB PLANTS, Volume 14, Issue 4, August 2022, plac031, https://doi.org/10.1093/aobpla/plac031
Willerslev E, Davison J, Moora M, Zobel M, Coissac E, Edwards ME, Lorenzen ED, Vestergård M, Gussarova G, Haile J, Craine J, Gielly L, Boessenkool S, Epp LS, Pearman PB, Cheddadi R, Murray D, Bråthen KA, Yoccoz N, Binney H, Cruaud C, Wincker P, Goslar T, Alsos IG, Bellemain E, Brysting AK, Elven R, Sønstebø JH, Murton J, Sher A, Rasmussen M, Rønn R, Mourier T, Cooper A, Austin J, Möller P, Froese D, Zazula G, Pompanon F, Rioux D, Niderkorn V, Tikhonov A, Savvinov G, Roberts RG, MacPhee RD, Gilbert MT, Kjær KH, Orlando L, Brochmann C, Taberlet P. Fifty thousand years of Arctic vegetation and megafaunal diet. Nature. 2014 Feb 6;506(7486):47-51. doi: 10.1038/nature12921. PMID: 24499916.
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