Extreme drought has limited effects on soil seed bank composition in desert grasslands

Prepared by Alejandro Loydi & Scott Collins

Climate change will impact all ecosystems across the globe, with particularly strong effects on mean annual temperature and rainfall. Temperature will increase worldwide, while changes in precipitation will depend on the region. In the southwest USA, predictions indicate that mean annual rainfall will become more variable, including more frequent and severe drought periods. This will not only affect aboveground vegetation, but also the plant seeds buried in the soil that constitute the soil seed bank. We know very little about the ecology of the seed bank in the region, and we know even less about how it may react to changes in annual rainfall. Therefore, we took advantage of an ongoing climate change experiment in the northern part of the Chihuahuan Desert of New Mexico, the Extreme Drought in Grasslands Experiment (EDGE), which started in 2012. Here grasslands had been subject to two different treatments, according to climate change predictions: a 66% reduction in growing season precipitation or a delay in the start of the rainy season from July to September. This experiment was replicated in two different grassland communities, Great Plains grassland dominated by blue grama and the Chihuahuan Desert grassland dominated by black grama. In 2017, aboveground vegetation was measured in spring and fall, and in mid-September, soil samples were collected and seed bank evaluated by germinating seeds in the greenhouse.

We found that rainfall manipulation treatments increased the number of species in aboveground vegetation and seed bank, but only at the Great Plains grassland. Also, while vegetation cover was reduced by both treatments at both sites, the number of seeds in the soil increased or remained the same, being not affected by rainfall changes. Moreover, the species present in the seed bank were similar among all treatments and mostly dominated by annuals and some perennial forbs. This is reflected in aboveground vegetation where perennial grasses reduced their cover due to drought and annual and perennial forbs increased.

In most grasslands around the world, seed banks are dominated by annual and short-lived species, and New Mexico grasslands were not the exception. But, when subjected to changes in precipitation, the seed bank was less affected by drought than aboveground vegetation. This “resistance” to drought may play an important role in maintaining ecosystem processes during and following drought periods, because seed banks can enhance long-term community stability. In the future, species loss due to drought might be compensated by an increase in drought-tolerant species that are already present in the seed bank. This might generate a shift in species composition from perennial-dominated grasslands to annual-dominated grasslands. Alternatively, since the dominant species in the study area rely less on the seed bank to survive, the high abundance of annual species might play an important role in facilitating the recruitment of perennial species. These may take longer to recover (from surviving individuals or from seeds), and during this transition, the establishment of new individuals from the seed bank will provide safe sites for the establishment of longer-lived species as grasslands recover from extreme climate events. In this way, the drought-resistant soil seed bank might contribute to vegetation recovery after drought.

This is a plain language summary for the paper of Loydi & Collins published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13089).