Climate Change and the Future of Vernal Pools

Vernal pools are ephemeral, isolated wetlands filled by rainwater during the wet season which evaporates during the dry season. These annual pools are vital to the health of forested ecosystems, sequestering toxins, recycling nutrients, retaining floodwaters, and renewing groundwater. Vernal pools also provide habitat for diverse floral and faunal communities that are heavily adapted to the annual cycle of pool formation and evaporation (Kenk 1949, Wiggins et al 1980, Brooks and Hayashi 2002). In California, 60 endemic taxa, both animal and plant – most of which are rare or in danger of extinction – rely upon vernal pools (Javornik and Collinge 2016). These wetlands allow for the formation of seed banks and provide predator-free areas for amphibian development. Habitat loss, invasive species, and climate change are the three main threats to the vernal pool ecosystem. The primary cause of habitat loss is agricultural development, and the secondary is the expansion of urban sprawl. Invasive plants outcompete native plant communities, which is catastrophic for the rare flora using vernal pools and the biodiversity of the local ecosystem. Climate change has caused weather variability which means uncertainty in rainfall, dry spells, change in local plant composition and abundance, and range shifts across all taxa (Walther et al 2002, Parmesan and Yohe 2003). All of these factors in combination drastically affect the biodiversity, health, and presence of vernal pool ecosystems. This paper will address the specific effects of climate change on these ephemeral wetlands.

Vernal pool hydrology affects the surrounding floral and faunal communities and, therefore, must be addressed first. Hydrology and annual hydroperiods are affected by long-term climatic conditions, size of the pool or depression, and physical properties of the site such as elevation and vegetation (Larson 1995, Lent et al. 1997, Kirkman et al. 1999, Brooks and Hayashi 2002). The source of water in a vernal pool ecosystem is mainly precipitation, with minimal contribution from snowmelt, and water is lost through evapotranspiration. Pools begin to accumulate water in the winter months and sustain water volume throughout the spring with frequent rainfall. Water volume is directly related to weekly precipitation, with weather patterns explaining over 50% of water variability in vernal pools (Brooks 2004). Figure 1 depicts the relationship between precipitation, evapotranspiration, and pool depth. Pool depth decreases when evapotranspiration is greater than precipitation, and it increases when precipitation is greater than evapotranspiration. Initial precipitation events recharge groundwater, and the following rainfalls and snowmelt fill the pool to capacity. Evapotranspiration is low during the winter and high during the summer because plants are in hibernation and not transpiring in the winter. In the summer, there is a negative water balance, meaning the amount of water lost is greater than the water gained. It is during this time vernal pools dry up. Lower elevations and increased annual precipitation can allow a vernal pool to retain its water throughout the year (Javornik and Collinge 2016, Brooks 2004). Pool size also significantly affects the length of the hydroperiod. The shallower the pool and greater the surface area, the faster the water will evaporate (Brooks and Hayashi 2002).


Figure 1 (Javornik and Collinge 2016)

The percentage of vernal pools with water is positively correlated with precipitation and negatively correlated with temperature (Brooks 2004). Figure 2 depicts predictions in climatic temperature changes in the United States’s within the next century as compared to the 20th century if the U.S. follows a low emissions plan. The northeastern United States is predicted to become wetter as well as warmer in the 21st century (New England Regional Assessment Group 2001). Precipitation events will become erratic with flooding interspersing weeks of drought, which will cause vernal pools to dry up and inundate multiple times annually in the northeast (Karl et al. 1995, Moore et al. 1997). These predictions are already coming true. The northeast region has seen an increase in annual temperature; the most dramatic seasonal change is a 2.1°C rise during the winter. The duration of winter shortens as autumn ends later and spring begins earlier. The predicted increase in precipitation has not happened yet (Hayhoe et al. 2007). This climatic change has devastating effects on the surrounding biodiversity.


Figure 2 (National Climate Assessment)

A study in California examined the possible effects of climate change on plant composition in vernal pool wetland ecosystems. Researchers altered the timing and amount of rainfall to which vernal pools were exposed. Vernal pool flora are typically annual, germinating in the wet season and reproducing in the dry season. Germination is solely triggered by first precipitation events which tend to occur in October-November, and growth can continue during periods of inundation (Bliss and Zedler 1998). Earlier first precipitation events resulted in a more native plant community composition, while later precipitation events favored invasive growth (Javornik and Collinge 2016). Native seeds seem to be adapted for early or on time first precipitation events and are therefore at a disadvantage during years with late rainfall, allowing invasive plants the opportunity to grow first. Early rainfall acts as a type of ecological filter, impairing the growth and establishment of invasive species. Increased rainfall over the whole wet season discouraged the viability of some invasive taxa, specifically those in the family Poaceae, however, it also affected the viability of a few native species. This ‘inundation filter’ proved effective in discouraging invasive plant growth overall, but Luzula multiflorae, an invasive species in California, flourished with or without inundation, bypassing the inundation filter and dominating local plant communities. The effects of the inundation filter increased with lower elevation. In general, early first precipitation events and inundation are beneficial for preserving the native floral biodiversity in a vernal pool ecosystem.

Amphibian biomass in a forest outweighs all other vertebrate biomass combined, and a number of them rely upon vernal pools for reproduction and larval development (Burton and Likens 1975; Petranka and Murray 2001). Because these wetlands are temporary and isolated, they are free of predators like fish. This provides a relatively safe place for tadpoles and larvae to metamorphose into adults. During years with long hydroperiods, vernal pools will see an increase in fauna biodiversity (Schneider 1999). Amphibians travel to vernal pools to mate and lay eggs in the spring, though some Ambystoma species start as early as November. A  10-year study in Massachusetts looked at how hydrology impacts two amphibian species: Lithobates sylvaticus and Ambystoma maculatum (Brooks 2004). L. sylvaticus requires 8-19 weeks to metamorphose, while A. maculatum requires 13-24 (DeGraaf and Yamasaki 2001). Longer hydroperiods allow the tadpoles and larvae time to develop more fully, resulting in a fitter organism (Semlitsch et al. 1988). However, the predicted increase in precipitation that climatologists are calling for will not extend hydroperiods. An earlier annual increase in temperature will allow plant growth to flourish, increasing rates of evapotranspiration. Because of increased temperature and evapotranspiration, there will be less water in vernal pools despite increased precipitation. A weekly precipitation greater than 2 cm is required for vernal pools to keep water volumes steady (Brooks 2004). Climate change will cause periods of drought interspersed by flooding which means vernal pools will dry up multiple times throughout amphibian breeding seasons. The pools will evaporate before the tadpoles and larvae have metamorphosed, reducing viable offspring for the season. If vernal pools do not fully dry up between rainfall events, the water volume will still significantly decrease, leading to increased competition, smaller adults, and a higher juvenile mortality rate (Wilbur 1987, Egan and Paton 2004). Interestingly, at low larval densities higher temperatures in vernal pools actually increase the survival rate (Govindarajulu and Anholt 2006). During the 10-year study, L. sylvaticus was able to breed for four years and A. maculatum for one. These success rates would decrease with climate change, causing massive declines in pool-breeding amphibian species.

Ongoing research is being conducted by Smithsonian researchers and citizen scientists from the Virginia Master Naturalist program on vernal pools in Newport News, Virginia. The Smithsonian is surveying for salamander species in the forests surrounding vernal pools and in the spring will survey for salamanders utilizing the pools, and the naturalists are taking an inventory of the number of vernal pools. Once they are completed, the combined data from these two projects will give land managers in Newport News an understanding of the sensitive salamander species using vernal pools and if the current amount of pools  is adequate to provide the habitat required for population stability of threatened species. The Smithsonian did not respond to efforts to get in touch, but a Master Naturalist said their target species for conservation is Ambystoma mabeei, a salamander found only in southeastern Virginia and part of North Carolina, which breeds in ephemeral pools.

The species most likely to be affected by climate change are those with low-mobility, small ranges, and reliance on hydroperiods (Rodenhouse et al 2009). Climate change is predicted to favor southern species and allow their ranges to expand northward (Logan and Powell 2005; Rodenhouse et al. 2008). The increasing temperatures have led to earlier call dates for some frog species. In 2015, spring peepers were reported to be calling as early as December in Virginia (Gibson and Sattler 2016). This might affect breeding dynamics in vernal pools, causing earlier mating which would be a waste of energy, because January and February bring long periods of freezing temperatures, destroying any eggs deposited. In contrast, Rodenhouse et al 2009 speculates the shortened winters will allow frogs to mature earlier and increase overall fitness. In the future, amphibian populations will decline significantly due to periods of drought between spring storms, but the individuals that mature may be large and physically fit. If predictions are correct, native flora will benefit from climate change if earlier first precipitation events and increased annual rainfall occur. In conclusion, predicted changes to climate will discourage invasive plant germination and growth at vernal pools but will also fluctuate hydroperiods and decrease success rates of amphibian reproduction.

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Thank you for reading. This was my final paper for my Wetland Ecosystems class. To learn more about conservation and ongoing research, follow my twitter.


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