Present-day California is notorious for having chronic hot droughts, the most recent occurring from 2012-2015, which can impact the vegetation in the area (Kirby, et al. 2019). Additionally, the Pacific southwest U.S. (pswUS) region is prone to several precipitation-intense scenarios a year amidst drought periods, resulting in cycles of climate-induced vegetative stress (Berg and Hall 2015). As climate change continues to run its course, the need for ecological protection of native plants vulnerable to these cycles becomes increasingly important. One lens that can help scientists devise solutions to this issue is the paleo perspective, which uses historical ecological analyses to understand the present.
One recent study by Kirby, et al. utilizes the paleo perspective to answer the question: What can the ecological history of the hydroclimates of the pswUS and its relationship to vegetation during the late-Wisconsin period tell us about the protection of native vegetation from drought and flood events in the present time? The authors chose to look at the late-Wisconsin period (32-10k yBP) because it represents a time period that is wetter and colder than the present, allowing them to observe the regional vegetation during a time period of heavy rainfall. Their study site was Lake Elsinore, California, shown in Figure 1, which has itself experienced several drying events. During the late-Wisconsin period, the lake was actually shallowed due to an approximately 2,000 year-long glacial mega-drought, making the site a great area to observe the impacts of rainfall, drought, and vegetation in the pswUS.
In this study, the scientists obtained core samples from the lake and observed the types of pollen that were present during the late-Wisconsin period, which they pinpointed using accelerator mass spectrometry radiocarbon aging on the levels of core sample soil. They found 6 main pollen types: Amaranthaceae, Asteraceae, and Cyperaceae within the lake’s immediate vicinity and Quercus, Pinus, and Juniperus-type (Cupressaceae) further beyond the lake. Figure 2 shows some of the specific plants in each family that can be found in the region.
According to the authors, the evolution of plant types at various sample points in and around the lake reflects changes in the location of the border of the lake, which is impacted by the lake’s depth, which becomes deeper as the climate is wetter. Interestingly, the scientists concluded that changes in the presence of Pinus in particular indicates moisture levels and Juinperus- type (Cupressaceae) pollen indicates temperature. Increased Pinus represents more total moisture, and increased Juniperus-type Cupressaceae represents colder conditions; these conclusions operate vise versa, too. Additionally, they concluded that total sand percentage of soil core samples is a proxy for the dryness of the climate. A higher sand percentage is acknowledged to represent higher runoff and thus higher precipitation, and vice versa. By combining these two criteria of pollen and sand, the scientists identified the time period within the late-Wisconsin that was characterized by a massive glacial mega-drought (27.6-25.7 yBP), which they conclude experienced a climate that was much drier than usual, even if not as dry as the present. They observed that during this time period, there are decreases in Pinus and a small increase in Quercus, suggesting less total moisture, and decreases in Juniperus-type (Cupressaceae), suggesting warmer temperatures (Figure 3). There were also overall increases in scrub vegetation including Cyperaceae, Amaranthaceae and Asteraceae, which indicate a more shallow lake. This specific time period within the late-Wisconsin period is significant because it mirrors a similar phenomenon in the region today where the climate is become drier and more prone to droughts.
So, how does this vegetative history inform us of the regional flora today? Today, we still see an abundance of Cyperaceae, Amaranthaceae, and Asteraceae in the region. My own plant collections in the San Gabriel Valley (~60 miles northwest of Lake Elsinore) contains some Asteraceae species including Encelia californica (Bush Sunflower) and Cirsium vulgare (Bull Thistle), an invasive species. Species distribution maps reveal a concentration of key species within the pswUS region including Amaranthus californicus, Helianthus annus, and Schoenoplectus californicus, which are also present throughout California and the coast. Additionally, Juniperus-type (Cupressaceae) is also in high abundance throughout southern California, which is noticeable via the species distribution map for Juniperus californica. Lastly, we see decreases in both Pinus and Quercus around the immediate Lake Elsinore area; some species such as Pinus coulteri and Quercus engelmannii are only present in the mountainous geography of the pswUS, specifically in the Santa Ana Mountains west of Lake Elsinore. In the context of this study, these current trends reflect a similar dry, warm climate that was seen in the late-Wisconsin glacial mega-drought. If the same trend continues and becomes more intense with the course of climate change, Lake Elsinore is expected to dry out and potentially desiccate, resulting in expected further increases in Cyperaceae, Amaranthaceae, and Asteraceae as the lake’s border diminishes.
Works Cited
Berg N., Hall A. 2015. Increased interannual precipitation extremes over California under climate change. Journal of Climate 28(16):6324-6334.
Kirby, M. E., Heusser, L., Scholz, C., Ramezan, R., Anderson, M. A., Markle, B., … Rangel, H. (2018). A late Wisconsin (32-10k cal a BP) history of pluvials, droughts and vegetation in the Pacific south-west United States (Lake Elsinore, CA). Journal of Quaternary Science, 33(2), 238–254. https://doi.org/10.1002/jqs.3018
Kirby, M. E., Patterson, W. P., Lachniet, M., Noblet, J. A., Anderson, M. A., Nichols, K., & Avila, J. (2019). Pacific Southwest United States Holocene Droughts and Pluvials Inferred From Sediment δ18O(calcite) and Grain Size Data (Lake Elsinore, California). Frontiers in Earth Science, 7. https://doi.org/10.3389/feart.2019.00074
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