Climate Change, Past and Future: A Vegetation History of Southern California, from 800 to 1600

     
      California is currently in the latest of a long series of droughts, which are by some models predicted to intensify with the progress of climate change (Diffenbaugh, Swain, & Touma, 2015). This is likely to impact plant survival and community composition in this critical biodiversity hotspot. On a more moderate scale, drought has always been a part of Southern California’s climate and has had a role in shaping the vegetation present. A recent study by Heusser, Hendy, and Barron (2015) provides new and detailed information on how plant communities changed in response to drought and moisture over 800 years (800-1600 AD) in Southern California. This time range covers two important climatic periods: the Medieval Climate Anomaly (MCA, 800-1200) and the Little Ice Age (LIA, 1300-1600). It investigates the forces influencing how these communities came to be and how they changed over time. In doing so, it provides needed indications of how plant communities in this area may respond and adapt to climate change and different moisture patterns in the area. 

                                              

Figure 1. Map from Heusser, Hendy, and Barron (2015) displaying the plant communities of the area and the locations of sediment extraction. Red lines indicate rivers and streams.

   

     This area is chaparral-dominated, due to the influence of Southern California’s seasonal rains, drought, and overall low moisture. The experiment used sediment samples from the Santa Barbara Basin (SBB), a shallow, open-water marine environment near the Channel Islands. Below is a picture from the article showing the Santa Barbara area’s biomes as well as the sources of the sediment cores.
     One portion of the experiment was aimed at making sure that using pollen deposited in marine sediment in the area is a valid way of reconstructing its historical climate and vegetation. Overall, they determined that the pollen composition in the sediment reflected what plant communities existed on the shore and their relative abundance

     To reconstruct historical plant communities, they studied a high-resolution (5-year time interval) sediment core containing pollen that had been deposited in the SBB from 800 to 1600 AD. This allowed them to find out whether mesic or xeric species dominated a particular time interval. This information in turn illustrates how the vegetation changed in response to the climatic periods studied. The authors also stated, based on other research (Barbour & Major, 1977), that they considered mesic vs. xeric species prevalence a proxy for soil moisture due to this being the limiting factor in their environment. This provided another way of measuring wet and dry periods. For the period studied, mesic species included oaks like Quercus agrifolia and Q. lobata and pines such as Pinus ponderosa and P. muricata. Xeric species included members of the broom-like genus Baccharis, members of the Asteraceae family, and coastal sagebrush (Artemisia californica). Examples of and further information on Southern California’s oak woodlands and chaparral, analogs to these mesic and xeric communities, respectively, from Las Pilitas Nursery.

Figure 2. Examples of xeric and mesic plant species found in Santa Barbara, all species, or members of families or genera, mentioned in the paper.

    During the period studied, two major dry periods occurred: one between roughly 850 and 1090, and the next between 1200 and 1300. The authors determined that each time a dry period occurred, xeric species dominated and oaks declined, evidenced by the increased amount of their pollen in the sediment. When the dry periods ended, mesic species like oaks and pines rebounded and xeric species declined but did not die out entirely. These dry periods roughly line up with the two climatic periods in question: the dry periods end when the MCA transitions to the LIA. The MCA was therefore dominated by these xeric species, especially chaparral species; the LIA witnessed the prevalence of oak-dominated mesic communities. That being said, the dry vs. wet period binary did not account for all of the variation between or within periods. Species’ pollen counts varied significantly within the MCA and LIA, displaying peaks and troughs that interrupted their declines and increases. This could indicate smaller moisture fluctuations or other variations in weather patterns. It is also important to emphasize the time scale for these vegetative changes: for example, oaks saw increases during wet periods over centuries, not decades. Therefore, any changes in response to climate change will be slow, and it will likely be centuries until the full effects are seen.

Figure 3. Timeline displaying the moist and dry periods, as well as the Medieval Climate Anomaly and Little Ice Age, adapted from the paper.

     One important aspect of their findings was the fact that there was a period during the MCA that was considered wet based on oak vs. xeric species prevalence: 1100-1200. This is interesting because it is a new finding: the authors state that previous, low-resolution studies using pollen samples from the area observed an uncomplicated and gradual transition from the moisture of the MCA to the LIA. Instead, this high resolution study found that the transition was rockier, and moisture levels did not always conform to climate-period-based expectations.
     This paper points towards the idea that the Southern California flora is dynamic, not static, even over relatively short periods of time, providing an example of how it may change in response to different weather patterns. Most likely, mesic species will further decrease- the authors cited research stating that oaks are likely to become restricted to areas near water (Kueppers, 2005; Sork et al., 2010; McLaughlin & Zaveleta, 2012)- and xeric species will increase. On a hopeful note, it also indicates the resilience and flexibility of the plant communities in this area, providing a welcome sign that they may be able to cope with the heat and moisture changes brought about by climate change.
 
                                                                    Works Cited
Barbour, M. G., & Major, J. (2007). Terrestrial vegetation of California. University of California Press.
Diffenbaugh, N. S., Swain, D. L., & Touma, D. (2015). Anthropogenic warming has increased drought risk in California. Proceedings of the National Academy of Sciences, 112(13), 3931–3936. https://doi.org/10.1073/pnas.1422385112
Heusser, L. E., Hendy, I. L., & Barron, J. A. (2015). Vegetation response to southern California drought during the Medieval Climate Anomaly and early Little Ice Age (AD 800–1600). Quaternary International, 387, 23–35. https://doi.org/10.1016/j.quaint.2014.09.032
Kueppers, L. M., Snyder, M. A., Sloan, L. C., Zavaleta, E. S., & Fulfrost, B. (2005). Modeled regional climate change and California endemic oak ranges. Proceedings of the National Academy of Sciences, 102(45), 16281–16286. https://doi.org/10.1073/pnas.0501427102
Mclaughlin, B. C., & Zavaleta, E. S. (2012). Predicting species responses to climate change: demography and climate microrefugia in California valley oak (Quercus lobata). Global Change Biology, 18(7), 2301–2312. https://doi.org/10.1111/j.1365-2486.2011.02630.x
Sork, V. L., Davis, F. W., Westfall, R., Flint, A., Ikegami, M., Wang, H., & Grivet, D. (2010). Gene movement and genetic association with regional climate gradients in California valley oak (Quercus lobata Née) in the face of climate change. Molecular Ecology, 19(17), 3806–3823. https://doi.org/10.1111/j.1365-294x.2010.04726.x
Plant Communities of California. Las Pilitas Nursery. https://www.laspilitas.com/nature-of-california/communities.
 
                                                                         Image Credits
California Polytechnic State University. (n.d.). A Tree Selection Guide. SelecTree.        https://selectree.calpoly.edu/.
Constancea nevinii. San Diego Botanic Garden. (n.d.). https://www.sdbgarden.org/plant-63.htm.
Native Plants. Native Plants - CSU Channel Islands. (n.d.). https://nativeplants.csuci.edu/.
Plant Communities of California. Las Pilitas Nursery. https://www.laspilitas.com/nature-of-california/communities.