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Thursday, June 23, 2016

Drought effects on chaparral shrublands on Tejon Ranch by Dr. R. Brandon Pratt, Department of Biology, California State University Bakersfield

Chaparral vegetation refers to a dense and typically impenetrable shrubland primarily found in low to mid elevations in California (Figure 1). Chaparral shrublands cover the most area of any vegetation type in California, and while these shrublands are found throughout the state they are most abundant in southern California (Parker et al. 2016). The climate where chaparral thrives is Mediterranean-type, which is characterized by hot dry summers and cool moist winters. This particular climate is only found in five areas of the globe: California, the Mediterranean Basin, central Chile, Western Australia, and the Cape Region of South Africa. Not only do all these regions have a Mediterranean-type climate, they also have shrublands that appear similar to chaparral, which has fascinated scientists for at least the last 150 years. One characteristic feature of the shrubs that inhabit such shrublands are their evergreen, thick and leathery leaves (Figure 1).

Figure 1. Chaparral shrubland on Tejon Ranch on the south slope of the Tehachapi Mountains (top photo). Common shrub species at this site are Ceanothus vestitus (California lilac), Adenostoma fasciculatum (chamise), Arctostaphylos spp. (manzanita; right photo), Quercus berberidifolia (scrub oak), and Cercocarpus betuloides (birch-leaf mountain mahogany). The bottomphoto shows a close up of the leaves and fruit of Arctostaphylos glauca “big-berry manzanita”. The name manzanita comes from the Spanish word for “little apple”, which refers to the fruit seen here.

The recent droughts in California have had a large effect on California’s ecosystems, including chaparral shrublands. In many areas, chaparral species have experienced substantial dieback and mortality (Paddock et al. 2013; Pratt et al. 2014). This is a concern for a number of reasons, but one important concern is how this dead material on the landscape affects flammability and the possibility of high intensity fires. Other concerns are the resilience of these systems to the droughts. In other words, will these systems retain their basic structure and function in response to drought or will they convert from closed canopy shrublands to more open savannah landscapes as shrubs continue to die (Figure 2). Such transformation has been documented in highly disturbed shrublands and is associated with loss of biodiversity and critical habitat for a range of important organisms that make their homes in chaparral shrublands (Parker et al. 2016).

Tejon ranch has chaparral communities on the mid-elevation southern slopes of the Tehachapi Mountains facing the Antelope valley (Figure 1). Beginning in fall of 2015, we began a study of how chaparral shrubs on Tejon Ranch are responding to drought. This began as a class field trip with my California State University, Bakersfield Plant Physiological Ecology class (Figure 2). Our group was joined by another Plant Physiological Ecology class from Pepperdine University being taught at the same time by Stephen Davis. As a group, we surveyed dieback and mortality of the site and made physiological measurements such as plant water status and photosynthetic rates (the rate of carbon dioxide uptake from the air that plants use to make carbohydrates using solar energy). Most of the energy and food for animals in these systems come from these shrubs, called primary producers, thus their health and function represents one of the most important aspects of how these ecosystems are functioning.

Figure 2. Chaparral shrubland (top photo) with considerable mortality and dieback among shrubs, particularly Ceanothus vestitus in this view (note the whitish/gray dead branches on the shrub in the foreground and throughout the landscape). When these shrubs die they open up space in the shrubland converting it from a closed canopy to a more open savannah type vegetation with abundant annual alien invasive species that are seen in the foreground of the top photo. In the bottom photo, graduate students Mitchell Coleman (right) and Alex Baer (left) are measuring photosynthesis of a scrub oak in the field.

Our initial survey in November 2015 indicated that most plants had greater than 50% dieback (half their crown was dead) and that the plants were experiencing water deficits and were not very physiologically active. These results would all be consistent with the ongoing droughts occurring in California. At this stage we are not certain what the exact cause of the dieback is, and it is going to be difficult to sort this out since most of the mortality occurred prior to the start of our study. This pattern of dieback and mortality is consistent with the effects of drought that have been observed at other chaparral sites in southern California (Paddock et al. 2013; Pratt et al. 2014).
One factor that is not particularly well studied is what happens to severely stressed plants when droughts cease. The winter of 2016 was forecast to be a wet one with a strong El Niño condition present. This provided a unique opportunity to follow these shrubs over time to see if they recover in response to a wet winter. In this context, we have continued to sample the shrubs at this field site on an approximately monthly basis at this field site through the winter of 2016. This work is ongoing, and in early measurements we have seen that the shrubs have been able to tap into the winter rains to improve their level of hydration. However, this has not translated into higher rates of photosynthesis for most of the species. The reasons for this may be drought related, but there are other interesting factors that could contribute as well. For example, there are exotic soil substrates present on the chaparral dominated slopes, such as limestone, and these may create mineral deficiencies that limit maximum photosynthetic rates. To examine this possibility, we have been measuring the nitrogen content of leaves since nitrogen is typically a key limiting nutrient for photosynthesis. We have found that some of them have rather low values, but not outside the range of previous studies. Another intriguing factor is related to the temperatures at this field site. The site has a Mediterranean-type climate, but it may experience colder winters than many other chaparral sites (we had abundant snow for our February sampling campaign). It is also a site that faces the Mojave Desert meaning it may be above average (compared to a typical chaparral site) in temperature in the summer and dry. This combination of colder winters and hotter and drier summers would place these evergreen shrubs under considerable strain. We are working with the Conservancy to acquire detailed weather data for this area to clearly determine the weather patterns at this site, but we do have some data suggesting some unusual adaptations of these shrubs.
The cells in the vascular system that transport water to keep the leaves hydrated have to function year round because evergreen chaparral leaves are active all year. Freezing temperatures and hot dry conditions create especially challenging conditions for water transport. For example, when water in these cells freezes solid during a hard frost (about 21 oF), bubbles form in the ice because gas is not as soluble in ice as it is in liquid water (check the ice cubes in your freezer to observe this phenomenon). When this water thaws during the day after a cold night, these bubbles can expand filling the transport cells with air and rendering them unable to transport water. If this repeatedly happens over the course of a winter the number of blocked cells can accumulate and lead to dehydration and death of the leaves and branches eventually causing dieback. The most challenging conditions occur when freezing and thawing occur when the plants are dehydrated, which may be common for Tejon chaparral in the fall when winter rains are late and early frosts occur.
One trait that can help avoid this freeze/thaw stress is to produce transport cells that have narrow diameters. This is because the bubbles that form in ice in small cells are smaller, and smaller bubbles are better able to dissolve during thawing. As an interesting aside, this is one reason why conifers do so well in cold habitats (high latitudes and mountains) as their transport cells are among the smallest diameters. We are currently examining the vascular traits of these shrubs to evaluate the anatomy of their vascular system in the context of vulnerability to freeze/thaw-induced vascular damage (Figure 3).
The long-term implications for Tejon chaparral are not easy to predict with the little data we presently have, but continued study of these shrublands is providing important insights into how these systems are unique. In particular, the juxtaposition of cold with hot and dry conditions may have driven these shrubs to develop specialized vascular adaptations. In the context of fire, the substantial dead biomass on the landscape means that when a fire ignites it may be one of high intensity. This may not be a bad thing for this ecosystem (chaparral vegetation is resilient to fires every 25-100 years), but it will make such a fire more dangerous and difficult to manage. The creation of gaps due to dead shrubs may lead to an expansion of non-native grasses and forbs, which can make fire more frequent and lead to degradation of these communities.

Figure 3. A cross section of big-berry manzanita vascular tissue (xylem) magnified 200x under a microscope. The large empty (white) cells are the ones specialized to transport water and are called vessels in flowering plants. The vessels of this species at Tejon Ranch are smaller than others that have been measured at other sites in southern California and this may relate to the cold fall and winter temperatures in Tejon chaparral sites. Image taken by Anna Jacobsen.

Literature Cited

Paddock III WAS, Davis SD, Pratt RB, Jacobsen AL, Tobin MF, López-Portillo J, Ewers FW. 2013. Factors determining mortality of adult chaparral shrubs in an extreme drought year in California. Aliso 31: 49-57.

Parker TV, Pratt RB, Keeley JE. 2016. Chaparral. H Mooney, and E Zavaleta, eds. Ecosystems of California. Univ of California Press.

Pratt RB, Jacobsen AL, Ramirez AR, Helms AM, Traugh CA, Tobin MF, Heffner MS, Davis SD. 2014. Mortality of resprouting chaparral shrubs after a fire and during a record drought: physiological mechanisms and demographic consequences. Global Change Biology 20: 893-907.