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.
