Growth, fire history, and browsing recorded in wood rings of shrubs in a mild temperate climate

Abstract

Abstract. Separate effects of abiotic and biotic factors on the structure and dynamics of ecological communities may be recorded in growth rings of woody plants. We used Ceanothus cuneatus rigidus and Arctostaphylos pumila to tease apart the roles of fire, rain, and herbivores on the histories and community structure of four areas in a coastal mediterranean-type climate in central California with mild winters and mild summers. Ring widths of both species were related to rainfall in two of the areas; heavy deer browsing on Ceanothus overwhelmed the climate signal in the others. Ceanothus germination was more closely related to heavy rainfall, especially during ENSO years, than to fire events. In a related greenhouse experiment that evaluated these observations, the same proportions of new Ceanothus seeds germinated after burning and after receiving regular water for several months, but germination of old seeds responded primarily to the fire treatment. In areas where heavy browsing by mammals reduces recruitment and growth of Ceanothus and increases mortality, the continuance of the Ceanothus population must rely heavily on germination from the persistent seed bank during unusually wet years or after occasional fires. Because Arctostaphylos can produce new stems from underground roots, individual plants may survive and produce seeds until another fire.

Browsing impacts on the stable isotope composition of chaparral plants

“…Many control Ceanothus suffered considerable dieback and some died completely, probably as a consequence of intense deer browsing over a long period of time (Coale et al. ). We replaced them as needed by sampling similar but healthy plants as close to the original control plot as possible.…”

Herbivory mediates direct and indirect interactions in long‐unburned chaparral

“…These webs (Figure 7a–c) can be viewed as hypotheses that show how two direct interactions—browsing on Ceanothus and facilitation between Ceanothus and nitrogen‐fixing bacteria—led to indirect linkages among component shrubs, herbaceous plants, open space (this paper), and soil N (L. R. Fox, unpublished). The interaction webs are predominantly based on data from this paper but are also informed by other observations and experiments at FONR (Coale et al, 2011; Deveny & Fox, 2006; Fox, 2007; Fox et al, 2006; Koch & Fox, 2017; Pittermann et al, 2014; and L. R. Fox, unpublished). These webs cannot legitimately be combined into one overall interaction web (Yang, 2020) because they represent different, mutually exclusive sets of interactions that do not occur at the same time or in the same space.…”

“…FONR (Coale et al, 2011;Deveny & Fox, 2006;Fox, 2007;Fox et al, 2006;Koch & Fox, 2017;Pittermann et al, 2014;and L. R. Fox, unpublished). These webs cannot legitimately be combined into one overall interaction web (Yang, 2020) because they represent different, mutually exclusive sets of interactions that do not occur at the same time or in the same space.…”

“…FONR is a fire‐dynamic system that probably has not burned in over 100 years. This assessment is based on counts of tree rings in a dominant shrub, A. pumila (Coale et al, 2011) and oak, Q. agrifolia (L. R. Fox, unpublished), together with analyses of A. pumila growth and aerial photographs since 1941 (B. Schulze, and L. R. Fox, personal observation). This burn interval is well within the expected 100–150 years for chaparral and oak woodlands in this region (Greenlee & Langenheim, 1980, 1990).…”

“…In our system, strong biotic interactions and abiotic effects continue to influence ongoing vegetation dynamics at a site that has probably not burned in at least ~100 years. Variations in composition and spatial structure of the plant community reflect demographic and physiological processes affecting shrubs, herbaceous plants, plant–plant interactions, and plant–herbivore interactions, all mediated in the context of changes in the abiotic environment (Coale et al, 2011; Deveny & Fox, 2006; Fox, 2007; Fox et al, 2006; Pittermann et al, 2014). This initial work indicated that browsing is a key interaction in this system (Coale et al, 2011; Fox, 2007; Koch & Fox, 2017), but the diverse biotic and abiotic impacts of browsing have not previously been integrated into causal pathways of interactions across the community.…”