Its range extends as far north as the Cascade mountains and south into Baja California, though it is most dominant in southern California shrublands . They frequently occur on exposed ridges and rocky outcroppings. In the chaparral shrublands of Santa Barbara County, it occurs from elevations of about 500- 1200m. A. glauca are obligate seeders, and must recruit from the seedbank following fire . Compared to resprouters, which regenerate from a carbohydrate-rich burl at their base following fire, seeders tend to be fairly shallow rooted , and are thus less able to access deep water sources . Seeders are generally considered to be more tolerant of seasonal drought than resprouters , possibly a mechanism for shallow rooted seedlings to survive summer drought in an open post-fire environment following germination . However, this strategy has also been linked to higher mortality during extreme drought . A. glauca are also known to exhibit anisohydric mechanisms of drought tolerance , and can exhibit extremely low water potentials and high resistance to cavitation during seasonal drought . In 2014, we observed sudden and dramatic dieback in A. glauca in the Santa Ynez mountain range of Santa Barbara, California during an historic drought . The drought that lasted from 2012 to 2018 in southern California was themost severe to hit the region in 1,200 years , stackable planters with 2014 being the driest year on record . Preliminary field observations indicated greater levels of canopy dieback at lower elevation stands compared to higher elevations. Dieback also seemed to be more prevalent on exposed and southwest-facing slopes, which in this region experience direct sunlight for most of the day.
Other studies have reported significant Arctostaphylos spp. dieback and even mortality during periods of extreme drought stress, further suggesting species in this genus are vulnerable to drought-related mortality. Additionally, we observed widespread symptoms of fungal infection – including branch cankers and brown/black leaf discoloration – later identified as members of the opportunistic Botryosphaeriaceae family , suggesting multiple factors may be driving canopy dieback in this species. Drought-related mortality has previously been associated with opportunistic fungal pathogens in A. glauca and other chaparral shrubs , yet few studies have sought to understand the relative levels of drought stress incurred by plants infected with these pathogens, or how stress is related to canopy dieback and/or mortality. A. glauca shrubs are important members of the chaparral ecosystem, providing habitat and food for wildlife through their nectar and berries . Their structure and fire-induced germination strategies also make them significant components of the chaparral fire regime and post-fire successional trajectories . Large-scale mortality of this species could reduce resource availability for wildlife, as well as alter fuel composition and structure in the region, resulting in an increased risk of more intense, faster burning fires. Therefore, the potential continued dieback of A. glauca is of great concern for both ecosystem functioning and human populations alike. Yet because of the heterogeneity of landscapes in this rugged region, it is possible that portions of the landscape will act as refugia for drought-susceptible species. We hypothesized that A. glauca dieback severity is associated with areas of increased water stress across the landscape. To better understand the patterns and trajectory of A. glauca stress and dieback across a topographically diverse region of coastal California, we asked the following specific questions: How severe is drought-related stress and dieback in this region?
How do plant stress and dieback severity vary with elevation and aspect across the landscape? How does dieback change across the landscape as a multi-year drought progresses? We chose xylem pressure potential as an indicator of plant water availability, and measured dark-adapted fluorescence and net photosynthesis as proxies for drought-related plant stress and physiological function. To address Question 1, we conducted an initial survey measuring general levels of canopy dieback, shrub water availability, and stress in the region. To address Questions 2 and 3, we conducted a more in-depth study of how shrub water relations and dieback vary with aspect and along an elevational gradient, and tracked changes in dieback severity for the four final years of the seven-year drought. We expected to find areas of low XPP correlated with greater physiological stress responses, and more severe dieback in lower elevation sites and on southwest aspects. Alternatively, shrub stress and dieback may be dependent on a wider variety of variables, particularly in a landscape as heterogeneous as this. Additionally, we predicted that dieback severity and individual shrub death would increase over time in lower elevations and exposed slopes compared to upper elevations and more mesic slopes.The specific area chosen for this study is located in the Santa Ynez mountains of Los Padres National Forest . Stands of A. glauca occur from approximately 400m to 1200m elevation, and are frequently mixed with other co-dominate woody evergreen shrub species including Adenostoma fasciculatum, Ceanothus megacarpus and, at lower elevations, Malosma laurina.
The landscape of this region is extremely heterogeneous, with unstable terrain composed largely of sandstone rock outcroppings and sandstone-derived soils , and steep slopes and ridges that are interrupted by deep canyons . These dramatic features, while common habitat for chaparral plant communities, were a limitation in our ability to choose field sites. Thus, we relied heavily on accessibility by road and trail in finding sites. The climate in this region is of a mediterranean-type, with cool moist winters and a hot, dry summer season. The majority of rainfall typically occurs from November to April, and mean annual rainfall, based on a 120-year average, is 47cm . 2.2 Precipitation data Three weather stations, equipped with real-time, self-recording data loggers and maintained by the Santa Barbara County Public Works Hydrology Division, were chosen to retrieve precipitation data during the drought based on proximity and similar elevation to study sites. The Trout Club , San Marcos Pass , and El Deseo Ranch stations represented low, intermediate, and high elevations, respectively. Data from these stations were retrieved from the Santa Barbara County Public Works Hydrology website.Average rainfall at these stations, based on 54-69 year means, increases with elevation from 68.3cm to 90.4cm . Annual rainfall data for this study are presented in “rainfall years” from November 1 of one year to October 31 of the next, to reflect the seasonal wet period preceding each sampling period. Consistent with these historical trends, annual rainfall at Trout Club was lowest between the 2014-15 and 2018-19 water years . However, during this same time period, rainfall totals were generally lower at El Deseo Ranch compared to San Marcos Pass .In 2015, an initial survey was conducted to assess dieback, as well as shrub water demand and physiological stress as the summer dry season progressed during the drought. Five sites were chosen for this survey representing variable elevation, slope, and aspect, but were also limited by access, safety and proximity to roads. Sites were defined as being composed of greater than 50% A. glauca cover, except for site C, which had lower than 50% A. glauca cover but favorable access. Boundaries were delineated using a combination of on-the-ground visual assessment and polygons drawn using 1m National Agriculture Imagery Program imagery within ArcGIS® . Later, they were refined using a Phantom 4 Pro Drone .Each site was initially assessed for A. glauca stand dieback severity and site mortality in winter, before new summer leaf-out occurred . Stand dieback severity was defined as the percent of non-green or defoliated canopy cover within the boundaries of thesite, and was estimated by the collective valuation of two-to-three people viewing the stand from different angles. If stands were not completely pure A. glauca , we did not include canopies of other species in our estimation of average percent dieback. Data on stand mortality were collected by counting the number of dead individuals within site boundaries using ArcGIS. Number of dead individuals per site was also recorded.To measure shrub water availability and physiological stress through the summer dry season, twenty individuals per site were selected based on similar size , accessibility, and representing different levels of health along a continuum. From these twenty, ten individuals were randomly chosen using a random number generator and tagged for collecting repeated data on stress and dieback as the dry season progressed. At one site only nine individuals fell within the size criteria and were readily accessible, therefore the sample size for this site was nine, stackable flower pots for a total of 49 shrubs used in this initial survey. All selected individuals were measured for height and canopy volume. Measurements included basal diameter , height, and canopy width in two directions.
Total plant volume was calculated as the volume of a pyramid height and the two canopy widths .Data on canopy dieback for each individual were collected in fall 2016. Dieback was assessed as the actual percent of “non-green” vegetation, defined as yellow, brown, and black/gray leaves, as well as bare/defoliated stems within the canopy. Percent dieback of each canopy was estimated by two-to-three researchers viewing multiple angles of each shrub, and final estimates were determined after thorough consultation. Entire stand dieback was also estimated using a combination of ground-level assessments and, when available, aerial drone photographs. These were used to confirm that canopy dieback of individual shrubs were collectively representative of whole stand dieback.Differences in means between sites for plant physiology and dieback were compared using one-way ANOVA with Tukey’s HSD post hoc analysis for means separations. Dieback data were square-root transformed for normality. Linear regression models were used to identify correlations of elevation and aspect between plant drought stress and dieback. Multiple regression models were developed using landscape factors and plant physiology as predictors for canopy dieback, and AIC values were calculated for choosing the best fit model. A repeated measures ANOVA was performed to compare dieback levels between sites and across four years of the study . Statistical analyses were performed using JMP Pro Statistical Software and R Studio .This study sought to understand spatial patterns of A. glauca canopy dieback across the landscape during an unprecedented drought, and track the progression of dieback and eventual mortality in this classically drought-tolerant shrub. Our data support the hypothesis that dieback is related to water stress from drought, and that this varied across the landscape, but there was considerable variation between sites and across time. While aspect was significantly correlated with dieback across all years, we found no consistent significant effect of elevation on dieback until 2019, late in drought period. Further, while dieback generally increased across all sites from 2016 to 2019, we observed no new incidences of mortality during this study, suggesting that in this landscape A. glauca individuals are resilient to this punctuated but overall prolonged drought.Consistent with our predictions, elevation and aspect appeared to both have significant correlations with XPP and Anet. While these correlations were weak, their significance lends support to our hypothesis that plant stress is related to these landscape variables. These relationships also provide evidence that elevation and southwestness may be used as tools for identifying areas where plant canopies are more vulnerable drought. The low correlation coefficients may be due to extreme landscape heterogeneity in the study region including heterogeneity in substrate rockiness, soil accumulation and topographic concavity, confounded by a relatively small sample size within each site. There are many microclimate variables that were not included in this study and that may contribute to variations in water stress, including site temperature, vapor pressure deficit, and fog patterns. Additionally, though we did not find plant size to be correlated with drought stress or dieback, we believe this is due to our efforts to select even-aged individuals for monitoring, while previous studies have found significant effects of plant canopy diameter on dieback and mortality . Also, because A. glauca recruit from seed after fire, individuals within a stand tend to be of similar size and age. Therefore, we recommend that future studies include greater variation in stand age so that relationships between age/size class and dieback can be better evaluated.Elevation was not shown to be highly correlated with dieback in all years of the study, and there did not appear to be a correlation between dieback annual precipitation. Yet, some important landscape patterns were revealed. Our hypothesis that dieback severity is related to landscape variables associated with water availability was most strongly supported in fall 2019, when dieback increased significantly with decreased elevation and increased southwestness.