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, nft hydroponic system 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 .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. Site dieback was then calculated as the mean of all selected shrubs within a site. 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.Data were again collected approximately every 4-6 weeks throughout the summer dry season, this time from June through October. XPP measurements were taken using the same methods as described above. Additionally, we measured daytime gas exchange to gain a better understanding of plant function throughout the dry season and as water availability declined.
Anet was measured using a Licor 6400XT . CO2 was set to 400ppm, and photosynthetically active radiation was set between 1400-1800, to reflect maximum photosynthesis conditions at peak daytime hours. All measurements were taken between 9am and 11am DST, typically on the morning after the predawn measurements were made. A single fully-expanded leaf was chosen per plant, and two readings were recorded and averaged later during data processing. Leaves were traced in the field, and tracings were brought back to the lab and scanned to calculate leaf area within the chamber using ImageJ software . Our FMS 2 system broke in August, and we were therefore unable to collect any Fv/Fm data after the July fieldwork.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, hydroponic nft system 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. Aspect was correlated with dieback severity across all years, suggesting thiscould be a significant variable to consider when generating models for future drought risk. Moreover, it was found that the increase in dieback severity from 2016-2019 was greatest at lower elevations compared to high. In other words, while most sites experienced an overall increase in dieback, the increase at lower elevations was much more dramatic. Collectively, these results suggest that while populations of A. glauca across the landscape in Santa Barbara county are susceptible to extreme drought, those occurring on exposed, southwest-facing slopes and/or at the lower edge of their range may be less capable of recovery, particularly during prolonged drought. This is consistent with studies showing that low elevations and exposed slopes can correlate with plant water stress and mortality . Interestingly, the dieback severity was greatest in fall 2019, despite the region experiencing above-average rainfall in both the 2016-17 and 2018-19 water years . Thus, a single year of above-average rainfall was not sufficient to restore shrubs to predrought canopy health levels.
One possible explanation for this is that 2011-2016 and the 2017-18 wet seasons were extremely dry, pushing A. glauca individuals towards a threshold of drought resistance that could not be counteracted by one or two wet years. Venturas et al. found A. glauca to have high levels of hydraulic failure, dieback, and whole shrub mortality in Malibu County during the drought year of 2014, and a study by Paddock et al. from an intense drought year in California yielded similarly high mortality for this species. This supports the significance of intense drought years that preceded higher rainfall years in our study. Additionally, Gill and Mahall and Mahall et al. found evidence that some chaparral shrubs do not respond to surplus water, and thus high-rainfall years may not be accurate predictors of shrub recovery. Shallow-rooted species like A. glauca may only be able to benefit from soil water availability near the surface, and/or they may be relying on water availability in rocky outcrops that is more dependent on outcrop cavity structure than on individual rain event totals. Therefore, short-term excessive rainfall may be irrelevant for the recovery of these shrubs. Another hypothesis is that opportunistic fungi, as identified in prior work , may have played a significant role in the sudden development and continuation of dieback. Biotic agents are known to exacerbate drought effects by amplifying hydraulic failure and/or carbon starvation . A. glauca, already weakened by drought, may have been further impacted by fungal pathogens emerging from their latency between 2012 and 2016. Drake-Schultheis et al. have documented synergistic interactions of drought and latent fungal pathogens on A. glauca in a greenhouse setting. However, more research is required to identify the exact role of latent fungal pathogens in A. glauca in the field. Interestingly, Mahall et al. report detailed phenological measurements of A. glauca individuals in a nearby population in the 1980s following individual leaves for up to three years. They do not report dieback or evidence of fungal disease in the canopy of any individuals despite detailed descriptions of leaf condition. Persona communication with Mahall and Thwing likewise confirm the absence of evidence for pathogenic fungi in their study plants. Drake-Schultheis et al. have evidence for the very recent introduction of the most virulent pathogen in this system, Neofusicoccum australe. While the Mahall et al. study was conducted during the drought of the late 1980s and this drought was similar in magnitude, itmay be cumulative drought and a new fungal pathogen together that are today causing canopy dieback.A surprising result of this study was that while mortality among A. glauca was noted across the landscape at the beginning of this study, no new mortality was observed over the following four years of drought, despite some very low XPP measurements in 2016. Indeed, the XPP obtained in Fall 2016 are almost twice as negative as those measured in A. glauca shrubs in this same region during an earlier drought in the late 1980s . Furthermore, a significant number of shrubs across sites reached extreme levels of canopy dieback yet survived and in some cases even showed evidence of recovery. These observations are indicative of an impressive resilience to drought stress in this chaparral shrub species. As previously reported, A. glauca are typically identified as anisohydric, exhibiting continued gas exchange during drought and high resistance to cavitation . Our data showing the continuation of gas exchange well into the dry season further supports this strategy, although Anet did shut down in the lower elevation populations. The Venturas et al. and Paddock et al. studies also found greater mortality in A. glauca than was recorded in our study, and in other chaparral shrubs with high cavitation resistance– a relationship suggested to be the result of greater susceptibility to high intensity drought. Therefore, why high mortality was not also observed in our study is unclear. It may be a result of Santa Barbara experiencing a slightly milder climate, compared to those of the aforementioned studies, which both took place in more arid regions of southern California, including in a chaparral-desert ecotone. Summer fog, a normal occurrence in Santa BarbaraCounty, may also play an important role in reducing vapor pressure deficit, thus providing critical drought stress relief during the summer months. Furthermore, our study areas were dominated by stands of relatively large, mature plants, which have been shown to exhibit greater survival rates during drought than smaller individuals .