Future studies which directly compare climatic variables and mortality of A. glauca – of various ages – across ecoregions during extreme drought would therefore be of great value, and may elucidate a better understanding of the driving factors and relative vulnerabilities of plants in high-risk ecosystems.Consistent with our hypothesis, plants located at lower elevations generally experienced more physiological stress and greater dieback severity than those at higher elevations. However, there was also considerable variability within sites and along the elevational gradient. We hypothesize this is due to the extreme heterogeneity of the Santa Ynez mountain landscape, and that certain landscape features outside the scope of this study may act as refugia for A. glauca resilience. Since A. glauca is a relatively shallowly-rooted species, variables such as slope, aspect, slope angle, concavity, and rockiness – all of which vary greatly across the region and influence temperature, water availability, or both – likely impact shrub functioning and vulnerability to drought on a very local scale. In considering long-term predictions, it is possible that while some stands of A. glauca may suffer high levels of dieback and even mortality during extreme drought, populations as a whole may be resilient because of microsite and landscape heterogeneity.Although higher elevations in our study area historically record greater rainfall, we found this trend to be slightly disrupted during our study period: rainfall at the highest elevation site was lower than at the intermediate site from 2015-16 to 2018-19 . Furthermore, nft channel while elevation is known to influence temperature such that lower elevations are generally hotter than higher ones within a region, human-induced climate warming has been shown in some cases to occur more rapidly at higher than at lower elevations.
For instance, Giambelluca et al. found more rapid warming at elevations above 800m in Hawaii, and a review by Pepin et al. found enhanced warming with elevation in high mountain regions globally . Therefore, higher elevation sites in our study may also be experiencing more warming. We do not have temperature records for our six sites so we cannot separate temperature trends or anomalies from other elevational impacts. Nonetheless, the lack of a consistent elevation effect could be due not only to the confounding effects of other topographic features , but also to lower rainfall and increased warming in some higher elevation sites. From the early studies of Whittaker ecologists have long recognized the influence of aspect on plant composition and vegetation dynamics in montane environments. Yet rarely have investigators tied performance or mortality of individuals within a species population to subtle differences in aspect that occur within a single population . Here we found that aspect was the strongest predictor of dieback over time, with plants in SW aspects showing greater dieback than plants on more north-facing aspects. This influence of aspect may be the result of direct negative effects of sun exposure and higher temperatures ina semi-arid environment or maybe indirect whereby influences of aspect are mediated through aspect-effects on soil and microbes. For example, Gilliam et al. 2015 demonstrated greater soil organic matter accumulation and different microbial community composition in N facing compared to S facing slopes. Regardless of mechanism, the finding that SW aspect is detrimental to these plants could be important for restoration practitioners seeking to identify promising sites on the landscape where restoration of individual species may be more successful .
Another source of variability both within the landscape region we studied, and across Southern California chaparral, which was not directly measured in this study, is the occurrence of fog. The Santa Barbara region experiences predictable fog in May and June, and late summer cloud shading and fog events have been demonstrated to reduce vapor pressure deficit , slow plant seasonal water loss and provide direct foliar uptake of water . We observed significant variation in fog intensity between sites, particularly at night during the early summer months , even between sites on a given night. Sites C and D in particular often experienced heavy fog drip, while other sites remained dry. We found plants saturated by fog to exhibit higher water potentials on such nights, similar to findings in Mahall et al. , suggesting that fog drip may be sufficient enough to percolate to shallow roots. As mentioned previously, A. glauca is considered to be a relatively shallowly rooted chaparral shrub species and has been shown to be sensitive to even small changes in rainfall . Additionally, some coastal California shrub species have been shown to exhibit foliar water uptake from fog , though this has not yet been demonstrated for A. glauca. Regardless, variations in summer fog patterns have previously been shown to influence late-season water potentials in other Arctostaphylos spp. , possibly causing populations with greater exposure to fog to have higher “safety margins” with which to avoid cavitation . Similarly, Emery et al. have shown that fog can reduce the rate of water loss in coastal shrublands in California. Mortality of A. glauca during drought documented in other studies occurred in more interior sites, with less fog influence.
Thus, we believe the role of local and regional variation in fog occurrence in A. glauca drought resistance warrants further research.While dieback severity increased over time, the fact that no new plant-level mortality was observed during the study supports previous findings that A. glauca are resilient to prolonged drought. The relatively more mesic climate of Santa Barbara may have contributed to resiliency in our study, compared to studies in more xeric shrubland regions that reported high mortality of A. glauca during the same drought . Landscape heterogeneity, while likely confounding some of our results, may also be advantageous in creating differential water availability across the landscape, thus providing refuges that can ultimately allow resilience of A. glauca populations in a region. However, severe and widespread dieback, like that which we observed, still represents a threat to healthy ecosystem functioning, and has implications for fuel management in regions that are already fire-prone. Lower elevations and exposed, southwesterly slopes may continue to seethe highest levels of dieback during future drought events, and thus should be identified as high-risk and potential focus areas for management.Supported metal catalysts consisting of finely dispersed metal nanoparticles on high-surface-area supports are key in realizing cleaner and more efficient chemical conversions by lowering the energy cost and increasing the selectivity to the desired product. Efficient use of precious metals is facilitated by dispersing the catalytically active metal in sub-10 nm parti-cles. Preventing the growth of these small NPs into larger agglomerates, for example, at elevated temperatures and pressures, is challenging and remains a major cause of catalyst deactivation. Traditionally, high surface-area supports are used to retard NP sintering by increasing the interparticle spacing. Most prominent approaches are to distribute the particles in well-defined mesoporous supports, to encapsulate the individual NPs in porous oxides, to induce a thin metal oxide coating via strong metal-support interactions, or by incorporating the NPs in a microporous framework, such as, for example, zeolites. These designs drastically increase the thermal stability of the NPs and successfully protect the particles from sintering and Oswald ripening during catalysis. However, the encapsulation can lead to a partial loss of catalytic activity due to blockage of the NP surface by the surrounding metal oxide. A key challenge is to mitigate this effect, hydroponic nft and to develop methodologies to probe the chemical accessibility of the NPs in these novel catalyst designs. An emerging class of materials developed by our group that addresses this challenge is raspberry-colloid-templated catalysts . The RCT materials exhibit exceptional thermal and catalytic stability, while maintaining high catalytic activity and selectivity. The material design consists of an ordered macroporous metal oxide framework with sub 10-nm metal NPs distributed at the pore walls. The colloidal preparation approach that involves infiltration of the metal oxide precursor into the assembled sacrificial colloids decorated with catalytic NPs offers high versatility of the structural design, that is, particle size, metal composition, metal oxide composition, and micro- and macro-porosity can be tuned independently. The robust, macroporous matrix allows facile mass transport throughout the catalyst. So far, the potential of the RCTs for thermal catalysis has been demonstrated in selective alkyne hydrogenation, HD exchange, CO oxidation, oxidative alcohol coupling, and the oxidation of volatile organic compounds. A unique feature of the RCT catalysts is their excellent stability during catalysis and thermal treatment, allowing long catalyst lifetimes and facile reactivation via thermal treatment. A particularly noteworthy finding by Shirman et al. is the observation that not only the stability but also the activity of the RCT catalysts can be strongly enhanced compared to commercial catalysts, allowing drastic reductions in precious metal use and in energy cost in oxidation catalysis.
The origin of the superior thermal stability of the RCT catalyst has remained an open question thus far. Furthermore, the crystal structure and accessibility of the catalytic NPs, in particular at their interface with the support, are still unknown despite the relevance of these properties to understanding the origin of sinter-resistance of these materials and their activity in catalysis. Here, by using theoretical modeling, 3D electron microscopy, and epitaxial overgrowth, we assess the NP embedding, the crystal structure, and the chemical accessibility of the metal NPs in RCT catalysts and discuss the implications of these structural characteristics for the catalysts’ functional properties, and in particular for NP stability.The RCT preparation relies on multi-step synthetic route . The first step comprised the preparation of the metal NPs and amidine-functionalized polystyrene colloids . Here, the metal particles consisted of 96 atom % Au and 4 atom % of Pd, abbreviated as Au96Pd4, and were capped with polyvinylpyrrolidone . However, the desired composition, size, and shape of both the NPs as well as the size and functional surface groups of the PS colloids can be readily altered, independent of the subsequent synthesis steps, allowing a tailored design of catalysts for specific chemical reactions. Next, the metal NPs were attached to the PS colloids, yielding so-called raspberry colloids . An ordered colloidal crystal of raspberry colloids assembled by solvent evaporation was infiltrated with a pre-hydrolyzed silica sol-gel solution . In the final step, the PS template and stabilizing ligands were removed via calcination , generating an ordered macroporous silica support with metal NPs decorating the pore walls. Additionally, the high-temperature calcination ensured the mixing of Pd and Au into a homogeneous alloy. The resulting RCT catalyst contained ≈5 wt.% bimetallic Au96Pd4 NPs as determined with inductively coupled plasma mass spectrometry . The weight loading closely matches the theoretical weight loading of 4.8 wt.% calculated based on the synthesis parameters, indicating that no metal leaching occurred during the RCT preparation. The sinter-resistance of the NPs in the RCT catalyst is evident from the overlapping size distributions before and after thermal treatment at 800 °C in static air . The average particle sizes before and after treatment closely match and are 7.5 ± 2.5 and 7.4 ± 2.4 nm, respectively. Additional long-term stability experiments show that thermally treating the catalyst for 10 h in either oxidizing or reducing gas atmospheres does not lead to particle growth . These findings are in line with previous reports in which particle growth was not pronounced in RCT catalysts at temperatures up to 950 °C, and in strong contrast to significant agglomeration and growth of NP in silica-supported catalysts in which the NPs were introduced after the macroporous silica framework was prepared. Thermal treatment of the latter already led to particle growth to > 20 nm after thermal treatment at 500 °C in static air. In addition to the thermal stability of the NPs during synthesis and in oxidizing and reducing atmospheres, the long-term stability and sinter resistance of the PdAu and Pd NP RCT catalysts have been demonstrated for a range of reactions under catalytic conditions: CO oxidation, methanol oxidation, 1-hexyne hydrogenation, and oxidation of simulated exhaust mixtures. It has been shown that the sinter resistance is maintained in RCT catalysts even at high metal loadings and small NP sizes. We postulated that the infiltration step in the synthesis procedure is crucial in determining the NP–silica interface and the degree of NP embedding into the matrix after calcination. More specifically, the geometry of the NP pocket within the silica support is defined by the wetting of the surfaces of the gold NPs and PS colloids by the infiltrating silica sol-gel solution.