Intensification of the global hydrological cycle, ranging from larger individual precipitation events to more extreme multiyear droughts, has the potential to cause widespread alterations in ...ecosystem structure and function. With evidence that the incidence of extreme precipitation years (defined statistically from historical precipitation records) is increasing, there is a clear need to identify ecosystems that are most vulnerable to these changes and understand why some ecosystems are more sensitive to extremes than others. To date, opportunistic studies of naturally occurring extreme precipitation years, combined with results from a relatively small number of experiments, have provided limited mechanistic understanding of differences in ecosystem sensitivity, suggesting that new approaches are needed. Coordinated distributed experiments (CDEs) arrayed across multiple ecosystem types and focused on water can enhance our understanding of differential ecosystem sensitivity to precipitation extremes, but there are many design challenges to overcome (e.g., cost, comparability, standardization). Here, we evaluate contemporary experimental approaches for manipulating precipitation under field conditions to inform the design of ‘Drought‐Net’, a relatively low‐cost CDE that simulates extreme precipitation years. A common method for imposing both dry and wet years is to alter each ambient precipitation event. We endorse this approach for imposing extreme precipitation years because it simultaneously alters other precipitation characteristics (i.e., event size) consistent with natural precipitation patterns. However, we do not advocate applying identical treatment levels at all sites – a common approach to standardization in CDEs. This is because precipitation variability varies >fivefold globally resulting in a wide range of ecosystem‐specific thresholds for defining extreme precipitation years. For CDEs focused on precipitation extremes, treatments should be based on each site's past climatic characteristics. This approach, though not often used by ecologists, allows ecological responses to be directly compared across disparate ecosystems and climates, facilitating process‐level understanding of ecosystem sensitivity to precipitation extremes.
Plant traits are useful for predicting how species may respond to environmental change and/or influence ecosystem properties. Understanding the extent to which traits vary within species and across ...climatic gradients is particularly important for understanding how species may respond to climate change. We explored whether climate drives spatial patterns of intraspecific trait variation for three traits (specific leaf area (SLA), plant height, and leaf nitrogen content (Nmass)) across 122 grass species (family: Poaceae) with a combined distribution across six continents. We tested the hypothesis that the sensitivity (i.e. slope) of intraspecific trait responses to climate across space would be related to the species' typical form and function (e.g. leaf economics, stature and lifespan). We observed both positive and negative intraspecific trait responses to climate with the distribution of slope coefficients across species straddling zero for precipitation, temperature and climate seasonality. As hypothesized, variation in slope coefficients across species was partially explained by leaf economics and lifespan. For example, acquisitive species with nitrogen‐rich leaves grew taller and produced leaves with higher SLA in warmer regions compared to species with low Nmass. Compared to perennials, annual grasses invested in leaves with higher SLA yet decreased height and Nmass in regions with high precipitation seasonality (PS). Thus, while the influence of climate on trait expression may at first appear idiosyncratic, variation in trait–climate slope coefficients is at least partially explained by the species' typical form and function. Overall, our results suggest that a species' mean location along one axis of trait variation (e.g. leaf economics) could influence how traits along a separate axis of variation (e.g. plant size) respond to spatial variation in climate.
Extreme drought decreases aboveground net primary production (ANPP) in most grasslands, but the magnitude of ANPP reductions varies especially in C₃-dominated grasslands. Because the mechanisms ...underlying such differential ecosystem responses to drought are not well resolved, we experimentally imposed an extreme 4-yr drought (2015–2018) in two C₃ grasslands that differed in aridity. These sites had similar annual precipitation and dominant grass species (Leymus chinensis) but different annual temperatures and thus water availability. Drought treatments differentially affected these two semiarid grasslands, with ANPP of the drier site reduced more than at the wetter site. Structural equation modeling revealed that community-weighted means for some traits modified relationships between soil moisture and ANPP, often due to intraspecific variation. Specifically, drought reduced community mean plant height at both sites, resulting in a reduction in ANPP beyond that attributable to reduced soil moisture alone. Higher community mean leaf carbon content enhanced the negative effects of drought on ANPP at the drier site, and ANPP–soil-moisture relationships were influenced by soil C:N ratio at the wetter site. Importantly, neither species richness nor functional dispersion were significantly correlated with ANPP at either site. Overall, as expected, soil moisture was a dominant, direct driver of ANPP response to drought, but differential sensitivity to drought in these two grasslands was also related to soil fertility and plant traits.
Many plant species can exhibit remarkable variation in leaf characteristics, depending on their abiotic and biotic environment. Environmental changes therefore have the potential to alter leaf ...traits, which in turn scale up to influence ecosystem processes including net primary productivity, susceptibility to fire, and palatability to herbivores. It is not well understood how consistent trait–environment relationships are among species, across sites and over time. This presents a fundamental challenge for functional ecology, because no study can measure all relevant species in all places at all times. Thus, understanding the limits of transferability is critical. We collected leaf trait measurements on 13 species of grass (family: Poaceae) across 11 sites and five years (n = 3091 individuals). Sites were arrayed along a spatial precipitation gradient in coastal northern California (annual precipitation of 590–1350 mm) with substantial interannual precipitation variability (from 60% below the 30‐year average to 100% above average). Temporal and spatial linear relationships between precipitation and specific leaf area (SLA) appear at first idiosyncratic, with each species sometimes displaying positive and sometimes negative responses. However, this variation arises from sampling different portions of an underlying hump‐shaped relationship, which was shared across most species. This hump‐shaped relationship was driven primarily by changes in leaf tissue density. These results suggest the potential for transferability among species, as well as between space and time, as long as the gradients are sufficiently long to capture the nonlinear response. Future work could explore the physiological basis of the nonlinear SLA response, including the possibility that distinct physiological mechanisms are operating at the two extremes of the gradient.
Recurrent droughts are an inevitable consequence of climate change, yet how grasslands respond to such events is unclear. We conducted a 6‐year rainfall manipulation experiment in a semiarid ...grassland that consisted of an initial 2‐year drought (2015–2016), followed by a recovery period (2017–2018) and, finally, a second 2‐year drought (2019–2020). In each year, we estimated aboveground net primary productivity (ANPP), species richness, community‐weighted mean (CWM) plant traits, and several indices of functional diversity. The initial drought led to reduced ANPP, which was primarily driven by limited growth of forbs in the first year and grasses in the second year. Total ANPP completely recovered as the rapid recovery of grass productivity compensated for the slow recovery of forb productivity. The subsequent drought led to a greater reduction in total ANPP than the initial drought due to the greater decline of both grass and forb productivity. The structural equation models revealed that soil moisture influenced ANPP responses directly during the initial drought, and indirectly during the subsequent drought by lowering functional diversity, which resulted in reduced total ANPP. Additionally, ANPP was positively influenced by CWM plant height and leaf nitrogen during the recovery period and recurrent drought, respectively. Overall, the greater impact of the second drought on ecosystem function than the initial drought, as well as the underlying differential mechanism, underscores the need for an understanding of how increased drought frequency may alter semiarid grassland functioning.
Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO
) represents one of the important interactions between anthropogenic emissions related to combustion and natural ...emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO
-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO
-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO
radical, the difficulty of characterizing the spatial distributions of BVOC and NO
within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO
-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO
-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.
Water‐limited ecosystems are highly sensitive to not only precipitation amount, but also precipitation pattern, particularly variability in the size and timing of growing season rainfall events. Both ...rainfall event size and timing are expected to be altered by climate change, but the relative responses of dryland ecosystems to changes in rainfall event size versus timing have not been resolved. Here, we disentangle the effects of these different aspects of precipitation pattern on ecosystem dynamics.
We experimentally assessed how these two aspects of rainfall variability impacted a semi‐arid grassland ecosystem by altering an ambient precipitation pattern to eliminate variability in (a) rainfall event size (all events were made the same size), (b) rainfall event timing (all events were uniformly spaced in time) and (c) both. Total precipitation amount was constant for all treatments. We measured responses of soil moisture, ecosystem carbon flux (e.g. net primary production and soil CO2 flux), plant community composition and physiological responses of the dominant C4 grass, Bouteloua gracilis.
Removing variability in rainfall event size altered ecosystem dynamics more than a pattern of uniform event timing, but the largest impact occurred when variability in both were removed. Notably, eliminating variability in both event size and timing increased above‐ground net primary productivity by 23%, consistent with reduced water stress in the dominant C4 grass, while also reducing seasonal variability in soil CO2 flux by 35%, reflecting lower seasonal variability in soil moisture.
Synthesis. Unique responses to different aspects of precipitation variability highlight the complexity of predicting how dryland ecosystems will be affected by climate change‐induced shifts in rainfall patterns. Our results provide novel support for the key roles of rainfall event size and timing, in addition to total precipitation amount, as determinants of ecosystem function.
We reproduced the exact precipitation pattern of a past growing season in a semi‐arid grassland and deconstructed this pattern to eliminate variability in (a) rainfall event size, (b) timing or (c) both. Unique ecological responses to different aspects of precipitation variability highlight the complexity of predicting how dryland ecosystems will be affected by climate change‐induced shifts in rainfall patterns.
We have reviewed the studies on radiation-induced vascular changes in human and experimental tumors reported in the last several decades. Although the reported results are inconsistent, they can be ...generalized as follows. In the human tumors treated with conventional fractionated radiotherapy, the morphological and functional status of the vasculature is preserved, if not improved, during the early part of a treatment course and then decreases toward the end of treatment. Irradiation of human tumor xenografts or rodent tumors with 5–10 Gy in a single dose causes relatively mild vascular damages, but increasing the radiation dose to higher than 10 Gy/fraction induces severe vascular damage resulting in reduced blood perfusion. Little is known about the vascular changes in human tumors treated with high-dose hypofractionated radiation such as stereotactic body radiotherapy (SBRT) or stereotactic radiosurgery (SRS). However, the results for experimental tumors strongly indicate that SBRT or SRS of human tumors with doses higher than about 10 Gy/fraction is likely to induce considerable vascular damages and thereby damages the intratumor microenvironment, leading to indirect tumor cell death. Vascular damage may play an important role in the response of human tumors to high-dose hypofractionated SBRT or SRS.
Alzheimer apolipoprotein E (APOE) ɛ4/ɛ4 carriers have earlier disease onset and more protein aggregates than patients with other APOE genotypes. Autophagy opposes aggregation, and important autophagy ...genes are coordinately regulated by transcription factor EB (TFEB) binding to “coordinated lysosomal expression and regulation” (CLEAR) DNA motifs.
Autophagic gene expression was assessed in brains of controls and Alzheimer's disease (AD) patients parsed by APOE genotype and in a glioblastoma cell line expressing either apoE3 or apoE4. Computational modeling assessed interactions between apoE and mutated apoE with CLEAR or modified DNA.
Three TFEB-regulated mRNA transcripts—SQSTM, MAP1LC3B, and LAMP2—were lower in AD ɛ4/ɛ4 than in AD ɛ3/ɛ3 brains. Computational modeling predicted avid specific binding of apoE4 to CLEAR motifs. ApoE was found in cellular nuclei, and in vitro binding assays suggest competition between apoE4 and TFEB at CLEAR sites.
ApoE4-CLEAR interactions may account for suppressed autophagy in APOE ɛ4/ɛ4 carriers and, in this way, contribute to earlier AD onset.
Questions
Plant traits can predict a species‘ relative abundance and its influence on ecosystem processes. However, trait expression and the relative abundance of a species are also influenced by its ...abiotic and biotic environment. Here, we ask whether the relationship between plant traits and relative abundance is modified by abiotic (e.g., climate and topography) and biotic factors (e.g., community taxonomic and functional diversity) across Californian grasslands.
Location
San Francisco Bay Area.
Methods
We measured specific leaf area (SLA; leaf area/dry mass) and plant height of 19 grass species (family: Poaceae) across 117 plots. We also quantified the relative abundance of each species as well as several biotic attributes of the neighboring grass community including total plant cover, species richness and evenness, community‐weighted mean (CWM) traits, and functional diversity. Using multiple linear regression, we assessed whether abundance could be predicted from traits and the interactions between traits and both biotic and abiotic factors. We fit similar models predicting traits from relative abundance.
Results
Grass species had higher relative abundance in plots where they were taller and had higher SLA. They were also more abundant in communities with low functional richness (FRic) and high functional evenness (FEve), perhaps because of the low resource‐use efficiency of their neighbors and a lack of dominant grasses. Neither abundance nor plant height were associated with abiotic variables, although SLA responded predictably to precipitation according to a bell‐shaped curve. Grasses were taller where they were more abundant, but the impact of abundance on SLA depended on community FEve. Finally, we show strong evidence for community trait similarity, whereby an individual‘s trait expression was positively correlated with the traits of its grass neighbors.
Conclusions
Taken together, these results imply that traits are predictive of abundance and vice versa, and these relationships depend on biotic interactions more than climate.
We measured the traits and relative abundance of California grasses. Our results suggest grasses are more abundant where they are taller and have higher specific leaf area. Grass traits tended to mirror the traits of their neighboring community. Overall, we find that traits are predictive of abundance and vice versa, and these relationships depend on biotic interactions more than climate.
PDF
