Periodic Reporting for period 2 - DIAGRASS (Differential adaptation capacity of dryland grasses to directional changes in water availability (DIAGRASS).)
Reporting period: 2020-04-01 to 2021-03-31
The contemporary global change is occurring at an unprecedentedly rapid pace, challenging organisms to maintain their fitness under novel biotic and abiotic conditions. For example, extreme rainfall and drought events are projected to increase in frequency and magnitude with different prevalence between continents and regions. Understanding the impacts of this intensification of the water cycle on grassland ecosystems will be critical for evaluating their future capacity to provide services and for developing sustainable management policies. This will be especially important in dryland regions, where water availability is the main driver of many ecological processes. The target species in the first phase of the project was Bouteloua eriopoda, considered a key forage species for both domesticated and wild animals in the Southwest States of USA. B. eriopoda is distributed in ten Southwest States of USA, and in rangelands where it is dominant, a net annual financial return of around $2/ha has been estimated. Also, its spread by stolons makes it one of the best erosion control plants in its distribution area. It is known that this species has a very negative response to the interaction between drought and grazing. Thus, understanding how will be the responses of this species to extreme events in drylands can improve predictions of vegetation dynamics and the future economic viability of most ranches located in its natural range. In the second phase of the project, Brachypodium hybridum was the target species. It offers a main direct economic and environmental benefit when it is used as a protective plant cover against erosion in human-managed lands and is an interesting species for ecological studies of typical Mediterranean plants and polyploid speciation. B. hybridum is also a close relative of B. distachyon, which is emerging as a genetic model for the study of key genes regulating economically important traits of temperate crop and forage grasses. Thus, the main objective of DIAGRASS is to evaluate the adaptation capacity and ecological memory (i.e. responses to current stress modulated by antecedent conditions) of dryland grass species, as well as basic ecosystem functions, to changes in water availability.
In the first phase of the project (two-year outgoing at Arizona State University) the experienced researcher (ER) used rainfall manipulation experiments (RMEs) at two different scales (population and plant phenotypic level) to monitor B. eriopoda responses to different rainfall legacies in a gradient of precipitation. Besides, she performed a litter matter decomposition trial to determine to what extent rainfall characteristics and legacies affect soil microorganisms and the carbon and nutrient cycles. The RMEs were implemented at The Jornada Long Term Ecological Research site (JRN LTER, New Mexico). At population scale, two variables were studied: patch size and total B. eriopoda cover. Mean patch size and total cover under extreme drought conditions (80% reduction of naturally occurring rainfall) during 3 and 12 years were significantly lower than in natural precipitation conditions. After 12 years of extreme drought, the total cover was near 0 m2. However, in the short-term drought, only the greatest patch sizes suffered a regression. Under an extreme regime of precipitation (80% increase), these variables were only significantly higher in the long term. These results pointed out the importance of the duration of extreme precipitation and drought events in the B. eriopoda population structure.
At the plant phenotypic level, 18 leaf and shoot traits and 6 sexual reproductive traits were analysed. The plasticity indexes of the traits related to sexual reproduction were located at the extreme ranges of the gradient of values. A three-year extreme drought event stimulated the seed production when summer precipitation returned close to its historical average value. Unexpectedly, this did not occur when after the extreme drought period the contemporaneous rainfall changed to an extreme rainfall event. An extreme precipitation legacy of three years did not cause significant changes in sexual reproduction investment, suggesting that the potential of fitness increase that greater water availability could cause is mediated by the asexual reproduction. The total number of leaves and the increase of alive leaves had low and high values of plasticity, respectively. This indicates that the responses to rainfall extremes related to photosynthetic tissues in this species are mainly driven by leaf span rather than to leaf morphology and quantity. The plastic potential of these traits modulated their resistance responses to extreme rainfalls and drought events. The morphological traits were in general the most resistant to change. The results of the decomposition trial pointed out an adaptation of the soil community to a scenario of lower soil water availability.
In the second phase of the project, another RME was established at the Garraf experimental station (south of Barcelona, Spain) with B. hybridum as the target species. In this experiment, a gradient of drought treatments was imposed (25% and 75% of natural rainfall reduction) through the use of rainfall exclusion shelters. The specific objective of this experiment is to evaluate the changes of response and effect traits caused by the drought treatments. The former will be used to predict how B. hybridum will respond to the projected increasing aridity in the study area, and the latter will be used to determine the impacts of the treatments on ecosystem processes (carbon and nutrient cycling). This projects also pursue to find links between the response traits and changes in the RNA expression of this species to identify the loci of its genome more subjected to selection or epigenetic processes due to drought stress gradient.
At the plant phenotypic level, 18 leaf and shoot traits and 6 sexual reproductive traits were analysed. The plasticity indexes of the traits related to sexual reproduction were located at the extreme ranges of the gradient of values. A three-year extreme drought event stimulated the seed production when summer precipitation returned close to its historical average value. Unexpectedly, this did not occur when after the extreme drought period the contemporaneous rainfall changed to an extreme rainfall event. An extreme precipitation legacy of three years did not cause significant changes in sexual reproduction investment, suggesting that the potential of fitness increase that greater water availability could cause is mediated by the asexual reproduction. The total number of leaves and the increase of alive leaves had low and high values of plasticity, respectively. This indicates that the responses to rainfall extremes related to photosynthetic tissues in this species are mainly driven by leaf span rather than to leaf morphology and quantity. The plastic potential of these traits modulated their resistance responses to extreme rainfalls and drought events. The morphological traits were in general the most resistant to change. The results of the decomposition trial pointed out an adaptation of the soil community to a scenario of lower soil water availability.
In the second phase of the project, another RME was established at the Garraf experimental station (south of Barcelona, Spain) with B. hybridum as the target species. In this experiment, a gradient of drought treatments was imposed (25% and 75% of natural rainfall reduction) through the use of rainfall exclusion shelters. The specific objective of this experiment is to evaluate the changes of response and effect traits caused by the drought treatments. The former will be used to predict how B. hybridum will respond to the projected increasing aridity in the study area, and the latter will be used to determine the impacts of the treatments on ecosystem processes (carbon and nutrient cycling). This projects also pursue to find links between the response traits and changes in the RNA expression of this species to identify the loci of its genome more subjected to selection or epigenetic processes due to drought stress gradient.
The results show a tradeoff between sexual and asexual reproduction investment in B. eriopoda that is modulated by the ecological memory to antecedent rainfall events. A previous study found an unexpected genetic diversity in a species that its main reproduction strategy is asexual. The results obtained in this project contribute to explain previous findings and to understand better the ecological strategies used by this species to survive in ecosystems with great rainfall variability. In conclusion, short-term droughts can promote the genetic diversity of their local populations, predisposing them to a greater capacity to overcome at population level medium-term droughts. However, this species does not possess the adequate set of traits to resist extreme drought events at a decadal scale.
DIAGRASS contributes to improving our ability to predict how more extreme rainfall regimes will affect future key ecosystem services (e.g. the stocking-density capacity) of grasslands located in dryland regions. This basic research aims to be a knowledge support tool for the stakeholders and to influence the sustainable management of grasslands that will enable their economic viability under new climatic scenarios. The introduction of genomic analyses will help us to unravel the adaptation capacity to climate change of species that supply a variety of economical services to society such as crop and forage grasses and to create a knowledge base to promote sustainable agriculture policies in water-limited ecosystems.
DIAGRASS contributes to improving our ability to predict how more extreme rainfall regimes will affect future key ecosystem services (e.g. the stocking-density capacity) of grasslands located in dryland regions. This basic research aims to be a knowledge support tool for the stakeholders and to influence the sustainable management of grasslands that will enable their economic viability under new climatic scenarios. The introduction of genomic analyses will help us to unravel the adaptation capacity to climate change of species that supply a variety of economical services to society such as crop and forage grasses and to create a knowledge base to promote sustainable agriculture policies in water-limited ecosystems.