Water Availability Controls on Above-Belowground Productivity Partitioning: Herbivory versus Plant Response
While precipitation is the major control of aboveground net primary production (ANPP) from deserts to grassland ecosystems, the effects on belowground net primary production (BNPP) has been less frequently studied because of methodological difficulties. However, BNPP is larger than ANPP in most arid to sub-humid ecosystems. Our lack of understanding of the controls of BNPP represents a void in our knowledge of the functioning of ecosystems, including food webs and nutrient cycling. Because few studies have explored the effect of precipitation on the fraction of total primary production allocated belowground (fBNPP), we asked how precipitation variability affects BNPP and what mechanisms, such as plant water allocation or belowground herbivory, may be driving these patterns. To test our hypotheses, we employed both field and microcosm experiments replicated in three locations along a gradient of mean annual precipitation. Field experiments manipulate incoming precipitation and observe fBNPP and soil biota composition. Microcosm experiments manipulate both precipitation and soil biota functional group community structure. These experiments, in combination, aim to elucidate the differential effects of temporal and spatial changes in water availability on fBNPP and the mechanisms behind those patterns.
Exotic Grass and Woody Plant Encroachment in Southwestern Rangelands
Although characterized by low and highly variable annual rainfall, arid and semi-arid landscapes represent a substantial percentage of terrestrial net primary productivity, contain >30% of the world’s human population, and support the majority of global livestock production. As such, rangelands play an important role in global biogeochemical cycles and human health. Rangelands thus have considerable, multi-dimensional conservation value. A key component of rangeland ecosystem management is maintaining vegetation within a desirable mix of herbaceous and woody plants. In recent decades, this balance has been disrupted and shifted in favor of unpalatable shrubs and “woody weeds.” Historic brush management treatments were relatively short-lived, ineffective in the long run, and unsustainable. These realizations led to the development of integrated brush management systems (IBMS), which are long-term planning processes that move away from a purely livestock production perspective and toward management of rangelands for multiple uses and values. Our research objectives and hypotheses are tested with a large-scale field experiment that manipulates in a factorial design: (a) incoming precipitation, (b) seed and seedling accessibility to ants, rodents, and small mammals, (c) levels of native grass defoliation, and (d) levels of N availability. These experiments take place at two Southwestern sites that differ in mean annual precipitation. Ultimately, this research explicitly addresses abiotic and biotic interactions and determine how livestock grazing interacts with precipitation, nitrogen availability and constraints imposed by native herbivores to influence the probability of shrub and non-native grass establishment. Results from our experiments will enable us to anticipate and forecast state-transitions so that proactive management can be adjusted to prevent them. As a result, this study will provide a basis for developing cost-effective, proactive management strategies aimed at preventing shrub encroachment and non-native grass invasion and the state-transitions that result.
Effects of changes in climate on the functioning of arid and semiarid ecosystems
Although precipitation is assumed to be the major control of the production of plants in arid and semiarid regions, annual precipitation explains only 20-40% of grass and shrub production variability among years and across ecosystems. This failure suggests that researchers are not accounting for key mechanisms that control the ability of arid ecosystems to track fluctuations in precipitation. Yet increased fluctuations are a prominent feature of climate predictions in arid regions. We put forward four hypotheses to explain observed lags in ecosystem response to changing precipitation. We are testing them by altering patterns of total precipitation and precipitation variability, with and without nitrogen manipulation. These manipulations, together with synthesis and simulation model analyses help us to evaluate the cause and magnitude of lags in the ecosystem response to precipitation change.
Effects of woody-plant encroachment on ecosystem services
Grasslands, shrublands, and savannas cover ca. 50% of the Earth’s land surface. Although characterized by low levels of precipitation, drylands represent 30-35% of terrestrial NPP, contain >30% of the world’s human population, and support the majority of global livestock production. These ecosystems produce a wealth of ecosystem services (ES), including food and fiber production, carbon sequestration, recreation, and conservation. A large fraction of the world’s grasslands and savannas are undergoing a rapid shift from herbaceous to woody-plant dominance. The objectiveof our work is to assess the effects of woody-plant encroachment on multiple ecosystem services, determine how this shift is modulated by long-term precipitation, and understand how it affects the stakeholders benefitting from these ecosystems. While encroachment is often assumed to be degradation, we believe that the ES framework lets us view this more objectively as a shift in the number, type, and amount of services provided. Here, we quantitatively assess the portfolio of 9 ecosystem services provided by grasslands before and after conversion into shrublands and savannas in two ecosystems with contrasting precipitation regimes (240-900 mmyr-1); Tallgrass prairie (KS) and Chihuahuan Desert Grassland NM).
Drought-Net Research Coordination Network
All ecosystems will be impacted to some extent by climate change, with forecasts for more frequent and severe droughts. likely to have the greatest impact on terrestrial ecosystems. Terrestrial ecosystems are known to vary dramatically in their responses to drought. However, the mechanistic basis underlying why some ecosystems respond more than others represents a critical knowledge gap, one that limits our ability to project drought impacts at regional and continental scales.
Traditional site-based approaches cannot fill this knowledge gap because site-specific experiments are conducted in ways that makes comparisons among ecosystems difficult. Coordinated experimental networks, however, are ideally suited for comparative studies at regional to global scales. Drought-Net will advance understanding of the determinants of terrestrial ecosystem sensitivity to drought by bringing together an international cadre of scientists with expertise that spans a wide range of terrestrial ecosystems, but with a common interest in drought, to design and coordinate three complementary research coordination activities:
- Planning and coordinating new research utilizing standardized measurements to leverage the value of existing drought experiments across the globe (Enhancing Existing Experiments, EEE)
- Finalizing the design and facilitating the establishment of a new international network of coordinated drought experiments (International Drought Experiment, IDE)
- Training highly motivated graduate students to conduct synthetic and network-level research through Distributed Graduate Seminars (DGS) focused on drought.
More information can be found at http://www.drought-net.org/