Kristina YoungUniversity of Texas at El Paso | UTEP · Ecology and Evolutionary Biology
· Masters of Science in Forestry
Research Items (5)
Arid and semiarid ecosystems play a significant role in regulating global carbon cycling, yet our understanding of the controls over the dominant pathways of dryland CO2 exchange remains poor. Substantial amounts of dryland soil are not covered by vascular plants and this patchiness in cover has important implications for spatial patterns and controls of carbon cycling. Spatial variation in soil respiration has been attributed to variation in soil moisture, temperature, nutrients and rhizodeposition, while seasonal patterns have been attributed to changes in moisture, temperature and photosynthetic inputs belowground. To characterize how controls over respiration vary spatially and temporally in a dryland ecosystem and to concurrently explore multiple potential controls, we estimated whole plant net photosynthesis (Anet) and soil respiration at four distances from the plant base, as well as corresponding fine root biomass and soil carbon and nitrogen pools, four times during a growing season. To determine if the controls vary between different plant functional types for Colorado Plateau species, measurements were made on the C4 shrub, Atriplex confertifolia, and C3 grass, Achnatherum hymenoides. Soil respiration declined throughout the growing season and diminished with distance from the plant base, though variations in both were much smaller than expected. The strongest relationship was between soil respiration and soil moisture. Soil respiration was correlated with whole plant Anet, although the relationship varied between species and distance from plant base. In the especially dry year of this study we did not observe any consistent correlations between soil respiration and soil carbon or nitrogen pools. Our findings suggest that abiotic factors, especially soil moisture, strongly regulate the response of soil respiration to biotic factors and soil carbon and nitrogen pools in dryland communities and, at least in dry years, may override expected spatial and seasonal patterns. This article is protected by copyright. All rights reserved.
Forecast increases in the frequency, intensity, and duration of droughts with climate change may have extreme and extensive ecological consequences. There are currently hundreds of published, ongoing, and new drought experiments worldwide aimed to assess ecological sensitivity to drought and identify the mechanisms governing resistance and resilience. To date, the results from these experiments have varied widely, and thus, patterns of drought sensitivities and the underlying mechanisms have been difficult to discern. Here we examined 89 published drought experiments, along with their associated historical precipitation records to (1) identify where and how drought experiments have been imposed, (2) determine the extremity of drought treatments in the context of historical climate, and (3) assess the influence of ambient precipitation variability on the magnitude of drought experiments. In general, drought experiments were most common in water-limited ecosystems, such as grasslands, and were often short-term, as 80% were 1–4 yr in duration. When placed in a historical context, the majority of drought experiments imposed extreme drought, with 61% below the 5th, and 43% below the 1st percentile of the 50-yr annual precipitation distribution. We also determined that interannual precipitation variability had a large and potentially underappreciated effect on the magnitude of drought treatments due to the co-varying nature of control and drought precipitation inputs. Thus, detecting significant ecological effects in drought experiments is strongly influenced by the interaction between experimental drought magnitude, precipitation variability, and key ecological thresholds. The patterns that emerged from this study have important implications for the design and interpretation of drought experiments and also highlight critical gaps in our understanding of the ecological effects of drought.
Climate change is expected to impact drylands worldwide by increasing temperatures and changing precipitation patterns. These effects have known feedbacks to the functional roles of dryland biological soil crust communities (biocrusts), which are expected to undergo significant climate-induced changes in community structure and function. Nevertheless, our ability to monitor the status and physiology of biocrusts with remote sensing is limited due to the heterogeneous nature of dryland landscapes and the desiccation tolerance of biocrusts, which leaves them frequently photosynthetically inactive and difficult to assess. To address this critical limitation, we subjected a dominant biocrust species Syntrichia caninervis to climate-induced stress in the form of small, frequent watering events, and spectrally monitored the dry mosses’ progression towards mortality. We found points of spectral sensitivity responding to experimentally-induced stress in desiccated mosses, indicating that spectral imaging is an effective tool to monitor photosynthetically inactive biocrusts. Comparing the Normalized Difference Vegetation Index (NDVI), the Simple Ratio (SR), and the Normalized Pigment Chlorophyll Index (NPCI), we found NDVI minimally effective at capturing stress in precipitation-stressed dry mosses, while the SR and NPCI were highly effective. Our results suggest the strong potential for utilizing spectroscopy and chlorophyll-derived indices to monitor biocrust ecophysiological status, even when biocrusts are dry, with important implications for improving our understanding of dryland functioning.
Background/Question/Methods Biological soil crusts-a community of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface of many drylands-are a fundamental component of arid and semiarid ecosystems. These photosynthetic soil communities play critical roles in dryand function; for example, carbon fixation, nitrogen fixation, and soil stabilization – and existing data suggest biocrusts can be quite sensitive to seemingly subtle changes in climate. In particular, previous research on the Colorado Plateau showed dramatic mortality of the common moss Syntrichea caninervis in response to altered precipitation treatments: Increased frequency of 1.2mm monsoonal rainfall events reduced moss cover from >25% to <2% after only one growing season. Yet our understanding of the ecosystem consequences of these large changes to the system remain notably poor. Here we explore how the moss mortality affects belowground biogeochemistry over the course of the lethal stress. Twice weekly for 5 months we added 1.2mm of simulated rainfall to S. caninervis, as well as maintained control mosses. Throughout the experiment, we assessed the soils beneath the moss for multiple forms of carbon, nitrogen and phosphorus; nitrogen mineralization rates; and aspects of moss photosynthetic capacity (Fv/Fm) to explore how belowground biogeochemistry is affected over the course of the mortality event. Results/Conclusions As expected, mosses were strongly, negatively affected by the increased frequency of small rainfall events. In concert with declining moss health, we found significant changes to soil biogeochemical cycling. For instance, during the first week of treatments we observed an increase in extractable NH4+ from soils associated with the stressed moss compared with the controls, however, this pattern switched such that soil extractable NH4+ for stressed moss was much lower than controls as moss decline progressed. In contrast, extractable NO3- remained elevated in the treated moss relative to controls until the last sampling event when no significant difference was observed. These patterns match well with assessments of nitrification rates, which showed nitrification was consistently elevated in soils beneath stressed moss relative to controls. In addition, the moss physiological decline was associated with a reduction in total available nitrogen and with changes in soil carbon chemistry. Taken together, our data suggest the stress mosses experience in response to altered precipitation results in significant changes to soil biogeochemical cycling. Due to the nature of these shifts, the data have important implications for soil fertility, as well as for the trajectory of biocrust recovery after the loss of a dominant community member.