Interannual Changes In Precipitation Research Articles

Dynamic Vegetation Responses to Climate and Land Use Changes over the Inner Mongolia Reach of the Yellow River Basin, China

Ecological protection and high-quality development of the Yellow River Basin (YRB), China, aroused remarkable concerns from China’s Central Government, and has been a major national strategy. The Inner Mongolia reach of the Yellow River Basin (IM-YRB) is a typical dryland with pervasive vegetation restoration through the actions of the ecological projects that have been conducted in recent years. However, how climate changes and human activities, such as land use and land cover (LULC) changes, jointly impact vegetation variations in this region remains poorly understood. Here, using an explainable machine learning technique, we evaluated linkages between the kernel normalized difference vegetation index (kNDVI) and air temperature, precipitation, soil moisture, and LULC changes, and relevant marginal contributions of these four drivers to the observed vegetation changes. The grassland fraction on a pixel level was selected as the quantitative LULC variable due to its key role in regional LULC. We found that interannual kNDVI changes in most areas of this study region were negatively sensitive to temperature, but positively sensitive to precipitation and soil moisture, with area fractions of 71.74%, 96.93%, and 89.33%, respectively. The area fraction of negative kNDVI sensitivity to LULC was roughly equivalent to that of positive kNDVI sensitivity. The contributions of air temperature, precipitation, soil moisture, and LULC to overall kNDVI changes were 21.54%, 33.32%, 32.19%, and 12.95%, respectively. Moisture conditions also play a critical role in vegetation changes, which was reflected by the fluctuating growth of kNDVI as interannual changes in precipitation. Nonetheless, kNDVI changes are also affected by LULC, and LULC became the dominant factor behind the kNDVI anomalies over the grassland restoration regions from barren over the IM-YRB. This research provides theoretical support for dryland vegetation restoration under the influence of climate change.

Changes in hydrology and pollutant loads from stream restoration in an urban headwater catchment

Stream restoration structures can alter the hydrology and biogeochemical processes of urban headwater streams in important ways. As a stream restoration design, regenerative stormwater conveyance (RSC) structures are built to reconnect floodplains to stream channels, raise the groundwater table, and increase streamwater retention times. Altered hydrology and flow dynamics within restored stream reaches can affect solute concentrations and loads in baseflow and stormflow runoff. We monitored interannual changes in precipitation, groundwater and stream runoff before and after the implementation of a stream restoration using an RSC design in order to evaluate changes in hydrology and to estimate changes in solute loads of an urban headwater catchment in the Rock Creek watershed of Washington DC. A Before After Control Impact (BACI) experimental design was used. Compared to the control catchment, average recharge of the groundwater system of the RSC riparian area increased by 0.76 m in the post-restoration period and contributed to a 6 % increase in baseflow as a percentage of total runoff (compared to a decrease of 6 % in the control catchment). Area yields from the pre- to the post-restoration monitoring period decreased in both the RSC and control catchments, but significantly larger decreases in total nitrogen (TN), total phosphorus (TP) and total suspended solids (TSS) occurred in the RSC catchment (i.e., −59 % vs −23 %, −54 % vs −28 %, and −76 % vs −40 %, respectively). Results of this study indicate that RSCs as stream restoration structures in degraded headwater catchments can result in important hydrological and biogeochemical changes that significantly reduce nutrient and sediment loads.

Divergent Climate Sensitivities of the Alpine Grasslands to Early Growing Season Precipitation on the Tibetan Plateau

Warming is expected to intensify hydrological processes and reshape precipitation regimes, which is closely related to water availability for terrestrial ecosystems. Effects of the inter-annual precipitation changes on plant growth are widely concerned. However, it is not well-known how plant growth responds to intra-annual precipitation regime changes. Here, we compiled reanalysis climate data (ERA5) and four satellite-based vegetation indices, including the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), the Solar-induced Chlorophyll Fluorescence (SIF), and the Modified Triangular Vegetation Index (MTVI2), to evaluate the response of alpine grasslands (including alpine meadow and alpine steppe) to the change of precipitation regimes, especially to the intra-annual precipitation regimes on the Tibetan Plateau. We found monthly precipitation over the alpine steppe significantly increased in the growing season (May–September), but precipitation over the alpine meadow significantly increased only in the early growing season (May–June) (MJP) during the past four decades (1979–2019). The inter-annual plant growth (vegetation indices changes) on the alpine meadow was dominated by temperature, but it was driven by precipitation for the alpine steppe. On the intra-annual scale, the temperature sensitivity of the vegetation indices generally decreased but precipitation sensitivity increased during the growing season for both the alpine meadow and steppe. In response to the increase in MJP, we found the temperature sensitivity of the vegetation indices during the mid-growing season (July–August) (MGNDVI, MGEVI, MGSIF, and MGMTVI2) in the alpine meadow significantly increased (p < 0.01) while its precipitation sensitivity significantly decreased (p < 0.01). We infer that more MJP over the meadow may be the result of enhanced evapotranspiration, which is at the expense of soil moisture and even induces soil “drought” in the early growing season. This may be to elevate community water acquisition capacity through altering root mass allocation and community composition, consequently regulating the divergent climate sensitivities of vegetation growth in the mid-growing season. Our findings highlight that it is inadequate to regard precipitation as an indicator of water availability conditions for plant growth, which may limit our understanding of the response and acclimatization of plants to climate change.

Drivers of drought-induced shifts in the water balance through a Budyko approach

Abstract. An inconsistent relationship between precipitation and runoff has been observed between drought and non-drought periods, with less runoff usually observed during droughts than would be expected based solely on precipitation deficit. Predictability of these shifts in the precipitation–runoff relationship is still challenging, largely because the underlying hydrologic mechanisms are poorly constrained. Using 30 years of data for 14 basins in California, we show how the Budyko framework can be leveraged to decompose shifts in precipitation versus runoff during droughts into “regime” shifts, which result from changes in the aridity index along the same Budyko curve, and “partitioning shifts”, which imply a change in the Budyko parameter ω and thus in the relationship among water balance components that governs partitioning of available water. Regime shifts are primarily due to measurable interannual changes in precipitation or temperature, making them predictable based on drought conditions. Partitioning shifts involve further nonlinear and indirect catchment feedbacks to drought conditions and are thus harder to predict a priori. We show that regime shifts dominate changes in absolute runoff during droughts but that gains or losses due to partitioning shifts are still significant. Low aridity, high baseflow, a shift from snow to rain, and resilience of high-elevation runoff correlate with higher annual runoff during droughts than would be predicted by the precipitation–runoff ratio during non-drought years. Differentiating between these shifts in the precipitation–runoff relationship using a Budyko approach will help water resource managers, particularly in arid, drought-prone regions, to better project runoff magnitudes during droughts based on available climate data and, furthermore, understand under what circumstances and to what extent their forecasts may be less reliable due to nonlinear basin–climate feedbacks.

Global evidence on the asymmetric response of gross primary productivity to interannual precipitation changes

Understanding gross primary productivity (GPP) response to precipitation (PPT) changes is essential for predicting land carbon uptake under increasing PPT variability and extremes. Previous studies found that ecosystem GPP may have an asymmetric response to PPT changes, leading to the inconsistency of GPP gains in wet years compared to GPP declines in dry years. However, it is unclear how the asymmetric responses vary among vegetation types and under different PPT variabilities. This study evaluated the global patterns of asymmetries of GPP response to different PPT changes using two state-of-science global GPP datasets. The result shows that under mild PPT changes (|ΔPPT| ≤ 25%), grasslands, savannas, shrublands, and tundra show positive asymmetric responses (i.e., larger GPP gains in wet years than GPP losses in dry years), while other vegetation types show negative asymmetric responses (i.e., larger GPP losses in dry years than GPP gains in wet years). Conversely, all vegetation types show negative GPP asymmetric responses to moderate (25% < |ΔPPT| ≤ 50%) and extreme (|ΔPPT| > 50%) PPT changes. Thus, we propose a new non-linear asymmetric GPP-PPT model that incorporates three modes with regards to vegetation types. Meanwhile, we found that the spatial patterns of asymmetry were mainly driven by PPT amount and variability. Stronger and negative asymmetries were found in areas with smaller PPT amount and variability, while positive asymmetries were found in areas with higher PPT variability. These findings promote our understanding of carbon dynamics under increased PPT variability and extremes and provide new insights for land models to better predict future carbon uptake and its feedback to climate change.

High-resolution interannual precipitation reconstruction of Southern California: Implications for Holocene ENSO evolution

The variability of El Niño-Southern Oscillation (ENSO) on centennial to millennial time-scales is poorly understood due to the insufficient length, continuity, or resolution of existing paleoclimate records. Here we present a new, continuous, sub-annually resolved scanning XRF Ti record from marine sediments collected from Santa Barbara Basin (SBB) to reveal interannual precipitation changes in Southern California for the past 9000 yrs. Interannual precipitation variability in Southern California is closely related to ENSO through an atmospheric teleconnection with the tropical Pacific. Thus, Southern California precipitation reconstructions provide an opportunity to explore ENSO variability through time. Wavelet analysis of the SBB Ti record demonstrates interannual (2–7 yrs) precipitation variance was relatively weak prior to 4.4 ka, and significantly increased after 4.4 ka. Our record demonstrates a relationship between Southern California precipitation and the tropical Pacific and extratropical climate. The inferred increase of ENSO variability during the late Holocene is generally consistent with the published tropical Pacific ENSO records, and could be associated with a southward shift of the Intertropical Convergence Zone (ITCZ). Meanwhile, a deeper, westward-shifted Aleutian Low after 4.4 ka may have strengthened the ENSO teleconnection between the tropical Pacific and Southern California, contributing to the amplified interannual precipitation variance in the SBB record. Community Earth System Model (CESM) simulations through the Holocene (0, 3, 6, 9 ka) support the role of both the ITCZ and the AL in the enhancement of interannual precipitation variability in Southern California.

Investigating the inter-annual precipitation changes of Iran

Abstract Precipitation is an important factor in the management of a variety of agricultural and industrial projects. This study investigated the temporal-spatial change of inter-annual precipitation of Iran from 1977 to 2007 by using the APHRODITE precipitation database. Statistical methods were applied, such as spatial auto-correlation, Global Moran's index, Local Moran's I index, and hotspots to acquire the variations in precipitation. The highest spatial anomalies belong to September (75.26) and October (45.02), based on the Dispersion index. Also, the size of the largest cluster of Iran's precipitation clusters is developed during winter, cited by the index's outputs, which indicates the relative regularity of Iran's precipitation. The results of the spatial statistics showed that inter-annual precipitation changes in Iran have an upward cluster model. The results of the Global Moran statistics showed that September, with the lowest number (0.712114), has the highest spatial precipitation anomalies throughout the year in Iran. Meanwhile, precipitation has a positive spatial autocorrelation on the Caspian Sea shores and western and south-western parts of the country (mainly Zagros highlands) and a negative spatial autocorrelation in parts of the central and south-eastern areas based on the Local Moran index and hotspots.

Connection between Antarctic Ozone and Climate: Interannual Precipitation Changes in the Southern Hemisphere

In this study, we explored the connection between anomalies in springtime Antarctic ozone and all-year precipitation in the Southern Hemisphere by using observations from 1960–2018 and coupled simulations for 1960–2050. The observations showed that this correlation was enhanced during the last several decades, when a simultaneously increased coupling between ozone and Southern Annular Mode (SAM) anomalies became broader, covering most of the following summer and part of the previous winter. For eastern Australia, the ozone–precipitation connection shows a greater persistence toward the following summer than for other regions. On the other hand, for South America, the ozone–precipitation correlation seems more robust, especially in the early summer. There, the correlation also covers part of the previous winter, suggesting that winter planetary waves could affect both parameters. Further, we estimated the sensitivity of precipitation to changes in Antarctic ozone. In both observations and simulations, we found comparable sensitivity values during the spring–summer period. Overall, our results indicate that ozone anomalies can be understood as a tracer of stratospheric circulation. However, simulations indicate that stratospheric ozone chemistry still contributes to strengthening the interannual relationship between ozone and surface climate. Because simulations reproduced most of the observed connections, we suggest that including ozone variability in seasonal forecasting systems can potentially improve predictions.

ENSO-induced groundwater changes in India derived from GRACE and GLDAS

This study aims to quantify the interannual variations of groundwater storage changes (GWSCs) over India. GWSCs are derived from the gravity recovery and climate experiment (GRACE) and global land data assimilation system (GLDAS)-Noah life safety model (LSM) for the period 2003–2015. Estimated GWSCs are validated with the satellite altimetry over the six lake stations. The variability of GWSC and altimetry water-level heights are assessed with the cross-correlation and plotting analysis. Annual trends of GWSC and GRACE in terrestrial water storage (TWS) were estimated using the non-parametric Mann–Kendall test and Sen’s slope method. Results suggest that GWSC and TWS have declined in northern India at the rate of \(\sim \)1.6 cm \(\hbox {yr}^{-1}\) and in southern and western central India at the rate of \(\sim \)0.5 cm \(\hbox {yr}^{-1}\). Impacts of short-term climate perturbations such as El Niño and La Niña for the GWSCs are assessed. During the El Niño period, the decline of GWSC over northern India enhanced, whereas during the La Niña period, the recovery of GWSC is evident. These interannual variations of GWSCs over India are attributed by interannual precipitation changes. Under the global warming scenario, the occurrences of El Niño events are likely to enhance in the future, and our findings help the water resource management policy makers for necessary actions during such short-term climate perturbations.

Monitoring ecosystem dynamics in northwestern Ethiopia using NDVI and climate variables to assess long term trends in dryland vegetation variability

Dryland ecosystems are highly vulnerable to environmental changes. Monitoring is vital in order to evaluate their response to fluctuating rainfall and temperature patterns for long-term ecosystem safeguarding. Monitoring of long term changes of normalized difference vegetation index (NDVI) and climate variables are fundamental for better understanding of change trajectories in dryland ecosystem, and to ascertain their potential interaction with anthropogenic drivers. In this study, we identify determinant factors of dryland changes by using MODIS NDVI, precipitation and temperature data for Breaks for Additive Seasonal and Trend (BFAST) and Mann Kendall test statistic. BFAST predicts iteratively time and number of changes within a time series data to depict the size and direction of changes. Analysis of NDVI, precipitation and temperature time series data showed substantial changes during the study period of 2000–2014. There is a reduction trend in vegetation showed by the decline in NDVI, with significant breakpoints till 2009 and recovery afterwards, without a significant change in annual trends of precipitation (α < 0.05) for the same study period. Furthermore 2 positive climate trends were founded: a) a significant positive trend on long term annual rainfall during the main rainy seasons and; 2) a significant (α < 0.05) annual increment of the long term mean minimum and mean maximum temperature of 0.03 °C/year and 0.04 °C/year, respectively. This assessment showed that climate variables cannot be considered as the main factors in explaining the observed patterns of vegetation dynamics. Seasonal and interannual precipitation changes have a lower weight as driving factors for the reduction in vegetation trends. Hence, the decline in vegetation productivity of the region can be attributed to the increasing pressure of human activities.

The role of drought on wheat yield interannual variability in the Iberian Peninsula from 1929 to 2012.

The production of wheat in the Iberian Peninsula is strongly affected by climate conditions being particularly vulnerable to interannual changes in precipitation and long-term trends of both rainfall and evapotranspiration. Recent trends in precipitation and temperature point to an increase in dryness in this territory, thus highlighting the need to understand the dependence of wheat yield on climate conditions. The present work aims at studying the relation between wheat yields and drought events in the Iberian Peninsula, using a multiscalar drought index, the standardized precipitation evapotranspiration index (SPEI), at various timescales. The effects of the occurrence of dry episodes on wheat yields were analyzed, on regional spatial scale for two subperiods (1929-1985 and 1986-2012). The results show that in western areas, wheat yield is positively affected by dryer conditions, whereas the opposite happens in eastern areas. The winter months have a bigger influence in the west while the east is more dependent on the spring and summer months. Moreover, in the period of 1986-2012, the simultaneous occurrence of low-yield anomalies and dry events reaches values close to 100% over many provinces. Results suggest that May and June have a strong control on wheat yield, namely, for longer timescales (9 to 12months). A shift in the dependence of wheat yields on climatic droughts is evidenced by the increase in the area with positive correlation and the decrease in area with negative correlation between wheat yields and SPEI, probably due to the increase of dry events.

Climate Regimes, El Niño-Southern Oscillation, and Meningococcal Meningitis Epidemics.

Meningococcal meningitis is a major public health problem that kills thousands annually in Africa, Europe, North, and South America. Occurrence is, however, highest during the dry seasons in Sahel Africa. Interannual changes in precipitation correlate with interannual changes in El Niño-Southern Oscillation (ENSO), while interdecadal changes in precipitation correlate with Pacific Decadal Oscillation (PDO). The objective of the study was to determine if there is spectral coherence of seasonal, interannual, and interdecadal changes in occurrence of meningococcal meningitis in Sahel, Central, and East Africa with interannual and interdecadal changes of PDO and ENSO. Time series were fitted to occurrence of meningococcal meningitis in Sahel, Central, and East Africa, to indices of precipitation anomalies in the Sahel, and to indices of ENSO and PDO anomalies. Morlet wavelet was used to transform the time series to frequency-time domain. Wavelet spectra and coherence analyses were performed. Occurrence of meningococcal meningitis showed seasonal, interannual, and interdecadal changes. The magnitude of occurrence was higher during warm climate regime, and strong El Niños. Spectra coherence of interannual and interdecadal changes of ENSO and PDO with occurrence of meningococcal meningitis in Sahel, Central, and East Africa were significant at p < 0.0001. Precipitation in Sahel was low during warm climate regimes. Spectra coherence of changes in precipitation in Sahel with ENSO was significant at p < 0.0001. ENSO and PDO are determinants of the seasonal, interannual, and interdecadal changes in occurrence of meningococcal meningitis. Public health management of epidemics of meningococcal meningitis should include forecast models of changes in ENSO to predict periods of low precipitation, which initiate occurrence.

ENSO impacts on Atlantic watersheds of South America

Interannual changes in precipitation cause significant effects on the Pacific watersheds of South America. Large rivers flowing to the Atlantic Ocean as the Paraná and Uruguay are also influenced by ENSO-triggered floods causing significant impacts on the economies of Argentina, Brazil and Uruguay. At the same time, the floodplains of small rivers are also sensitive to strong ENSOs. Floods occur approximately simultaneously in subtropical watersheds of Brazil and high-latitude watersheds of North Patagonia. ENSO-triggered floods were recorded during the years 1941-42, 1982-83 and 1997-98. Several floods impacted along the Itajaí River affecting the higher (Blumenau city) and lower estuary (Itajaí city). The Guaiba River, at the headlands of Lagoa dos Patos, has repeatedly flooded Porto Alegre. At the Río de la Plata basin, significant increases in the discharges were assigned to the Paraná and the Uruguay rivers. The small basin of the Quequén Grande River without extended floodplains is subject to periodic failure of its margins. The Colorado River, the northern limit of Patagonia, is allochthonous and its watershed was artificial and progressively modified; the ENSO event of 1983 caused an increase in its salinity due to the sudden reactivation of the Desaguadero-Curacó system.

THE IMPACT OF PRECIPITATION ON WET DEPOSITION OF SULPHUR AND NITROGEN COMPOUNDS

Abstract Atmospheric transport model FRAME has been used in this study to estimate the influence of precipitation on the patterns of wet deposition of oxidised sulphur, oxidised nitrogen and reduced nitrogen in Poland during the years 1981-2005. A constant wind and emission data and year-specific spatially interpolated precipitation data was used in the model. The results show that the correlation coefficient between mean annual precipitation totals and mean wet deposition is above 0.9 for all examined compounds. The spatial patterns of pollutant deposition are similar for all years, with the north-western part of Poland receiving the lowest and the southern, mountainous part, the highest pollutant load. The largest precipitation-induced changes in wet deposition budgets are observed for oxidised sulphur (53% of the average amount between wet and dry year), and smaller for oxidised and reduced nitrogen (30%). Inter-annual precipitation changes cause large variations in the amount of wet deposition of pollutants. This means that the emission abatements may not cause immediate environmental effects, eg reductions in deposition of pollutants and, further ecosystems areas of exceeded critical loads.

Desertification alters the response of vegetation to changes in precipitation

Summary 1. Desertification is of critical concern because it may affect 40% of the global land area inhabited by more than 1 billion people. During the process of desertification, defined as land degradation in arid, semi‐arid and dry subhumid areas, drylands shift to a state of reduced biological productivity that may lead to widespread loss of human well‐being. Despite recent advances, we need a better understanding of the response of ecosystems to desertification to improve the assessment and monitoring of desertification. 2. We used a published physiognomic description, MODIS monthly NDVI data for 2000–2005 and rain gauge data to characterize the long‐term effects of degradation for an area of 128 000 ha located in western Patagonia. 3. We focused on three aspects of vegetation dynamics: radiation interception, precipitation use efficiency (PUE) and the sensitivity of vegetation to interannual changes in precipitation (i.e. the slope of the relationship between the above‐ground net primary productivity and precipitation, the precipitation marginal response, PMR). In particular, we analysed the response of PMR and PUE to long‐term changes in vegetation structure due to grazing. 4. On average, NDVI decreased by 28%, ranging between 35% (grass or grass–shrub steppes to semi‐deserts) and 22% (grass or grass‐shrub steppes to low cover grass steppes) suggesting that, in Patagonia, desertification may imply a reduction in the above‐ground net primary productivity. 5. Additionally, PMR and PUE captured the functional modifications associated with vegetation structure caused by desertification. In general, grass and grass‐shrub steppes had the highest average PUE and PMR. Shrub steppes and semi‐deserts had the lowest PMR and PUE. These results support the hypothesis that PUE is more sensitive to changes in total plant cover and PMR to changes in plant functional type composition. 6. Synthesis and applications. Our results indicate that the precipitation marginal response could complement current desertification assessments based only on precipitation use efficiency thereby improving our ability to monitor desertification. Enhanced monitoring programmes could provide an early warning signal for the onset of desertification allowing for timely management action.

Interannual variations in vegetation activities and climate variability caused by ENSO in tropical rainforests

Interannual variations in terrestrial carbon cycle over tropical rainforests affect the global carbon cycle. Terrestrial ecosystem models show the interannual relationship between climate changes due to El Niño‐Southern Oscillation (ENSO) and net primary production over tropical rainforests. However, we need an independent analysis using satellite‐based vegetation index and climate parameters. In the present study, we extracted the ENSO‐related interannual variations from time‐series in Normalized Difference Vegetation Index (NDVI) and climate data from 1981 to 2000, and analysed their relevance. We detected relationships among NDVI, ENSO, and climate parameters from long‐term data with negative NDVI–ENSO, NDVI–temperature, and positive NDVI–precipitation relations. These correlations suggest that interannual variability in vegetation activities over tropical rainforests could be extracted from NDVI time‐series despite noise components in NDVI data, and that interannual changes in precipitation and temperature caused by ENSO play a more important role in vegetation activities over tropical rainforests than in incoming surface solar radiation.

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.

Quantifying Early Indicators of Global Climate Change

One of the more significant voids remaining in our scientific understanding of global climate change is the relationship between climate change and the resulting changes expected in ecological communities. Because a large proportion of the North American landscape has been modified by human activities, it is difficult to assess whether ecological changes are being caused by human activities or climate change. Thus, we must look to landscapes where the modification has been less severe. One of the most pristine landscapes in North America where scientists can study natural processes is that of the Greater Yellowstone Ecosystem. Within this system some of the more sensitive habitats are the montane meadows. These habitats exist along a continuum from very dry (xeric) sagebrush meadows, to flowering (mesic) meadows, to wet (hydric) sedge meadows. Because of the relatively short growing season, species in these meadows can exhibit quick changes in distribution and abundance relative to climatic changes. My research uses satellite images and field surveys to evaluate how meadow habitats and their associated species respond to interannual changes in precipitation and soil moisture. I am examining the plant and butterfly communities to measure the response. Over 100 species of butterflies occur in this area and many are closely associated with specific types of meadows. This research is significant because it will provide an early warning system for assessing the effects of climate change. Documenting changes in montane meadows will assist in understanding how climate change may affect more highly managed areas of the globe.

Seasonal and interannual variation in evapotranspiration, energy balance and surface conductance in a northern temperate grassland

The eddy covariance technique was used to measure evapotranspiration and sensible heat flux continuously for 2.5 years (three growing seasons) in a northern grassland near Lethbridge, Alta., Canada. The 1999 growing season had precipitation (226.9 mm) near the 30-year average value (1971–2000; 206.4±77.5; mean±S.D.). In contrast, the 1998 growing season precipitation (295.3 mm) was significantly above average and almost four times greater than the 2000 growing season precipitation (75.1 mm), which was significantly below average. The 1998 growing season had higher peak evapotranspiration (4.5 mm per day) than values observed in 1999 and 2000, when the highest evapotranspiration was approximately 3 mm per day. Evapotranspiration during the winter periods was normally <0.5 mm per day. In 1998, maximum latent heat flux was higher than sensible heat flux during June and early July. In contrast, sensible heat flux dominated the energy budget at midday during the entire 1999 and 2000 growing seasons. The soil heat flux component was quite similar during all 3 years, with maximum daily values reaching approximately 80–100 W m −2. The decoupling coefficient (ranged between 0.2 and 0.3) indicated that evapotranspiration was strongly controlled by surface conductance in this grassland. The highest λE/ λE eq values (0.7–0.8) occurred in June and July 1998, before the exhaustion of soil moisture reduced values to <0.4 in late August. Lower values were recorded in 1999 and 2000, often below 0.5. A non-linear model of surface conductance that included functions for vapor pressure deficit, photon flux density, and available soil moisture explained a large proportion of the variation in the surface conductance data. To model surface conductance accurately, separate model coefficients need to be used for the different years. Therefore, our data indicate that evapotranspiration and its physiological control vary in response to interannual changes in precipitation.