Article

Seasonality of soil moisture mediates responses of ecosystem phenology to elevated CO2 and warming in a semi-arid grassland

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Abstract

Vegetation greenness, detected using digital photography, is useful for monitoring phenology of plant growth, carbon uptake, and water loss at the ecosystem level. Assessing ecosystem phenology by greenness is especially useful in spatially extensive, water-limited ecosystems such as the grasslands of the western United States, where productivity is moisture dependent and may become increasingly vulnerable to future climate change.We used repeat photography and a novel means of quantifying greenness in digital photographs to assess how the individual and combined effects of warming and elevated CO2 impact ecosystem phenology (greenness and plant cover) in a semi-arid grassland over an 8-year period.Climate variability within and among years was the proximate driver of ecosystem phenology. Individual and combined effects of warming and elevated CO2 were significant at times, but mediated by variation in both intra- and inter-annual precipitation. Specifically, warming generally enhanced plant cover and greenness early in the growing season but often had a negative effect during the middle of the summer, offsetting the early season positive effects. The individual effects of elevated CO2 on plant cover and greenness were generally neutral.Opposing seasonal variations in the effects of warming and less so elevated CO2 cancelled each other out over an entire growing season, leading to no net effect of treatments on annual accumulation of greenness. The main effect of elevated CO2 dampened quickly, but warming continued to affect plant cover and plot greenness throughout the experiment. The combination of warming and elevated CO2 had a generally positive effect on greenness, especially early in the growing season and in later years of the experiment, enhanced annual greenness accumulation. However, interannual precipitation variation had larger effect on greenness, with 2-3 times greater greenness in wet years than in dry years.Synthesis. Seasonal variation in timing and amount of precipitation governs grassland phenology, greenness, and the potential for carbon uptake. Our results indicate that concurrent changes in precipitation regimes mediate vegetation responses to warming and elevated atmospheric CO2 in semi-arid grasslands. Even small changes in vegetation phenology and greenness in response to warming and rising atmospheric CO2 concentrations, such as those we report here, can have large consequences for the future of grasslands.This article is protected by copyright. All rights reserved.

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... If moisture is not limiting, carbon storage can increase significantly in response to warmer conditions and rising atmospheric CO 2 (see Section 10.3.3, p. 410). In part, this increase results from flexible timing of grassland plant growth and photosynthesis (Ryan et al., 2016;Zelikova et al., 2015). For example, drought decreased the growing season length and led to reductions in NPP and carbon sequestration in the Canadian Great Plains (Flanagan and Adkinson 2011). ...
... The lengthening of the growing season was dependent on a mix of C 3 and C 4 species adapted to different climate conditions. In the same experiment, greenness was enhanced (i.e., indicating increased aboveground biomass and cover) with warming and elevated CO 2 , but the effects of seasonal and interannual rainfall variability were much stronger (Zelikova et al., 2015). High-precipitation years had two to three times greater vegetation greenness than dry years. ...
... Plant nutrient uptake can decrease soil nutrients, which may be made available during SOM decomposition. [Figure conceptionderived from numerous studies, includingHufkens et al., 2016;Morgan et al., 2011; Mueller et al., 2016;Reich and Hobbie 2013;Reyes-Fox et al., 2014;and Zelikova et al., 2015.] U.S. Global Change Research Program November 2018 ...
... Alternatively, when temperatures are favorable and water is not limited, warming may instead lead to increased transpiration as D v increases (Berry & Bjorkman, 1980;Morison & Gifford, 1983). Thus, the magnitude and direction of these effects are likely to depend on plant cover and soil moisture, which vary throughout the growing season (Gray et al., 2016;Zelikova et al., 2015). This seasonal variation is particularly strong in the semiarid grasslands of southeastern Wyoming (Bachman et al., 2010;Lauenroth & Bradford, 2009), where soil moisture is the highest early in the growing season followed by progressive drying during summer and autumn (Blumenthal et al., in press;Kurc & Small, 2007). ...
... Ecosystem gas exchange measurements of H 2 O and CO 2 were collected during most seasons of the 7-year PHACE experiment (Bachman et al., 2010;Pendall et al., 2013). Percentage of green vegetation (greenness) in each plot was measured with time series repeat photography and quantification of greenness in digital photographs (Zelikova et al., 2015). We included this greenness measurement as a proxy of phenological activity, which is directly linked to plant canopy gas exchange (Kurc & Benton, 2010). ...
... To measure greenness, pixel values were converted to a hue, saturation, and value scale, where boundaries were selected to define a range of hue, saturation, and value vales for "green" (Zelikova et al., 2015). ...
Article
Global climate change is expected to alter seasonal patterns and rates of evapotranspiration in dry regions. While climate change will involve elevated CO2 and increased temperatures, independently these factors may have different impacts on actual evapotranspiration (AET) due to their opposing effects on transpiration. We used canopy gas exchange chambers to quantify AET in a semi-arid grassland experimentally altered by elevated CO2 and warming over three years with contrasting ambient precipitation. Seasonal and interannual variations in AET due to background climate variability were larger than the effects of climate manipulation treatments. However, in a year with average precipitation, cumulative growing season AET was suppressed by warming by 23%. Across years, warming increased AET early in the growing season and suppressed it later in the growing season. By contrast, elevated CO2 suppressed AET early in the growing season and enhanced it later, but only in years with average or above-average precipitation. Vegetation greenness (a proxy for photosynthetically active leaf area) was consistently the strongest predictor of AET, while soil moisture and vapor pressure deficit were secondary drivers. Our research demonstrates that effects of increased atmospheric CO2 and temperature on AET will be mediated by plant phenological development and seasonal climatic conditions.
... weeks) (Migliavacca et al., 2011;Morgan et al., 2004;Pathare et al., 2017). Consequently, high-frequency measures of ecosystem productivity are required to adequately capture climate and eCO 2 effects on grassland productivity (Migliavacca et al., 2011;Zelikova et al., 2015), and will provide greater value for the development and testing of process-based ecosystem models (Hufkens et al., 2016). Near-surface remote sensing using time-lapse digital photography provides an ideal tool for the quantification of trends in grassland growth at fine temporal scales (Hufkens et al., 2016;Migliavacca et al., 2011;Zelikova et al., 2015). ...
... Consequently, high-frequency measures of ecosystem productivity are required to adequately capture climate and eCO 2 effects on grassland productivity (Migliavacca et al., 2011;Zelikova et al., 2015), and will provide greater value for the development and testing of process-based ecosystem models (Hufkens et al., 2016). Near-surface remote sensing using time-lapse digital photography provides an ideal tool for the quantification of trends in grassland growth at fine temporal scales (Hufkens et al., 2016;Migliavacca et al., 2011;Zelikova et al., 2015). Greenness indices derived from digital photographs are highly correlated with projected foliage cover and gross primary productivity in grasslands (Hufkens et al., 2016;Migliavacca et al., 2011;Moore et al., 2016;Toomey et al., 2015;Zelikova et al., 2015). ...
... Near-surface remote sensing using time-lapse digital photography provides an ideal tool for the quantification of trends in grassland growth at fine temporal scales (Hufkens et al., 2016;Migliavacca et al., 2011;Zelikova et al., 2015). Greenness indices derived from digital photographs are highly correlated with projected foliage cover and gross primary productivity in grasslands (Hufkens et al., 2016;Migliavacca et al., 2011;Moore et al., 2016;Toomey et al., 2015;Zelikova et al., 2015). Stereo imaging techniques are also effective at characterising vegetation structure and can provide estimates of vegetation biomass (Yoon & Thai, 2010). ...
Article
Rising atmospheric [CO2] and associated climate change are expected to modify primary productivity across a range of ecosystems globally. Increasing aridity is predicted to reduce grassland productivity, though rising [CO2] and associated increases in plant water use efficiency may partially offset the effect of drying on growth. Difficulties arise in predicting the direction and magnitude of future changes in ecosystem productivity, due to limited field experimentation investigating climate and CO2 interactions. We use repeat near-surface digital photography to quantify the effects of water availability and experimentally manipulated elevated [CO2] (eCO2) on understorey live foliage cover and biomass over three growing seasons in a temperate grassy woodland in south-eastern Australia. We hypothesised that (i) understorey herbaceous productivity is dependent upon soil water availability, and (ii) that eCO2 will increase productivity, with greatest stimulation occurring under conditions of low water availability. Soil volumetric water content (VWC) determined foliage cover and growth rates over the length of the growing season (August - March), with low VWC (< 0.1 m³ m⁻³) reducing productivity. However, eCO2 did not increase herbaceous cover and biomass over the duration of the experiment, or mitigate the effects of low water availability on understorey growth rates and cover. Our findings suggest that projected increases in aridity in temperate woodlands are likely to lead to reduced understorey productivity, with little scope for eCO2 to offset these changes.
... c o m / l o c a t e / e n v e x p b o t in C 3 plants (Ainsworth and Long, 2005;Bloom et al., 2010;Zhou et al., 2011;Xu et al., 2013Xu et al., , 2014van der Kooi et al., 2016). Although this stimulation occurs regardless of water conditions in C 3 species, C 4 plants benefit from elevated CO 2 only under a water-deficit rather than when the water supply is sufficient (Leakey et al., 2006;Morgan et al., 2004;Xu et al., 2013Xu et al., , 2014Xu et al., , 2015Manderscheid et al., 2014;Zelikova et al., 2015;van der Kooi et al., 2016). Based on the results of Morgan et al. (2011), CO 2 enrichment does not significantly increase the aboveground biomass of C 3 grasses when precipitation is sufficient precipitation, but the increase is notable under drought. ...
... Grass-dominated, dry rangelands cover over 30% of the terrestrial surface of the earth and provide most of the forage for the world's domestic livestock (Morgan et al., 2011). These grasslands are primarily limited by water because they are mainly located in arid and semi-arid regions (Weltzin et al., 2003;Zelikova et al., 2015). Stipa grandis, a C 3 perennial bunch grass of the Graminaceae, is a dominant species in the typical steppe in northern China, which is an area that has experienced severe degradation during recent decades and is also sensitive to climate change (Bai et al., 2004;Zhang et al., 2007;Xu et al., 2014;Seddon et al., 2016). ...
... The effects of elevated CO 2 on P n , WUE and R n also varied with the precipitation condition. Precipitation is a critical environmental factor in arid and semi-arid temperate grasslands (Weltzin et al., 2003;Bai et al., 2004;Zhang et al., 2007;Zelikova et al., 2015). In this region, the annual precipitation often varies greatly, indicating that the benefits of increasing CO 2 might not be as optimal as expected. ...
... Most terrestrial systems have already undergone some sort of phenological shift in response to climate over the past several decades (Buitenwerf et al. 2015), but changes in autumn phenology have received relatively little attention (Gallinat et al. 2015). Autumn phenology is thought to be regulated by several factors including precipitation (Panchen et al. 2014, Zelikova et al. 2015, since ample soil moisture is critical to maintaining photosynthetic capacity and leaf longevity (Grassi and Magnani 2005). However, the impacts of IRV on autumn phenology are unclear (Cleland et al. 2006, Dragoni andRahman 2012). ...
... The wetter conditions associated with IRV supported the retention of leaves later into the autumn and delayed the completion of plant senescence. While others have shown that greater water availability has delayed autumn phenology in other systems (Leuzinger et al. 2005, Zeppel et al. 2013, Zelikova et al. 2015, to our knowledge, no previous studies have demonstrated that this effect can be caused by IRV. One possible explanation for the correlation between IRV and the later senescence is that seedlings grown under IRV were taller, and thus had more leaves and consequently took longer to complete litterfall. ...
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Although encroaching woody plants have reduced the global extent of grasslands, continuing increases in soil nitrogen availability could slow this trend by favoring resident herbaceous species. At the same time, projected increases in rainfall variability could promote woody encroachment by aligning spatiotemporal patterns of soil moisture availability with the needs of woody species. We evaluated the responses of two deciduous woody species to these simulated environmental changes by planting seedlings of Quercus palustris and Lonicera maackii into tallgrass prairie communities grown under a factorial combination of increased rainfall variability and nitrogen addition. Lonicera maackii growth was reduced 20% by nitrogen addition, and increased rainfall variability led to 33% larger seedlings, despite greater competition for light and soil resources. In contrast, Q. palustris growth showed little response to either treatment. Increased rainfall variability allowed both species to retain their leaves for an additional 6.5 days in autumn, potentially in response to wetter end-of-season shallow soils. Our findings suggest increases in rainfall variability will counteract the inhibitory effect of nitrogen deposition on growth of L. maackii, extend autumn phenology, and promote the encroachment of some woody species into grasslands. This article is protected by copyright. All rights reserved.
... In alpine tundra, forbs responded to warming by flowering earlier and responded to increase snowpack and nitrogen addition by flowering later, but interactions of warming, snowpack and nitrogen addition advanced flowering (Smith et al., 2012). Commonly, in sub-arid grasslands, interannual variation in precipitation controls the direction of warming effects (delay or advance) on plant phenology (Zelikova et al., 2015). ...
... Hence, preseason water availability is an important factor regulating the warming effects on plant phenology. In the USA, field experiment has showed that seasonal variation in the timing and amount of precipitation governs grassland green up of great plain ecosystems (Zelikova et al., 2015). After conducting three years of year-round warming and spring precipitation addition experiment, we found that warming delayed plant green up and flowering in a warm and dry year (2016) while advanced plant green up in a cold and wet year (2015). ...
Article
Temperature and precipitation are primary regulators of plant phenology. However, our knowledge of how these factors might interact to affect plant phenology is incomplete. The Qinghai-Tibetan Plateau, a cold and high region, has experienced no consistent changes in spring phenology, despite a significant warming trend. We conducted a manipulative experiment of warming and precipitation addition in an alpine meadow on the Qinghai-Tibetan Plateau in 2015 (cold and wet), 2016 (warm and dry) and 2017 (mild and very wet). We found that warming increased annual variability of plant spring phenology. Warming delayed green up of all monitored species in 2016, advanced green up of early flowering species in 2015, and did not alter green up in 2017. For example, green up of the shallow rooted Kobresia pygmaea advanced 8 (± 2) days in 2015 and was delayed by 23 (± 3) days in a dry year (2016) under warming compared with control. Early spring precipitation addition can offset the delaying effects of warming in a dry year on the Qinghai-Tibetan Plateau. Under warming plus precipitation addition, community average green up advanced compared to control plots in 2015 and 2016, and community average flowering advanced for all three years. In 2016, flowering of K. pygmaea (an early flowering species) advanced under warming plus precipitation addition compared to control while flowering of other species did not change. Our results highlight that annual variation of soil moisture condition plays a critical role in determining the magnitude and direction of spring phenology response to warming. We provide insights in how plant spring phenology might change in a warmer future in the presence or absence of precipitation increase .
... While seasonal swings in the timing and amount of precipitation directly affect the timing of plant production (e.g. phenology, Zelikova et al., 2015), such swings can also indirectly drive production responses via effects on plant community composition (Suttle et al., 2007). As substantial shifts in precipitation regimes including increased seasonal rainfall variability and the frequency of extreme events such as severe storms and droughts is expected globally (Tebaldi et al 2006;IPCC 2007;Groisman et al. 2012), a better understanding of intra-annual precipitation's effects on phenology, community composition, and consequently, withinsite productivity is needed. ...
... Intra-annual precipitation patterns directly determine soil water availability to plants during key growth stages, affecting ecosystem productivity in ways not reflected by total annual precipitation (Knapp et al., 2008). Specifically, water availability during particular windows of time may affect annual net primary productivity (ANPP; Epstein et al., 1999), as well as when peak productivity occurs and how long it lasts (Zelikova et al., 2015). For example, early growing season precipitation impacts ANPP (Chelli et al., 2016;Craine et al., 2012;Hossain & Beierkuhnlein, 2018) due to the critical need for water to initiate and support early growth. ...
Article
Within ecosystems, intra‐annual precipitation patterns – the variability and timing of rainfall – may be a stronger driver of net primary productivity than total annual precipitation. In particular, the amount and timing of precipitation directly affects the amount and timing of plant production, but also indirectly affects productivity via changes to plant community composition. Community response patterns may either buffer or amplify productivity responses to precipitation, as different species respond to different conditions. In a semi‐arid California grassland, we experimentally tested how plant communities respond to intra‐annual precipitation using rainout shelters in which we manipulated drought amount and timing (early‐season drought, late‐season drought, continuous drought and ambient precipitation) over three years and assessed plant responses: aboveground net primary production (ANPP), phenological timing of peak production and senescence, and community composition. Overall, early‐season and consistent drought treatments had lowest productivity, while late‐season and consistent drought treatments senesced earlier. Plots with functionally diverse communities shifted community composition and had a significant ANPP response to precipitation treatments. In contrast, communities dominated by a single resource‐acquisitive grass species did not change in community composition over time and had no ANPP response to precipitation treatments. The timing of production also differed by community, however, where functionally diverse communities remained green longer (particularly under the early‐season drought treatment) compared to communities dominated by one grass species, which senesced earlier (particularly under the late‐season drought treatment). Synthesis. Our study demonstrates that drought patterns may indirectly drive ANPP via plant community responses in composition and phenology. This suggests that the combination of species composition and vegetation phenology could jointly alter ecosystem‐level sensitivity to precipitation seasonality under future climate change. We show that both functional diversity and dominant stability mechanisms are in operation simultaneously, highlighting the need to understand both the context and variation in community structure to predict ANPP responses to intra‐annual precipitation.
... In the first year (2006), an additional 160 mm of water was added (20 mm 9 8 dates during the growing season) to establish growth. Further details can be found in Morgan et al. (2011), Pendall et al. (2013, Ryan et al. (2015), and Zelikova et al. (2015). Mueller et al. (2016) present a comprehensive summary of the ecosystem responses over the duration of the PHACE experiment. ...
... Vegetation greenness was inferred from biweekly digital photographs taken between March and October. In 2008, photographs were obtained monthly (see Zelikova et al. (2015) for details). Phenology leaf-on and leaf-off dates for different species were obtained by direct observation (Reyes-Fox et al., 2014). ...
Article
Multi-factor experiments are often advocated as important for advancing terrestrial biosphere models (TBMs), yet to date such models have only been tested against single-factor experiments. We applied 10 TBMs to the multi-factor Prairie Heating and CO2 Enrichment (PHACE) experiment in Wyoming, USA. Our goals were to investigate how multi-factor experiments can be used to constrain models, and to identify a road map for model improvement. We found models performed poorly in current ambient conditions; there was a wide spread in simulated above-ground net primary productivity (range: 31-390 g C m(-2) yr(-1) ). Comparison with data highlighted model failures particularly in respect to carbon allocation, phenology, and the impact of water stress on phenology. Performance against observations from single-factors experiments was also relatively poor. In addition, similar responses were predicted for different reasons across models: there were large differences among models in sensitivity to water stress and, among the N cycle models, N availability during the experiment. Models were also unable to capture observed treatment effects on phenology: they over-estimated the effect of warming on leaf onset and did not allow CO2 -induced water savings to extend the growing season length. Observed interactive (CO2 x warming) treatment effects were subtle and contingent on water stress, phenology and species composition. Since the models did not correctly represent these processes under ambient and single-factor conditions, little extra information was gained by comparing model predictions against interactive responses. We outline a series of key areas in which this and future experiments could be used to improve model predictions of grassland responses to global change. This article is protected by copyright. All rights reserved.
... The experiment was a factorial manipulation of CO 2 (ambient and 600 ppm) and canopy temperature (ambient and warmed; + 1.5°C during the day, + 3°C at night), achieved using free-air CO 2 enrichment and infrared heaters . Results from the first few years of the experiment have been reported for plant production and community composition, soil moisture, and nitrogen (N) in plants and soils (Dijkstra et al. 2010b(Dijkstra et al. , 2012aMorgan et al. 2011;Carrillo et al. 2012Carrillo et al. , 2014Zelikova et al. 2014Zelikova et al. , 2015. Using data for all these ecosystem properties across the entire 7-year experiment, we present a novel quantification of how the impacts of eCO 2 and warming, and their interactions, were mediated by meteorological conditions and experiment duration. ...
... The specific temporal variables were: (1) experiment duration, (2) mean ambient air temperature during each growing season, and (3) mean ambient soil moisture during each growing season. To focus on meteorological conditions that most strongly influence aboveground plant growth in this ecosystem (Derner & Hart 2007;Morgan et al. 2011;Zelikova et al. 2015), we averaged ambient soil moisture and air temperatures between day of year 100 (early April) and harvest of plant biomass . With each of these temporal variables in the same model, the modelled effects of each temporal variable (e.g. ...
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It is unclear how elevated CO2 (eCO2 ) and the corresponding shifts in temperature and precipitation will interact to impact ecosystems over time. During a 7-year experiment in a semi-arid grassland, the response of plant biomass to eCO2 and warming was largely regulated by interannual precipitation, while the response of plant community composition was more sensitive to experiment duration. The combined effects of eCO2 and warming on aboveground plant biomass were less positive in 'wet' growing seasons, but total plant biomass was consistently stimulated by ~ 25% due to unique, supra-additive responses of roots. Independent of precipitation, the combined effects of eCO2 and warming on C3 graminoids became increasingly positive and supra-additive over time, reversing an initial shift toward C4 grasses. Soil resources also responded dynamically and non-additively to eCO2 and warming, shaping the plant responses. Our results suggest grasslands are poised for drastic changes in function and highlight the need for long-term, factorial experiments.
... However, in a semiarid creosote shrubland in Arizona, an index derived from phenocam data tracked the "green-up" of the evergreen vegetation, showing good agreement with increases in the net ecosystem exchange (NEE) of CO 2 (Kurc & Benton, 2010). Also, Zelikova et al. (2015) found significant associations between an index of the "greening" of vegetation and total biomass in a temperature and CO 2 enrichment experiment in a C3-dominated prairie in Wyoming. Thus, information produced by phenocams can be used as an input for process-based models to estimate gross primary productivity (GPP). ...
... We found a moderate agreement between the Ig and GPP EC (Figure 2c), supporting the trend to apply a vegetation index from RGB intensities of repeat digital photographs as a proxy to monitor the timing and level of primary production. This proxy has been tested in various temperate ecosystems, some with noticeable changes in leaf coloration and defoliation, but there are few examples of this application of phenocams in drylands (Kurc & Benton, 2010;Moore et al., 2017;Zelikova et al., 2015). Phenocams are a feasible technology for terrestrial monitoring of vegetation despite the logistical challenges of drylands (Browning et al., 2017). ...
Article
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Although drylands cover >40% of the land surface, models of ecosystem gross primary productivity (GPP) generally have been designed for mesic temperate ecosystems. Arguably, GPP models often lack a good representation of vegetation phenology, particularly not estimating the ecosystem effects of the prolonged foliage senescence which may be common in drylands. To estimate daily GPP for a water‐limited Mediterranean shrubland, we propose a simple framework (GPPmod) using light use efficiency, a spectral vegetation index derived from digital cameras, and five meteorological variables, including an index of functional senescence of foliage (i.e., heat degree‐days). We tested the model with different combinations of meteorological variables but without senescence, using 1 year's data. The best formulation showed good agreement with GPP derived from eddy covariance (GPPEC; r² = 0.53, RMSE = 0.77). However, including the foliage senescence parameter significantly improved model performance (r² = 0.74, RMSE = 0.49), especially during the fall season. In the following year, we validated the parameters: The overall GPPmod and GPPEC comparison yielded an r² = 0.78. We postulate that models that mainly rely on meteorological variables or greenness indices could yield an overestimation of annual GPP between 24% and 90%, while models including the foliage senescence parameter reduced that bias by 10% to 34%. Our results highlight the importance of incorporating the phenology of foliage senescence in models regarding productivity in drylands or dry sclerophyll ecosystems.
... Compared with precipitation, soil moisture was a more straightforward index affecting grassland phenology, because it directly reflects the available water for plant seeds and roots (Yuan et al. 2007;Liu et al. 2013). The soil moisture also mediated the phenological response of grass to warming in a semi-arid grassland (Zelikova et al. 2015). ...
... The onset of spring and subsequent seasonal patterns of plant growth depended on both soil temperature and soil moisture in a semi-arid steppe in the western Great Plains of the USA (Moore et al. 2015). Similarly, soil moisture influenced the phenology of grasslands both directly (greenness was two to three times greater in wet years than in dry years) and indirectly by mediating the magnitude and direction of warming effects (Zelikova et al. 2015). In the Canadian Prairies, fewer water deficits favored an earlier start date of the growing season . ...
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Investigating grassland phenology and its relationship with climatic factors is crucial for understanding ecosystem responses to climate change. However, there have been few studies on the impact of soil moisture on grassland phenology. In this study, we extracted the green-up date in the Inner Mongolia grasslands, China, using the normalized difference vegetation index (NDVI) data from the Global Inventory Modeling and Mapping Studies (GIMMS) NDVI3g dataset (1982–2015) and the Moderate Resolution Imaging Spectroradiometer (2001–2015). We investigated the spatiotemporal pattern of the green-up date and its relationship with four climatic factors, including growing degree days, chilling days, soil moisture, and solar radiation. Most areas (67.1%) exhibited a trend towards earlier green-up date, but the significant trends (p < 0.05) were only found in 13.4% of the areas. For both datasets, the pixels with a negative regression coefficient of soil moisture accounted for more than 87% of the total area (more than 21% significantly, p < 0.05). Regarding the other factors, the sign of regression coefficients was not consistent among pixels. Thus, soil moisture correlated more significantly with the green-up date in temperate grasslands compared with the other climatic factors. This study provides the scientific basis for better understanding the mechanism of phenological response to climate change in temperate grasslands.
... Determining FVC from digital photographs is often simpler, faster and more economical than measuring LAI [1,15,17]. FVC values derived from ground and near-ground remotely sensed images for validation of FVC estimated from satellite images is vital in ensuring the quality of FVC estimates derived from satellite images [9,[21][22][23][24][25]. However, there are often significant problems with current FVC estimation methodologies. ...
... The fact that ACE has been shown to accurately estimate the FVC of different crop types from photographs taken at heights of up to 7 m emphasizes its utility as more than a ground-based FVC estimation tool. The use of near-ground remotely sensed images provides the opportunity to increase sample collection, survey efficiency, spatial and temporal resolution as well as decrease the unit cost of sampling [22,25,39,40]. ACE, therefore, makes a contribution to the existing approach and offers the prospect of advancing the emerging approach of near-ground remote sensing. ...
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The use of automated methods to estimate fractional vegetation cover (FVC) from digital photographs has increased in recent years given its potential to produce accurate, fast and inexpensive FVC measurements. Wide acceptance has been delayed because of the limitations in accuracy, speed, automation and generalization of these methods. This work introduces a novel technique, the Automated Canopy Estimator (ACE) that overcomes many of these challenges to produce accurate estimates of fractional vegetation cover using an unsupervised segmentation process. ACE is shown to outperform nine other segmentation algorithms, consisting of both threshold-based and machine learning approaches, in the segmentation of photographs of four different crops (oat, corn, rapeseed and flax) with an overall accuracy of 89.6%. ACE is similarly accurate (88.7%) when applied to remotely sensed corn, producing FVC estimates that are strongly correlated with ground truth values.
... Types of land use are different in different climatic conditions. For example, wet areas are dominated by forest [2], arid and semi-arid areas are dominated by grassland [3], while arid areas are dominated by desert [4]. Climate warming and wetting changes will correspondingly result in further land use changes [5]. ...
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In this study, we analyzed the temporal-spatial variations of the characteristics of land use change in central Asia over the past two decades. This was conducted using four indicators (change rate, equilibrium extent, dynamic index, and transfer direction) and a multi-scale correlation analysis method, which explained the impact of recent environmental transformations on land use changes. The results indicated that the integrated dynamic degree of land use increased by 2.2% from 1995 to 2015. The areas of cropland, water bodies, and artificial land increased, with rates of 1047 km2/a, 39 km2/a, and 129 km2/a, respectively. On the other hand, the areas of forest, grassland, and unused land decreased, with rates of 54 km2/a, 803 km2/a, and 359 km2/a, respectively. There were significant increases in cropland and water bodies from 1995 to 2005, while the amount of artificial land significantly increased from 2005 to 2015. The increased areas of cropland in Xinjiang were mainly converted from grassland and unused land from 1995 to 2015, while the artificial land increase was mainly a result of the conversion from cropland, grassland, and unused land. The area of cropland rapidly expanded in south Xinjiang, which has led to centroid position to move cropland in Xinjiang in a southwest direction. Economic development and the rapid growth of population size are the main factors responsible for the cropland increases in Xinjiang. Runoff variations have a key impact on cropland changes at the river basin scale, as seen in three typical river basins.
... Our goal was to evaluate these relationships under relatively realistic conditions. Thus, DETECT was parameterized based on the well-studied Prairie Heating and CO 2 Enrichment (PHACE) study in Wyoming, United States, (Bachman et al., 2010;Pendall et al., 2013;Zelikova et al., 2015) and run with driving data representative of the PHACE site. We then applied a CWC analysis to the model output to evaluate variability in the temporal relationship between S Total and R soil and between these CO 2 fluxes and environmental driving variables at subdaily to monthly time scales over the course of a single growing season. ...
Article
Inferences about subsurface CO2 fluxes often rely on surface soil respiration (Rsoil) estimates because directly measuring subsurface microbial and root respiration (collectively, CO2 production, STotal) is difficult. To evaluate how well Rsoil serves as a proxy for STotal, we applied the nonsteady state DEconvolution of Temporally varying Ecosystem Carbon componenTs model (0.01-m vertical resolution), using 6-hourly data from a Wyoming grassland, in six simulations that cross three soil types (clay, sandy loam, and sandy) with two depth distributions of subsurface biota. We used cross-wavelet coherence analysis to examine temporal coherence (localized linear correlation) and offsets (lags) between STotal and Rsoil and fluxes and drivers (e.g., soil temperature and moisture). Cross-wavelet coherence revealed higher coherence between fluxes and drivers than linear regressions between concurrent variables. Soil texture and moisture exerted the strongest controls over coherence between CO2 fluxes. Coherence between CO2 fluxes in all soil types was strong at short (~1 day) and long periods (>8 days), but soil type controlled lags, and rainfall events decoupled the fluxes at periods of 1-8 days for several days in sandy soil, up to 1 week in sandy loam, and for a month or more in clay soil. Concentrating root and microbial biomass nearer the surface decreased lags in all soil types and increased coherence up to 10% in clay soil. The assumption of high temporal coherence between Rsoil and STotal is likely valid in dry, sandy soil, but may lead to underestimates of short-term STotal in semiarid grasslands with fine-grained and/or wet soil.
... However, we also found positive effects of warming at the central site during the winter and spring of the wet year (2017) and at the northern site during the winter, spring, and fall of both wet and average years. Thus, for the central and northern sites, there appear to be interactions between the effect of warming and annual rainfall on live biomass across parts of theyear, a phenomenon that has been previously documented(Mueller et al., 2016;Zelikova et al., 2015). At the southern site, however, this interaction is absent; warming is consistently negative. ...
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Plant phenology will likely shift with climate change, but how temperature and/or moisture regimes will control phenological responses is not well understood. This is particularly true in Mediterranean climate ecosystems where the warmest temperatures and greatest moisture availability are seasonally asynchronous. We examined plant phenological responses at both the population and community levels to four climate treatments (control, warming, drought, and warming plus additional precipitation) embedded within three prairies across a 520 km latitudinal Mediterranean climate gradient within the Pacific Northwest, USA. At the population level, we monitored flowering and abundances in spring 2017 of eight range‐restricted focal species planted both within and north of their current ranges. At the community level, we used normalized difference vegetation index (NDVI) measured from fall 2016 to summer 2018 to estimate peak live biomass, senescence, seasonal patterns, and growing season length. We found that warming exerted a stronger control than our moisture manipulations on phenology at both the population and community levels. Warming advanced flowering regardless of whether a species was within or beyond its current range. Importantly, many of our focal species had low abundances, particularly in the south, suggesting that establishment, in addition to phenological shifts, may be a strong constraint on their future viability. At the community level, warming advanced the date of peak biomass regardless of site or year. The date of senescence advanced regardless of year for the southern and central sites but only in 2018 for the northern site. Growing season length contracted due to warming at the southern and central sites (~3 weeks) but was unaffected at the northern site. Our results emphasize that future temperature changes may exert strong influence on the timing of a variety of plant phenological events, especially those events that occur when temperature is most limiting, even in seasonally water‐limited Mediterranean ecosystems.
... Most previous studies suggest that a warmer spring results in an advancement of spring phenological events of grassland vegetation [11,[31][32][33][34]. However, under conditions where concurrent precipitation is limiting, increasing temperature may have no significant effects on spring phenology [35,36]. It has been shown that increasing precipitation had no significant effects on flowering dates in two temperate grasslands of North America [37,38], while reduced precipitation induced an earlier green-up and flowering of herbaceous species in field experiments [39][40][41]. ...
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Vegetation phenology in temperate grasslands is highly sensitive to climate change. However, it is still unclear how the timing of vegetation phenology events (especially for autumn phenology) is altered in response to climate change across different grassland types. In this study, we investigated variations of the growing season start (SOS) and end (EOS), derived from Moderate Resolution Imaging Spectroradiometer (MODIS) data (2000–2016), for meadow steppe, typical steppe, and desert steppe in the Inner Mongolian grassland of Northern China. Using gridded climate data (2000–2015), we further analyzed correlations between SOS/EOS and pre-season average air temperature and total precipitation (defined as 90-day period prior to SOS/EOS, i.e., pre-SOS/EOS) in each grid. The results showed that both SOS and EOS occurred later in desert steppe (day of year (doy) 114 and 312) than in meadow steppe (doy 109 and 305) and typical steppe (doy 111 and 307); namely, desert steppe has a relatively late growing season than meadow steppe and typical steppe. For all three grasslands, SOS was mainly controlled by pre-SOS precipitation with the sensitivity being largest in desert steppe. EOS was closely connected with pre-EOS air temperature in meadow steppe and typical steppe, but more closely related to pre-EOS precipitation in desert steppe. During 2000–2015, SOS in typical steppe and desert steppe has significantly advanced by 2.2 days and 10.6 days due to a significant increase of pre-SOS precipitation. In addition, EOS of desert steppe has also significantly advanced by 6.8 days, likely as a result from the combined effects of elevated preseason temperature and precipitation. Our study highlights the diverse responses in the timing of spring and autumn phenology to preceding temperature and precipitation in different grassland types. Results from this study can help to guide grazing systems and to develop policy frameworks for grasslands protection.
... Previous studies demonstrated that canopy regreening can occur after rain pulses in grasslands (e.g. Zelikova et al., 2015;Zhou et al., 2017) and this process was even observed at the study site in 2009 (Migliavacca et al., 2011). However, despite the favorable conditions observed in August 2015, canopy senescence continued and re-greening was not detected. ...
Article
Climate extremes can have tremendous impacts on the terrestrial biosphere and their frequency is very likely going to increase in the coming years. In this study we examine the impact of the 2015 summer heat wave on a mountain grassland in the Western European Alps by jointly analyzing phenocam greenness (GCC) trajectories, proximal sensing, CO2 flux data and structural canopy traits. Phenocam effectively tracked the impact of the heat wave, showing 39% of reduction in maximum canopy greenness and a senescence advance of 32 days compared to mean values. The same patterns (i.e. reduction of maximum values and senescence advance) were observed for all considered canopy traits and photosynthetic ecosystem functional properties, in particular the maximum light-saturated rate of CO2 uptake (Amax), LAI and PRI. Pixel-level analysis of phenocam images allowed us to further highlight that forbs were more heavily impacted than grasses. Moreover the effect of the extreme event on greenness seasonal course was evaluated testing new formulations of the Growing Season Index (GSI) model. Results demonstrate that a combination of water and high temperature stress was responsible for the observed reduction of canopy greenness during the heat wave.
... Temperate grasslands in northern China, primarily in arid and semiarid regions, are dominated by Stipa species, which are an integral part of Eurasian grasslands. Precipitation is one important environmental factor that determines the community structure and productivity in these grasslands (Zhang et al. 2007;Xu et al. 2014;Zelikova et al. 2015). Historical records suggest that the frequency of drought over northern China is much higher than that of flood, and that the frequency of extreme drought is increasing (Wu et al. 2011;Zhang and Zhou 2015a). ...
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Precipitation is a primary environmental factor in the semiarid grasslands of northern China. With increased concentrations of atmospheric greenhouse gases, precipitation regimes will change, and high-impact weather events may be more common. Currently, many ecophysiological indicators are known to reflect drought conditions, but these indicators vary greatly among species, and few studies focus on the applicability of these drought indicators under high CO2 conditions. In this study, five precipitation levels (− 30%, − 15%, control, + 15%, and + 30%) were used to simulate the effects of precipitation change on 18 ecophysiological characteristics in Stipa bungeana, including leaf area, plant height, leaf nitrogen (N), and chlorophyll content, among others. Two levels of CO2 concentration (ambient, 390 ppm; 550 ppm) were used to simulate the effects of elevated CO2 on these drought indicators. Using gray relational analysis and phenotypic plasticity analysis, we found that total leaf area or leaf number (morphology), leaf water potential or leaf water content (physiology), and aboveground biomass better reflected the water status of S. bungeana under ambient and elevated CO2 than the 13 other analyzed variables. The sensitivity of drought indicators changed under the elevated CO2 condition. By quantifying the relationship between precipitation and the five most sensitive indicators, we found that the thresholds of precipitation decreased under elevated CO2 concentration. These results will be useful for objective monitoring and assessment of the occurrence and development of drought events in S. bungeana grasslands.
... For example, after six years of soil warming (at +4°C) of alpine vegetation near the tree line, wood biomass increased greatly, especially for Pinus uncinata, whereas the biomasses of graminoids, forbs, and nonvascular plants (mosses and lichens) all decreased (Dawes et al. 2015). Differential daytime/night-time warming (1.5/3°C) for eight years in a semi-arid grassland generally increased plant vegetation cover and greenness early in the growing season, but often had a negative effect during the middle of the summer, thereby offsetting the positive early-season effects (Zelikova et al. 2015). Warming by 1.2 ± 0.1°C for 10 and 13 years in a subarctic heath only increased the cover of the dominant deciduous dwarf shrub (Vaccinium myrtillus), and other plant groups showed no significant response (Rinnan et al. 2009). ...
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Rising temperatures and precipitation are important climate change processes around the world. The responses of plants to these trends are still unclear in semi-arid regions, especially in areas with degraded sandy grassland. To provide insights into the response in these regions, we investigated responses of vascular plants to warming and increased precipitation in mobile dunes, fixed dunes and grassland, which represent the series of sand dune stabilization by plants in semi-arid northeastern China. Plant biomass, especially the aboveground biomass, varied significantly (P < 0.05) among dune categories. Total plant density in the fixed dunes and grassland was 1.9 and 1.7 times that in the mobile dunes. Species richness differed slightly but significantly (P < 0.05) among the habitats. Increasing precipitation in a drought year (65.5% of the long-term average annual precipitation) by 30% did not significantly affect any plant variable. By contrast, warming significantly decreased the belowground biomass, total biomass, species richness and plant total density. In summary, in semi-arid region with sandy soil, additional precipitation slightly improved plant performance, but increased temperature decreased plant performance. Soil texture, which determines the balance between moisture retention and evaporation, may be a key factor in determining these responses when precipitation is unusually low.
... community composition, maximum photosynthetic production, precipitation repackaging, semi-arid grasslands, vegetation greenness repackaging have been difficult to discern through long-term observations due to large interannual precipitation variability, legacy and lag effects, and complex impacts of simultaneous variations in natural precipitation amount and temporal packaging (Sala et al., 2012;Wu et al., 2015). However, high degrees of interannual and intra-annual variability in peak vegetation greenness timing suggest that precipitation variability at various time-scales may drive the magnitude and timing of GPP max (Zelikova et al., 2015). ...
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Against a backdrop of rising temperature, large portions of the western United States are experiencing fewer, larger, and less frequent precipitation events. How such temporal ‘repackaging’ of precipitation alters the magnitude and timing of seasonal maximum gross primary productivity (GPPmax) remains unknown. Addressing this knowledge gap is critical, since changes to GPPmax magnitude and timing can impact a range of ecosystem services and management decisions. Here we used a field‐based precipitation manipulation experiment in a semi‐arid mixed annual/perennial bunchgrass ecosystem with mean annual precipitation ~ 384 mm to investigate how temporal repackaging of a fixed total seasonal precipitation amount impacts seasonal GPPmax and its timing. We found that temporal repackaging of precipitation profoundly influenced the seasonal timing of GPPmax. Many/small precipitation events advanced the seasonal timing of GPPmax by ~13 days in comparison with climatic normal precipitation. Conversely, few/large events led to deeper soil water infiltration, which delayed the timing of GPPmax by up to 16 days in comparison with climatic normal precipitation, and altered end‐of‐season community composition by increasing the diversity of shallow‐rooted annual plants. While GPPmax magnitude did not differ across precipitation treatments, it was positively correlated with the abundance and biomass of deeper‐rooted perennial bunchgrasses. The sensitivity of plant growth, biomass accumulation, and plant life histories to the timing and magnitude of precipitation events and the resulting temporal patterns of soil moisture regulated ecosystem responses to altered precipitation patterns. Our results highlight the sensitivity of semi‐arid grassland ecosystem to the temporal repackaging of precipitation. We find that already‐observed and model‐forecasted shifts toward few/large precipitation events could drive significant delays in the timing of peak productivity for this ecosystem. Adaptive land management frameworks should consider these findings since shifts in peak ecosystem productivity would have major implications for multiple land user communities. Additional research is needed to better understand the role of climate, community composition, and soil properties in mediating variability in the seasonal timing of maximum ecosystem productivity.
... In April and May, the maximum leaf temperature measured during gas exchange measurements in the T ELEV treatment was 25 and 28°C, respectively, which is within the assumed range of optimum temperature for A net of a temperate C 3 species (15 to 30°C; e.g. temperate trees: Dreyer et al. 2001 andMedlyn et al. 2002;C 4 This assumption is in line with recent findings from the PHACE experimental site in Wyoming, USA, where warming increased spring and fall greenness, resulting in an increase in plant growth (Zelikova et al. 2015). In general, the reduction in net photosynthesis and plant growth with a 2°C increase in temperature was surprising, given that our study site is located in the middle of the range of this species (which extends from southern Canada to northern Mexico; http://plants.usda.gov/). ...
Article
Drylands represent our planet's largest terrestrial biome and, due to their extensive area, maintain large stocks of carbon (C). Accordingly, understanding how dryland C cycling will respond to climate change is imperative for accurately forecasting global C cycling and future climate. However, it remains difficult to predict how increased temperature will affect dryland C cycling, as substantial uncertainties surround the potential responses of the two main C fluxes: plant photosynthesis and soil CO2 efflux. In addition to a need for an improved understanding of climate effects on individual dryland C fluxes, there is also notable uncertainty regarding how climate change may influence the relationship between these fluxes. To address this important knowledge gap, we measured a growing season's in situ photosynthesis, plant biomass accumulation and soil CO2 efflux of mature Achnatherum hymenoides (a common and ecologically important C3 bunchgrass growing throughout western North America) exposed to ambient or elevated temperature (+2 °C above ambient, warmed via infrared lamps) for 3 years. The 2 °C increase in temperature caused a significant reduction in photosynthesis, plant growth and soil CO2 efflux. Of important note, photosynthesis and soil respiration appeared tightly coupled and the relationship between these fluxes was not altered by the elevated temperature treatment, suggesting C fixation's strong control of both above-ground and below-ground dryland C cycling. Leaf water use efficiency was substantially increased in the elevated temperature treatment compared to the control treatment. Taken together, our results suggest notable declines in photosynthesis with relatively subtle warming, reveal strong coupling between above- and below-ground C fluxes in this dryland and highlight temperature's strong effect on fundamental components of dryland C and water cycles.
... The ecological consequences of drought, loosely defined as a decline in the expected quantity of precipitation, is explored in some fashion by each of the papers featured in this special issue. Less explored and understood are the ecological consequences of shifts in rainfall seasonality (Zelikova et al. 2015), rainfall frequency (Liu et al. 2017), and rainfall intensity (Kulmatiski and Beard 2013). The fact that all four of these elements of precipitation variability may shift in response to anthropogenic climate change underpins the urgency to understand their ecological consequences despite the logistical limitations of experimental design (Owens 2003;Beier et al. 2012). ...
... In some species, phenological changes caused by warming were detected within the first year of warming, whereas phenological changes in most other species occurred after 2-4 years of warming treatment. Warming effects on the phenology of plants in grasslands interact with other climate change factors (e.g., elevated CO 2 , Reyes-Fox et al., 2014) or are mediated by variation in both intraannual and interannual precipitation (Zelikova et al., 2015). ...
... In the present study, warming facilitated significantly community NEP and WUE in May and August, but inhibited them significantly between June and July ( Figure 5), which partly supports hypothesis 2 that warming would inhibit these factors. The result found in this study is similar to that found by Zelikova et al. (2015). They showed that differential daytime/night-time warming (1.5/3°C) for 8 years increased vegetation cover and greenness early in the growing season, but often had a negative effect during the middle of the summer in a semiarid grassland. ...
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Abstract Temperature increasing and precipitation alteration are predicted to occur in arid and semiarid lands; however, the response mechanism of carbon and water exchange at community level is still unclear in semiarid sandy land. We investigated the responses of carbon and water exchanges to warming and precipitation enhancement along a sand dune restoration gradient: mobile sand dunes (MD), semifixed sand dunes (SFD), and fixed sand dunes (FD). The average net ecosystem productivity (NEP) and evapotranspiration (ET) between May and August increased by 98% and 59%, respectively, from MD to SFD, while they had no significant differences between FD and the other two habitats. Warming inhibited ecosystem NEP, ET, and water use efficiency (WUE) by 69%, 49% (p
... An earlier onset of spring and significant extension of the growing season, particularly, have been documented due to climate warming in the Northern Hemisphere [16][17][18] . Apart from changes in temperature, ecosystem processes are influenced by other factors like elevated CO 2 and N in soil in different seasons 19,20 . ...
... Resource managers need effective options to address increasing conservation challenges as key rangelands are subjected to changing fire regimes (Balch et al. 2013;Kelly et al. 2013), the spread of non-native invasive plant species (Watkins et al. 2007;Dufresne et al. 2009;McCary et al. 2016), urban expansion (Soulsbury and White 2015), energy development (Sawyer et al. 2006) and changing climates (Dufresne et al. 2009;Zelikova et al. 2015). Particularly in western sagebrush rangelands such as the Kaibab, conservation and restoration efforts have the potential to promote not only mule deer populations, but other seasonal or year-round sagebrush obligates in Arizona, such as sage thrashers (Oreoscoptes montanus), Brewer's sparrows (Spizella breweri), and American pronghorn (Antilocapra americana), as well as pygmy rabbits (Brachylagus idahoensis) and greater sage-grouse (Centrocerus urophasianus) in other western states (Copeland et al. 2014). ...
Article
Context Wildfire and vegetation treatments affect mule deer (Odocoileus hemionus) populations across the western United States. However, the relative influence of fire and treatments on habitat use by mule deer in Arizona is not well defined. Aims We examined locations of mule deer on the Kaibab Plateau in northern Arizona, so as to determine the influence of vegetation treatments and wildfire severity on deer habitat-use patterns across their winter range where fires and treatments had occurred previously. Methods We used locations (n = 11297) from 21 adult female mule deer fitted with global positioning system collars to model probability of use as a function of habitat covariates. Key results The best model describing winter-range habitat use by mule deer on the Kaibab Plateau included covariates describing the age of vegetation treatments and fire severity. Increased deer use in winter was associated with areas of lower terrain ruggedness and reduced snow depths. Deer use also increased in areas that experienced a higher average fire severity, resulting in decreased vegetation heights. Among treatment age classes, deer use was greatest in areas containing vegetation treatments that were ≤6 years old, but negatively associated with treatments that were >6 years old. Conclusions Vegetation treatments designed to remove or reduce less palatable tree and shrub species to improve forage conditions may increase the use of winter habitats by deer on the Kaibab Plateau. Similarly, prescribed fire and rangeland treatments designed to return areas to a more natural fire regime and, thereby, generate new plant growth, may improve winter-range habitat conditions for mule deer. Implications Similar treatment strategies may also benefit mule deer populations throughout the western USA, by improving forage conditions on critical habitats and reducing the potential for catastrophic wildfire.
... The dominating factors for vegetation activities are complicated in cold and semi-arid mid-latitude regions such as northeastern Asia, where plant life cycle is likely limited by temperature and also water shortage. Previous studies of vegetation dynamics in the mid-latitude regions had concluded that enhanced vegetation activity were related to regional climate warming (Fensholt et al 2012, Cong et al 2013, Zelikova et al 2015. However, the positive effect of climate warming on vegetation dynamics was insignificant despite the pronounced increase in temperature (Garonna et al 2014). ...
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Long-term (1982–2013) datasets of climate variables and Normalized Difference Vegetation Index (NDVI) were collected from Climate Research Union (CRU) and GIMMS NDVI3g. By setting the NDVI values below the threshold of 0.2 as 0, NDVI_0.2 was created to eliminate the noise caused by changes of surface albedo during non-growing period. TimeSat was employed to estimate the growing season length (GSL) from the seasonal variation of NDVI. Statistical analyses were conducted to reveal the mechanisms of climate-vegetation interactions in the cold and semi-arid Upper Amur River Basin of Northeast Asia. The results showed that the regional climate change can be summarized as warming and drying. Annual mean air temperature (T) increased at a rate of 0.13 °C per decade. Annual precipitation (P) declined at a rate of 18.22 mm per decade. NDVI had an insignificantly negative trend, whereas, NDVI_0.2 displayed a significantly positive trend (MK test, p < 0.05) over the past three decades. GSL had a significantly positive rate of approximately 2.9 days per decade. Correlation analysis revealed that, NDVI was significantly correlated with amount of P, whereas, GSL was highly correlated with warmth index (WMI), accumulation of monthly T above the threshold of 5°C. Principal regression analysis revealed that the inter-annual variations of NDVI, NDVI_0.2 and GSL were mostly contributed by WMI. Spatially, NDVI in grassland was more sensitive to P, whereas, T was more important in areas of high elevation. GSL in most of the areas displayed high sensitivity to T. This study examined the different roles of climate variables in controlling the vegetation activities. Further studies are needed to reveal the impact of extended GSL on the regional water balance and the water level of regional lakes, providing the habitats for the migratory birds and endangered species.
... The management such as removal of biomass for hay required rainfall for the recovery. The harvesting of biomass or grazing followed by rainfall events stimulated the growth of vegetation causing higher productivity (Zelikova et al., 2015;Zhou et al., 2017b). However, drought following harvesting of biomass impedes the productivity. ...
Article
Future weather and climates, especially rainfall, are expected to have larger variability in the Southern Plains of the United States. However, the degree and timing of environmental variability that affect productivity of pastures managed differently have not been well studied. We examined the impacts of environmental variability on grassland productivity using 17 years of gross primary productivity (GPP) data for co-located native and managed prairie pastures in Oklahoma. We also considered the interactive effects of management factors and environmental variability into the regression models and identified the critical temporal windows of environmental variables (CWE) that influence annual variability in GPP. Managed pasture (MP) showed greater variability of GPP than did native pasture (NP), particularly with reduced GPP in drought years. The resilience of native prairies under unfavorable climate extremes was evident by lower GPP anomalies in NP than MP during the 2011–2012 drought. Although both pastures experienced the same degree of environmental variability, the CWE affecting GPP was significantly different between NP and MP due to the modulating impact of management practices on the responses of GPP. Not only the range but also the timings of the CWE were different between NP and MP as MP was more responsive to the spring temperature and fall rainfall. Our findings warrant the incorporation of MP as a different commodity from NP when accounting for the ecosystem responses to environmental variability in global climate models.
... Changes in green up can then subsequently alter phenological events, like flowering, fruiting and withering . However, in some arid regions, precipitation, rather than temperature, determines the onset of green up (Zelikova et al., 2015;Cleverly et al., 2016). Moreover, some studies show plant phenology is controlled by the interaction of temperature and precipitation Chen et al., 2015). ...
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1. Phenology is an important indicator of plant responses to environmental changes and is closely correlated with biomass production. However, how changes of phenological events affect plant biomass production when exposed to changing temperature and precipitation remains unclear. 2. We conducted a four-year manipulative experiment of warming and precipitation addition to explore phenology-biomass interactions under climate change in a dry alpine meadow on the central Qinghai-Tibetan Plateau from 2015 to 2018. 3. In dry and warm years, warming delayed phenology and precipitation addition advanced them. Warming decreased biomass of Kobresia pygmaea in 2018 and biomass of Poa pratensis in 2015, 2017 and 2018. However, precipitation addition significantly increased the biomass of Poa pratensis and Potentilla multifida in most of the experimental years. Phenological changes regulated the responses of biomass to treatments. Specifically, delay of green up of P. pratensis and delay of withering of K. pygmaea induced by warming can increase biomass production, but it can be offset by the direct negative effects of warming on biomass. 4. Synthesis. Here we show how warming induced drought tend to decrease biomass production of graminoids and the negative effects of warming on biomass of P. pratensis and K. pygmaea were partially offset by green up postponement and withering postponement, respectively. Our results highlights phenology is a crucial regulator for biomass production under climate change. Hence, both direct and indirect effects of warming and precipitation addition on phenology and biomass cannot be ignored when predicting biomass responses to climate change.
... Ghazanfar, 1997;Olivares and Squeo, 1999;Seghieri and Simier, 2002). In addition to the effect of precipitation, some authors found that in water-limited ecosystems plant growth (Bisigato et al., 2013b;Thoma et al., 2016;Campanella et al., 2018) and also plant phenology (Friedel et al., 1993;Zelikova et al., 2015) are better correlated to soil water availability than to direct precipitation. That was attributed to the fact that, instead of bulk precipitation the most important element is effective precipitation (Fernández, 2007). ...
Article
The identification of the main abiotic variables influencing the seasonal development of plant phenology contributes to our knowledge of how arid and semi-arid ecosystems function. In this study, we addressed the following questions: 1. Is soil water content the most important variable determining plant phenophases? 2. Are phenophases across different life forms associated with the soil water content of different layers? We evaluated the relationships between environmental variables (i.e. precipitation, air temperature, soil moisture, and day length) and plant phenophases, using variable-length time periods preceding each measurement. We selected five representative evergreen shrubs, four deciduous shrubs, and three dominant perennial grasses. All phenophases related to vegetative and reproductive growth, and senescence were registered monthly during three years. The relationships between plant phenophases and environmental variables were evaluated using Spearman's correlation. We found that plant phenophases showed stronger association with soil water content and air temperature than precipitation. In most species, vegetative and reproductive phenophases were positively related to soil water content while leaf senescence was negatively associated. Soil water contents of layers 2 (10–20 cm) and 3 (20–40 cm) were more frequently related to plant phenophases than those of the first (0–10 cm) and fourth (40–100 cm) layers. Significantly correlated environmental variables encompassed previous periods of variable length depending on the species. Our results highlighted that soil water and air temperature were tightly correlated to plant phenophases and that all species seem to use the water contained in the same soil layers.
... In order to evaluate changes in cover and community composition over time, images were analysed from seven time-points covering the beginning, middle and end of the experiment. Images were analysed using the software SamplePoint as described inZelikova et al. (2015). Each photograph was cropped to the rim of the pot and then a grid of one-points was superimposed onto the image. ...
Article
Aims Given the key functional role of understorey plant communities and the substantial extent of forest cover at the global scale, investigating understorey community responses to elevated CO2 (eCO2) concentrations, and the role of soil resources in these responses, is important for understanding the ecosystem-level consequences of rising CO2 concentrations for forest ecosystems. Here, we evaluated how experimentally manipulated the availabilities of the two most limiting resources in an extremely phosphorus-limited eucalypt woodland in eastern Australia woodland (i.e. water and phosphorus) can modulate the response of the understorey community to eCO2 in terms of germination, phenology, cover, community composition, and leaf traits. Methods We collected soil containing native soil seed bank to grow experimental understorey plant communities under glasshouse conditions. Important findings Phosphorus addition increased total plant cover, particularly during the first four weeks of growth and under high-water conditions, a response driven by the graminoid component of the plant community. However, the treatment differences diminished as the experiment progressed, with all treatments converging at ~80% plant cover after ~11 weeks. In contrast, plant cover was not affected by eCO2. Multivariate analyses reflected temporal changes in the composition of plant communities, from pots where bare soil was dominant to high-cover pots dominated by a diverse community. However, both phosphorus addition and the interaction between water availability and CO2 affected the temporal trajectory of the plant community during the experiment. Elevated CO2 also increased community-level specific leaf area, suggesting that functional adaptation of plant communities to eCO2 may precede the onset of compositional responses. Given that the response of our seedbank-derived understorey community to eCO2 developed over time and was mediated by interactions with phosphorus and water availability. Our results suggest that a limited role of eCO2 in shaping plant communities in water-limited systems, particularly where low soil nutrient availability constrains productivity responses.
... N availability can influence plant leaf biochemistry, as well as photosynthetic capacity, and ultimately, plant growth (Novriyanti et al. 2012). Furthermore, plant growth is rather sensitive to water availability, and the interactive effects of environmental factors are particularly important in order to ascribe their effects on water use (Zelikova et al. 2015). ...
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Although plant performance under elevated CO2 (EC) and drought has been extensively studied, little is known about the leaf traits and photosynthetic performance of Stipa bungeana under EC and a water gradient from water deficit to wellirrigated. In order to investigate the effects of EC, watering, and their combination, S. bungeana seedlings were exposed to two CO2 regimes (ambient: 390 ppm; elevated: 550 ppm) and five levels of watering (–30%,–15%, control, +15%, +30%) from June 1 to August 31 in 2011, where the control water level was 240 mm. Gas exchange and leaf traits were measured after 90-d treatments. Gas-exchange characteristics, measured at the growth CO2 concentration, indicated that EC significantly decreased the net photosynthetic rate, water-use efficiency, nitrogen concentration based on mass, chlorophyll and malondialdehyde content, while increased stomatal conductance, intercellular CO2 concentration, dark respiration, photorespiration, carbon concentration based on mass, C/N ratio, and leaf water potential. Compared to the effect of EC, watering showed an opposite trend only in case of the net photosynthetic rate. The combination of both factors showed little influence on these physiological indicators, except for stomatal conductance, intercellular CO2 concentration, and malondialdehyde content. Photosynthetic acclimation to EC was attributed to the N limitation, C sink/source imbalance, and the decline of photosynthetic activity. The watering regulated photosynthesis through both stomatal and nonstomatal mechanisms. Our study also revealed that the effects of EC on photosynthesis were larger than those on respiration and did not compensate for the adverse effects of drought, suggesting that a future warm and dry climate might be unfavorable to S. bungeana. However, the depression of the growth of S. bungeana caused by EC was time-dependent at a smaller temporal scale.
... Some studies have shown that precipitation in temperate grasslands of North America has no significant effect on the flowering date of vegetation [66]; however, other experiments have shown that reduced water supply advances SOS and flowering date to some extent. Moreover, in semiarid grasslands, precipitation largely controls the direction of phenological changes [67]; however, in meadow grasslands, precipitation is dominated by temperature, which indicates the variability among different geographical regions. To better represent the relationship between climate change and SOS, researchers have also used modeling and other means to estimate phenology [68]. ...
Article
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Vegetation phenology is an important indicator of global climate change, and the response of grassland phenology to climate change is particularly sensitive in ecologically fragile areas. To enhance the ecological security of the Tibetan Plateau, it is crucial to determine the relationship between fluctuations in the start of the growing season (SOS) and the response to environmental factors. We investigated the trends of the intra-annual (ten-day) and interannual spatiotemporal dynamics of the SOS on the Northeast Qinghai-Tibet Plateau (NQTP) from 2000–2020 with MOD09GA data. We identified the response relationships with environmental factors (climate, terrain) using the maximum value composite method and the Savitzky–Golay filtering and dynamic threshold method. The SOS was concentrated from the 110th to 150th days; the average annual SOS was on the 128th day, with a spatial pattern of “early in the east and late in the west”. The overall trend of the SOS was advanced (45.48%); the regions with the advanced trend were mainly distributed in the eastern part of the NQTP. The regions with a delayed SOS were mainly concentrated in the higher-altitude regions in the southwest (38.31%). The temperature, precipitation and SOS exhibited a reverse fluctuation trend around the midpoint of 2010. Precipitation affected the SOS earlier than temperature. When temperature became a limitation of the SOS, precipitation had a more significant regulatory effect on the SOS. The SOS and aspect, slope and altitude were distributed in axisymmetric, pyramidal and inverted pyramidal shapes, respectively. The SOS on shaded slopes was earlier and more intensive than that on sunny slopes. With increasing slope, the area of the SOS decreased, and it occurred later. The SOS area was largest at 4500–5000 m and decreased at lower and higher altitude intervals. The SOS occurred later as altitude increased.
Article
Non-steady state chambers are often employed to measure soil CO2 fluxes. CO2 concentrations (C) in the headspace are sampled at different times (t), and fluxes (f) are calculated from regressions of C versus t based a limited number of observations. Variability in the data can lead to poor fits and unreliable f estimates; groups with too few observations or poor fits are often discarded, resulting in “missing” f values. We solve these problems by fitting linear (steady state) and non-linear (non-steady state, diffusion based) models of C versus t, within in a hierarchical Bayesian framework. Data are from the Prairie Heating and CO2 Enrichment (PHACE) study that manipulated atmospheric CO2, temperature, soil moisture, and vegetation. CO2 was collected from static chambers bi-weekly during five growing seasons, resulting in >12,000 samples and >3100 groups and associated fluxes. We compare f estimates based on non-hierarchical and hierarchical Bayesian (B vs HB) versions of the linear and diffusion-based (L vs D) models, resulting in four different models (BL, BD, HBL, HBD). Three models fit the data exceptionally well (R2 ≥ 0.98), but the BD model was inferior (R2 = 0.87). The non-hierarchical models (BL, BD) produced highly uncertain f estimates f (wide 95% CIs), whereas the hierarchical models (HBL, HBD) produced very precise estimates. Of the hierarchical versions, the linear model (HBL) underestimated f by ~33% relative to the non-steady state model (HBD). The hierarchical models offer improvements upon traditional non-hierarchical approaches to estimating f, and we provide example code for the models.
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Knowledge of the response of grassland phenology towards climatic factors is essential to improve our understanding of ecological processes under global warming. To date, however, it remains unclear how climate change and associated changes in vegetation dynamics might affect autumn phenology of grasslands at the global scale. In this study, the trends in start of growing season (SOS) and end of growing season (EOS) dates were explored using remote sensing data (1981-2014). The responses of EOS to preseason temperature, rainfall, SOS, and net primary productivity (NPP) were then investigated for the mid-latitude (30°N~55°N) grasslands of the Northern Hemisphere. The remotely-sensed SOS/EOS and PhenoCam-based SOS/EOS were first compared and a good correlation was observed. Trend analysis revealed that the time span of SOS/EOS (from the earliest SOS/EOS to the last SOS/EOS) and the growing season length (from SOS to EOS) have extended for the entire study region. Furthermore, a forward shift in all SOS pixels was observed in Central-West Asian grasslands, whereas no such significant trend was observed for North American grasslands and East Asian grasslands. The duration of EOS completion had shortened within North American grasslands but lengthened in Asian grasslands. Next, correlation analysis uncovered a stronger relationship between EOS and previous rainfall than between EOS and temperature, indicating the key role of water availability in controlling autumn phenology. The sensitivity of EOS to both temperature and rainfall was higher in drier and warmer locations. Moreover, a significant negative correlation between EOS and SOS was observed in part of the study region, but no significant relationship between NPP and EOS was observed. Overall, this study highlights the spatially intensified heterogeneity of spring and autumn phenology in northern grasslands and that climatic changes in precipitation might act as key drivers for modifying autumn phenology of grassland vegetation in the Northern Hemisphere.
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