Relationship between variability in aboveground net primary ...

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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L23710, doi:10.1029/2008GL035408, 2008

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Relationship between variability in aboveground net primary production and precipitation in global grasslands Yuanhe Yang,1 Jingyun Fang,1 Wenhong Ma,2 and Wei Wang1 Received 22 July 2008; revised 10 October 2008; accepted 14 October 2008; published 11 December 2008.

[1] Aboveground net primary production (ANPP) is strongly correlated with annual precipitation (AP) in grassland ecosystems. However, the relationship between the interannual variation in ANPP and the variability in precipitation remains controversial. In this study, we used long-term data of biomass and precipitation from 118 sites across global grasslands to examine the relationship between variability in ANPP and AP. Our results showed that ANPP increased with precipitation, but leveled off in humid regions, and that increased variation in precipitation led to an increase in the variability in ANPP. The relative ANPP maxima significantly increased with relative AP maxima and the relative ANPP minima also positively correlated with relative AP minima. These suggest that the fluctuations in precipitation can alter the growth of grasslands, which should be incorporated into the prediction and modeling of climate changes. Citation: Yang, Y., J. Fang, W. Ma, and W. Wang (2008), Relationship between variability in aboveground net primary production and precipitation in global grasslands, Geophys. Res. Lett., 35, L23710, doi:10.1029/2008GL035408.

1. Introduction [2] During the past several decades, the intensity of precipitation and the frequency of extreme events have increased throughout the world [Easterling et al., 2000; Weltzin et al., 2003]. These changes could exert great effects on temperate grasslands [Knapp and Smith, 2001; Knapp et al., 2002; Fay et al., 2003, 2008; Weltzin et al., 2003; Fang et al., 2005; Nippert et al., 2006; Yahdjian and Sala, 2006; Chou et al., 2008; Niu et al., 2008]. Therefore, the assessment of the relationship between interannual variation in aboveground net primary production (ANPP) and temporal variability in precipitation is critical for predicting the responses of growth in grassland ecosystems to global environmental changes, particularly to changes in rainfall patterns [Knapp and Smith, 2001; Knapp et al., 2002; Fang et al., 2005]. However, the relationship between ANPP and precipitation is debated among researchers [e.g., Goward and Prince, 1995; Le Houerou et al., 1988; Knapp and Smith, 2001; Fang et al., 2001, 2005]. Based on 11 Long Term Ecological Research (LTER) sites across North America, Knapp and Smith [2001] reported that interannual variability in ANPP was not related to temporal variability in precipitation. In contrast, using time series remote sens-

ing data, Fang et al. [2001] suggested that interannual variability in production increased with that in precipitation. A critical limitation to explore this relationship is the lack of long-term data in ANPP [Fang et al., 2001; Knapp and Smith, 2001]. Since the 1950s, the long-term observations of vegetation production in grasslands have been available worldwide. In this study, we used these long-term observations to quantify the relationship between interannual variability in ANPP and in precipitation.

2. Data and Methods 2.1. Long-Term Data of Aboveground Primary Production [3] To examine the relationship between interannual variability in aboveground net primary production (ANPP) and annual precipitation (AP), we collected long-term data of ANPP and AP from 118 sites across the world’s grasslands, of which 77 were are from Le Houerou et al. [1988], 31 from North America (23 from Scurlock et al. [2002] and 8 from Knapp and Smith [2001]), and 10 from temperate China (Table S1 of the auxiliary material).1 The ANPP ranged from 16.8 to 563.7 g m2, with a mean of 162.0 ± 11.1 g m2 (±SE), and AP from 125.0 to 1572.2 mm, with a mean of 398.6 ± 23.2 mm (±SE). 2.2. Data Analysis [4] We used coefficient variation (CV) of ANPP and AP during the sampling intervals as the proxy of their interannual variability [Le Houerou et al., 1988; Fang et al., 2001]. Similar to Knapp and Smith [2001], the relative precipitation maxima (Relative AP maxima), minimum (Relative AP minima), relative ANPP pulses (Relative ANPP maxima) and declines (Relative ANPP minima) were calculated using equations (1) – (4). We then quantified the relationship between relative ANPP maxima and relative AP maxima, and between relative ANPP minima and relative AP minima. Finally, we examined the relationship between relative ANPP maxima and relative ANPP minima.

1 Department of Ecology and Key Laboratory for Earth Surface Processes of the Ministry of Education, Peking University, Beijing, China. 2 College of Environmental and Urban Sciences, Shenzhen Graduate School, Peking University, Shenzhen, China.

Copyright 2008 by the American Geophysical Union. 0094-8276/08/2008GL035408$05.00

Relative AP maxima ¼ ð PPTmax PPTmean Þ=PPTmean

ð1Þ

Relative AP minima ¼ ð PPTmean PPTmin Þ=PPTmean

ð2Þ

1 Auxiliary materials are available in the HTML. doi:10.1029/ 2008GL035408.

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YANG ET AL.: RELATIONSHIP BETWEEN VARIATION IN PRODUCTION AND PRECIPITATION

Figure 1. Relationships between (a) aboveground net primary production (ANPP) and annual precipitation (AP), and between (b) variation in ANPP and in AP.

Relative ANPP maxima ¼ ð ANPPmax ANPPmean Þ=ANPPmean ð3Þ

Relative ANPP minima ¼ ð ANPPmean ANPPmin Þ=ANPPmean ð4Þ

where PPTmax, PPTmin and PPTmean are maximum, minimum, and average of precipitation; ANPPmax, ANPPmin and ANPPmean are maximum, minimum, and average of production during the sampling intervals.

3. Results and Discussion 3.1. Relationship between Variation in ANPP and Precipitation [5] It was generally recognized that ANPP in grasslands increased with precipitation [e.g., Sala et al., 1988; Jobba´gy et al., 2002; Fang et al., 2005]. However, our data showed that ANPP increased with precipitation but leveled off in rainy regions for global grasslands (Figure 1a). The maximum of ANPP (329.3 g m2) occurred at about 1035 mm of AP. The relationship between ANPP and AP was well fitted by an empirical function proposed by Schuur [2003] (ANPP = 0:01249AP1:7131 , r2 = 0.38, P < 0.001), which described the expð0:001655APÞ revisited relationship between global vegetation productivity and annual precipitation. [6] The slight changes in ANPP under humid environments may be resulted from the following three aspects. Firstly, precipitation may not be primary growth-limiting factor for humid grasslands. Other factors, such as nutrient availability, determine the growth of humid grasslands [Fang et al., 2005; Grosso et al., 2008]. Secondly, increased precipitation may inhibit plant growth by decreasing solar radiation, increasing nutrient leaching, and reducing soil

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oxygen availability in humid grasslands [Schuur, 2003]. Although soil oxygen availability does not appear to limit plant growth directly, slower decomposition rates induced by low oxygen may decrease nutrient availability and thus reduce the supply of nutrients for plant growth [Schuur and Matson, 2001]. Thirdly, biotic factors such as species composition may also contribute to such a ANPP-precipitation pattern [Bai et al., 2008]. The nonlinear relationship between ANPP and precipitation suggests that grasslands may respond differently to alterations in precipitation. Specifically, arid grasslands may be more sensitive to precipitation fluctuations than those under humid environments. [7] Coefficient of variation (CV) in ANPP increased with that in AP (Figure 1b), possibly because AP could result in spatial and temporal variations in ANPP for grassland ecosystems [Sala et al., 1988; Lauenroth and Sala, 1992; Jobba´gy et al., 2002; Bai et al., 2004; Swemmer et al., 2007]. The positive relationship between variation in ANPP and AP was consistent with Le Houerou et al. [1988] and Fang et al. [2001], but not to Knapp and Smith [2001]. These differences may be caused by the different biomes or scales involved in the previous studies [Boisvenue and Running, 2006]. For example, ANPP in grasslands responds strongly to alterations in precipitation regimes [Knapp and Smith, 2001], but in other biomes (such as forests) it is insensitive to precipitation swings due to vegetational or biogeochemical constraints [Paruelo et al., 1999; Knapp and Smith, 2001; Huxman et al., 2004a]. In addition, limited data used in the previous analyses may also be a factor of this discrepancy [Fang et al., 2001]. [8] The positive relationship between variation in ANPP and precipitation suggests that growth of grasslands responds strongly to increased variation in AP [Knapp et al., 2002; Fay et al., 2003; Nippert et al., 2006], while large difference may exist among different grasslands [Bai et al., 2008]. In our study, variation in ANPP was negatively correlated with AP (r = 0.3, P < 0.05), suggesting that arid grasslands may be more sensitive to precipitation fluctuations than humid grasslands. In addition, it should be noted that the response of grasslands to precipitation fluctuations may be scale-dependent. For example, short-term manipulation experiments showed that increased variability in soil moisture led to a decrease in the ANPP [Knapp et al., 2002; Fay et al., 2003], while long-term ANPP datasets suggested a positive relationship between soil moisture variability and ANPP [Nippert et al., 2006]. 3.2. Relationship between Relative Magnitude of ANPP and Precipitation [9] The relative ANPP maxima significantly increased with relative AP maxima (Figure 2a). Similarly, the magnitude of relative ANPP minima showed a positive correlation with that of relative AP minima (Figure 2b). The former was agreement with Knapp and Smith [2001], while the latter exhibited the opposite pattern. The positive relationship between relative magnitude of ANPP and AP in global grasslands was probably due to the effect of water availability on grassland growth [Sala et al., 1988; Lauenroth and Sala, 1992; Jobba´gy et al., 2002; Bai et al., 2004; Fang et al., 2005]. Precipitation pulses could prompt leaf area development and leaf-level photosynthetic capacity, and

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Moreover, growth of grasslands responded strongly to both the dry and wet years. Our results have two important implications for understanding the responses of grasslands’ growth to climate changes. Firstly, the revisited relationship between ANPP and AP suggests an insensitivity of grassland growth to increased rainfall under humid environments. Secondly, the positive correlation between interannual variation in ANPP and temporal variability in precipitation predicts that altered precipitation regimes will exert strong impacts on the growth of grasslands. These findings should be incorporated into the prediction and modeling of how grassland ecosystems respond to climate changes and to short-term fluctuations of rainfall. [12] Acknowledgments. This work was supported by the National Natural Science Foundation of China (90711002, 90211016, and 40638039). We also thank three anonymous reviewers for their constructive comments on the earlier draft of this manuscript.

References

Figure 2. Relationships between (a) relative ANPP maxima and relative AP maxima, between (b) relative ANPP minima and relative AP minima, and between (c) relative ANPP maxima and relative ANPP minima. maintain plant activity under water-limited conditions, and thus could stimulate plant production in arid ecosystems [Huxman et al., 2004b]. In contrast, declined precipitation would limit plant cover, plant density, species diversity, and leaf growth, and therefore reduce plant production in grasslands [Yahdjian and Sala, 2006]. [10] Consistent with Knapp and Smith [2001], our data depicted a positive correlation of relative ANPP pulses with ANPP declines for most sites (Figure 2c), but some data did not support that all of herbaceous dominated sites exhibited larger pulses in ANPP than declines. Our results showed that pulses in ANPP were greater than declines in dry years for some sites, but not for all. These results suggest that the growth of grasslands respond strongly to extreme precipitation [Knapp et al., 2002; Ciais et al., 2005; Reichstein et al., 2007; Fay et al., 2008]. It should be noted that the responses of grassland ecosystems to extreme rainfall regimes may also depend on the interval between rainfall events and individual event size [Fang et al., 2005; Fay et al., 2008].

4. Conclusions [11] Using 118 long-term datasets of ANPP and precipitation records worldwide, we evaluated the relationship between interannual variation in ANPP and temporal variability in precipitation. ANPP increased with precipitation, but leveled off under humid environments. Increased variation of precipitation led to a greater variability of ANPP.

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J. Fang, W. Wang, and Y. Yang, Department of Ecology, Peking University, Beijing 100871, China. ([email protected]) W. Ma, College of Environmental and Urban Sciences, Shenzhen Graduate School, Peking University, Shenzhen 518055, China.

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