Long‐Term Forage Production of North American Shortgrass Steppe

Abstract

Abstract. We evaluated the relationship between annual forage production and annual and seasonal precipitation and temperature at a shortgrass steppe site in north-central Colorado using a long-term data set (52 yr). We also constructed a relationship between forage production and aboveground net primary production (ANPP). Precipitation fluctuated randomly, but temperature had clear warming and cooling trends including a 17-yr warming trend from 1974 to 1990.Forage production was significantly related to both annual and seasonal precipitation but not temperature. Precipitation events between 15 and 30 mm accounted for most of the variability in production because they accounted for most of the variability in precipitation and because they wetted the soil layers that have the largest effect on production. Forage production amplified variability in annual precipitation.Production showed time lags of several years in responding to increases in precipitation. Change in vegetation structure has a characteristic response time, which contrains production responses in wet years. Constraint caused by vegetation structure is the reason why regional ANPP-precipitation models have a steeper slope than long-term models and point out a weakness of exchanging space for time in predicting production patterns.

Do Grasslands Have a Memory: Modeling Phytomass Production of a Semiarid South African Grassland

“…from the previous 1 or 3 months if the grassland is in medium or poor condition (dominated by shorter-lived grasses); (d) the memory effect both buffers and amplifies the responses of production to changes in precipitation, depending on the sequence of dry or wet years/month; and (e) phytomass production in the South African grassland amplifies the variability in seasonal rainfall. The latter result is consistent with many previous studies of arid and semiarid ecosystems (Le Houé rou and others 1988), but it is contrary to findings in some grassland and shrubland systems (for example, Lauenroth and Sala 1992;Paruelo and others 1998) where variability in phytomass production was smaller than that of rainfall.…”

“…More specifically, this mechanism was caused by a positive correlation of precipitation at the scale of the memory of the grassland that amplified the variability in precipitation: Production was higher during a sequence of several wet years or months and lower during a sequence of several dry years or months. This mechanism could explain the contradictory results of other studies in South and North America (Ferná ndez and others 1991;Lauenroth and Sala 1992;Paruelo and others 1998) in which the CV of production was smaller than that of precipitation. At the shortgrass steppe, which has a memory at an annual scale (Oesterheld and others 2001), precipitation is only rarely consistently above or below the mean for more than 2 or 3 years (Lauenroth and Sala 1992).…”

“…This mechanism could explain the contradictory results of other studies in South and North America (Ferná ndez and others 1991;Lauenroth and Sala 1992;Paruelo and others 1998) in which the CV of production was smaller than that of precipitation. At the shortgrass steppe, which has a memory at an annual scale (Oesterheld and others 2001), precipitation is only rarely consistently above or below the mean for more than 2 or 3 years (Lauenroth and Sala 1992). In areas where there are no prolonged periods of dry and wet years, a dry year after a wet year will yield more production than expected by precipitation alone, and a wet year after a dry year will yield less production than expected by precipitation alone -a pattern that will buffer the variability in precipitation.…”

Seasonal variation in carbon dioxide exchange over a Mediterranean annual grassland in California

“…Depending on precipitation, more carbon uptake occurs during wet years and vice versa (Flanagan et al, 2002;Meyers, 2001;Suyker et al, 2003). These physiological results, based on the eddy covariance method, are consistent with numerous ecological studies, which have shown that aboveground net primary production (ANPP) of grasslands growing in the continental region of North America is correlated linearly with annual precipitation (Sims and Singh, 1978;Webb et al, 1978;Sala et al, 1988;Paruelo et al, 1999;Lauenroth and Sala, 1992;Knapp and Smith, 2001).…”

Asymmetric Responses of Primary Productivity to Altered Precipitation Simulated by Ecosystem Models across Three Longterm Grassland Sites

“…The P-ANPP sensitivities obtained from spatial relationships are usually higher than those obtained by temporal relationships (Estiarte et al, 2016;Fatichi and Ivanov, 2014;Sala et al, 2012). Possible mechanisms behind the steeper spatial relationship may be (1) a 'vegetation constraint' reflecting the adaptation of plant communities over long time scales in such a way that grasslands make the best use of the water received from rainfall for growth, and (2) the spatial variation in structural and functional traits of ecosystems (soil properties, nutrient pools, plant and 15 microbial community composition) that constrain local P-ANPP sensitivities (Lauenroth and Sala, 1992;Smith et al, 2009;Wilcox et al, 2016). For projecting the effect of climate change on grassland productivity in the coming decades, inter-annual relationships are arguably more informative than spatial relationships because spatial relationships reflect long-term adaptation of ecosystems, and because P-ANPP relationships from spatial gradients are confounded by the co-variation of gradients in other environmental variables (e.g.…”