Biol Invasions https://doi.org/10.1007/s10530-018-1668-5 ORIGINAL PAPER Mechanisms of influence of invasive grass litter on germination and growth of coexisting species in California Bao-Ming Chen . Carla M. D’Antonio . Nicole Molinari . Shao-Lin Peng Received: 16 May 2017 / Accepted: 13 January 2018 Ó Springer International Publishing AG, part of Springer Nature 2018 Abstract In grasslands, litter has been recognized as and two native (the grass S. pulchra, and the forb an important factor promoting grass persistence and Clarkia purpurea) herbaceous plants in a greenhouse the suppression of forbs. The invasive European setting. Our results showed that B. diandrus litter annual grass Bromus diandrus (ripgut brome) is cover hindered seedling establishment of the four widespread throughout California, where it produces species tested, but that the degree and mechanism of a persistent and thick litter layer. The native grass, inhibition was dependent on which species was tested, Stipa pulchra, is also common in some grassland life form (e.g. monocot/dicot) and seed size. Seedling settings and can also produce persistent litter, yet it is emergence of the two forb species was more vulner- typically associated with more forbs. Very little is able to litter cover than either grass species and both known about the mechanisms through which these two forbs had smaller seed size. After germination, only common grass species influence seedling establish- seedling biomass of B. diandrus itself was reduced by ment of both exotic invasive and native herbs. Here, litter (both B. diandrus and S. pulchra). We found no we evaluated the effect of B. diandrus and S. pulchra significant effects of leachate of either grass species on litter on seedling establishment of two invasive (the seedling emergence of any species, while a high grass B. diandrus and the forb Centaurea melitensis) concentration of B. diandrus leachates inhibited root growth of all species including B. diandrus seedlings. Stipa pulchra litter leachates did not affect S. pulchra B.-M. Chen (&) Á S.-L. Peng (&) or C. melitensis seedlings although it did suppress B. State Key Laboratory of Biocontrol, Guangdong Key diandrus and C. purpurea seedling growth. Our Laboratory of Plant Resources, School of Life Sciences, Sun Yat-Sen University, Guangzhou 510275, China findings provide direct experimental evidence for the e-mail: [email protected]; mechanism of effect of litter on these coexisting [email protected] invasive and native species. Such evidence helps S.-L. Peng advance our understanding of role of B. diandrus and e-mail: [email protected] S. pulchra litter in California grassland. C. M. D’Antonio Á N. Molinari Ecology Evolution and Marine Biology, University of Keywords Allelopathy Á Bromus diandrus Á Bunch California, Santa Barbara, CA 93106-4160, USA grass Á Ripgut brome Á Soil nitrogen Á Thatch Present Address: N. Molinari U.S. Department of Agriculture (USDA) Forest Service, Pacific Southwest Region, Goleta, CA 93117, USA 123 B.-M. Chen et al. Introduction allelopathy (Foster and Gross 1998; Bonanomi et al. 2011; Cummings et al. 2012). The accumulation and decomposition of plant litter Exotic invasive species have the potential to has long been identified as an important factor influence plant composition and nutrient cycling of influencing both vegetation structure (Facelli and the invaded ecosystem via their living and dead (litter) Pickett 1991; Xiong and Nilsson 1997, 1999) and plant matter (Ehrenfeld 2003, 2010; Chen et al. 2013; ecosystem functioning (Wardle et al. 1997; Handa Eppinga and Molofsky 2013; Jo et al. 2016, 2017). In et al. 2014; Barbe et al. 2017). Accumulations of grasslands, the abundance of recalcitrant grass litter recalcitrant plant litter can reduce seed germination can be enhanced following invasion (Evans et al. and alter species composition and productivity (Ham- 2001; Molinari and D’Antonio 2014). Litter has been rick and Lee 1987; Amatangelo et al. 2008; Wolk- recognized as an important factor promoting exotic ovich et al. 2009). In harsh environments, litter can grass persistence (Lenz et al. 2003; Cox and Allen facilitate the establishment and growth of plants and 2008; Molinari 2014; Molinari and D’Antonio in enhance species diversity by improving moisture review), suggesting that it may function as a positive conditions (Fowler 1986; Willms et al. 1986). Plant feedback mechanism (Molinari and D’Antonio, in litter also plays a critical role in nutrient cycling, review). In the last few decades, it has been docu- organic matter turnover and community structure and mented that litter of invasive plants has the potential to dynamics (Gessner et al. 2010; van der Putten et al. influence species composition by modifying nutrient 2016). Interestingly, both the effects of grass-litter (in availability, reducing light levels, creating a physical contrast to other litter types) and the effects of litter in barrier and releasing allelochemicals (Bergelson grasslands (in contrast to other ecosystem types) could 1990; Hierro and Callaway 2003; Callaway and be positive or negative depending on the setting Ridenour 2004; Amatangelo et al. 2008; Yelenik and (Xiong and Nilsson 1999). In fact, a meta-analysis by Levine 2011; Kaproth et al. 2013; Loydi et al. 2015). Loydi et al. (2013) found an overall neutral effect of Bergelson (1990) found that the dead grass blades of litter presence on seedling emergence and survival and invasive grass Poa annua decreased seedling emer- a positive effect on seedling biomass, and they pointed gence and survival of the two studied annual weeds out that litter effects depend on many variables and changed the population dynamics of annual plants. including litter amount, study condition, grassland Litter accumulation of an exotic perennial species type and the seed size of species influenced by the (Holcus lanatus) was found to inhibit seed germina- litter. In addition, different litter types had differential tion of itself, with no significant effects on seed effects of on woodland and grassland species, and the germination of the native perennial species Stipa different effects were probably related to litter struc- pulchra (formerly Nassella pulchra) (Reynolds et al. ture (Donath and Eckstein 2008). 2001). Dead material of the invasive plant black The mechanisms through which litter influences mustard has potential to depress seedling emergence plant species and ecosystems are both direct and and growth of other native and exotic species through indirect (Facelli and Pickett 1991; Xiong and Nilsson allelopathy (Bell and Muller 1973). While several 1997; Bonanomi et al. 2011). The presence of litter studies evaluate these physical, chemical and biolog- affects the exchange of water between the soil and the ical effects of litter (e.g.) in isolation, the mechanisms atmosphere, an effect frequently observed in grass- have rarely been studied simultaneously. lands (Weaver and Rowland 1952; Fowler 1986) In California, exotic annual grasses have widely where litter increases water availability through shad- invaded grassland and shrubland habitats. The Eur- ing of the soil surface. Litter also constitutes a physical asian annual grass Bromus diandrus (ripgut brome), is barrier for seedling establishment because it may keep widespread throughout California (D’Antonio and seeds from reaching the soil, as well as physically Vitousek 1992; Malmstrom et al. 2005), New Zealand inhibit the emergence of seedlings (Bosy and Reader (Tozer et al. 2007) and Australia (Kleemann and Gill 1995; Olson and Wallander 2002). Moreover, litter 2009), and forms dense stands that when not grazed by may have negative effects on seed germination, and livestock develop a persistent and thick leaf litter layer plant growth through chemical inhibition, termed (Molinari and D’Antonio 2014). Although seed ger- mination and seedling establishment are two key 123 Mechanisms of influence of invasive grass litter on germination stages of a plant’s lifecycle that seem to be particularly predicted that the effects of litter leachates might be sensitive to the presence of litter, the chemical and stronger for B. diandrus litter because of its low physical mechanisms through which B. diandrus litter association with forbs in the field. influences seedling establishment of exotic invasive and native herbs has not been studied. In addition, native grassland species, like the widespread perennial Materials and methods bunchgrass Stipa pulchra can also create a persistent litter layer. However, in contrast to B. diandrus, S. Study sites and species pulchra provides habitat conditions that support a broad spectrum of native grassland organisms (Strom- The study site is Sedgwick Reserve (34°42004.3800N, berg et al. 2007), including a diversity of native herbs 120°02050.8100W), a 2358 ha reserve that is part of the (Molinari and D’Antonio 2014). Allelopathic poten- University of California Natural Reserve System tial of aqueous leachates of S. pulchra leaf litter was (UCNRS) and located in the Santa Ynez Valley. The demonstrated on a common exotic grass Avena fatua reserve is 29 km from the coast, receives approxi- in California (Hull and Muller 1977), yet we know mately 400 mm per year of precipitation and is nothing about how S. pulchra and B. diandrus transitional between wetter coastal prairies and the leachates compare or whether allelopathic potential drier grasslands of the Central and San Joaquin will be realized against a wider range of target species. Valleys (Bartolome et al. 2007). The climate in this Here, we selected two common exotic annual region is Mediterranean, with hot dry summers and species and two native herbaceous species to study cool wet winters. In this reserve, both the exotic the influence of B. diandrus and S. pulchra litter on species Bromus diandrus and native dominated grass- seedling establishment within a controlled greenhouse land patches of Stipa pulchra (purple needle grass) are setting. These species were chosen for study because scattered (Molinari 2014), which spurred interest in of their commonness and their ecological importance comparing the effect of grass litter of two dominant (see Materials and methods).