Exotic plants get a little help from their friends

doi:10.1126/science.abc3587

GSTDTAP > 气候变化

DOI	10.1126/science.abc3587
	Exotic plants get a little help from their friends
	Carlos Urcelay; Amy T. Austin
	2020-05-29
发表期刊	Science
出版年	2020
英文摘要	Terrestrial ecologists have identified multifaceted controls—climate, biogeography, disturbances, and their interactions—that shape how plant communities in natural ecosystems organize in space and time. Multiple documented interactions directly link plant diversity with other biotic guilds (herbivores, root symbionts, bacteria, and pathogens) and ecosystem processes \[carbon (C) and nutrient cycling\] ([ 1 ][1]). However, all appears to go awry when exotic (non-native) plant species invade and establish themselves without human intervention; such changes affect the functioning and diversity of natural ecosystems ([ 2 ][2]). On page 967 in this issue, Waller et al. ([ 3 ][3]) provide insight into pathways that explain the underlying relationship between plant invasions and acceleration of a crucial ecosystem process: C turnover. Changes in plant diversity, species dominance, or nutrient cycling as a result of exotic plant invasion can alter ecosystems so substantially that they may be virtually unrecognizable from their original state. So what makes a non-native species a successful invader? Can scientists predict which species will have large impacts on ecosystems? It might seem straightforward to identify characteristics of success for exotic invader species, including faster growth rate, thinner leaves, or root anatomy ([ 4 ][4]–[ 6 ][5]), often with consequences for accelerated C turnover ([ 2 ][2]). However, definitive conclusions as to what traits predict invasive success remain elusive; this is due in part to the interactions between above- and belowground components of an invaded ecosystem (including plants with contrasting life-history strategies) seldom having been examined simultaneously. Using a comprehensive, manipulable, outdoor experimental system (mesocosm), Waller et al. revealed that as the proportion of exotic plants increases, interactive effects from herbivore and soil-microorganism communities alter C cycling and other processes. Thus, changes in C cycling are indirectly mediated by the plant invaders through changes in these biotic interactions. In an extensive experimental effort, the authors used plants and soils from subalpine grassland in New Zealand to establish 160 experimental outdoor ecosystems (mesocosms) that contained 20 singular combinations of eight plant species that varied in their proportions of exotic and native woody plants (see the photo). The plant communities were grown in soils previously conditioned by them (“home”) and in soils conditioned by plant species not present in the community (“away”), a technique that is commonly used to evaluate the importance of plant-soil feedbacks ([ 6 ][5]). Last, herbivore interactions were evaluated by adding invertebrate herbivores (such as leafhoppers, aphids, moths, beetles, and slugs) to half of the mesocosms. With plant traits as covariates, linear mixed-effects models were used to test how the proportion of exotics, herbivores, and soil treatments affected biotic interactions and ecosystem processes. In addition, structural equation modeling was used to explore how exotic-plant functional types (those with different growth forms such as grasses, shrubs, and trees) and various other plant traits influence C cycling directly and indirectly (through modifying herbivore and soil-microorganism biomass). A noteworthy aspect of the new study is the use of different functional types of plants, including woody species with various symbiotic strategies. This is relevant because the success of exotic plant species and their effects on new environments might be strongly determined by their symbiotic interactions ([ 7 ][6]). Chief among them are mycorrhizal fungi and nitrogen-fixing bacteria that associate with plant roots, providing access to soil nutrients and stress alleviation ([ 8 ][7]). The insights observed by Waller et al. likely derive not from C-cycle changes that occur with exotic-plant dominance but from the mechanistic underpinnings that reveal why these changes occur. Whereas plant richness (number of species) decreased with an increasing proportion of exotics, likely through competitive exclusion of natives, total plant biomass remained constant. Key to understanding the impacts of exotic plants on biotic interactions and C cycling is the widely explored plant trait that expresses the thickness of the leaves [called specific leaf area (SLA)], which is linked to herbivore palatability and nutrient-conservation strategies ([ 9 ][8]). Community-weighted SLA increased, at the community scale, with a growing presence of exotic plants and appears to have mediated some of the exotic-plant effects on biotic interactions and C cycling. For example, the increasing proportion of exotic species was accompanied by an increase in herbivore biomass, which could be explained by the presence of exotic nitrogen (N)–fixing herbs of high nutrient content and higher SLA, both of which imply enhanced leaf palatability ([ 10 ][9]). Soil microbial guilds also were affected. The biomass of arbuscular mycorrhizal fungi (AMF) decreased with an increase in the proportion of nonmycorrhizal exotic plant species with high SLA. An increase in bacterial biomass was associated with symbiotic N-fixing exotic woody species. There is an emerging understanding of how the composition of soil fungi and bacteria can affect soil respiration (carbon dioxide production from soil microorganisms and roots) ([ 11 ][10]). In the Waller et al. study, the increased proportion of exotics triggered an increase in total soil respiration but only in the presence of herbivores and microbial guilds in the away soil. This process was positively influenced by the presence of exotic N-fixing woody plants and high community-weighted SLA. In turn, basal plant respiration (that which occurs without plant roots) increased in the presence of away soil biota, which was attributed to the activity of saprophytic fungi (non-AMF). The combination of exotic-plant traits and changes in the relative abundance of soil microbial guilds boosted nutrient and C cycling. Why total respiration increased only with aboveground herbivory remains unknown but reveals the complex pathways that drive plant-soil interactions. The authors also showed that the increasing proportion of exotic plants accelerated decomposition of a standard substrate of black tea leaves indirectly through the exotic plants' effects on soil fungi and herbivory. The increases in non-AMF biomass could be explained by the increasing presence of exotic N-fixing woody plants, together with reductions in nonsaprophytic AMF biomass by exotic non–N-fixing plants. This evidence provides support for the importance of life-history strategies in structuring below-ground fungal communities ([ 12 ][11]) and their interactions with herbivores. However, it is difficult to draw conclusions about direct links between ecosystem changes and exotic-plant traits because litter decomposition was evaluated with tea leaves from species not included in the experiments. The chemical and morphological characteristics of plant-litter identity ([ 13 ][12]) could contribute to the complexity of the patterns observed in the new study. Thus, the complex linkages among plant invasions, soil biota, and litter decomposition deserve further exploration. The new work by Waller et al. highlights the fundamental role of biotic interactions in C-cycling processes in an experimental setting. It remains unclear how these results will scale up to field communities with more complex biological networks. Future research will unearth whether these interactive pathways are maintained or accentuated with adult woody species in invaded natural ecosystems or in other invasive scenarios, such as those lacking N-fixing invaders or those dominated by other mycorrhizal types. What seems evident from this study is that linkages between plant invasions and ecosystem processes are of pivotal importance for understanding the interplay of how plants interact with other organisms ([ 14 ][13]). Researchers must delve further into this complexity to better predict how these links will respond to global changes. 1. [↵][14]1. W. W. Weisser et al ., Basic Appl. Ecol. 23, 1 (2017). [OpenUrl][15] 2. [↵][16]1. J. G. Ehrenfeld , Annu. Rev. Ecol. Evol. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/271742
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Carlos Urcelay,Amy T. Austin. Exotic plants get a little help from their friends[J]. Science,2020.
APA	Carlos Urcelay,&Amy T. Austin.(2020).Exotic plants get a little help from their friends.Science.
MLA	Carlos Urcelay,et al."Exotic plants get a little help from their friends".Science (2020).