Effect of drought and nutrient availability on invaded plant communities in a semi-arid ecosystem

Ecosystem functions are heavily dependent on the functional composition of the plant community, i.e., the functional traits of plants forming the community. This, on the one hand, depends on plant occurrence, but on the other hand depends on the intraspecific variability of functional traits of the species, which are influenced by climate and nutrient availability and affected by plant-plant interactions. To illustrate that, we studied the effects of drought and nitrogen addition (+ N), two important abiotic variables which are changing with ongoing global change, as well as their combined effect on the functional responses of grassland communities in semi-arid environments of Northern Africa comprising of natural and invasive species. We conducted a plot plant experiment where we planted three native species and one invasive plant species in artificial communities as of 5 individuals per species per plot. We exposed these communities to four different treatments: a drought treatment, a N-addition treatment, the combination between drought and N-addition as well as a control. To assess the performance of plants within treatments, we measured selected plant functional traits (plant height, specific leaf area (SLA), leaf dry matter content (LDMC), N content of the leaves (Nmass), specific root length (SRL) and root diameter) for all individuals occurring in our plots, and additionally assessed the above and belowground biomass for each plant individual. We found that the invasive species showed a higher performance (higher biomass accumulation, taller plants, higher SLA, Nmass, SRL and root diameter as well as lower LDMC) than the native species under drought conditions. The invasive species was especially successful with the combined impact of drought + N, which is a likely scenario in ongoing global change for our research area. Thus, plant functional traits might be a key factor for invasion success of plant species which will be even more pronounced under ongoing global change.

Methods
We conducted a plot plant experiment where we planted three native species and one invasive plant species in artificial communities as of 5 individuals per species per plot. We exposed these communities to four different treatments: a drought treatment, a N-addition treatment, the combination between drought and N-addition as well as a control. To assess the performance of plants within treatments, we measured selected plant functional traits (plant height, specific leaf area (SLA), leaf dry matter content (LDMC), N content of the leaves (Nmass), specific root length (SRL) and root diameter) for all individuals occurring in our plots, and additionally assessed the above and belowground biomass for each plant individual.

生态系统功能高度依赖植物群落的功能组成，即构成群落的植物的功能性状。这一方面取决于植物的定居与分布，另一方面则取决于物种功能性状的种内变异——该变异受气候与养分有效性调控，并受到植物种间互作的影响。为阐明这一科学问题，本研究聚焦干旱与氮添加（+N）这两种随全球变化持续发生改变的重要非生物因子，及其交互作用对北非半干旱草原群落（包含本土物种与入侵物种）的功能响应的影响。

我们开展了样地植物实验：以每物种每样地5株个体的密度，将3种本土植物与1种入侵植物配置为人工群落。将这些人工群落分为4组处理：干旱处理组、氮添加处理组、干旱与氮添加交互处理组，以及对照组。为评估各处理组内植物的生长表现，我们测定了样地内所有个体的若干关键植物功能性状：株高、比叶面积（specific leaf area, SLA）、叶干物质含量（leaf dry matter content, LDMC）、叶片氮含量（leaf N content, Nmass）、比根长（specific root length, SRL）以及根直径；同时还测定了每株植物的地上与地下生物量。

研究结果显示，在干旱条件下，入侵物种的生长表现优于本土物种：其生物量积累更多、株高更高，比叶面积、叶片氮含量、比根长与根直径均更大，而叶干物质含量更低。在干旱与氮添加的交互作用下，入侵物种的竞争优势尤为显著——这正是本研究区域在当前及未来全球变化背景下的潜在场景。由此可见，植物功能性状可能是决定植物入侵成功的关键因素，且该效应在持续的全球变化背景下将更为突出。

方法
我们开展了样地植物实验：以每物种每样地5株个体的密度，将3种本土植物与1种入侵植物配置为人工群落。将这些人工群落分为4组处理：干旱处理组、氮添加处理组、干旱与氮添加交互处理组，以及对照组。为评估各处理组内植物的生长表现，我们测定了样地内所有个体的若干关键植物功能性状：株高、比叶面积（SLA）、叶干物质含量（LDMC）、叶片氮含量（Nmass）、比根长（SRL）以及根直径；同时还测定了每株植物的地上与地下生物量。