Paz Esquivias M, Zunzunegui María, Díaz Barradas Mari Cruz, Álvarez-Cansino Leonor
Departamento de Biología Vegetal y Ecología, Universidad de Sevilla, PO Box 1095, 41080, Sevilla, Spain,
Oecologia. 2015 Jan;177(1):133-46. doi: 10.1007/s00442-014-3106-4. Epub 2014 Oct 28.
The canopy shade of the Retama species has been widely reported to ameliorate the environmental conditions in the understory, thus facilitating other species' establishment. The shading effect of the native-invasive leguminous shrub Retama monosperma (L.) Boiss on the endangered Thymus carnosus Boiss was analysed to determine a positive or negative net effect. Data was taken in all four seasons, representing contrasting light and water availability in a Mediterranean coastal dune ecosystem (SW Spain). The morphological and physiological status of sun-exposed T. carnosus plants growing in open areas versus shaded plants growing under R. monosperma were measured seasonally. Leaf mass area, leaf area index and pigment content showed typical sun-shade responses. In contrast, sun-exposed T. carnosus displayed higher stem water potential, transpiration rate and water use efficiency, both intrinsic and integrated, denoting low tolerance to the presence of R. monosperma. Five years after the measurements, canopy cover had decreased and mortality was higher in shaded plants, thus confirming the competitive effect of R. monosperma on T. carnosus. R. monosperma arises as a competitor for endangered T. carnosus communities, consequently reinforcing its invasive behaviour. This species-specific shrub study demonstrates that eventual beneficial effects of Retama canopy may be overridden by competition in the understory, particularly in the case of species well-adapted to high light and low water levels.
据广泛报道，刺木属植物的树冠遮荫可改善林下环境条件，从而促进其他物种的生长。分析了本地入侵豆科灌木单籽刺木（Retama monosperma (L.) Boiss）对濒危的肉质百里香（Thymus carnosus Boiss）的遮荫效果，以确定其净效应是正还是负。数据采集于四季，代表了西班牙西南部地中海沿岸沙丘生态系统中不同的光照和水分条件。季节性测量了生长在开阔地的肉质百里香植株与生长在单籽刺木林下的遮荫植株的形态和生理状态。叶质量面积、叶面积指数和色素含量表现出典型的阳生叶与阴生叶响应。相比之下，暴露在阳光下的肉质百里香叶显示出更高的茎水势、蒸腾速率和水分利用效率（包括内在水分利用效率和综合水分利用效率），表明其对单籽刺木的存在耐受性较低。测量五年后，遮荫植株的树冠覆盖率下降且死亡率更高，从而证实了单籽刺木对肉质百里香的竞争效应。单籽刺木成为濒危肉质百里香群落的竞争者，因此强化了其入侵行为。这项针对特定物种灌木的研究表明，刺木树冠最终的有益影响可能会被林下竞争所抵消，特别是对于适应高光和低水位的物种而言。
