Data from: Pine afforestation decreases the long-term performance of understory shrubs in a semiarid Mediterranean ecosystem: a stable isotope approach.

1. Plant-plant interactions shape the structure and composition of plant communities, but shifts in interaction outcomes might occur in the face of ongoing climate change. 2. We assessed the influence of Pinus halepensis plantations on the long-term ecophysiological performance of understory vegetation, by conducting a retrospective comparison (1989-2007) of growth-ring widths, δ13C and δ18O between Rhamnus lycioides shrubs from two contrasting vegetation types: P. halepensis plantations vs. open woodlands. We also measured the leaf δ13C, δ18O, δ15N, and nutrient concentrations of understory R. lycioides shrubs growing at varying distances from planted trees within pine plantations. 3. Dendroecological and stable isotope data revealed strong competitive effects of planted P. halepensis trees on R. lycioides. Shrubs in pine plantations showed lower radial growth and higher growth-ring δ13C and δ18O than those in open woodlands, indicating lower stomatal conductance and photosynthesis in the former. The strong competitive effects of P. halepensis on understory R. lycioides were most evident in wet, productive years. Conversely, in very dry years there were indications of a facilitative effect of planted P. halepensis canopies on understory shrubs. 4. Within pine plantations, understory R. lycioides shrubs growing at shorter distances from planted trees were forced to rely on more superficial and ephemeral soil water sources, which reduced their stomatal conductance (higher leaf δ18O) and interfered with nutrient uptake (lower leaf N and P concentrations and more negative δ15N). 5. The intrinsic water use efficiency of R. lycioides shrubs growing in open woodlands has increased during recent decades as a result of their ability to adjust their stomatal conductance in response to increasing temperature and atmospheric CO2. However, this adaptive response was much weaker or absent in the severely drought-stressed shrubs from pine plantations. 6. Pine afforestation strongly reduces water and nutrient availability for understory shrubs in drylands, with potential long-term consequences for ecosystem biodiversity, structure and functioning. Competition by P. halepensis on R. lycioides clearly outweighed facilitation in the long-term, especially during wet years, thus compromising the ability of understory shrubs in semiarid pine plantations to cope with climate change.

1. 植物间相互作用塑造植物群落的结构与组成，但在持续气候变化的背景下，种间相互作用的结局可能发生偏移。2. 本研究通过对两种生境（阿勒颇松（Pinus halepensis）人工林与开阔林地）中的药鼠李（Rhamnus lycioides）灌木开展1989-2007年的回溯性对比分析，测定其年轮宽度、碳同位素比值δ¹³C及氧同位素比值δ¹⁸O，以此评估阿勒颇松人工林对林下植被长期生理生态性能的影响。此外，本研究还测定了人工林内与种植松树距离各异的林下药鼠李灌木的叶片δ¹³C、δ¹⁸O、氮同位素比值δ¹⁵N及养分浓度。3. 树木年代生态学与稳定同位素数据显示，种植的阿勒颇松对药鼠李具有显著的竞争抑制作用。与开阔林地中的药鼠李相比，人工林内的灌木径向生长更慢，年轮δ¹³C与δ¹⁸O值更高，表明前者的气孔导度与光合速率更低。阿勒颇松对林下药鼠李的强竞争效应在湿润且生产力较高的年份最为显著。反之，在极端干旱年份，则有迹象表明阿勒颇松冠层对林下灌木存在促进作用。4. 在阿勒颇松人工林内，与种植松树距离更近的林下药鼠李灌木不得不依赖更浅层且更短暂的土壤水源，这降低了其气孔导度（表现为叶片δ¹⁸O值升高），并干扰了养分吸收过程（表现为叶片氮、磷浓度降低，且δ¹⁵N值更偏负）。5. 近几十年来，开阔林地中的药鼠李灌木因可通过调节气孔导度以响应气温升高与大气CO₂浓度上升，其内在水分利用效率得以提升。然而，对于人工林内遭受严重干旱胁迫的药鼠李而言，这种适应性响应则显著减弱甚至完全缺失。6. 旱地地区的松树造林会显著降低林下灌木可获取的水分与养分资源，进而对生态系统的生物多样性、结构与功能产生潜在的长期影响。长期来看，阿勒颇松对药鼠李的竞争效应明显强于促进作用，尤其在湿润年份，这削弱了半干旱区松人工林内林下灌木应对气候变化的能力。