Data from: Effects of dry-season irrigation on leaf physiology and biomass allocation in tropical lianas and trees

Lianas are more abundant in seasonal forests than in wetter forests and are thought to perform better than trees when light is abundant and water is limited. We tested the hypothesis that lianas perform better than trees during seasonal drought using a common garden experiment with 12 taxonomically diverse species (6 liana and 6 tree species) in 12 replicated plots. We irrigated six of the plots during the dry season for four years, while the remaining 6 control plots received only ambient rainfall. In year 5, we measured stem diameters for all individuals and harvested above- and belowground biomass for a subset of individuals to quantify absolute growth and biomass allocation to roots, stems, and leaves, as well as total root length and maximum rooting depth. We also measured photosynthesis, intrinsic water use efficiency (iWUE), pre-dawn and midday water potential, and a set of functional and hydraulic traits. During the peak of the dry season, lianas in control plots had 54% higher predawn leaf water potentials (ΨPD), and 45% higher photosynthetic rates than trees in control plots. By contrast, during the peak of the wet season, these physiological differences between lianas and trees become less pronounced and, in some cases, even disappeared. Trees had higher SLA than lianas; however, no other functional trait differed between growth forms. Trees responded to the irrigation treatment with 15% larger diameters and 119% greater biomass than trees in control plots. Liana growth, however, did not respond to irrigation; liana diameter and biomass were similar in control and irrigation plots, suggesting that lianas were far less limited by soil moisture than were trees. Contrary to previous hypotheses, lianas did not have deeper roots than trees; however, lianas had longer roots per stem diameter than did trees. Our results support the hypothesis that lianas perform better and experience less physiological stress than trees during seasonal drought, suggesting clear differences between growth forms in response to altered rainfall regimes. Ultimately, better dry-season performance may explain why liana abundance peaks in seasonal forests compared to trees, which peak in abundance in less seasonal, wetter forests.

藤本植物（Liana）在季节性森林中的丰度高于湿润森林，且被认为在光照充足、水分受限的条件下，生长表现优于树木。本研究通过设置于12个重复样地的同质园试验，检验了“季节性干旱期间藤本植物生长表现优于树木”这一假说，试验共选取12个分类学多样的物种（6种藤本植物与6种树木）。研究期间，我们在旱季对其中6个样地进行了为期4年的灌溉，剩余6个对照样地仅接受自然降水。第5年，我们测定了所有个体的茎直径，并收获部分个体的地上与地下生物量，以量化绝对生长量以及生物量向根、茎、叶的分配比例，同时测定总根长与最大根深。此外还测定了光合速率、内在水分利用效率（intrinsic water use efficiency, iWUE）、黎明前叶水势与正午叶水势，以及一系列功能性状与水力性状。
旱季高峰期，对照样地中的藤本植物黎明前叶水势（ΨPD）较树木高54%，光合速率较树木高45%。与之相反，在湿季高峰期，藤本与树木间的此类生理差异趋于减弱，部分情况下甚至完全消失。树木的比叶面积（specific leaf area, SLA）高于藤本植物，但二者在其余功能性状上无显著差异。灌溉组树木的茎直径较对照组树木大15%，生物量较对照组高119%。但藤本植物的生长并未响应灌溉处理：对照样地与灌溉样地中的藤本茎直径及生物量无显著差异，这表明藤本植物受土壤水分的限制远小于树木。
与既往假说相悖的是，藤本植物的根深并未深于树木，但其单位茎直径的根长高于树木。本研究结果支持“季节性干旱期间藤本植物生长表现更优、生理胁迫程度更低”的假说，表明两类生长型植物在响应降水格局改变时存在显著差异。综上，旱季更优的生长表现或可解释：为何藤本植物在季节性森林中丰度达到峰值，而树木则在季节性更弱的湿润森林中占据优势。