Abstract Fragmentation and increased summer drought are two main threats to Mediterranean forests. Forest fragmentation has many negative impacts on forests but could attenuate water stress on Mediterranean species by reducing intraspecific competition or improving soil properties at forest edges. However, little is known about the combined effects of drought and fragmentation on tree functioning. We evaluated the effect of forest fragmentation on tree functioning under severe drought periods in continental holm oak forests (Quercus ilex). We monitored the functional response of focal trees before and during summer drought in two regions of the Iberian Peninsula with contrasting climates. Forest interiors, edges and small forest fragments were compared. Predawn leaf water potential (Ψpd), leaf stomatal conductance to water vapor (gs), maximum leaf photochemical efficiency (Fv/Fm), ground (Fo) and maximum (Fm) fluorescence in dark adapted, fully-developed leaves were assessed in the field. Trees located at forest interiors and edges showed strong stomatal closure and clear symptoms of photochemical inhibition, revealing that the trees were operating at predawn water potential beyond tolerance thresholds. In contrast, trees located in small forest fragments were able to maintain high levels of functionality for all ecophysiological parameters during the drought. These differences in tree functioning among fragments were more noticiable in the driest region. Our results prove that forest fragmentation attenuates drought impacts on tree functioning, and that these positive effects become more important during extremely dry periods. These novel findings are essential for realistic predictions of the functionality of fragmented forests under increasing and more severe droughts.

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碎片化减少了严重干旱对圣栎林树木功能的影响

摘要 破碎化和夏季干旱加剧是地中海森林面临的两大威胁。森林破碎化对森林有许多负面影响，但可以通过减少种内竞争或改善森林边缘的土壤特性来减轻地中海物种的水分压力。然而，人们对干旱和破碎对树木功能的综合影响知之甚少。我们评估了森林破碎对大陆圣栎林（Quercus ilex）严重干旱时期树木功能的影响。我们监测了伊比利亚半岛两个气候截然不同的地区在夏季干旱之前和期间重点树木的功能响应。对森林内部、边缘和小森林碎片进行了比较。黎明前叶水势 (Ψpd)，叶气孔对水汽的导度 (gs)，在田间评估了适应黑暗、完全发育的叶片的最大叶片光化学效率 (Fv/Fm)、地面 (Fo) 和最大 (Fm) 荧光。位于森林内部和边缘的树木表现出强烈的气孔关闭和明显的光化学抑制症状，表明树木在黎明前水势超出耐受阈值的情况下运行。相比之下，位于小森林碎片中的树木能够在干旱期间保持所有生态生理参数的高水平功能。这些碎片之间树木功能的差异在最干燥的地区更为明显。我们的研究结果证明，森林破碎化会减弱干旱对树木功能的影响，而且这些积极影响在极端干旱时期变得更加重要。