CREAF, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, Catalonia E081193, Spain.
CREAF, Cerdanyola del Vallès, Catalonia E08193, Spain.
Tree Physiol. 2023 Sep 6;43(9):1533-1547. doi: 10.1093/treephys/tpad059.
The Mediterranean is among the regions predicted to be most affected by climate change due to rising temperatures and increasing frequency as well as intensity of extreme weather events, such as drought. These shifts in climatic conditions might lead to changes in species community composition by enabling the increase of drought-tolerant species at the expense of less tolerant ones. This hypothesis was tested in the current study using chlorophyll fluorescence data from a 21-year precipitation exclusion experiment in a Mediterranean forest for two co-dominant Quercus ilex L. and Phillyrea latifolia L. species with contrasting low and high levels of drought tolerance, respectively. The maximum potential quantum efficiency of photosystem II (PSII) (Fv/Fm), photochemical efficiency of PSII (yield) and non-photochemical quenching (NPQ) varied seasonally. The Fv/Fm and NPQ levels were positively correlated with air temperature and standardized precipitation-evapotranspiration index (SPEI), whereas yield, which was greater under drought treatment, was negatively associated with vapor pressure deficit and SPEI. The Fv/Fm values showed similar increase in the two species over the 21-year study period regardless of treatment and in parallel to progressive warming. By contrast, yield values were higher in Q. ilex than in P. latifolia, while NPQ values were higher in P. latifolia than in Q. ilex. Notably, high yield values were also observed in the drought-treated plots. Throughout the study, plants in the drought-treated plots exhibited decreased basal area, leaf biomass and aerial cover due to high stem mortality. In addition, a continuous increase in temperature was detected in summer and autumn, which might explain the observed increase in Fv/Fm values over the study period. Higher yield and lower NPQ detected in Q. ilex could be attributed to less competition for resources in the drought-treated plots and acclimation of Q. ilex plants over the study period. Our results indicate that reduction in stem density could improve forest resilience to climate change-induced drought conditions.
地中海由于气温上升以及极端天气事件（如干旱）的发生频率和强度增加，预计是受气候变化影响最严重的地区之一。这些气候条件的变化可能会导致物种群落组成的变化，使耐旱物种增加，而不耐旱物种减少。本研究通过对地中海森林进行了为期 21 年的降水排除实验，利用叶绿素荧光数据对此假说进行了检验。该实验中选取了两种优势树种，即栎属 Quercus ilex L. 和滨柃 Phillyrea latifolia L.。这两个树种的耐旱性存在差异，前者耐旱性低，后者耐旱性高。结果表明，最大光化学量子产量（Fv/Fm）、光系统 II （PSII）的光化学效率（yield）和非光化学猝灭（NPQ）的季节变化。Fv/Fm 和 NPQ 水平与空气温度和标准化降水蒸散指数（SPEI）呈正相关，而在干旱处理下产量更高的 yield 则与水汽压亏缺和 SPEI 呈负相关。在 21 年的研究期间，无论处理方式如何，两种树种的 Fv/Fm 值都呈现出相似的增加趋势，且与气温的升高呈正相关。相反，Q. ilex 的 yield 值高于 P. latifolia，而 P. latifolia 的 NPQ 值高于 Q. ilex。值得注意的是，干旱处理的样地中也观察到了较高的 yield 值。在整个研究过程中，由于高茎死亡率，干旱处理的样地中植物的基面积、叶生物量和空中覆盖度都减少了。此外，夏季和秋季的温度持续升高，这可能解释了研究期间 Fv/Fm 值的增加。在 Q. ilex 中检测到较高的 yield 和较低的 NPQ，可能是由于在干旱处理的样地中资源竞争较少，以及 Q. ilex 植物在研究期间的适应。我们的研究结果表明，减少茎密度可以提高森林对气候变化引起的干旱条件的适应能力。
