Abstract

•Seedlings of New Zealand’s treeline forming Fuscospora cliffortioides (Hook.f.) perform poorly beyond the established canopy, limiting treeline advance. To test the long-standing assumption that photoinhibition impairs regeneration in the subalpine belt of New Zealand’s Southern Alps, we assessed photosystem II performance of seedling-sized individuals and microclimate variation.•We performed diurnal, non-invasive chlorophyll-a-fluorescence measurements on exposed and canopy-sheltered individuals at two sites in New Zealand’s Southern Alps during summer and winter. Diurnal recordings of the effective (ΦPSII) and optimal (F_v/F_m) photosynthetic quantum yield were supplemented with light response curves and micro-temperature recordings.•ΦPSII returned to near-optimal values around 0.8 after 30 minutes of shading which rules out accumulative or long-term photoinhibition effects. The maximum electron transport rate (ETR_max) derived from rapid light curves was significantly higher (+12%) in exposed compared to canopy-shaded individuals. Summer temperature fluctuated widely on the scree (−0.5 to 60.5°C) and near seedlings (−2 to 26.5°C).•Our results revealed a remarkable level of light adaptation and contradict previous studies hinting at high light-induced photoinhibition as treeline-limiting factor in the Southern Alps. By linking low ΦPSII on winter mornings, and large, sudden temperature drops in summer, we suspect that cold-induced photoinhibition might occur but the rapid recovery of ΦPSII seen across a wide temperature range makes lethal photo-oxidative damage rather unlikely. Given the demonstrably low summer frost tolerance of F. cliffortioides, cold-related damage resulting from frost events during the growing season or embolism induced by frost drought may offer more plausible explanations for the poor seedling establishment. Duration and frequency of these events could diminish with global warming, which may promote treeline advance.

中文翻译：

---

高光诱导的光抑制并不限制新西兰突然的树线交错带的幼苗建立

摘要

• 新西兰树线形成Fuscospora Clinton (Hook.f.) 的幼苗在已建立的树冠之外表现不佳，限制了树线的前进。为了验证长期以来光抑制会损害新西兰南阿尔卑斯山亚高山带再生的假设，我们评估了幼苗大小的个体的光系统 II 性能和小气候变化。•我们对在夏季和冬季，新西兰南阿尔卑斯山两个地点的暴露和树冠遮蔽的个体。有效 (ΦPSII) 和最佳 (F _v /F _{m ) 的昼夜记录}) 光合量子产率补充了光响应曲线和微温记录。•ΦPSII 在遮光 30 分钟后恢复到接近最佳值 0.8 左右，排除了累积或长期光抑制效应。最大电子传输速率（ETR _max) 与树冠阴影个体相比，暴露于快速光变曲线的个体显着更高 (+12%)。夏季温度在碎石（-0.5 至 60.5°C）和幼苗附近（-2 至 26.5°C）波动很大。•我们的结果显示了显着的光适应水平，与先前暗示高光诱导的光抑制作为树线的研究相矛盾- 南阿尔卑斯山的限制因素。通过将冬季早晨的低 ΦPSII 与夏季的大而突然的温度下降联系起来，我们怀疑可能会发生冷诱导的光抑制，但在较宽的温度范围内看到的 ΦPSII 的快速恢复使得致命的光氧化损伤不太可能发生。鉴于F.clifortioides对夏季霜冻的耐受性明显较低生长季节的霜冻事件或霜冻干旱引起的栓塞导致的与寒冷相关的损害可能为不良的幼苗建立提供了更合理的解释。这些事件的持续时间和频率可能会随着全球变暖而减少，这可能会促进树线的前进。