Tree growth is generally considered to be temperature-limited at upper elevation treelines. Yet, climate factors controlling tree growth at semiarid treelines are poorly understood. We explored the influence of climate on stem growth and stable isotopes for Polyepis tarapacana, the world’s highest elevation tree-species found only in the South American Altiplano. We developed tree-ring width index (RWI), oxygen (δ¹⁸O) and carbon (δ¹³C) chronologies for the last 60 years at four P. tarapacana stands located above 4,400 meters in elevation, along a 500-km latitude-aridity gradient. Total annual precipitation decreased from 300 to 200 mm from the northern to the southern sites. We used RWI as a proxy of wood formation (carbon sink) and isotopic tree-ring signatures as proxies of leaf-level gas exchange processes (carbon source). We found distinct climatic conditions regulating carbon-sink processes along the gradient. Current-growing season temperature regulated RWI at wetter-northern sites, while prior-growing season precipitation determined RWI at arid-southern sites. This suggests that the relative importance of temperature to precipitation in regulating tree growth is driven by site-water availability. In contrast, warm and dry growing-seasons resulted in enriched tree-ring δ¹³C and δ¹⁸O at all study sites, suggesting that similar climate conditions control carbon-source processes. Site-level δ¹³C and δ¹⁸O chronologies were significantly and positively related at all sites, with the strongest relationships among the southern-drier stands. This indicates an overall regulation of intercellular carbon dioxide via stomatal conductance for the entire P. tarapacana network, with greater stomatal control when aridity increases. The manuscript also highlights a coupling and decoupling of physiological processes at leaf level versus wood formation depending on their respectively uniform and distinct sensitivity to climate. This study contributes to better understand and predict the response of high-elevation Polylepis woodlands to rapid climate changes and projected drying in the Altiplano.

中文翻译：

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径向生长的气候敏感性不同，但在世界上海拔最高的树种 Polylepis tarapacana 沿干旱梯度的年轮稳定同位素是一致的

通常认为树木生长在高海拔林线处受温度限制。然而，对控制半干旱林线树木生长的气候因素知之甚少。我们探讨了气候对Polyepis tarapacana茎生长和稳定同位素的影响，Polyepis tarapacana是世界上海拔最高的树种，仅在南美高原地区发现。我们在四个P. tarapacana 上开发了过去 60 年的树轮宽度指数 (RWI)、氧 ( δ ¹⁸ O) 和碳 ( δ ¹³ C) 年表位于海拔 4,400 米以上，沿着 500 公里的纬度-干旱梯度。从北到南，年总降水量从 300 毫米减少到 200 毫米。我们使用 RWI 作为木材形成（碳汇）的代表，使用同位素树轮特征作为叶级气体交换过程（碳源）的代表。我们发现不同的气候条件沿梯度调节碳汇过程。当前生长季节温度调节了较湿润的北部地区的 RWI，而先前生长季节的降水决定了干旱南部地区的 RWI。这表明温度对降水在调节树木生长方面的相对重要性是由场地可用水量驱动的。相比之下，温暖和干燥的生长季节导致丰富的树轮δ ¹³所有研究地点的C 和δ ¹⁸ O，表明类似的气候条件控制着碳源过程。站点级δ ¹³ C 和δ ¹⁸ O 年代学在所有站点都呈显着正相关，其中南部干燥林分之间的关​​系最强。这表明整个P. tarapacana网络通过气孔导度对细胞间二氧化碳进行整体调节，当干旱增加时，气孔控制能力更强。手稿还强调了叶级生理过程与木材形成之间的耦合和解耦，这取决于它们对气候的分别统一和独特的敏感性。这项研究有助于更好地了解和预测高海拔Polylepis林地对快速气候变化和 Altiplano 预计干燥的响应。