Data from: Mediterranean and temperate treelines are controlled by different environmental drivers

The growth limitation hypothesis (GLH) is the most accepted explanation for treeline formation, but it has been scarcely examined in Mediterranean treelines, which are located at lower elevations than temperate treelines. The GLH states that low temperature is the ultimate environmental driver for treeline formation, constraining C-sinks (i.e. tissue formation) more than C-sources. The GLH predicts similar or increasing (but not decreasing) non-structural carbohydrate (NSC) concentrations with elevation throughout the course of the growing season. We hypothesized that elevational trends in growth and NSC in Mediterranean regions are not determined by only low temperature. We tested the GLH by examining the mean annual basal area and NSC concentrations in developing (new twigs) and ripened tissues (branch, stem) of trees growing at three elevations in three Mediterranean and three temperate treelines in the Chilean southern Andes (33ºS, Kageneckia angustifolia, 36ºS, 40ºS, and 46ºS, Nothofagus pumilio) and in Spain (36ºN, Pinus sylvestris, 42ºN, P. uncinata). Samples for NSC were taken at the onset of summer and autumn, which represent periods of contrasting drought intensities in Mediterranean regions. Tree growth decreased significantly with elevation in temperate treelines but not in Mediterranean treelines. In Mediterranean treelines, new twig NSC concentrations increased significantly with elevation in the early summer but not in the early autumn. In temperate treelines, in contrast, no elevational or seasonal variation (or interaction between them) was observed in new twig NSC concentrations. The NSC concentrations of the branches and stems from both climates showed no elevational trends. The soluble sugars’ NSC fraction increased over the season in Mediterranean treelines and decreased in temperate treelines. Synthesis. Although we found support for the growth limitation hypothesis (GLH) in temperate and Mediterranean treelines, our study shows that the tree growth and C balance in Mediterranean treelines are not controlled by low temperature alone. We suggest that environmental factors other than temperature explain the lower global elevation of Mediterranean treelines when compared to temperate treelines.

生长限制假说（growth limitation hypothesis, GLH）是当前被广泛认可的林线形成机制，但针对地中海林线的相关研究仍较为匮乏——这类林线的分布海拔低于温带林线。GLH提出，低温是驱动林线形成的终极环境因子，其对碳汇（即组织构建过程）的抑制作用强于碳源。该假说预测，在整个生长季中，非结构性碳水化合物（non-structural carbohydrate, NSC）的浓度会随海拔升高呈现相似或上升（而非下降）的变化趋势。我们提出假说：地中海区域内树木生长与NSC的海拔变化趋势并非仅由低温决定。我们通过检验智利南部安第斯山脉（33°S，窄叶 Kageneckia（Kageneckia angustifolia）；36°S、40°S与46°S，假山毛榉（Nothofagus pumilio））以及西班牙（36°N，欧洲赤松（Pinus sylvestris）；42°N，细叶松（P. uncinata））的3处地中海林线与3处温带林线的3个海拔梯度样地中树木的年均胸高断面积，以及新生组织（新梢）与成熟组织（枝条、茎干）的NSC浓度，以此对GLH进行验证。NSC样本分别于夏季初与秋季初采集，这两个时段对应地中海区域干旱强度差异显著的阶段。研究结果显示：温带林线的树木生长随海拔升高显著下降，而地中海林线未出现该规律。在地中海林线中，新梢的NSC浓度在初夏随海拔升高显著上升，但初秋未观察到该趋势；与之相反，温带林线的新梢NSC浓度未呈现海拔、季节变化或二者的交互作用。两类林线的枝条与茎干NSC浓度均未表现出随海拔的变化趋势。可溶性糖占NSC的比例在地中海林线中随季节上升，而在温带林线中随季节下降。综合来看，尽管我们在温带与地中海林线中均找到了支持GLH的证据，但本研究表明地中海林线的树木生长与碳平衡并非仅受低温调控。我们提出，相较于温带林线，地中海林线全球分布海拔更低的现象，可由温度以外的其他环境因子解释。