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）是目前学界公认的林线形成核心解释理论，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显著低于温带林线。该假说提出，低温是驱动林线形成的最终环境因子，其对碳汇（即组织形成过程）的约束作用强于碳源。生长限制假说预测，在整个生长季内，非结构性碳水化合物（non-structural carbohydrate, NSC）浓度会随海拔升高呈现相似或上升（而非下降）的变化趋势。我们提出研究假说：地中海区域内，树木生长与非结构性碳水化合物的海拔梯度格局并非仅由低温单一调控。

我们通过对智利南部安第斯山脉（33°S、36°S、40°S及46°S，分别对应物种Kageneckia angustifolia、Nothofagus pumilio）及西班牙（36°N分布有Pinus sylvestris、42°N分布有P. uncinata）的3处地中海林线与3处温带林线开展实验，检测了三个海拔梯度下树木的年均基面积，以及新生组织（新梢）与成熟组织（枝条、茎干）的非结构性碳水化合物浓度，以此对生长限制假说进行验证。非结构性碳水化合物样本分别于夏季初与秋季初采集——这两个时期代表了地中海区域干旱强度迥异的两个阶段。

研究结果显示：温带林线的树木生长随海拔升高显著降低，而地中海林线未呈现该变化规律；在地中海林线中，新梢的非结构性碳水化合物浓度在夏季初随海拔升高显著上升，但秋季初未出现该趋势。与之相反，温带林线的新梢非结构性碳水化合物浓度未表现出海拔、季节变化及其交互作用。两种气候区的枝条与茎干非结构性碳水化合物浓度均未呈现海拔梯度趋势。非结构性碳水化合物中的可溶性糖组分在 Mediterranean？不，地中海林线中随季节升高，而温带林线则随季节降低。哦，不对，修正为：非结构性碳水化合物中的可溶性糖组分在地中海林线中随生长季推移升高，而在温带林线中随季节降低。

综合结论：尽管我们在温带与地中海林线中均找到了支持生长限制假说的证据，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，地中海林线在全球范围内更低的分布海拔，可由温度以外的其他环境因子解释。