Data from: An assessment of carbon and nutrient limitations in the formation of the southern Andes treeline

Although the principal mechanism determining tree line formation appears to be carbon (C)-sink limitations due to low temperatures, few studies have assessed the complementary role of reduced soil nutrient availability with elevation. We tested the hypothesis that nutrient (especially nitrogen, N) limitations at tree line may directly (via C-source) or indirectly (via C-sink) reduce the growth of a winter deciduous tree line species. If a shortage of soil N with elevation is involved in tree line formation, it should occur in two alternative ways: (i) through sink limitations because N is required for tissue formation, which would indirectly limit C investments (N decreases and C reserves increase with elevation), and (ii) through C limitations because this would lead directly to a reduction of photoassimilates (N and C reserves decrease with elevation). In testing our hypothesis, we analysed tree growth rates (basal area increment), twig non-structural carbohydrate (NSC) and N concentrations, leaf N, phosphorus (P), N:P ratio concentrations, and soil nutrient levels (NO3−, NH4+, Olsen–P) in four disparate climate and soil Nothofagus pumilio tree lines spanning 18 degrees of latitude in the southern Andes of Chile. We found a significant decrease in tree growth with elevation. Twig NSC concentrations pooled across locations also decreased significantly with elevation (starch constituted most of the NSC and was highly responsible for the negative trend), although this trend was mostly driven by the northernmost locations. Contrary to soil N availability, leaf N and P concentrations increased significantly with elevation. Twig N concentrations, soil P and leaf N:P ratios did not change with elevation. Synthesis. The elevational decrease in NSC concentrations supports C-source limitation in N. pumilio trees at tree line elevation. In the light of this, we assert that the current global explanation for tree line formation (C-sink-limitation driven by low temperatures) must be revisited. Given that leaf N and P concentrations increased and twig N concentrations did not change with elevation, nutrient limitation is not likely to be involved in the C-limitations and could not therefore be an explanation for tree line formation.尽管调控林线形成的核心机制似乎为低温驱动的碳（C）汇限制，但鲜有研究探究随海拔升高土壤养分有效性降低的补充作用。我们验证了如下假说：林线处的养分（尤其是氮，N）限制可通过直接（经由碳源途径）或间接（经由碳汇途径）两种方式，抑制冬季落叶林线树种的生长。 若随海拔升高的土壤氮亏缺参与林线形成，其作用路径可分为两类：（i）碳汇限制途径：由于植物组织构建需要氮素，这会间接限制碳投入——随海拔升高，植株氮含量下降而碳储备增加；（ii）碳源限制途径：这会直接导致光合产物合成减少——随海拔升高，植株氮与碳储备均下降。 为验证上述假说，我们分析了智利南部安第斯山脉跨18个纬度梯度的4处气候与土壤条件各异的假山毛榉（Nothofagus pumilio）林线的相关指标，包括树木生长速率（基面积增量）、小枝非结构性碳水化合物（non-structural carbohydrate, NSC）与氮浓度、叶片氮（N）、磷（P）及N:P比值浓度，以及土壤养分水平（硝酸根离子NO3−、铵根离子NH4+、奥尔森磷Olsen–P）。 研究结果显示，树木基面积增量随海拔升高显著下降。合并所有采样点数据后，小枝非结构性碳水化合物浓度同样随海拔升高显著降低（淀粉占非结构性碳水化合物的绝大部分，是该负向趋势的主要贡献因子），不过这一趋势主要由最北部的采样点主导。与土壤氮有效性的变化趋势相反，叶片氮与磷浓度随海拔升高显著上升。而小枝氮浓度、土壤磷含量及叶片N:P比值均未随海拔发生显著变化。 综合分析表明，小枝非结构性碳水化合物浓度随海拔升高的下降趋势，支持了林线海拔处假山毛榉受到碳源限制的结论。据此我们提出，当前关于林线形成的全球通用解释——即低温驱动的碳汇限制——需要被重新审视。鉴于叶片氮与磷浓度随海拔升高而上升，且小枝氮浓度未发生显著变化，养分限制不太可能参与碳限制过程，因此无法作为林线形成的合理解释机制。