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（tree line）形成的核心机制被认为是低温诱导的碳（C）汇限制，但目前鲜有研究探讨随海拔升高土壤养分有效性降低所发挥的补充作用。本研究验证了如下假说：林线处的养分（尤其是氮，N）限制可通过碳源途径直接抑制，或通过碳汇途径间接抑制冬季落叶林线物种的生长。
若海拔梯度下土壤氮素亏缺参与林线形成，则其作用机制存在两种路径：（i）碳汇限制路径：组织合成过程依赖氮素，这会间接限制碳投入（随海拔升高，土壤氮含量下降而碳储备上升）；（ii）碳源限制路径：这将直接导致光合产物减少（随海拔升高，氮与碳储备均下降）。
为验证上述假说，我们针对智利南部安第斯山脉沿18个纬度梯度分布的4处气候与土壤条件各异的假山毛榉（Nothofagus pumilio）林线，分析了以下指标：树木生长速率（断面积增长量）、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）及氮磷比浓度，以及土壤养分水平（硝酸根NO3−、铵根NH4+、Olsen-P）。
研究结果显示，树木生长速率随海拔升高显著下降。合并所有采样点的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多数，是该负相关趋势的主要驱动因素），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、土壤磷含量及叶片氮磷比则未随海拔发生显著变化。
综合分析：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存在碳源限制的结论。据此，我们认为当前关于林线形成的全球主流解释——低温驱动的碳汇限制——亟需重新审视。鉴于叶片氮、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。