Pinus ponderosa alters nitrogen dynamics and diminishes the climate footprint in natural ecosystems of Patagonia

1. Evaluating climate effects on plant-soil interactions in terrestrial ecosystems remains challenging due to the fact that floristic composition co-varies with climate, particularly along rainfall gradients. It is difficult to separate effects of precipitation per se from those mediated indirectly through changes in species composition. As such, afforestation (the intentional planting of woody species) in terrestrial ecosystems provides an ecological opportunity to assess the relative importance of climate and vegetation controls on ecosystem processes. 2. We investigated the impacts of 35 years of afforestation on ecosystem N dynamics, in ecosystems ranging from arid shrub-steppe to closed-canopy forest in Patagonia, Argentina. Site of natural vegetation and adjacent sites planted with a single exotic species, Pinus ponderosa, were identified in five precipitation regimes along a continuous gradient of 250 to 2200 mm mean annual precipitation (MAP). We evaluated C and N parameters of vegetation and soil, as well as natural abundance of 13C and 15N, in leaves, roots, ectomycorrhizae (EcM), and soils. 3. In natural vegetation, most leaf traits (%N, C:N ratios, leaf mass per area, δ15N values) demonstrated strong significant relationships with MAP, while these relationships were nearly absent in afforested sites. In addition, the EcM of native southern beech and pine trees were significantly enriched in 15N relative to leaves at all sites where they were present. While soil C and N pools in both vegetation types increased with MAP, overall pool sizes were significantly reduced in afforested sites. 4. Synthesis Observed relationships between leaf traits and precipitation in natural vegetation may be driven largely by shifts in species composition and plant-soil interactions, rather than direct effects of precipitation. Our results suggest that a change in species composition of the dominant vegetation is sufficient to alter C and N cycling independently of climate constraints: pine afforestation homogenized N dynamics across sites spanning an order of magnitude of MAP. These results highlight the important control of ectomycorrhizal associations in affecting C and N dynamics. Additionally, it serves to demonstrate that altering natural species composition alone is sufficient to cause large, detectable impacts on N turnover independently of direct climate effects.

1. 由于植物区系组成（floristic composition）随气候发生协同变化，尤其沿降水梯度分布时这一特征更为显著，因此评估陆地生态系统（terrestrial ecosystem）中气候对植物-土壤相互作用（plant-soil interactions）的影响仍颇具挑战。直接区分降水本身的效应与通过物种组成变化间接介导的效应难度极大。在此背景下，陆地生态系统中的造林活动（afforestation，即有意栽植木本植物的举措）为评估气候与植被对生态系统过程的相对调控重要性提供了生态学契机。
2. 本研究针对阿根廷巴塔哥尼亚地区的一系列生态系统——从干旱灌丛草原到密闭冠层森林，探究了35年造林对生态系统氮（N）动态的影响。我们在沿年均降水量（mean annual precipitation, MAP）连续梯度（250~2200 mm）划分的5个降水区域中，选取了天然植被样地与相邻的、栽植单一外来种美国黄松（Pinus ponderosa）的造林样地。我们测定了植被与土壤的碳（C）、氮参数，以及叶片、根系、外生菌根（ectomycorrhizae, EcM）和土壤中碳13（¹³C）与氮15（¹⁵N）的自然丰度。
3. 在天然植被样地中，多数叶片性状（全氮含量、碳氮比、比叶质量、δ¹⁵N值）与年均降水量呈显著强相关关系；而在造林样地中，这类关联几乎消失。此外，在所有存在原生南山毛榉与松树的样地中，二者的外生菌根（ectomycorrhizae, EcM）的δ¹⁵N值均显著高于对应叶片。尽管两种植被类型下的土壤碳、氮库均随年均降水量增加而增大，但造林样地的总库规模显著低于天然植被样地。
4. 综合结论 天然植被中叶片性状与降水的关联，可能主要由物种组成变化与植物-土壤相互作用驱动，而非降水的直接效应。本研究结果表明，优势植被的物种组成变化足以在不受气候约束的情况下改变碳、氮循环：美国黄松造林使得跨越近一个数量级年均降水量梯度的样地的氮动态趋于同质化。这些结果凸显了外生菌根共生关系对碳、氮动态的重要调控作用。同时，本研究也证实，仅改变天然物种组成即可独立于直接气候效应，对氮周转产生显著且可检测的影响。