Data from: Novel forests maintain ecosystem processes after the decline of native tree species

The positive relationship between species diversity (richness and evenness) and critical ecosystem functions, such as productivity, carbon storage, and nutrient cycling, is often used to predict the consequences of extinction. At regional scales, however, plant species richness is mostly increasing rather than decreasing because successful plant species introductions far outnumber extinctions. If these regional increases in richness lead to local increases in diversity, a reasonable prediction is that productivity, carbon storage, and nutrient cycling will increase following invasion, yet this prediction has rarely been tested empirically. We tested this prediction in novel forest communities dominated by introduced species (~ 90% basal area) in lowland Hawaiian rainforests by comparing their functionality to that of native forests. We conducted our comparison along a natural gradient of increasing nitrogen availability, allowing for a more detailed examination of the role of plant functional trait differences (specifically, N2-fixation) in driving possible changes to ecosystem function. Hawai'i is emblematic of regional patterns of species change; it has much higher regional plant richness than it did historically, due to > 1000 plant species introductions and only ~ 71 known plant extinctions--resulting in an approximately 100% increase in richness. At local scales, we found that novel forests had significantly higher tree species richness and higher diversity of dominant tree species. We further found that aboveground biomass, productivity, nutrient turnover (as measured by soil-available and litter-cycled nitrogen and phosphorus), and belowground carbon storage either did not differ significantly or were significantly greater in novel relative to native forests. We found that the addition of introduced N2-fixing tree species on N-limited substrates had the strongest effect on ecosystem function--a pattern found by previous empirical tests. Our results support empirical predictions of the functional effects of diversity, but they also suggest basic ecosystem processes will continue even after dramatic losses of native species diversity if simple functional roles are provided by introduced species. Because large portions of the Earth's surface are undergoing similar transitions from native to novel ecosystems, our results are likely to be broadly applicable.

物种多样性（丰富度与均匀度）与关键生态系统功能（如生产力、碳储量及养分循环）之间的正向关联，常被用于预测物种灭绝所带来的生态后果。然而在区域尺度上，植物物种丰富度整体呈上升趋势而非下降，这是因为成功引入的植物物种数量远多于灭绝的物种。若区域尺度上的丰富度提升会推动局地多样性增加，那么可合理推测：生物入侵后，生态系统生产力、碳储量及养分循环能力均会上升，但该假说极少得到实证检验。本研究以夏威夷低地热带雨林中以引入物种为主（基盖度(basal area)约占90%）的新型森林群落为研究对象，通过将其生态功能与原生林进行对比，验证了上述假说。我们沿着氮有效性(nitrogen availability)逐渐升高的自然梯度开展对比研究，以此更细致地探究植物功能性状差异（尤其是固氮作用(N₂-fixation)）在驱动生态系统功能变化中所扮演的角色。夏威夷群岛是物种变化区域格局的典型代表：由于引入了超过1000种植物，仅记录到约71种植物灭绝，其区域植物丰富度较历史水平大幅提升，增幅约达100%。在局地尺度上，本研究发现新型森林的树木物种丰富度显著更高，优势树种的多样性也更为突出。进一步分析显示，相较于原生林，新型森林的地上生物量、生产力、养分周转（以土壤有效态及枯落物循环的氮、磷含量为衡量指标）以及地下碳储量要么无显著差异，要么显著更高。我们还发现，在氮受限的生境中引入固氮树种对生态系统功能的提升效果最为显著——这一结果与此前的实证研究结论一致。本研究结果既验证了多样性功能效应的实证预测，同时也表明：若引入物种能够填补原生物种丧失后的核心生态功能位，则即便原生物种多样性出现大幅流失，生态系统的基本过程仍可维持。由于全球大部分地表区域正经历着从原生生态系统向新型生态系统的类似转型，本研究结果具备广泛的应用前景。