Data from: Woodland ectomycorrhizal fungi benefit from large-scale reduction of nitrogen deposition in the Netherlands

1.Woodland ectomycorrhizal (ECM) fungal species declined considerably in the Netherlands in the late 20th century, mainly due to raised levels of atmospheric nitrogen deposition. Environmental measures have been taken to reduce this deposition, but it remains unclear whether and to what extent ECM species have benefitted from this.

2.We hypothesised that ECM species, especially those species that are known to be nitrophobic, i.e. sensitive to nitrogen loading, have recovered to some extent from the reduction in nitrogen deposition after 1994. We further hypothesised that, due to legacy effects of deposition, recovery has been stronger in regions where deposition levels have previously been lower.

3.To test these hypotheses, we analysed long-term opportunistic data, i.e., observations collected without standardised field method. We applied data filtering and a modified List Length method to adjust for potential biases in these data. The removal of bias left us with two periods to examine ECM species trends: before (1965-1985) and after (1994-2013) deposition reduction started [in 1994].

4.We compared trends in ECM species in 1965-1985 with those in 1994-2013. Multispecies indicators were used to summarise the findings of ECM species, and to compare these with results of litter saprotrophic species and wood saprotrophic and wood parasitic species.

5.We found that (i) most trends switched in direction from negative to positive after the reduction of nitrogen deposition began, (ii) these trends were more pronounced for nitrophobic ECM species than for nitrotolerant ECM species; (iii) trends for ECM species differed from those of the other functional groups; and (iv) recovery was stronger in the region with a history of lower deposition.

6.Policy implications. Our results suggest that woodland ectomycorrhizal species benefit substantially from environmental measures to reduce nitrogen deposition. Our study is one of few scientific studies to date documenting evidence of success of large-scale (nation-wide) environmental measures. We have demonstrated that opportunistic citizen-science data can be used for the detection of species trends, but it is essential to examine and control for potential bias in the data.

1. 20世纪后期，荷兰林地外生菌根（ectomycorrhizal, ECM）真菌物种种群大幅下降，其主要诱因是大气氮沉降水平升高。目前虽已采取环境管控措施以降低氮沉降，但目前仍不清楚外生菌根真菌物种是否能从该举措中获益，以及获益程度如何。
2. 本研究提出如下假说：外生菌根真菌物种，尤其是已知的厌氮性（nitrophobic）物种（即对氮负荷敏感的物种），在1994年后氮沉降降低的背景下已出现一定程度的种群恢复。本研究进一步假设：由于氮沉降的遗留效应，在历史氮沉降水平较低的区域，外生菌根真菌的种群恢复更为显著。
3. 为验证上述假说，本研究分析了长期偶遇观测数据（opportunistic data）——即未采用标准化野外方法采集的观测数据。研究采用数据过滤法与改进后的列表长度法（List Length method）对这些数据中潜在的偏差进行校正。经过偏差校正后，本研究可将研究时段划分为两个阶段以分析外生菌根真菌物种的种群趋势：氮沉降管控启动前（1965-1985年）与启动后（1994-2013年）[注：氮沉降管控始于1994年]。
4. 本研究对比了1965-1985年与1994-2013年两个时段外生菌根真菌物种的种群趋势。研究采用多物种种群指标对其结果进行汇总，并将该结果与枯落物腐生真菌、木材腐生真菌以及木材寄生真菌的研究结果进行对比。
5. 本研究结果显示：（i）氮沉降管控启动后，多数外生菌根真菌物种的种群趋势由负转正；（ii）相较于耐氮性（nitrotolerant）外生菌根真菌物种，厌氮性物种的种群趋势变化更为显著；（iii）外生菌根真菌物种的种群趋势与其他功能群存在显著差异；（iv）在历史氮沉降水平较低的区域，种群恢复效果更为突出。
6. 政策启示：本研究结果表明，林地外生菌根真菌物种可从降低氮沉降的环境管控措施中获得显著益处。本研究是目前为数不多可证实大规模（全国范围）环境管控措施取得成效的科学研究之一。本研究证实，偶遇式公民科学观测数据可用于物种种群趋势的检测，但对数据中潜在偏差进行检验与校正仍是必不可少的环节。