Emerging stability of forest productivity by mixing two species buffers temperature destabilizing effect

The increasing disturbances in monocultures around the world are testimony to their instability under global change. Many studies have claimed that temporal stability of productivity increase with species richness, although the ecological fundaments have mainly been investigated through diversity experiments. To adequately manage forest ecosystems, it is necessary to have a comprehensive understanding of the effect of mixing species on the temporal stability of productivity and the way in which this it is influenced by climate conditions across large geographical areas.

Here, we used a unique dataset of 261 stands combining pure and two-species mixtures of four relevant tree species over a wide range of climate conditions in Europe to examine the effect of species mixing on the level and temporal stability of productivity. Structural equation modelling was employed to further explore the direct and indirect influence of climate, overyielding, species asynchrony and additive effect (i.e. temporal stability expected from the species growth in monospecific stands) on temporal stability in mixed forests.

We showed that by adding only one tree species to monocultures, the level (overyielding: +6%) and stability (temporal stability: +12%) of stand growth increased significantly. We identified the key effect of temperature on destabilizing stand growth, which may be mitigated by mixing species. We further confirmed asynchrony as the main driver of temporal stability in mixed stands, through both the additive effect and species interactions, which modify between-species asynchrony in mixtures in comparison to monocultures.

Synthesis and applications. This study highlights the emergent properties associated with mixing two-species, which result in resource efficient and temporally stable production systems. We reveal the negative impact of mean temperature on temporal stability of forest productivity and how the stabilizing effect of mixing two species can counterbalance this impact. The overyielding and temporal stability of growth addressed in this paper are essential for ecosystem services closely linked with the level and rhythm of forest growth. Our results underline that mixing two species can be a realistic and effective nature-based climate solution, which could contribute towards meeting EU climate target policies.

Methods
The research unit is the forest stand. We used data from a total of 261 forest stands belonging to three triplet-transects across Europe. Each triplet consists of a plot established in a two species mixed stands, and two plots on the respective monospecific stands; the three stands are located close to each other under similar environmental conditions. The species composition of the mixtures changes in the three triplet-transects. The first transect covers monospecific and mixed stands of Fagus sylvatica and Pinus sylvestris (32 sites, 96 stands), the second of Quercus petraea and Pinus sylvestris (35 sites, 105 stands), and the third of Picea abies and Pinus sylvestris (20 sites, 60 stands). Plot sizes varies from 0-02 to 0.15 ha depending on stand density a local site characteristics.

In each plot the diameter of all trees was measured, and two increment cores per tree were taken at a 1.3 m stem height in a sample of approximately 20 trees per species and plot. Annual ring widths were measured and cross-dated using standardized dendrochronological techniques. The studied period was 2000-2013 for the beech-pine transect and 2004-2017 for the oak-pine and spruce-pine transects (except in five triplets where the period was 2000-2013), the last year corresponding to triplet establishment. Using data from cored trees, tree diameter increment-diameter models were fitted by year, species and plot to estimate diameter increments of noncored trees for the studied period. Dead trees during the last 14 years were estimated using stumps, standing and lying dead trees, and their decomposition status. Based on measured tree diameters and annual diameter increments we estimated species and stand annual basal area (BA) and basal area growth (BAI), which conforms the dataset.

Annual climate data were obtained from meteorological weather stations located in the proximity of each triplet (50 triplets). When local station data were not available, national digital climatic atlas data (24 triplets) or more general gridded data (13 triplets) were used (mostly CRU gridded database). For each triplet mean and standard deviation of annual precipitation (P) and mean annual temperature (T) for the studied period were calculated.

全球范围内单一栽培林分日益频发的干扰事件，印证了其在全球变化背景下的不稳定性。诸多研究表明，群落生产力的时间稳定性随物种丰富度提升而增强，尽管相关生态学机制主要通过生物多样性实验展开探究。为实现森林生态系统的精细化管理，亟需全面解析物种混交对生产力时间稳定性的影响，以及大地理尺度下气候条件对该效应的调控路径。
本研究依托一套独特的数据集，涵盖欧洲跨多种气候条件下的261个林分，包含4种目标树种的纯林与双树种混交林，以此探究物种混交对生产力水平及时间稳定性的影响。本研究采用结构方程模型（Structural Equation Modelling），进一步解析气候、超产效应（overyielding）、物种异步性（species asynchrony）及加性效应（additive effect，即单一林分中物种生长所预期的时间稳定性）对混交林生产力时间稳定性的直接与间接调控作用。
研究结果显示，仅向单一栽培林分中引入1个树种，即可显著提升林分生长的水平（超产效应提升6%）与稳定性（时间稳定性提升12%）。本研究明确了温度对林分生长的去稳定化关键作用，而物种混交可缓解该效应。此外，本研究进一步证实，物种异步性是混交林生产力时间稳定性的核心驱动因子，其通过加性效应与物种互作调控混交林相较于纯林的种间异步性水平。
综合与应用。本研究揭示了双树种混交所带来的涌现性特征，该特征可构建资源高效且时间稳定的生产系统。本研究明确了年平均温度对森林生产力时间稳定性的负面影响，以及双树种混交的稳定效应如何抵消该负面影响。本文所探讨的超产效应与林分生长时间稳定性，对于与森林生长水平及节律紧密相关的生态系统服务至关重要。本研究结果表明，双树种混交是一种切实可行且高效的基于自然的气候解决方案，有助于欧盟（EU）达成其气候目标政策。
研究方法
研究单元为森林林分。本研究采用欧洲境内3条样带三联体共261个林分的观测数据。每个样带三联体包含1个双树种混交林样地，以及对应两个单树种纯林样地；三个林分地理位置邻近且环境条件相似。三条样带三联体的混交林物种组成各不相同：第一条样带涵盖欧洲山毛榉（Fagus sylvatica）与欧洲赤松（Pinus sylvestris）的纯林及混交林（32个样点，96个林分）；第二条样带涵盖无梗花栎（Quercus petraea）与欧洲赤松的纯林及混交林（35个样点，105个林分）；第三条样带涵盖挪威云杉（Picea abies）与欧洲赤松的纯林及混交林（20个样点，60个林分）。样地面积介于0.02公顷至0.15公顷之间，具体数值取决于林分密度与当地立地特征。
在每个样地中，测定所有树木的胸径，并在每个物种约20株树木的样本中，于树高1.3米处采集每株树木的2个树芯。采用标准化树木年代学技术测定年轮宽度并进行交叉定年。山毛榉-赤松样带的研究时段为2000-2013年，栎树-赤松与云杉-赤松样带的研究时段为2004-2017年（其中5个三联体的研究时段为2000-2013年），最后一年对应三联体样地的建立年份。基于采芯树木的数据，按年份、物种与样地拟合树木胸径生长量-胸径模型，以估算研究时段内未采芯树木的胸径生长量。通过伐桩、枯立木与倒木及其分解状态，估算近14年间的死亡树木数量。基于实测的树木胸径与年胸径生长量，我们估算得到各物种及林分的年胸高断面积（Basal Area, BA）与胸高断面积生长量（Basal Area Increment, BAI），以此构建本研究数据集。
年度气候数据来源于每个三联体附近的气象站（共50个三联体）。若无本地气象站数据，则采用国家数字气候图集数据（24个三联体）或更通用的格点数据（13个三联体，主要采用CRU格点数据库）。针对每个三联体，计算研究时段内年降水量（Precipitation, P）的均值与标准差，以及年平均温度（Mean Annual Temperature, T）。