Historical contingency in ecology and restoration: year effects, priority effects, and climate change in California grasslands

INTRODUCTION There are two opposing ideas about how natural communities develop after disturbance: 1) communities will tend to return to their pre-disturbance state, regardless of the vagaries of early establishment; and 2) species that make up communities are largely determined by the order of arrival of colonizers. This study seeks to address these ideas while asking whether year-to-year variation affects the communities in the long term. In this multi-year research program we examine the plant community ecology of California grasslands, controlling for colonization order of plants in the context year-to-year variation in rainfall. This research will also test whether rare climatic events are more important for plant communities than gradual changes in average climate, with implications for how we respond to climate change. We are carrying out a series of replicated experiments that test central questions shared by both conceptual ecology and ecological restoration, in the context of climate change, and we will implement these experiments identically for at least five consecutive years. At the heart of these experiments is the hypothesis that the year of establishment is a major determinant of long-term community structure. There are two sets of core experiments being carried out at three sites in California grasslands. The Grass/Forb Priority Experiment asks whether short-term stochastic effects (arrival order across years) can override longer-term deterministic effects (succession) in perennial grasslands undergoing restoration. A similar Native/Weed Priority Experiment asks how much the competitive suppression of native perennial grasses by exotic annual grasses could be overcome by a single year of weed control. For each of these two core experiments we have established five replicate plots at each of three grassland sites in northern California. Treatments were initiated in 2011, and we will do this repeatedly for (at least) five consecutive years total, in a formal test of the importance of Year Effects in ecological systems. Each year we will also initiate five replicate plots of watering experiments at each of the three sites. These watering experiments initially will be based on our a priori understanding of the limiting factors in these systems, but as the experiments progress, we will use these targeted experiments to test hypotheses arising from inter-annual differences observed in our core experiments. These experiments also allow us to test the degree to which short-term variation in current climate can be used to predict the long-term responses of species and communities to climate change. HYPOTHESES H1) Simple Competition: Guilds grown in monocultures will perform better than when grown in competition with other guilds, and this competition will differ in years with different rainfall patterns (see below). Even replicating this simple experiment over several years at several sites will be both unique and revealing. H2) Priority (Grass/Forb): Guilds planted a year earlier with have greater initial success in competition with guilds planted later, but this effect will be greater with grass priority over forbs than with forb priority over grasses. H3) Priority (Grass/Forb): Over several years, priority effects will become muted, and different treatments will become more similar in species composition (convergence). However, the priority advantage of grasses will last longer than the priority advantage of forbs, and exclusion of forbs may occur (divergence). H4) Priority (Grass/Forb): There will be distinct "grass years" and "forb years", expressed even in plots where guilds are planted alone, and these will correspond to different rainfall patterns (c.f. Lulow 2004). Variation in the success of forb and grass plantings will be more strongly correlated with patterns of rainfall than with total rainfall (c.f. Pitt & Heady 1978). H5) Priority (Grass/Forb): Supplemental watering will alter the community structure in these plantings. For example, based on the patterns reported by Pitt & Heady (1978), adding water to plots in a dry fall may change a forb year into a grass year. Conversely, adding water during a mid-winter drought may change a forb year into a grass year. Both of these experimental treatments will alter the results of the priority experiments in ways similar to natural interannual variation in rainfall. H6) Priority (Native/Weed): When exotic annual grasses are planted one year later than native perennial grasses their competitive advantage will be significantly reduced. H7) Priority (Native/Weed): This competitive change will initially be asymmetrical: the effect of one year of priority will be more beneficial to the perennial grasses than it will be detrimental to the annual grasses. Later, as perennial grass biomass increases, the priority advantage will translate into significantly reduced weed biomass. H8) Priority (Native/Weed): There will be distinct "native years" and "weed years", expressed even in plots where guilds are planted alone, and these will correspond to different rainfall patterns (c.f. Bakker et al. 2004). H9) Priority (Native/Weed): Supplemental watering will alter the community structure in these plantings. For example, based on the patterns reported by Hamilton et al. (1999), adding water to plots during a mid-winter drought may favor perennial grasses more than exotic annual grasses. Conversely, adding water to plots in a dry fall may favor exotic annual grasses more than perennial grasses. Both of these experimental treatments will alter the results of the priority experiments in ways similar to natural interannual variation in rainfall. H10) Year effects: Success of individual plantings will vary significantly among years, and our watering experiments will be able to reverse these year effects. H11) Year Effects: The relative advantage gained by site priority will differ significantly from year to year, with those years favoring a particular guild in monoculture also producing stronger priority effects for that guild. H12) Climate: Sites differing in mean historical climate will produce communities that differ significantly, averaged across all years. H13) Climate: Population and community variation associated with climate in the year(s) of restoration will exceed variation associated with site differences. METHODS Within each site (Davis Research Fields 38° 32.61' N, 121° 47.19' W; Hopland Research and Extension Center 38° 58.99' N, 121° 5.18' W; and McLaughlin Natural Reserve 38° 52.17' N, 122° 25.31' W), we selected areas of uniform topography on similar soils (relatively fertile clay loams) to minimize within-site heterogeneity. We chose species mixes that represent appropriate native perennials and common exotic annual grasses, already present at these sites, and known to germinate and establish well in restoration settings. Species planted were as follows (with those sites in parentheses indicating particular species that were not planted at all three sites, in order to make adjustments at the species level to match local sites): Native Perennial Grasses - Stipa (Nassella) pulchra, Hordeum brachyantherum, Elymus glaucus, Bromus carinatus Native Perennial Forbs - Achillea millefolium, Artemisia douglasiana, Eschscholzia californica, Croton setigerus Exotic Annual Grasses - Avena fatua (Davis), Avena barbata (Hopland and McLaughlin), Vulpia myuros (Davis and McLaughlin), Vulpia bromoides (Hopland), Bromus hordeaceus, Hordeum murinum One experiment will use the native perennial grasses and forbs (Grass/Forb Priority Experiment), and the other will use the native perennial grasses and the exotic annual grasses (Native Perennial/Annual Weed Priority Experiment). All seed was purchased from native plant growers who have documented local provenances, or when available, collected locally from remnant populations. We will aggressively control all unplanted species in unused plots and buffer areas between the plots, as well as within the planted native grass/forb experiment. For the Grass/Forb Priority Experiment there will be eight treatment combinations, representing all combinations of the two guilds planted over a two-year period. Each of the eight treatments are described as follows, giving with species planted in year 1 (2011) and species planted in year 2 (2012) if any (if no planting for that year, listed as "none"): Treatment Planting Year 1, Year 2 1 Native grasses, none 2 Native forbs, none 3 Native grasses and forbs, none 4 Native grasses, Native forbs 5 Native forbs, Native grasses 6 none, Native grasses 7 none, Native forbs 8 none, Native grasses and forbs While treatments 6, 7 and 8 from the Year One design are identical to treatments 1, 2 and 3 in the Year Two design, they must be independently replicated in order to test for interactions between priority and year effects. The entire design will be replicated in each of at least five years. Each year, the appropriate plot will be sown with a mix of native perennial grasses, native forbs, or both according to the assigned treatment. Prior to sowing, each plot will be cleared of all vegetation by herbicide, if needed, and shallow tilling. We will sow both grass and forb mixes at a rate of 800 live seeds/m2/guild in all treatments. We will plant at the onset of the rainy season each year, and rake seeds into the soil. Plots will be hand-weeded for obvious non-sown species. For the Perennial/Annual Priority Experiment we repeated the above design, replacing the native perennial forbs with a mix of common exotic annual grasses. Five main treatments were applied as follows: Treatment Planting 1 Native grasses sown alone 2 Native and exotic grasses sown together 3 Native grasses sown, followed by exotic grasses two weeks later (2-week priority) 4 Exotic grasses sown alone, two weeks after other initial plantings 5 Natives grasses sown alone, followed by exotic grasses one year later (1-year priority) All of these exotic grasses are already common at each of the experimental sites, so there will be no risk of releasing new invasions from our study plots into the surrounding land. Rather than relying on natural invasion, seeding the exotic grasses will give control in a way that will simulate real-world conditions, where exotic seed often saturates many grassland sites. Both of the above experiments will be also include a set of replicates that will receive a supplemental irrigation treatment; each block is split in half and receive watering. These replicates will be the subject of a different watering experiment each year, for which the remaining intact replicates ("core experiment") will serve as controls. For both experiments, core treatments were randomly assigned within each half block. Each experimental unit (plot) was 1.25 m on a side, and each was separated from adjacent plots by 1 m. Each year's watering experiment will initially be based on what we can infer from previous descriptive studies as likely climatic drivers of community structure. Later, we will use data from our own replicate core experiments to generate hypotheses about which aspect of rainfall may be drivers of community structure. In each given year, the particular rainfall pattern for that year will determine which modifications are likely to be experimental tests of weather drivers. For example, if in a given year the fall rains are late, or there is a sustained midwinter drought (both common events, and putative drivers of community structure), we will add supplemental aerial applications of water by hand to the targeted replicates in amounts representative of normal rainfall years (~10 cm). By their nature, this watering will be reactive (with a lag) to actual conditions, but we believe they can nonetheless provide tests of previously described patterns based on natural climatic variation, with strong covariance among several rainfall and temperature variables. After each year's targeted treatment, these plots will be left intact and monitored for the duration of the experiments (at least 5 years). We have carried out an analysis of the last 54 years of local rainfall (unpublished data). In fully half of those years there was a mid-winter drought of at least three weeks, while in nearly 40% of years a substantial fall rain (2.5 cm) did not occur until after December 1st. Therefore, we can expect that one (or both) of these drought events will occur in a majority of years. Our analysis further shows that in years with either of these drought types, total yearly rainfall is approximately 10 cm less, and that the effects of the two types of drought on total rainfall are simply additive. That is, years with both late onset rain and a sustained midwinter drought average 20 cm less rainfall than years with neither. Therefore, our supplemental watering during a natural drought will both eliminate the drought effect directly and increase overall rainfall in a way that mimics non-drought years. Every three months, we will measure cover using a ten-pin point frame, placed 20 times in each plot, counting all hits by species and by plant part (living and dead). We will estimate plant density by counting all individuals (to species) in four 25 cm x 25 cm subplots in each plot. STATISTICAL ANALYSES Each hypothesis will be tested with a mixed model (Type III) ANOVA, with experimental treatments and guild as fixed effects, and site and (when applicable) year of initiation as random effects. Multiple measurements will be incorporated as repeated measures. We will test for differences among sites in general (H12), but do not have enough sites to test specific climatic or other correlates of differences across study sites. After five years, we believe that we will have enough power to test Hypothesis H13. The number of replicates in these experiments represents a compromise between the power to answer individual questions in a given year, and the power to ask questions across and between years. Based on the preliminary study, we believe that these replicates will be sufficient for both tasks, especially with the potential added power of multiple sites and (for some analyses) repeated measures. We will compare similarity indices based on Euclidean distance in the first four principal components of PCA to test hypotheses about convergence and divergence, and to test whether variation in community structure associated with climate in the year(s) of species establishment exceeds variation associated with site differences. CAVEAT In addition to priority effects and year effects, we recognize that there are other drivers of ecosystem function in these grasslands, including herbivory, soil disturbance, fire, microbial interactions, and other forms of competition. We are not seeking at this time a comprehensive description (or literature review) of how all of these factors drive community structure, but rather how these particular historical factors do.# 研究背景 关于干扰后自然群落的演替模式，学界存在两种对立假说：其一，无论早期定居过程存在何种随机性波动，群落均倾向于恢复至干扰前的初始状态；其二，群落的物种组成在很大程度上由定植者的到达顺序决定。本研究旨在验证这两种假说，并探讨年际波动是否会对群落产生长期影响。本长期多年度研究以加利福尼亚草原的植物群落生态学为研究对象，在降雨年际变异的背景下控制植物定植顺序变量。此外，本研究还将检验极端气候事件是否较气候平均态的缓慢变化对植物群落的影响更为显著，该结论将为气候变化应对策略提供理论参考。我们开展了一系列重复受控实验，旨在验证概念生态学与生态修复领域共同关注的核心科学问题，并将在连续至少5个年度内采用完全一致的实验设计，核心假说为：定植年份是决定群落长期结构的关键因素。 本研究在加利福尼亚草原的3个样地开展两组核心实验。**禾草/非禾草本优先性实验**（Grass/Forb Priority Experiment）旨在探究：在正在进行生态修复的多年生草原中，短期随机效应（跨年度的到达顺序）是否能够抵消长期确定性效应（演替过程）。另一项**本土/杂草优先性实验**（Native/Weed Priority Experiment）则旨在明确：通过1年的杂草防控，能否抵消外来一年生禾草对本土多年生禾草的竞争抑制作用。针对这两组实验，我们在北加利福尼亚的3个草原样地各设置5个重复样方。实验处理始于2011年，我们将连续重复开展至少5个年度，以正式检验年际效应在生态系统中的重要性。每年我们还会在3个样地各设置5个浇水实验的重复样方。初始的浇水实验设计将基于前期对研究系统限制因子的先验认知，但随着实验推进，我们将利用核心实验中观测到的年际差异衍生新的假说，并通过靶向浇水实验加以验证。此类实验还可帮助我们检验：当前气候的短期波动能否用于预测物种与群落对气候变化的长期响应。 # 研究假说 H1）单作竞争假说：单一功能群的单作表现优于其与其他功能群混作的表现，且该竞争效应会随降雨模式不同的年份发生变化。即便仅在多个样地、多个年度开展实验，其结果也将兼具独特性与启发性。 H2）优先性效应（禾草/非禾草）：提前1年定植的功能群在与后定植功能群的竞争中初始优势更显著，且禾草相较于非禾草的优先定植优势更强。 H3）优先性效应（禾草/非禾草）：在多个年度的观测周期内，优先定植效应会逐渐减弱，不同处理组的物种组成将趋于一致（趋同）。但禾草的优先优势持续时间将长于非禾草，甚至可能出现非禾草被完全排除的情况（趋异）。 H4）优先性效应（禾草/非禾草）：存在显著的“禾草年”与“非禾草年”，即使在单一功能群定植的样方中也会表现出该特征，且该特征与特定降雨模式相对应（参照Lulow 2004）。非禾草与禾草定植成功率的变异与降雨格局的相关性强于与总降雨量的相关性（参照Pitt & Heady 1978）。 H5）优先性效应（禾草/非禾草）：补充灌溉会改变这些定植样地的群落结构。例如，基于Pitt & Heady（1978）报道的模式，在干燥秋季对样地补水可将“非禾草年”转换为“禾草年”；反之，在冬季中期干旱期补水亦可实现相同效果。上述两种处理方式均会通过类似自然降雨年际波动的方式改变优先性实验的结果。 H6）优先性效应（本土/杂草）：当外来一年生禾草较本土多年生禾草晚定植1年时，其竞争优势将显著降低。 H7）优先性效应（本土/杂草）：该竞争效应初始呈现不对称性：1年的优先定植对本土多年生禾草的益处大于对外来一年生禾草的损害。后续随着本土多年生禾草生物量增加，该优先定植优势将转化为杂草生物量的显著降低。 H8）优先性效应（本土/杂草）：存在显著的“本土种年”与“杂草年”，即使在单一功能群定植的样方中也会表现出该特征，且该特征与特定降雨模式相对应（参照Bakker et al. 2004）。 H9）优先性效应（本土/杂草）：补充灌溉会改变这些定植样地的群落结构。例如，基于Hamilton et al.（1999）报道的模式，在冬季中期干旱期补水将更有利于本土多年生禾草而非外来一年生禾草；反之，在干燥秋季补水则更有利于外来一年生禾草而非本土多年生禾草。上述两种处理方式均会通过类似自然降雨年际波动的方式改变优先性实验的结果。 H10）年际效应假说：不同年度的单个定植样地的定植成功率存在显著差异，且浇水实验可逆转该年际效应。 H11）年际效应假说：样地优先定植带来的相对优势会随年度发生显著变化，在单作中有利于特定功能群的年份，也会使该功能群的优先定植效应更强。 H12）气候假说：历史平均气候存在差异的样地，其跨所有年度的平均群落结构存在显著差异。 H13）气候假说：与修复年份气候相关的种群与群落变异，大于与样地差异相关的变异。 # 研究方法 在每个样地（戴维斯研究田：38° 32.61' N, 121° 47.19' W；霍普兰研究与推广中心：38° 58.99' N, 121° 5.18' W；麦克劳克林自然保护区：38° 52.17' N, 122° 25.31' W）中，我们选取地形均一、土壤类型相似（相对肥沃的黏壤土）的区域，以最小化样地内异质性。我们选用的物种组合涵盖该区域已存在的本土多年生植物与常见外来一年生禾草，且这些物种已被证实可在生态修复场景中良好萌发与定植。定植物种如下（括号内标注的物种为未在全部3个样地定植的物种，用于根据样地本地情况进行物种水平调整）： - 本土多年生禾草：**Stipa (Nassella) pulchra**、**Hordeum brachyantherum**、**Elymus glaucus**、**Bromus carinatus** - 本土多年生非禾草本：**Achillea millefolium**、**Artemisia douglasiana**、**Eschscholzia californica**、**Croton setigerus** - 外来一年生禾草：**Avena fatua**（戴维斯样地）、**Avena barbata**（霍普兰与麦克劳克林样地）、**Vulpia myuros**（戴维斯与麦克劳克林样地）、**Vulpia bromoides**（霍普兰样地）、**Bromus hordeaceus**、**Hordeum murinum** 其中一组实验采用**禾草/非禾草本优先性实验**，即使用本土多年生禾草与非禾草本；另一组实验采用**本土/杂草优先性实验**，即使用本土多年生禾草与外来一年生禾草。所有种子均购自具备本地种源证明的本土植物供应商，或在可行时从样地周边的残存种群就地采集。我们将对未使用样地、样区间缓冲区以及本土禾草/非禾草实验样地内的所有非目标物种进行严格管控。 针对**禾草/非禾草本优先性实验**，共设置8种处理组合，涵盖两个功能群在2个年度内的所有定植组合，具体如下： 1. 处理1：第1年定植本土禾草，第2年无定植 2. 处理2：第1年定植本土非禾草本，第2年无定植 3. 处理3：第1年同时定植本土禾草与非禾草本，第2年无定植 4. 处理4：第1年定植本土禾草，第2年定植本土非禾草本 5. 处理5：第1年定植本土非禾草本，第2年定植本土禾草 6. 处理6：第1年无定植，第2年定植本土禾草 7. 处理7：第1年无定植，第2年定植本土非禾草本 8. 处理8：第1年无定植，第2年同时定植本土禾草与非禾草本 尽管处理6、7、8分别对应处理1、2、3，但为了检验优先性与年际效应的交互作用，仍需独立重复设置。每年我们将在当年雨季来临之际播种，播种量为每个功能群800活种子/平方米，播种后将种子耙入土中。播种前，若有必要，将通过除草剂清除样地内所有植被，并进行浅耕。样地将定期人工拔除非播种物种。 针对**本土/杂草优先性实验**，我们沿用上述实验设计框架，仅将本土非禾草本替换为常见外来一年生禾草的混合种植群落。共设置5种核心处理组合，具体如下： 1. 处理1：仅定植本土禾草 2. 处理2：同时定植本土禾草与外来禾草 3. 处理3：先定植本土禾草，2周后定植外来禾草（2周优先处理） 4. 处理4：先定植其他初始植物，2周后仅定植外来禾草 5. 处理5：先定植本土禾草，1年后定植外来禾草（1年优先处理） 所有上述外来禾草均已在各实验样地中广泛存在，因此不会导致新的外来物种入侵风险。相较于依赖自然入侵，人工播种外来禾草能够更精准地控制实验条件，模拟现实世界中外来禾草种子广泛饱和草原生境的实际情况。 两类实验均设置了补充灌溉对照组：每个实验小区被均分为两半，其中一半接受补充灌溉，另一半作为对照。每年的浇水实验设计将根据当年的实际气候条件调整，初始实验设计将基于前期的研究结果，即基于我们对研究系统限制因子的先验认知，但随着实验推进，我们将利用核心实验中观测到的年际差异衍生新的假说，并通过靶向浇水实验加以验证。此类实验设计还可帮助我们检验：当前气候的短期波动能否用于预测物种与群落对气候变化的长期响应。 我们分析了过去54年的当地降雨数据（未发表）：近半数年份存在至少3周的冬季中期干旱，近40%的年份首场秋季降雨（2.5cm）直到12月1日后才出现。因此，多数年份至少会出现其中一种干旱事件。上述两种干旱事件对总降雨量的影响呈加性效应：同时出现秋季降雨延迟与冬季中期干旱的年份，总降雨量较无干旱年份平均减少20cm，而仅出现其中一种干旱事件的年份平均减少10cm。因此，补充灌溉可直接消除干旱影响并增加总降雨量，模拟非干旱年份的气候条件。 每年我们将采用10针采样框架，在每个样地内放置20次，统计所有物种与植物部分（活与死）的覆盖度。同时，在每个样地内设置4个25cm×25cm的小样方，统计所有个体的密度（按物种分类）。 # 统计分析方法 本研究设计将采用混合效应模型，采用Type III方差分析（ANOVA），其中实验处理、功能群作为固定效应，样地与定植年份作为随机效应。重复测量数据将采用重复测量设计分析。我们将检验不同样地间的总体差异（H12），但由于样地数量有限，无法检验具体的气候或其他与样地差异相关的关联。在5年的实验周期后，我们将有足够的统计效力检验H13。基于前期预实验，我们认为当前的重复样方数量足够兼顾单年度与跨年度的实验需求，尤其是结合多样地与重复测量设计后，统计效力将进一步提升。 我们将基于主成分分析（PCA）前4个主成分的欧氏距离计算相似性指数，以检验趋同与趋异假说，并检验定植年份气候相关的群落变异是否大于样地差异相关的变异。 # 研究局限与说明 除优先效应与年际效应外，我们认识到该草原生态系统中还存在其他驱动生态系统功能的因素，包括草食动物、土壤扰动、火灾、微生物互作以及其他形式的竞争。本研究目前仅关注历史定植因素对群落结构的影响，而非全面涵盖所有驱动因素。