Abstract In the past decades, the expansion and modernisation of agriculture in the mountainous areas of Southeast Asia has had severe impacts on biodiversity, as the once species-rich forests were turned into homogeneous fields receiving ample external inputs. A common feature of permanent cropping with annual crops is the frequent change of crop choice, depending on market opportunities or other motives. However, the precise effect of crop shifts on weeds in tropical areas is largely unknown. In this study, we investigated the short-term effect of crop sequences on the diversity of weed communities in smallholder fields of Northern Thailand. Crop choices were upland rice, maize, fallow and young tree plantations with or without intercrop. We counted the number of crop shifts and the number of crops involved during a 3-years period preceding weed sampling. We showed that the number of crop shifts did not affect weed density and biomass. However, herbaceous species number and diversity (measured as Shannon index) increased by 36% and 46% respectively, while herbaceous species dominance decreased by 38%, in fields with yearly crop shifts compared to fields with no shifts in the previous three years. The effect of a particular crop on diversity, or the effect of intercropping with young trees, was weaker. It was likely due to the more variable resources (especially light) in fields with two crop shifts, allowing species with different niches to co-exist. Crop type and frequent crop shifts did not affect shrub and tree species number, diversity or dominance. Some species were strongly associated with fields with no crop shift in the sequence (e.g. the tree Antidesma velutinosum) or to fields with two crop shifts in the sequence (e.g. the herb Centella asiatica, the C4 grass Digitaria radicosa). Overall, this study showed that in this agronomical system, maintaining yearly crop shifts does not significantly affect weed abundance, but supports in-field plant species diversity, which is likely to impact the services provisioned by tropical mountainous agro-ecosystems.

中文翻译：

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年复一年的作物变化促进了热带永久雨养种植中的杂草多样性

摘要 在过去的几十年中，东南亚山区农业的扩张和现代化对生物多样性产生了严重影响，曾经物种丰富的森林变成了接受大量外部投入的均质田。一年生作物永久种植的一个共同特征是作物选择的频繁变化，这取决于市场机会或其他动机。然而，作物变化对热带地区杂草的确切影响在很大程度上是未知的。在这项研究中，我们调查了作物序列对泰国北部小农田杂草群落多样性的短期影响。作物选择是旱稻、玉米、休耕和有或没有间作的幼树种植园。我们计算了杂草采样前 3 年期间的作物轮换次数和所涉及的作物数量。我们表明，作物轮班的数量不会影响杂草密度和生物量。然而，与前三年没有变化的田地相比，每年轮作的田地的草本物种数量和多样性（以香农指数衡量）分别增加了 36% 和 46%，而草本物种的优势度下降了 38%。特定作物对多样性的影响，或与幼树间作的影响较弱。这可能是由于在两次作物轮换的田间资源（尤其是光）的可变性更大，允许具有不同生态位的物种共存。作物类型和频繁的作物轮换不影响灌木和树种的数量、多样性或优势。一些物种与序列中没有作物变化的田地（例如树 Antidesma velutinosum）或序列中有两个作物变化的田地（例如草本积雪草、C4 草 Digitaria radicosa）密切相关。总体而言，这项研究表明，在该农艺系统中，保持每年的作物轮换不会显着影响杂草丰度，但会支持田间植物物种多样性，这可能会影响热带山区农业生态系统提供的服务。