Abstract The trees in agroforestry plots create spatial heterogeneity of high interest for adaptation, mitigation, and the provision of ecosystem services. But to what distance, exactly, from the tree? We tested a novel approach, based upon geostatistics and Unmanned Aerial Vehicle (UAV) sensing, to infer the distance at which a single agroforestry tree affects the surrounding under-crop, to map yield, litter (i.e. stover) and compute crop-partial Land Equivalent Ratio (LERcp) at the whole-plot level. In an agro-silvo-pastoral parkland of semi-arid western Africa dominated by the multi-purpose tree Faidherbia albida, we harvested the pearl-millet under-crop at the whole-plot scale (ca. 1 ha) and also in subplot transects, at three distances from the trunks. We observed that the yield was three times higher below the tree crown (135.6 g m−2) than at a distance of five tree-crown radii from the trunk (47.7 g m−2). Through geostatistical analysis of multi-spectral, centimetric-resolution images obtained from an UAV overflight of the entire plot, we determined that the ‘Range’ parameter of the semi-variogram (assumed to be the distance of influence of the trees on the Normalized difference vegetation index (NDVI)) was 17 m. We correlated the yield (r2 = 0.41; RRMSE = 48 %) and litter production (r2 = 0.46; RRMSE = 35 %) in subplots with NDVI, and generated yield and litter maps at the whole-plot scale. The measured whole-plot yield (0.73 t ha-1) differed from the one estimated via the UAV mapping by only 20 %, thereby validating the overall approach. The litter was estimated similarly at 1.05 tC ha-1 yr-1 and mapped. Using a geostatistical proxy for the sole crop, LERcp was estimated 1.16, despite the low tree density. This new method to handle heterogeneity in agroforestry systems is a first application. We also propose strategies for extension to the landscape level.

中文翻译：

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树木对农林业作物的影响有多大？Faidherbia 公园的新空间分析方法

摘要 农林业地块中的树木创造了空间异质性，对适应、减缓和生态系统服务的提供具有重要意义。但是，离树到底有多远？我们测试了一种基于地质统计学和无人机 (UAV) 传感的新方法，以推断单个农林业树影响周围作物的距离，绘制产量、凋落物（即秸秆）和计算作物部分土地的地图整区水平的等效比率 (LERcp)。在以多用途树 Faidherbia albida 为主的半干旱西部非洲农林牧草地上，我们在全地规模（约 1 公顷）和子样地样带中收获了珍珠粟，距离树干三个距离。我们观察到，树冠以下的产量高出三倍 (135. 6 gm-2) 比距离树干 5 个树冠半径 (47.7 gm-2)。通过对从整个地块的无人机飞越获得的多光谱、厘米分辨率图像的地统计分析，我们确定了半变异函数的“范围”参数（假设是树木对归一化差异的影响距离）植被指数 (NDVI)) 为 17 m。我们将子地块中的产量（r2 = 0.41；RRMSE = 48 %）和凋落物产量（r2 = 0.46；RRMSE = 35 %）与 NDVI 相关联，并生成了全区尺度的产量和凋落物图。测得的整区产量 (0.73 t ha-1) 与通过无人机测绘估计的产量仅相差 20%，从而验证了整体方法。凋落物的估计值类似为 1.05 tC ha-1 yr-1，并绘制了地图。使用单一作物的地统计代理，尽管树木密度低，但 LERcp 估计为 1.16。这种处理农林业系统异质性的新方法是第一个应用。我们还提出了扩展到景观水平的策略。