Empirical and simulated biomass, species composition, and grazing intensity in Laikipia, Kenya

We compared grazing intensity estimated by the InVEST beta Rangeland Production model to empirical grazer density measured empirically in Laikipia, Kenya. Modeled grazing intensity was estimated through calibration to empirically measured biomass. Empirical measures were collected on 25 properties across the region, and more detailed data were collected on one property, OPC. Biomass and animal dung were measured at 100 m transects. Biomass was measured with a pasture disk meter (PDM) which was dropped once every 10 m along the transect to estimate vegetation height. PDM measurements were transformed to dry weight biomass with a calibration coefficient estimated by regression. Animal dung on each transect was counted if it fell within 0.5 m of the transect centerline. When possible, animal dung was identified to species; the dung of cattle and buffalo could not be reliably distinguished in the field and so their density was combined into a single bovid class. Simulated grazing intensity was estimated by the InVEST beta Rangeland Production model through calibration to empirical biomass at a given location and date. Simulated grazing intensity was compared to empirical biomass that was averaged within 2 km of weather stations on OPC and within regional properties. Dung on OPC were averaged across transects within months of sampling for comparison with modeled grazing intensity.

我们将InVEST beta牧场生产力模型（InVEST beta Rangeland Production model）估算的放牧强度，与肯尼亚莱基皮亚地区实测的放牧者密度进行了对比。模型估算的放牧强度通过对实测生物量进行校准得到。本次研究在该区域的25处研究地块收集了实测数据，其中针对编号为OPC的地块采集了更为详尽的数据集。生物量与动物粪便的测量沿100米样带开展，生物量采用牧场圆盘测草仪（pasture disk meter, PDM）进行测定，沿样带每10米放置一次该仪器以估算植被高度；通过回归分析得到的校准系数，可将PDM测量值转换为干重生物量。对于每条样带上的动物粪便，若其位于样带中心线0.5米范围内则予以计数。若条件允许，可对粪便进行物种鉴定；但牛与水牛的粪便难以在野外可靠区分，因此将二者的密度合并为一个牛科（bovid）类别。InVEST beta牧场生产力模型通过对特定地点与日期的实测生物量进行校准，估算得到模拟放牧强度。随后将模拟放牧强度与两类实测生物量数据进行对比：一类为OPC地块内气象站周边2公里范围内的平均生物量，另一类为区域内各研究地块的平均生物量。为与模型估算的放牧强度进行对比，OPC地块的粪便数据按采样月份在所有样带间取平均值后参与比对。