求翻译：宁夏林木引种大致经过了林场生产引种、管理科研部门引种、国家地方个人引种、有序管理主导引种几个时期。前后引种了大量的新树种、新品种，在林业生产中广泛推广应用，改变了宁夏树种少的现状，为林业生态建设起到了基础工作。目前林木引种主要问题是缺乏系统管理，重复引进的多，在生产中造成了一定的损失。应加强管理、规范，把握好试验与推广的界限，服务于农民，减少农民在栽种中的损失。同时要密切注意防止外来有害生物进入宁夏。是什么意思？

The introduction of Ningxia trees generally after the introduction of a forest production, management, scientific research departments, introduction, personal introduction of the national local and orderly management of the dominant periods of the introduction. Before and after the introduction of a

Ningxia forestry plantations after the planting was planting, production management and research departments, the national local planting ornamentals personal orderly management-led, and planting several times. After planting a large number of new species, new species in the forest, widely used in t