Threshold change in forest understory vegetation as a result of selective fuelwood extraction in Nairobi, Kenya

We examined whether heavy fuelwood collection can cause threshold change in understory forest community and evaluated how selective wood extraction might lead to delayed forest recovery in an urban forest of Nairobi, Kenya. Piecewise regression which represents strongest support for threshold change provided the best fit for the relationships between understory floristic composition (i.e. DCA axis 1) and human disturbance gradients (i.e. canopy cover, and distance from the slum), where threshold changes were detected at c.a. 350 m from the slum and c.a. 30% canopy cover. Only one tree species significantly indicated communities beyond the threshold while an aggressive invasive alien plant (IAP) Lantana camara was strongly represented. Total species diversity along the two human disturbance gradients peaked before the threshold was reached, suggesting that decline in species diversity along the prevailing disturbance gradient might be able to forecast threshold change. Tree species richness in the understory rapidly declined as the threshold was surpassed while other growth forms (i.e. shrubs, herbs and climbers) were relatively unaffected. The effect of selective tree cutting was indirectly impacting the forest understory as species richness pattern of preferred and non-preferred species paralleled that of trees and shrubs, respectively. Thickets of L. camara can negatively affect indigenous flora and its establishment was favored under selective fuelwood extraction removing certain tree species while leaving the IAP untouched. Shading can readily eliminate the IAP, but weak tree regeneration beyond the threshold suggested forest recovery might be delayed for longer than expected because of the interaction between selective fuelwood use and the IAP.     

同源重组介导的克隆策略研究进展

随着基因功能的深入研究,构建表达载体是研究各基因的功能及其相互关系的第一步。近年来涌现的多种同源重组介导的高效克隆策略极大地方便了载体的构建。简要综述了同源重组介导的克隆方法的研究进展,及其在载体构建中的应用。     

Comparison of machine learning algorithms for mapping mango plantations based on Gaofen-1 imagery

Mango is a commercial crop on Hainan Island, China, that is cultivated to develop the tropical rural economy. The development of accurate and up-to-date maps of the spatial distribution of mango plantations is necessary for agricultural monitoring and decision management by the local government. Pixel-based and object-oriented image analysis methods for mapping mango plantations were compared using two machine learning algorithms (support vector machine (SVM) and Random Forest (RF)) based on Chinese high-resolution Gaofen-1 (GF-1) imagery in parts of Hainan Island. To assess the importance of different features on classification accuracy, a combined layer of four original bands, 32 gray-level co-occurrence (GLCM) texture indices, and 10 vegetation indices were used as input features. Then five different sets of variables (5, 10, 20, and 30 input variables and all 46 variables) were classified with the two machine learning algorithms at object-based level. Results of the feature optimization suggested that homogeneity and variance were very important variables for distinguishing mango plantations patches. The object-based classifiers could significantly improve overall accuracy between 2–7% when compared to pixel-based classifiers. When there were 5 and 10 input variables, SVM showed higher classification accuracy than RF, and when the input variables exceeded 20, RF showed better performances. After the accuracy achieved saturation points, there were only slightly classification accuracy improvements along with the numbers of feature increases for both of SVM and RF classifiers. The results indicated that GF-1 imagery can be successfully applied to mango plantation mapping in tropical regions, which would provide a useful framework for accurate tropical agriculture land management.     

无线通信系统在小型遥控机耕船控制中的应用—基于nRF905无线收发芯片

提出了一种无线操控机耕船的设计方法。采用挪威Nordic VLSI公司最新推出的单片无线收发芯片nRF905进行设计,主要是通过发送端发送指令控制接收端机耕船的直流电机,从而控制离合器以及转向系统。此系统主要由接收终端和发送终端组成,各部分主要由ARM7-LPC2132单片机主芯片、液晶显示系统、RS232串口等各功能模块组成。重点说明了无线通信系统的硬件设计和软件设计,对nRF905的内部寄存器配置和收发流程进行了重点介绍,并且给出了实验测试分析。实验表明,该无线传输系统抗干扰性良好,能够很好地传输各种指令,从而较精确地控制机耕船的转动方向。     

New monitoring concept of moisture content distribution in wood during RF/vacuum drying

A new method for monitoring moisture content during radio-frequency (RF)/vacuum drying was developed by measurement of temperature and pressure in wood. Temperature and pressure inside the wood were measured simultaneously during RF/vacuum drying at the same point. The relative humidity (RH) and moisture content (MC) below the fiber saturation point (FSP) were calculated based on temperature and pressure, and the relationship between the temperature, RH, and equilibrium moisture content (EMC) at the measurement point. When the moisture content was below the FSP, the calculated MC was slightly greater than the value given by oven drying. The absolute error was within 0.8% near the open cross side, and was within 1.8% at another measurement point. Thus, we concluded that it was practicable to monitor the moisture content below the FSP according to the temperature and pressure inside the wood. Part of this study was presented at the 15th Annual Meeting of the Chugoku Shikoku Branch of the Japan Wood Research Society, Higashi-Hiroshima, Japan, September 2003     

基于单片机的非接触式IC卡消费系统设计

基于单片机的非接触式IC卡消费系统设计以射频技术为核心,以单片机为控制器的非接触式IC卡读写器在消费系统中的应用.文章介绍其在实际消费系统中的应用,详细介绍了该系统的硬件组成、软件单片机数据处理和一些问题的处理方法.     

基于Landsat 8数据的荒漠土壤水分遥感反演

[目的]分析荒漠土壤水分变化特征,为南疆干旱区荒漠土壤水分遥感监测提供理论依据和方法支持。[方法]以Landsat 8数据构建干旱地区荒漠土壤水分建模指示因子,通过优选的26个光谱指数、地表温度(Ts)和地形数据(DEM)为建模因子,分别以偏最小二乘(PLSR)、支持向量机(SVM)和随机森林(RF)3种方法构建土壤水分反演模型,对模型进行验证和对比,选取最优模型反演空台里克土壤水分空间分布。[结果]①TVDI,NR,GLI等26个优选的光谱指数中,Ts和DEM与土壤水分均达极显著相关,可作为南疆干旱区荒漠土壤水分遥感建模的指示因子;②对比3种模型,RF模型建模集和预测集的R²分别为0.93,0.91,预测集RPD为3.90,各评价指标均为最高,PLSR模型精度次之,SVM模型精度最低;③以RF模型反演研究区表层土壤水分,在不同土地利用分类中土壤水分分布特征存在明显差异,特别在盐结皮区域的差异尤为突出。[结论]利用光谱指数、环境因子和地形数据构建的多因子、多指数综合的模型能较高精度地反演干旱区荒漠表层土壤水分,对研究该地区土地荒漠化和生态环境治理具有一定参考价值。     

低湿玉米籽粒的射频加热模拟与试验

为了准确预测射频加热过程低湿(含水率13.0%～20.0%)玉米籽粒的温度分布变化,该研究结合玉米籽粒的多组分结构特征、热物理特性及介电特性的异质性,建立了玉米籽粒的三维二组分物理几何模型和射频加热数学模型,并利用验证后的模型研究不同姿态和含水率玉米籽粒的射频选择性加热。结果表明:射频加热过程中平放玉米籽粒温度的模拟值与试验值最大相对误差仅为3.47%,玉米籽粒中胚的温度高于胚乳,该模型能很好地预测籽粒内部的选择性加热现象。射频加热过程直立玉米籽粒的几何效应最强,显著提高了胚的电场强度和功率密度,导致其优先加热程度最大,其次是斜放和侧立玉米籽粒的,平放玉米籽粒中胚的优先加热程度最小。射频加热过程含水率为13.0%的玉米籽粒中胚与胚乳的温差逐渐增大,而含水率为16.5%和20.0%的玉米籽粒中胚与胚乳的温差先增大后减小;当玉米籽粒被加热至55℃时,含水率为16.5%的玉米籽粒中胚的优先加热程度最大,其次是含水率为13.0%玉米籽粒的,含水率为20.0%的玉米籽粒中胚的优先加热程度最小。研究结果揭示了玉米籽粒的姿态和含水率对其射频选择性加热的影响规律,可为射频加热技术应用于低湿玉米籽粒的热处理过程提供有益借鉴。     

土壤不同频率无线地下传感器网络信号传播特性试验

为探究土壤环境中无线地下传感器网络节点部署要求、信号传输及其应用环境的特性,分别以240、433和868 MHz为载波频率,研究了无线信道下地上至地下、地下至地上和地下至地下3种通信方式中无线射频信号的传播特性和节点埋藏深度、水平节点间距离以及土壤含水率等影响因素之间的关系,获得了接收信号强度、误码率等数据,并进行了统计分析。试验结果表明,节点埋藏深度和土壤含水率对AG-UG和UG-AG通信中RSSI和BER影响的24种模型拟合优度,R2最大为0.997,最小为0.910。水平节点间距离和土壤含水率对UG-UG通信中RSSI和BER影响的12种模型拟合优度,R2最大为0.971,最小为0.866。此外,建立了433 MHz频率下RSSI变化的三维曲面,可直观反映土壤环境下无线信号的传播特性,并采用SPSS软件对模型进行了验证。AG-UG和UG-AG通信中,拟合优度R2最小为0.954,最大为0.998,均方根误差RMSE在0.729~3.198 d Bm之间。UG-UG通信中,拟合优度R2最小为0.854,最大为0.960。均方根误差RMSE在3.238~6.553 d Bm之间。验证试验结果表明,该模型可以较好地预测不同条件通信中的接收信号强度,为土壤环境中无线地下传感器网络的部署和系统的建立提供了技术支持。     

基于加热均匀性的射频干燥系统结构优化与试验

针对现有射频干燥系统存在装料量大时射频加热不均匀、干燥过程无法实时称重等问题,该文设计了一种热风对流加热装置、装料装置和称量装置。热风对流加热装置主要由热源供给系统、风温风速控制系统和气流分配室组成;装料装置由特制的多层料盘组成;称量装置由称量传感器、称量托盘和支柱等组成。采用计算流体力学软件对气流分配室进行模拟,结果显示改变气流分配室为喇叭口形状及增加分流圆柱体后,条形出风口处风速均匀性提高,其速度偏差比从31%降为10.5%,速度不均匀系数从19%降为7.6%。以玉米种子为例进行了优化后的性能验证试验,结果表明:热风对流加热装置可以提供均匀的温度和风速;自动称量装置能够实时监测物料的质量,其最大量程为24 kg,分层后的薄层物料最大处理量为18 kg,偏差范围为1~5 g;在热风和射频合适的匹配参数下,采用射频干燥结合热风对流技术,以及分层装料的方法,可解决射频干燥大装载量玉米种子产生的热偏移问题和边角效应问题,进而提高射频加热均匀性。该研究避免了干燥过程中外界温湿度的影响,减少了10%的干燥时间,为改善射频加热均匀性提供了参考。