具有根级别导入的Angular2应用程序模块

我有以下root / app模块：

import { BrowserModule } from '@angular/platform-browser';
import { NgModule,ApplicationRef } from '@angular/core';
import { CommonModule } from '@angular/common';
import { FormsModule } from '@angular/forms';
import { AppComponent } from './app.component';
import { HttpModule } from '@angular/http';
import { RouterModule } from '@angular/router';
import { appRouterProviders,routing } from './app.routes';
import { DashboardModule } from './dashboard/dashboard.module';
import { DecisionModule } from './decision/decision.module';
import { MdCoreModule }           from '@angular2-material/core';
import { MdButtonModule }         from '@angular2-material/button';
import { MdCardModule }           from '@angular2-material/card';
import { MdListModule }           from '@angular2-material/list';
import { MdSidenavModule }        from '@angular2-material/sidenav';
import { MdToolbarModule }        from '@angular2-material/toolbar';
import { MdIconModule }           from '@angular2-material/icon';

@NgModule({
  declarations: [
    AppComponent                    
  ],imports: [
    BrowserModule,CommonModule,FormsModule,HttpModule,RouterModule,routing,DashboardModule,MdCoreModule,MdButtonModule,MdCardModule,MdListModule,MdSidenavModule,MdToolbarModule,MdIconModule
  ],providers: [
    appRouterProviders
  ],entryComponents: [ AppComponent ],bootstrap: [ AppComponent ]
})
export class AppModule {

}

如果我尝试在我的子模块中使用它们不显示的材质元素,这就是子模块的样子：

import { NgModule }               from '@angular/core';
import { CommonModule }           from '@angular/common';
import { FormsModule }            from '@angular/forms';
import { dashboardRouting }       from './dashboard.routes';
import { DashboardComponent }     from './dashboard.component';
import { ActionsDialogComponent } from './actions-dialog';

@NgModule({
  imports: [
    CommonModule,dashboardRouting,],declarations: [
    DashboardComponent,ActionsDialogComponent    
  ],providers: [    
  ]
})
export class DashboardModule {}

但是,如果我导入它们显示的子模块中的材料模块.这是材料设计组件工作时子模块的样子：

import { NgModule }               from '@angular/core';
import { CommonModule }           from '@angular/common';
import { FormsModule }            from '@angular/forms';
import { dashboardRouting }       from './dashboard.routes';
import { DashboardComponent }     from './dashboard.component';
import { ActionsDialogComponent } from './actions-dialog';
import { MdCoreModule }           from '@angular2-material/core';
import { MdButtonModule }         from '@angular2-material/button';
import { MdCardModule }           from '@angular2-material/card';
import { MdListModule }           from '@angular2-material/list';
import { MdSidenavModule }        from '@angular2-material/sidenav';
import { MdToolbarModule }        from '@angular2-material/toolbar';
import { MdIconModule }           from '@angular2-material/icon';

@NgModule({
  imports: [
    CommonModule,MdIconModule,providers: [    
  ]
})
export class DashboardModule {}

如果材料模块已经在根级别导入,为什么必须再次在子级别上导入材料模块？