problem in updatepanel , timer and trigger

See more:

C#

ASP.NET

my problem ,, two update pane and one of them had timer , the second had dropdown list and other controls when i choose from dropdownlist , it affect with timer !! and drop down close ..
how to stop effect of dropdown ??

my code ?

<h2>
    Welcome to ASP.NET!
</h2>
<p>
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
</p>
<asp:UpdatePanel ID="UpdatePanel1" runat="server" ChildrenAsTriggers="false" UpdateMode="Conditional">
    <ContentTemplate>
        <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="Timer1" />
    </Triggers>
</asp:UpdatePanel>
<asp:UpdatePanel ID="UpdatePanel2" runat="server">
    <ContentTemplate>
        <asp:Label ID="Label2" runat="server" Text="Label"></asp:Label>
        <br />
        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
            <asp:ListItem Value="0">jerusalem Capital</asp:ListItem>
            <asp:ListItem Value="1">Haifa</asp:ListItem>
            <asp:ListItem Value="2">Jaffa</asp:ListItem>
            <asp:ListItem Value="3">Gaza</asp:ListItem>
        </asp:DropDownList>
    </ContentTemplate>
</asp:UpdatePanel>
<asp:Timer ID="Timer1" runat="server" Interval="2000" ontick="Timer1_Tick">
</asp:Timer>