UpdatePanel Timer causes all panels to update
===

I have an ASP.NET web form with several UpdatePanels and I would like to have a timer that triggers only one of these UpdatePanels to update. I then want a separate UpdatePanel to update on the OnSelectedIndexChanged event of a DropDownList. The code I have will update the timer-triggered panel just fine, but when I change the index, the drop down-triggered panel will wait until the next timer tick to update. How can I have the drop down-triggered panel update instantaneously? Code below.

C#:

    public partial class WebForm2 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Timer1_Tick(object sender, EventArgs e)
        {
            Label1.Text = "UpdatePanel1 refreshed at: " +
          DateTime.Now.ToLongTimeString();
            
        }

        protected void dropDown_indexChange(object sender, EventArgs e)
        {
            Label2.Text = "UpdatePanel2 refreshed at: " +
              DateTime.Now.ToLongTimeString();
            //UpdatePanel2.Update(); -- Didn't help
        }
    }

ASP:

    <form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <br />
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:Label ID="Label1" runat="server" Text="not updated yet"></asp:Label>
                <asp:Timer ID="Timer1" runat="server" Interval="3000" ontick="Timer1_Tick"></asp:Timer>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="Timer1" EventName="Tick" />
            </Triggers>
        </asp:UpdatePanel>
        <br />
        <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:Label ID="Label2" runat="server" Text="also not updated yet"></asp:Label>
            </ContentTemplate>
            <Triggers>
                <asp:AsyncPostBackTrigger ControlID="DropDownList1"  EventName="SelectedIndexChanged" />
            </Triggers>
        </asp:UpdatePanel>
        <br />
        <br />
        <asp:DropDownList OnSelectedIndexChanged="dropDown_indexChange" ID="DropDownList1" runat="server">
            <asp:ListItem>This</asp:ListItem>
            <asp:ListItem>That</asp:ListItem>
            <asp:ListItem>The Other</asp:ListItem>
        </asp:DropDownList>
    </div>
    </form>