Tidyverse data frame conversion from monthly to annual

I have a data frame in which the first column is a sequence of monthly 
dates and the other columns are variables. There are a great many 
variables. I want to create another data frame similar to the first
one, 
but with annual values instead of monthly, created by summing the
months 
within each year.

I am able to do this as shown in this reprex:

library(tidyverse)
REF_DATE <- seq(as.Date("2000/1/1"),by="month",length.out=36)
set.seed(57)
df <- data.frame(REF_DATE,
                 x=sample(1:100,size=36),
                 y=sample(1:100,size=36),
                 z=sample(1:100,size=36),
                 Year=year(REF_DATE))
df1 <- df %>%
  group_by(Year) %>%
  summarise(x_a=sum(x),y_a=sum(y),z_a=sum(z)) %>%
  ungroup()

However, while this works for the simple case with only three
variables, 
I actually have many more than three, so I am looking for a more
general 
approach. I have no clue as to how to proceed. Any advice will be much 
appreciated.

Philip