1. A study examined how nitrogen (N) and phosphorus (P) limitation affects aboveground growth in northern hardwood forests of different ages. 2. The study applied N, P, and N+P fertilizer treatments to young, mid-age, and mature forest stands over 5 years and measured tree growth responses. 3. The results showed that aboveground growth responded more to P fertilization in mid-age and mature forests, while growth in young forests responded more to N fertilization, indicating that P limits older stands while N limits younger stands.