Variability in needle lifespan and foliar biomass along a gradient of soil fertility in maritime pine plantations on acid soils rich in organic matter

• Growth of maritime pine plantations in acidic and organic matter-rich soils is not limited by N nutrition.
• Mg and K are the key resources in determining needle survival, biomass and area, affecting directly site index.
• K:N and Mg:N ratios are good indicators of nutrient status.
• Growth efficiency increases with foliar K concentration and needle longevity.

The variability in needle lifespan, foliar biomass and leaf area index was studied in 31 Pinus pinaster Ait. plots in Galicia (NW Spain), established along a gradient of soil productivity in plantations of similar age (11–14 years). The soils under study were acidic (pH 3.7–5.3) and very rich in organic matter (6.9–34.3%). All needles were classified according to age class, and presence/absence of needles was recorded to evaluate survival. Specific leaf area and needle biomass were calculated for each plot. The foliage nutrient concentrations were also measured and site index was calculated. The maximum needle age was 5 years. Survival of needles older than 2 years declined rapidly. Needle longevity was positively affected by site index and foliar nutrient concentrations, particularly of Mg and K. This suggests that maritime pine responds to low soil fertility by decreasing needle lifespan at local scales. Needle biomass and leaf area index were significantly higher in 1 and 2-year-old needles. Total needle biomass and LAI values were very variable (average values of 4.89 t ha−1 and 1.64 m2 m−2, standard deviation 3.1 and 1.1 and CV of 64% and 69%, respectively) and were positively related to site productivity and foliar nutrients. The average litterfall rate was estimated to be 0.31 year−1. The mean growth efficiency (0.36 kg m−2 year−1) was also rather variable (CV = 50%) and was positively related to foliar K and needle longevity. The relationships between all variables associated with high levels of pine productivity and the mean shrub height were always significant and negative, indicating a reduction in the risk of ground fire.