Soil fertility induces coordinated responses of multiple independent functional traits

Abstract

Summary A central goal of functional ecology is to determine how independent functional traits are selectively filtered by environmental conditions to improve our understanding of the mechanisms of community assembly. Soil fertility clearly influences community composition, but it is unclear which plant functional traits are most strongly associated with gradients of increasing nutrient limitation. We hypothesized that leaf economic traits and stem tissue density would be strongly associated with soil fertility given their direct relationship to soil resource acquisition and use. In contrast, we hypothesized that functional traits that are commonly associated with competition for light (maximum height), shade tolerance (seed mass) and resistance to disturbance (bark thickness) would be unrelated to soil fertility. We measured 13 functional traits from 30 tree species occurring in 40 plots across a soil fertility gradient in a mature warm temperate rain forest in Northland, New Zealand. Principal component analysis was used to assess the dimensionality and independence of the functional traits and the soil properties, and regression was used to determine the relationships between community‐weighted mean traits and the soil fertility gradient. We observed a coordinated response of multiple independent traits to soil fertility. Consistent with our hypothesis, species associated with low‐fertility soils had comparatively ‘slower’ leaves (i.e. low SLA and leaf N and P, and high LDMC and thickness) and higher stem tissue density than species associated with high‐fertility soils. Unexpectedly, we observed that species associated with low‐fertility soils had larger maximum heights, thicker bark and lower seed mass. Tall trees can persist on poor soils. Thick bark may be a defensive strategy for trees growing in resource‐limited sites and large‐seeded shade‐tolerant species can persist in fertile soils where light is more limiting. Synthesis. Species sorting can occur over short distances in ecosystems where topographically driven variation in soil fertility leads to complete compositional turnover. Inferences about species distributions based on single‐trait spectrums can be misleading when environmental gradients sort species by filtering multiple independent traits simultaneously. Identifying the multidimensional trait combinations that promote fitness will advance both theory development and ecological restoration.