Photosynthesis, growth, and survival of native and invasive woody species: Trade-offs and plasticity across open and understory environments
Non-native, invasive plants often exhibit high photosynthesis and growth rates relative to native species. Growth in open environments typically varies inversely with understory survival, however. Do invasive and native species achieve growth by similar means? Are they subject to similar trade-offs between growth and survival? We compared photosynthesis, growth, and survival of two invasive trees (Acer platanoides and Ailanthus altissima ) and two invasive shrubs (Rhamnus frangula L. = Frangula alnus Miller and Elaeagnus umbellata), each paired with a native species of similar life form and shade tolerance. Seedlings were planted in the field, with and without exclosures, in open and understory environments.
Invaders exhibited greater light-saturated photosynthesis (Pmax) and leaf N concentration than natives, and greater plasticity in leaf N across environments. Net assimilation rates (NAR) in the open were greater in invaders and closely correlated with Pmax. The high NAR of invaders did not produce more rapid growth, however, because invasive shrubs had lower leaf area and leaf biomass ratios than natives. Natives and invaders thus achieved similar growth rates, but by different means.
In general, similar trade-offs constrained native and invasive species. Growth in the open was negatively correlated with understory survival, but this relationship did not differ between natives and invaders. The effects of herbivory on growth and survival were not consistently greater in natives, although the invader E. umbellata was notably unaffected by browsing. Invaders exhibited greater Pmax than natives of similar dark respiration or leaf N in the understory, but not in the open.
In our initial experiment, leaf N and leaf mass per area were inversely correlated among species, but often positively correlated across canopy treatments within species, implying that intraspecific plasticity may not be subject to the same trade-offs as interspecific adaptation. In further experiments, we measured leaf-level responses to nitrogen manipulation within existing populations of R. frangula. In contrast with interspecific patterns, leaf mass per area (LMA) and leaf longevity increased with leaf N in the open, and were uncorrelated with leaf N in the understory. Thus, acclimation within this species differed qualitatively from interspecific patterns of adaptation to N availability.