A leaf-height-seed (LHS) plant ecology strategy scheme

Abstract

A leaf-height-seed (LHS) plant ecology strategy scheme is proposed. The axes would be specific leaf area SLA (light-capturing area deployed per dry mass allocated), height of the plant's canopy at maturity, and seed mass. All axes would be log-scaled. The strategy of a species would be described by its position in the volume formed by the three axes. The advantages of the LHS scheme can be understood by comparing it to Grime's CSR scheme, which has Competitors, Stress-tolerators and Ruderals at the corners of a triangle. The CSR triangle is widely cited as expressing important strategic variation between species. The C-S axis reflects variation in responsiveness to opportunities for rapid growth; in the LHS scheme, SLA reflects the same type of variation. The R axis reflects coping with disturbance; in the LHS scheme, height and seed mass reflect separate aspects of coping with disturbance. A plant ecology strategy scheme that permitted any species worldwide to be readily positioned within the scheme could bring substantial benefits for improved meta-analysis of experimental results, for placing detailed ecophysiology in context, and for coping with questions posed by global change. In the CSR triangle the axes are defined by reference to concepts, there is no simple protocol for positioning species beyond the reference datasets within the scheme, and consequently benefits of worldwide comparison have not materialized. LHS does permit any vascular land plant species to be positioned within the scheme, without time-consuming measurement of metabolic rates or of field performance relative to other species. The merits of the LHS scheme reside (it is argued) in this potential for worldwide comparison, more than in superior explanatory power within any particular vegetation region. The LHS scheme avoids also two other difficulties with the CSR scheme: (a) It does not prejudge that there are no viable strategies under high stress and high disturbance (the missing quadrant in the CSR triangle compared to a two-axis rectangle); (b) It separates out two distinct aspects of the response to disturbance, height at maturity expressing the amount of growth attempted between disturbances, and seed mass (inverse of seed output per unit reproductive effort) expressing the capacity to colonize growth opportunities at a distance. The advantage of LHS axes defined through a single readily-measured variable needs to be weighed against the disadvantage that single plant traits may not capture as much strategy variation as CSR's multi-trait axes. It is argued that the benefits of potential worldwide comparison do actually outweigh any decrease in the proportion of meaningful variation between species that is captured. Further, the LHS scheme opens the path to quantifying what proportion of variation in any other ecologically-relevant trait is correlated with the LHS axes. This quantification could help us to move forward from unprofitable debates of the past 30 years, where CSR opponents have emphasized patterns that were not accommodated within the scheme, while CSR proponents have emphasized patterns that the scheme did account for.