Being John Harper: Using evolutionary ideas to improve understanding of global patterns in plant traits

This review summarizes current understanding of five key plant traits: seed mass, plant height, wood density, leaf mass per unit area and leaf size, emphasizing ways in which our understanding of large-scale patterns in plant traits have improved over the last two decades. Notable advances include: (1) large-seeded species have greater seed dispersal distances than do small-seeded species, (2) leaf mass per unit area is not strongly or consistently related to plant traits outside the leaf economics spectrum, or to broad gradients in environmental conditions, and (3) fleshy fruit could not have first evolved for seed dispersal, as the first fleshy fruit appeared millions of years before the first potential seed dispersers. While quantifying large-scale patterns in plant traits has yielded many important discoveries, it is clear that the next major leap in understanding will not come from simply including ever more variables in our analyses. I suggest that we build upon Harper's Darwinian approach to plant ecology and apply evolutionary ideas to large-scale trait ecology. For example, quantifying trait impacts on lifetime fitness rather than on particular stages of plant regeneration can allow us to understand the coordination between seemingly disparate traits. I use this approach to bring seed mass and plant height together as integrated parts of a species' life-history spectrum. I then point out problems associated with the implicit assumption that selection acts on species' mean trait values and show how considering the way selection acts can improve our understanding of the effects of climate on plant traits. A goal for the future is to quantify the full suite of biotic and abiotic factors that shape plant strategy in complex, real-world situations.Synthesis. Enormous data availability and ever more powerful computational and statistical tools have given ecologists unprecedented power to quantify large-scale patterns in plant ecology. However, there is a limit to how far big data alone can take us. The time is ripe for a new generation of hypotheses and ecological theory built on strong evolutionary foundations. Let the creativity begin!