Shoot senescence in herbaceous perennials of the temperate zone: Identifying drivers of senescence pace and shape

Abstract

1. Perennial herbaceous species form their above-ground parts every year anew and discard them before the advent of winter. The senescence of above-ground structures is thus an inevitable part of their life cycle. This is also a key process that determines photosynthetic gain late in the season and the economy of soil-borne nutrients.
2. Here we address patterns and drivers of the shoot senescence of perennial herbaceous plants. We present a comparative study of 231 temperate species, ranging from spring ephemeroids to species senescing in late autumn, in a common botanical garden collection. We assessed senescence by measuring size decline in the autumn part of the season.
3. There were two main directions of variation in senescence trajectories: the pace–date axis, separating early and fast senescing species from late and slowly senescing species, and the shape-asynchrony axis, separating species with accelerating and synchronised senescence from constant senescence asynchronous among individual shoots. While accelerating senescence late in the season can be due to passive effects of the environment (e.g. frost), accelerating senescence early in the season is likely to be an indication of an active process driven by the enzymatic activity of the plant.
4. The pace and shape of shoot senescence were associated with both leaf- and shoot-level traits. Species having leaves with high dry matter content senesced linearly and with higher asynchrony. Species with a larger specific leaf area senesced earlier and faster, while tall plants and plants with monocyclic shoots senesced later and in a more synchronous and accelerating manner.
5. Species from different habitats varied in their senescence patterns. Forest species postpone their senescence relative to open-habitat species, presumably to boost their photosynthetic balance. We did not confirm the hypothesis that plants from nutrient-poor habitats senesce earlier to retain soil-borne nutrients before the winter.
6. Synthesis. Shoot senescence in herbaceous plants is a neglected phenomenon in its own right, which bears only superficial similarity to autumn leaf shedding in trees. Individual species differ strongly in the pace, shape and synchrony of their senescence trajectories, with a potential bearing on the carbon and nutrient dynamics of their habitats.