The more stems the merrier: advantages of multi‐stemmed architecture for the demography of understorey trees in a temperate broadleaf woodland

1. Multi‐stemmed trees are an understudied but common component of temperate broadleaf forests that can provide insight into how plants persist and regenerate within communities. In particular, multi‐stemmed architecture may be an important trait for the growth and survival of trees in forest understoreys that have to cope with low light levels, but this idea has been rarely tested using long‐term individual‐level data. 2. We use measurements of 8527 individual woody stems from 1985, 1996 and 2008 to model the growth, survival and recruitment of hazel (Corylus avellana) and hawthorn (Crataegus laevigata and C. monogyna) trees as a function of neighbourhood competition in the understorey of a minimum‐intervention, mixed‐ash woodland in England. We test the effects of browsing by deer (Muntiacus reevesi) on woodland dynamics by comparing demographic rates during a period of high deer densities (1985–1996) with rates recorded from 1996 to 2008, when sustained culling substantially reduced deer densities. 3. Growth and survival of hazel and hawthorn trees increased with the number of stems they possessed, demonstrating clear benefits to multi‐stemmed architecture. Surviving trees continued to accumulate basal area and stems without any clear upper limit after 23 years. However, increasing numbers of stems generally reduced the growth and recruitment of stems within multi‐stemmed trees. 4. Temporal differences in deer browsing appeared to influence the strength of intra‐specific (i.e. inter‐stem) competition and, consequently, the growth, survival and recruitment of multi‐stemmed trees. Most notably, stem survival declined with the number of stems in a tree only after deer culling, but not during a period of high deer densities, suggesting that intense deer browsing reduced resource competition among stems. Inter‐specific neighbourhood competition had no detectable effect on hazel or hawthorn demography. 5. Synthesis. Multi‐stemmed architecture is an advantageous trait for understorey trees in temperate woodlands relative to the allocation of resources towards the growth of a single stem. We suggest that the low light levels of forest understoreys favour ‘persistence’, through multi‐stemmed growth, rather than ‘regeneration’ niches (i.e. periodic recruitment through seed), with the advantage of this life‐history strategy influenced by herbivory and intra‐specific competition.