Above- and below-ground competition effects of two heathland species: implications for growth and response to herbivory in birch saplings

We examined experimentally the effect of competition from two common heathland plant species (Calluna vulgaris or Molinia caerulea) on Betula pubescens saplings subjected to simulated mammalian browsing damage. We tested two hypotheses: that B. pubescens saplings alter their growth allocation in response to different patterns of competition from the two species in order to maximise resource acquisition; and that, when only B. pubescens saplings are damaged, herbivory reduces its ability to compete with both species. In an ex-situ experiment we grew B. pubescens saplings in the presence of below- or above- and below-ground interactions from C. vulgaris or M. caerulea. Saplings were also subjected to simulated browsing by clipping (50% of current year's growth), either pre-senescence or at bud-burst. We measured the morphology and dry mass allocation response of the saplings over a period of two years. We found that competition reduced sapling dry mass by approximately 50%, but C. vulgaris reduced dry mass to a greater extent than did M. caerulea. The total competition intensity of C. vulgaris was greater than that of M. caerulea, due to an apparent facilitative effect of M. caerulea shoots on birch growth. Saplings compensated for browsing damage, resulting in no difference in dry mass one year after damage. However, sapling morphological responses to browsing damage were dependent on the competing species. Despite the large competitive effect of below-ground interactions, saplings did not increase allocation to root growth as predicted. Additionally, in response to above-ground interactions from M. caeruela, saplings increased allocation to root growth. This study highlights the importance of patterns, as well as intensity, of competition in determining plant responses to inter-specific interactions. Sapling responses did not follow the pattern predicted by the balanced-growth hypothesis. We suggest that this is due to the multi-functionality of plant component parts and the balance between competitive and facilitative effects of interacting plants

How do different competing species influence the response of Betula pubescens Ehrh. to browsing?

When attempting to expand existing woodland through natural regeneration, herbivory and competition from the existing vegetation may impede the regenerating saplings. This work addresses how browsing and competition with other vegetation interact to drive sapling growth and morphology of the widespread tree species B. pubescens. We took above-ground morphological measurements of B. pubescens saplings within an intimate mosaic of Calluna vulgaris and Molinia caerulea, comparing saplings growing with each of the two plant species under three different red deer (Cervus elaphus) densities, allowing comparison of different levels of both past and present levels of browsing damage. Saplings growing in M. caerulea dominated vegetation responded to reduced browsing with faster growth than those growing in C. vulgaris dominated vegetation. However, we found that when natural browsing levels were high, browsing masked any differences in inter-specific interactions between plant species. We propose that, in regeneration schemes where deer densities are reduced, these differences should confer a competitive advantage to saplings growing with M. caerulea over those growing with C. vulgaris. Additionally, our results highlight the importance of browsing history, rather than just current browsing levels, in determining sapling growth responses under different herbivore management regimes. This study highlights the importance of multi-factor interactions in determining plant growth and morphology under different conditions. In particular we identify the prevalence of interactions between competition, herbivory and time, as determining the potential growth and morphology of B. pubescens saplings in regeneration areas. This has important implications for the management of sites where browsing impedes the natural regeneration of trees and shrubs, or where herbivore densities have been reduced to encourage woodland regeneration

Do competition and herbivory alter the internal nitrogen dynamics of birch saplings?

• Deciduous trees recycle nitrogen (N) within their tissues. The aim of this study was to test the hypothesis that reductions in plant growth, due to competition and herbivory, reduce the sink strength for N during autumn nutrient withdrawal and reduce the storage capacity and hence the amount of N remobilised in the following spring. • We used 15N labelled fertiliser to quantify N uptake, leaf-N withdrawal and remobilisation. Betula pubescens saplings were grown with either Molinia caerulea or Calluna vulgaris, and subjected to simulated browsing damage. • Competition reduced B. pubescens leaf-N withdrawal and remobilisation, with C. vulgaris having a greater effect than M. caerulea. However, simulated browsing had no significant effect on sapling N dynamics. The patterns of leaf-N withdrawal and remobilisation closely followed sapling dry mass. • We conclude that the effect of competition on sapling mass reduces their N storage capacity. This reduces sink-strength for leaf-N withdrawal and the source-strength for remobilised N. The ability of saplings to compensate for browsing damage removed any potential effect of browsing on N dynamics