Influence of canopy openness, ungulate exclosure, and low‐intensity fire for improved oak regeneration in temperate Europe

Abstract

Failed oak regeneration is widely reported in temperate forests and has been linked in part to changed disturbance regimes and land‐use. We investigated if the North American fire–oak hypothesis could be applicable to temperate European oaks (Quercus robur, Quercus petraea) using a replicated field experiment with contrasting canopy openness, protection against ungulate browsing (fencing/no fencing), and low‐intensity surface fire (burn/no burn). Survival, relative height growth (RGRH), browsing damage on naturally regenerated oaks (≤300 cm tall), and changes in competing woody vegetation were monitored over three years. Greater light availability in canopy gaps increased oak RGRH (p = .034) and tended to increase survival (p = .092). There was also a trend that protection from browsing positively affected RGRH (p = .058) and survival (p = .059). Burning reduced survival (p < .001), nonetheless, survival rates were relatively high across treatment combinations at the end of the experiment (54%–92%). Most oaks receiving fire were top‐killed and survived by producing new sprouts; therefore, RGRH in burned plots became strongly negative the first year. Thereafter, RGRH was greater in burned plots (p = .002). Burning altered the patterns of ungulate browsing frequency on oaks. Overall, browsing frequency was greater during winter; however, in recently burned plots summer browsing was prominent. Burning did not change relative density of oaks, but it had a clear effect on competing woody vegetation as it reduced the number of individuals (p < .001) and their heights (p < .001). Our results suggest that young, temperate European oaks may respond similarly to fire as their North American congeners. However, disturbance from a single low‐intensity fire may not be sufficient to ensure a persistent competitive advantage—multiple fires and canopy thinning to increase light availability may be needed. Further research investigating long‐term fire effects on oaks of various ages, species‐specific response of competitors and implications for biodiversity conservation is needed.

The influence of fencing on seedling establishment during reforestation of oak stands: a comparison of artificial and natural regeneration techniques including costs

“…This is in line with several other studies on the subject in Europe and elsewhere (e.g. Jensen et al 2012, Churki et al 2016, Dey et al 2019, Ramirez et al 2020, Petersson et al 2020, Barrere et al 2021). In the long run, however, the protective effect from fences can be complicated by competition from other woody vegetation (Leonardsson et al 2015).…”

“…In addition, and in line with previous research in the same region (Petersson et al 2020), more seedlings in our study seemed to have been browsed during the winter and spring (included in the May measurements) than in the summer and autumn (included in the October measurements). Short et al (1975) found that the digestibility and nutrient content are optimal during the early spring, which might explain why deer preferentially browse deciduous shoots at that time.…”

“…The mean basal area was 12 m 2 ha −1 at the end of the study. This corresponds to ca 42% of above canopy light in oak forests according to Petersson et al (2020). In the beginning of the experiment in 2014, the ground vegetation was already dominated by a dense layer of herbaceous vegetation.…”

“…abundant light and natural or anthropogenic disturbances that maintain such habitats (Vera 2000;Bobiec et al 2018;Petersson et al 2020). Furthermore, the recruitment strategy of light-demanding temperate tree species such as oaks, growing in open habitats in the presence of thorny shrubs, may be viewed as an adaptation to now extinct large browsers and grazers (Bakker et al 2004).…”

Sympatric genome size variation and hybridization of four oak species as determined by flow cytometry genome size variation and hybridization

“…Quercus L. is a genus that contains many economically and ecologically important tree species found in the northern hemisphere (Aldrich & Cavender‐Bares, 2011). From an evolutionary point of view, Quercus is a good material for studying the species boundaries and adaptive evolution (Porth et al., 2016; Yuan et al., 2018; Manuel et al, 2020; Petersson et al., 2020). Quercus is famous for its remarkable natural hybridization, which makes it difficult to identify species owing to the formation of hybrids (Song et al., 2015).…”