Abstract
Large, trees have survived for a long time, and their complex crown structure can serve as habitat for epiphytic plants. Canopy plants are not as well studied in the temperate zones as in the tropics, because many of them are accidental epiphytes, epiphytic individuals of normally terrestrial species. We hypothesized that the canopy can serve as a refuge for terrestrial species that have difficulty establishing on the ground (e.g., insufficient light, deer over-browsing), promoting and conserving forest species diversity. Terrestrial species may also vary in their ability to adapt to a wide range of canopy growth conditions. Here, we investigated canopy vascular plants hosted on a large Cercidiphyllum japonicum tree in a temperate old-growth forest, Japan. The canopy plant community was diverse comprising 39 vascular plant species (6 of which were threatened species), including 31 accidental epiphytes and 8 obligate epiphytes. We found at least 14 species could escape deer over-browsing by establishing in the canopy. Most accidental epiphytes were found on multiple and large reiterated trunks bifurcated from the main trunk as well as on large horizontal branches. Leaves of canopy plants had higher nitrogen concentration compared to those of plants on the ground, and higher water-use efficiency with increasing height was achieved by controlling transpirational water loss. Our results show one large tree crown can be defined as a local hotspot for current and future plant species diversity in a temperate old-growth forest, reinforcing its ecological value for conservation purposes.
Highlights
Large old trees in old-growth forests, among the biggest and most long-lived organisms on earth, are important for their rarity and limited spatial distribution in the current global environment (Lindenmayer et al 2012)
Our results show that various vascular plants utilize a single large host tree as their habitat while acclimating to the unique canopy environment, suggesting one large crown can function as a local hotspot for plant species diversity in a forest ecosystem
In addition to maintaining habitat for endemic epiphytes including endangered species that can only survive in the crown of large, old trees, the many accidental epiphyte species found temporary refuge from deer browsing in our study tree
Summary
Large old trees in old-growth forests, among the biggest and most long-lived organisms on earth, are important for their rarity and limited spatial distribution in the current global environment (Lindenmayer et al 2012). In tropical montane or rain forests, vascular epiphytes especially orchids and bromeliads are dominant and important for species richness, as well as for their role in water and nutrient cycling of forest ecosystems In temperate forests of the Pacific Northwest Coast of North America, high diversity of epiphytes especially ferns, bryophytes and lichens were documented in the crowns of large trees in old-growth conifer forests by directly accessing the canopy using rope-climbing techniques In boreal forests, epiphytic lichens on large, old trees greatly contribute to increasing local biodiversity and nutrient cycling, facilitating establishment by other organisms In boreal forests, epiphytic lichens on large, old trees greatly contribute to increasing local biodiversity and nutrient cycling, facilitating establishment by other organisms (e.g. Essen et al 1996; Lie et al 2009)
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