Andrew Cortese, from Michigan State University, discusses his article: Ectomycorrhizal tree islands in arbuscular mycorrhizal forests: hotspots of fungal inoculum important for seedling establishment of historically dominant trees
Present-day forests echo past disturbance history
Northeastern North American forests are comprised of a mixture of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) trees. However, by the turn of the 20th century, about half of the original forest cover was cleared and converted to agriculture from European settlers. With the onset of the Great Depression in the 1930s, many of these farms were abandoned, which initiated old-field succession that developed into secondary forests present throughout the region today. Many of these forests shifted from EM to AM tree dominance, partly because of agricultural land use legacies, among other factors, ultimately limiting their functional diversity and ability to store carbon.
EM patches are inoculum sources for establishing seedlings
Seedling establishment is a critical stage in trees’ life cycles, which can be hampered by a lack of compatible mycorrhizal fungi. Past research under primary and secondary succession has shown that EM fungi are often restricted to patches of EM vegetation. EM tree seedling establishment may be enhanced near these patches through access to EM fungal inoculum, in addition to established EM fungal mycelial networks. However, the influence of EM patches on fungal associations and performance of tree seedlings in AM-tree dominated secondary forests remains largely unexplored.
A secondary forest stand at the Mianus River Gorge, NY, USA dominated by AM Acer saccharum (right). The lack of pit and mound topography, coarse woody debris, or native herbaceous understory plants, single age class of trees, and a stone wall, are all indications of past agricultural land use. A nearby old-growth forest is mostly dominated by EM trees, including multiple age classes of Tsuga canadensis and Fagus grandifolia, with coarse woody debris, pit and mound topography, and abundant native herbaceous understory plants (left). Photo by A. Cortese.
Investigating the role of EM patches and in EM tree seedling establishment
We investigated whether EM fungi are widespread or restricted to scattered EM trees in AM tree-dominated secondary forests, and whether seedling establishment of historically dominant EM trees, such as Tsuga canadensis, Pinus strobus, or Quercus species, may be influenced by proximity to existing EM trees. We planted these seedlings next to scattered EM Betula species or the dominant AM Acer species and inoculated some of the seedlings at each focal tree with soil from local EM tree-dominated forests.
Proximity to EM patches may enhance EM tree seedling establishment
We found that seedling growth was enhanced through proximity to EM trees and inoculation with EM forest soil. Both positively influenced the second-year height growth of Tsuga and Pinus seedlings, while soil inoculum alone enhanced Quercus shoot mass. However, we suspect that seedling growth responses were muted, at least in part, from competition from the overstory dominant AM Acer trees and low light levels in most of our plots. We found more pronounced responses for Tsuga and Pinus seedlings in a related growth chamber experiment.
In my opinion, one of the most interesting findings came from examining the life history of the EM fungi colonizing seedlings planted near AM Acer trees. Some EM fungi like Tuber, Hymenogaster, and Suillus species, can be spread by mammals and form resistant spore banks (like a seed bank) which may persist for many years and serve as important inoculum for EM tree seedlings following a disturbance. However, the sparse levels of EM colonization of seedlings planted near AM Acer suggest that even in the over 80 years since agricultural land abandonment, spore banks have yet to fully accumulate in many secondary forests. It is in these locations, with a paucity of native EM fungal propagules, where soil inoculum appears to be the most effective!
Many seedlings planted near EM Betula were colonized by Amanita, Russula, and Cortinarius species, which usually cannot form EM with seedlings via spore germinants. These fungi probably colonized seedlings through EM fungal mycelium associated with focal EM Betula trees. Of course, the next question is: how did the EM Betula establish in these forests? We think that their highly dispersible seeds and ability to persist as seedlings without EM fungal inoculum may have enabled this, with new EM fungi slowly accumulating as the trees matured through spore dispersal from nearby residual EM forest patches.
Main takeaways
Our findings suggest that a lack of EM fungal inoculum is a cryptic and pervasive land use legacy across many secondary forests in northeastern North America. This, in conjunction with seed dispersal limitations and other factors, may hinder seedling establishment of EM trees outside of established EM patches. However, local soil inoculum can be an effective restoration tool to enhance establishment of planted EM tree seedlings in areas devoid of existing EM trees.
