There is no doubt that scientists like to make predictions. How the forest floor changes over time is a thought-provoking ecological exercise that I frequently entertain. By using chronosequences, our predictions become more valid as the hypotheses we create are put to the test without building a time machine. In ecology, a chronosequence is a tool used to examine how the composition of living organisms’ change throughout time. For example, the Hawaiian archipelago are made up of 8 major islands with the largest and main island being the youngest. Over millennia, Hawaii has been formed as the Pacific plate slowly moves over a major hotspot, where magma is forced to the surface, creating islands. The oldest islands furthest away from the hotspot are made from the same parent material as the youngest ones, which makes this American archipelago a great chronosequence to gain some ecological insight. Over the past 50 years, scientists have flocked here to study how soil, communities of flora and fauna, as well geologic processes have changed over vast expanses of time. This chronosequence spans just under 5 million years. Depending on what you’re looking into, this amount of time is not required. A group of Scandinavian scientists used 10 sites of managed Pinus sylvestris stands that spanned just 158 years to tackle their fungal queries.
The Hawaiian archipelago is used as a chronosequence.
In 2016, Julia Kyaschenko and her team were interested in how the fungal community changes from clear-cutting, to forest establishment and development, all the way to forest maturity. Throughout Scandinavia and, well almost everywhere, forested areas are managed into even-age stands. Only in unperturbed, old-growth forests that were never logged do we see diverse assemblages of trees of various ages. For this reason, the 10 sites of various aged Pinus sylvestris stands are perfect for a chronosequence study, since a wide variation in tree diversity would bring convoluted species interactions. This study showed noteworthy trends which help paint a pretty ecological picture from underneath the forest floor.
In the boreal Scandinavian ecosystems studied here, the soil is made up by two distinct layers. The top layer called the litter layer, is made up of fresh organic material that recently fell off of the trees above. Saprobic fungi heavily inhabit these areas of the soil horizon, because this is where most of their nutrient resources reside. The lower, more decomposed humus layer is dominated by biotrophic mycorrhizal fungi that receive carbohydrates from their plant hosts. Although mycorrhizal fungi have a reduced ability to enzymatically break down organic material compared to their decomposer counterpart, many of these fungi use the energy given by their tree host to mobilize complex organic matter riddled throughout the humus layer. This process is increasingly important when soil nutrients become limited in older forest stands.
Pinus sylvestris boreal forest in Leivonmäki National Park Finland.
Initially, after clear-cutting, the amount of organic litter skyrockets. The now dead roots and other plant debris promotes saprobic species, as their resource pool is vastly deepened. Through the loss of plant hosts, the mycorrhizal fungi are nudged out of ecosystems that have been recently cut down. With plenty of woody debris to consume, saprobic fungi proliferate after the first decade of clear-cutting. Soon after clear-cutting, the saprobic fungi within these chronosequences are unperturbed by competition, and efficiently release digestive enzymes that rapidly reduce soil nutrients, leading to high net carbon losses. It is not too far down the road that new seedlings of Pinus sylvestris become established. With a new generation of trees gaining a foothold on a recently obliterated ecosystem, a new generation of mycorrhizal fungi germinates, and begins sequestering soil nutrients driven by a sugar reward.
Right after clear-cutting, there is almost a complete loss of mycorrhizal fungal richness. Once saplings become established, mycorrhizal species in the family Atheliaceae dominate the system because they disperse well and are adapted to high levels of nitrogen, outcompeting the later successional species that specialize in mobilizing scant soil nutrients. This research which is supported by other work explains that fungal richness in the hummus layer increases in the 60 years that follow clear-cutting. Though, given enough time, more soil nutrients become bound into large complex organic molecules that are largely inaccessible. Species from the genera Cortinarius and Russula tend to dominate older stands because it is these species that have evolved to exist in nutrient poor habitats through their enzymatic adaptations. Within the deeper layers of humus, fungal species richness in stands older than 60 years decreases, as these few genera outcompete other species.
Cortinarius sp. A later successional mycorrhizal fungus in boreal Scandinavian ecosystems.
Clear-cutting has only been around since man started exploiting the forest floor, so these ecological patterns are actually quite new to Earth’s scene. By utilizing extremely similar habitats dominated by Pinus sylvestris, these scientists worked out how the fungal community as well as their functioning changes through time after a clear-cutting disturbance. Once the trees are removed, these Scandinavian ecosystems lose almost all of their mycorrhizal species, and become dominated by voracious decomposers. Once young saplings establish, the deeper humus layer becomes colonized by mycorrhizal species adapted to a high nutrient load. For 60 years, the hummus layer increases in species diversity only to be reduced once nutrients become limited. The few enzymatically vigorous mycorrhizal species begin to dominate and outcompete others in more mature stands. Once again, we see that succession doesn’t take place by species getting along. The forest floor is a place of competition, that is ever changing due do intense land management practices by us humans. How does boreal forest return after a major human disturbance? Well, thanks to chronosequences, we have a pretty good idea.