Determining the timespan and ecological conditions necessary for afforested farmlands to function like old-growth forests: A view from the understorey

Background/Question/Methods

Resource extraction occasionally necessitates removal of mature natural woodlands without replacement onsite due to extreme landscape alterations. Regulators increasingly require creation of new forests to help offset such losses, but it is uncertain how well mitigation forests reflect the ecosystem goods and services of the original or how long development of sufficient functionality may take. Habitat provision for conservative understorey biodiversity is one essential service that may be easily lost, especially since most planted forests begin as homogeneous, low-diversity conifer plantations optimized for timber production. Here we assessed the capacity for afforested farmland in southern Ontario to develop habitat suitable for the understorey vegetation of mature hardwood forests. We crossed phytometer and chronosequence approaches (plantations were 30-90 years old) to determine how survival of conservative forest herbs Allium triccocum (wild leek) and Asarum canadense (wild ginger) following relocation from old-growth hardwood stands to conifer plantations depended on plantation age, the composition of trees planted, and the extent of managed stand-thinning which had occurred. Bare roots were transplanted either alone, alongside sterilized soil from the home environment, or alongside unsterilized home soil to test the importance of biotic and abiotic soil development in generating suitable habitat.

Results/Conclusions

Logistic regression showed survival of both phytometers was significantly related to plantation age and soil treatment but Asarum survival was additionally related to trees planted and thinning history. Phytometer translocations within the home forests set benchmarks against which survival in plantations were compared. From the perspective of Allium, a spring ephemeral, habitat conditions equivalent to those experienced within old-growth hardwood forests emerged in conifer plantations after 80 years, irrespective of stand-thinning or which trees were initially planted. From the perspective of Asarum, an all-summer species, old-growth habitat will emerge after 100 years in conifer-only plantations but much longer is needed for mixed hardwood-conifer plantations. Stand thinning accelerated this convergence in mixed conifer plantations but inhibited it monocultures. Responses to soil treatments indicate 10-15% of the time needed for Allium habitat to emerge was associated with development of suitable biotic soil properties while a similar proportion of the time needed for Asarum habitat depended on development of suitable abiotic soil properties. Collectively, these patterns suggest old-growth understorey conditions can be attained in mitigation forests within a century, though efforts to accelerate this process should perhaps focus more on assisting soil development than planting hardwoods or creating canopy gaps.