Varying harvest gap size may do little to increase tree diversity in northern hardwood forest constrained by low tree seedling diversity

Background/Question/Methods

Increasing evidence of low tree regeneration density and diversity in harvest gaps in some selection-managed northern hardwood forests (NHF) suggests the sustainability and resilience of these forests are threatened.  We are investigating the multiple possible causes of these patterns with stand-scale experiments in NHF dominated by Acer saccharum (hereafter As, with similar acronyms for other species) with large components of Fraxinus americana, Fagus grandifolia and Tilia americana, in Michigan, USA. Via quantifying the relative impact of multiple factors on the dynamics of natural and planted tree regeneration, we intend to identify management strategies for increasing tree diversity and density. Here, we report the impacts of harvest gap size (n= 45, 0- 955 m²),  soil resources, fencing (white-tailed deer excluded), and advanced regeneration removal on natural tree regeneration characteristics measured in sub-plots within 10 x 10 m plots in each harvest gap over six years following the creation of gaps. Three classes of tree regeneration are discussed: ADVANCED  (established before harvest gap creation),  GAP ORIGIN (established from seed the first three years following gap creation), and DOMINANT (tallest individuals of either GAP ORIGIN or ADVANCED in several 1 x 1 m subplots per gap six years following gap creation).   

Results/Conclusions

Abundant ADVANCED regeneration dominated the DOMINANT class in gaps where it wasn’t removed, with larger individuals mostly Ostrya virginiana and Fg.  In ADVANCED-removed gaps, 96% of GAP ORIGIN seedlings (2007-2009 summed) were As, Fa, and Prunus pensylvanica (214,000/ ha). GAP ORIGIN-DOMINANT individuals were approximately 55% Fa and 15% each of Pp, As and Others. However, after six years many DOMINANT individuals were still shorter than the maximum height of competing vegetation and deer’s reach (1.5 m). In 2012, the 2007 germination cohort dominated DOMINANT Fa (tallest cohort, high survival) and As (large cohort), whereas the 2009 cohort dominated DOMINANT Pp stems (large cohort, low survival in older cohorts).  Factors affecting the density (+/-) of DOMINANT stems were:  gap size Pp+, As+; deer presence, Pp-, Fa+; soil water, Pp-, Fa+.  Thus, for all regeneration classes, density was high and diversity low.  Increasing diversity in NHF may depend on removing low-diversity advanced regeneration and/or enhancing local seed and/or germinant diversity as a prerequisite to realizing the effects of other factors (e.g. gap size) on diversity. High Fa regeneration density and positive deer effects on Fa suggest future management challenges for NHF with a Fa component, emerald ash borer, and high deer populations.