Successional trends and apparent Acer saccharum regeneration failure in an oak-hickory forest in central Missouri, USA

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Permanent vegetation plots established in 1968 at the Baskett Research and Education Area (BREA) were remeasured in 2004/2005 to assess trends in canopy composition, vegetation–environment relationships, and regeneration patterns. Quercus spp. [particularly Quercus alba (white oak)] remained dominant in BREA forests. However, Nonmetric Multidimensional Scaling Ordination of a dataset composed of tree importance values from both samples indicated the two most common community types (Dry Ridge and Slope; Mesic Slope) were shifted away from Quercus spp. abundance toward greater importance of Acer saccharum (sugar maple) in ordination space. Plots in glade-like environments shifted toward greater Juniperus virginiana (eastern redcedar) importance. Environmental analysis indicated that soil properties (pH, organic matter, Ca, Mg, and K) and slope position were highly correlated with ordination axes. Saplings of Quercus spp. exhibited greatly reduced abundance in the most recent sample and the reduction in Quercus spp. density was highest in plots with greatest canopy cover. While Acer saccharum saplings were still quite abundant in BREA forests, smaller sapling size classes were becoming notably depleted, reflecting growth of a cohort of plants established before the 1968 sample toward and into tree-sized individuals. Seedlings surveys emphasized that A. saccharum seedlings have become very rare in BREA forests, while Quercus spp. seedling densities are higher, but likely below those needed for eventual adequate recruitment to the canopy. Although a number of mechanisms to explain apparent A. saccharum regeneration failure can be developed, herbivory, recent climate trends (warming late-winter conditions) and increased litter layer depth are the most plausible explanations for the observed scarcity of A. saccharum seedlings. Although increasing importance of A. saccharum in the canopy of BREA forests appears inevitable in coming decades, longer-term trends are unclear given the shortage of seedling regeneration.