Thirty eight years of autogenic, woody understory dynamics in a mature, temperate pine-oak forest

In 1935, 32 ha of a pine-hardwood forest were set aside from future timber management in southern Arkansas, U.S.A. Old-growth timber had been cut to a 36-cm stump diameter before 1915. Between 1952 and 1993, four inventories were made of the overstory and midstory components (number of live trees greater than or equal to 9 cm in diameter breast height (DBH) taken at 1.37 m, by 2.54-cm DBH classes). Between 1954 and 1992, four corresponding inventories were made of the woody vegetation (>15 cm tall but <9 cm DBH) by counting rootstocks within 52 systematically spaced 8-m(2) circular quadrats. Understory species importance values (relative density + relative height + relative frequency) were used to compute diversity and similarity indices. During 38 years of assessment, no catastrophic disturbances occurred within the 32-ha forest. Although loblolly pine (Pinus taeda L.) and shortleaf pine (Pinus echinata Mill.) dominated the overstory (64% of basal area in 1954 and 63% in 1993), pines were absent from the understory in all but seedling size classes for the last 38 years. Woody understory diversity indices were essentially stable for 38 years, but the similarity of understory species tended to decline as the time between inventories increased. Relative importance values for woody understory species tended to increase for the more shade-tolerant genera and decrease for less tolerant genera. Survival and height growth of woody understory species were also found to be positively correlated with shade tolerance.