Stand dynamics over 18 years in a southern mixed hardwood forest, Texas, USA

Abstract

Summary 1 Using 18 years of permanent‐plot data from a 4‐ha stand, we investigated whether stand dynamics is predictable, i.e. driven by competition (inferred from species shade tolerances), or unpredictable (driven by frequent exogenous disturbance or other factors). We also considered whether small disturbance might accelerate or retard succession. The study involved dynamics of stems ≥ 4.5 cm diameter at breast height (d.b.h.) for 10 important species in a southern mixed hardwood forest 80 years after selective removal of Pinus taeda. 2 Shade‐tolerant species (Ilex opaca, Fagus grandifolia, Magnolia grandiflora, Acer rubrum, Nyssa sylvatica) increased in small d.b.h. classes, whereas intermediate (Quercus alba) and shade‐intolerant species (Q. nigra, Q. hemisphaerica, Liquidambar styraciflua, Pinus taeda) did not. 3 Per‐capita ingrowth was higher for shade‐tolerant species (0.016–0.024) than for intermediate and shade‐intolerant species (0–0.006), whereas mortality was lower ((0.006–0.015 vs. 0.019–0.035). 4 Over 18 years, ingrowth exceeded mortality loss slightly, resulting in an increase in number of stems of about 1%, from 995 to 1029 stems ha−1; average live basal area (BA; 35.9 m2 ha−1) varied by less than 2% over eight surveys spanning 18 years. Basal area increased substantially for three species (Ilex opaca, Pinus taeda, Acer rubrum), and declined strongly for three others (Quercus nigra, Q. hemisphaerica and Fagus grandifolia). 5 The stand d.b.h. distribution had a rotated sigmoid form which became flatter over time. Species d.b.h. distributions were mostly bell‐shaped for intolerant species, and irregular‐to‐monotonic declining for shade‐tolerant species. Over time, bell‐shaped distributions moved to the right and irregular distributions remained irregular. 6 A hurricane in 1986 caused loss of 4.5% of standing BA. Compositional change was most rapid during the hurricane interval and ingrowth was highest in the two intervals following the hurricane. Effects on species trends in density, basal area, mortality or recruitment were minor. 7 Temporal trends in species abundances mostly supported the hypothesis of predictable successional change related to competition for light. However, d.b.h. distributions of shade‐tolerant species and rapid declines in BA of Quercus nigra and Fagus indicated that other processes are also important and will limit predictability of change.