Cg. Lorimer et al., TALL UNDERSTOREY VEGETATION AS A FACTOR IN THE POOR DEVELOPMENT OF OAK SEEDLINGS BENEATH MATURE STANDS, Journal of Ecology, 82(2), 1994, pp. 227-237
1 Replacement of oaks (Quercus spp.) by other species after natural an
d human caused disturbance is recognized as a common problem on averag
e and productive sites in many parts of eastern North America. Oak see
dlings are often numerous beneath mature stands, but seedlings are usu
ally too small to compete effectively with tall saplings of other spec
ies. Reasons for the poor development of oak seedlings are not well un
derstood. 2 An experiment was designed to evaluate the impact of tall
and low understorey vegetation on oak seedling development beneath mat
ure stands on two sites in southwestern Wisconsin. Understorey stems t
aller than 1.5 m and scattered small canopy trees were removed on half
of the main plots, with the other plots retained as controls. Half of
the nested subplots within each main plot were also sprayed with foli
ar herbicide to reduce the density of vegetation shorter than 1.5 m. T
he development of planted Quercus rubra and natural Q. rubra and Q. al
ba seedlings was monitored for 5 years. 3 On undisturbed control plots
, more than 70% of the planted oak seedlings died within 5 years, and
survivors showed a net decrease in height. On plots with the tall unde
rstorey vegetation removed, more than 90% of the planted seedlings sur
vived and average total height increased 50-96%. 4 Understorey removal
plots had 10-140 times as many natural oak seedlings after 5 years as
undisturbed plots. Height growth of natural oak seedlings under heavy
partial shade (87% crown cover) was slow, however, averaging 4-6 cm/y
ear. 5 Results suggest that tall understorey trees of Acer, Ostrya, Ti
lia, and other species are a major obstacle to the development of oak
seedlings. Disturbances such as fire that reduce this understorey laye
r can probably improve the prospects that oak will be self-perpetuatin
g, but development of competitive natural oak seedlings is a slow proc
ess that may take several decades.