Structure of Isolated <i>Acer grandidentatum</i> (Bigtooth Maple) Communities and Potential Population Changes

Abstract

Isolated, relic populations of <i> Acer grandidentatum </i> (bigtooth maple, Sapindaceae) were found in deep canyons in central Texas. Associated with them on hilltops were communities of <i> Juniperus ashei </i> (ash juniper). We determined the structure of relic <i> A. grandidentatum </i> populations and compared them to adjacent <i> J. ashei </i> populations. The overstory and understory density was counted and basal area was determined for all woody species in both communities using the quadrat procedure. <i> Acer grandidentatum </i> was the highest density (788 ± 964 or 52%) and basal area (29 ± 35 or 52%) overstory species in the canyon communities while <i> J. ashei </i> had the highest density (1589 ± 146 or 92%) and basal area (51 ± 13 or 88%) in the hilltop communities. In the understory of the canyon communities, there were five juvenile tree species including <i> A. grandidentatum </i> which had the fourth highest density (13% or 176 ± 110 plants/ha). In the hilltop communities, <i> J. ashei </i> was the high density understory tree species (52% or 994 ± 400 plants/ha). There were no <i> A. Grandidentatum </i> overstory or understory plants in the hilltop <i> Juniperus </i> communities. <i> Acer grandidentatum </i> had an inverse quadratic size distribution with few juveniles. Although we counted a number of seedlings, there was little recruitment over the last 16 years and very little potential recruitment in the previous 20 years. <i> Juniperus ashei </i> had an inverse exponential size distribution with many understory plants and considerable recruitment. Weibull age/size distributions showed one <i> A. grandidentatum </i> established in the 1670’s and 2 in the 1760’s, with peak establishment in the 1970’s. Causes of the lack of recent recruitment are uncertain, but it could be episodic due to changes in herbivory, management or other environmental conditions.