SPATIAL AND TEMPORAL ABIOTIC CHANGES ALONG A CANOPY TO INTERCANOPY GRADIENT IN CENTRAL TEXAS JUNIPERUS ASHEI WOODLANDS

摘要

Juniperus ashei (ashe juniper), in the southern Edwards Plateau region of central Texas, exhibits both spatial and temporal trends in seedling demography, emergence, growth and physiology which vary in relation to patterns of woodland overstory: the canopy patches of woody plants vs. the intercanopy patches of grasses and herbs between them. This study reports gradients of abiotic factors found from below J. ashei canopy trees into associated intercanopy patches. There were significant differences in soil organic content, soil field capacity, soil temperature, soil water content and surface light levels along this gradient from April through December 1997, but not soil depth. Mean soil organic content was highest under the canopy (32.0 +- 6.9%) and lowest in the intercanopy patch (12.5 +-0.8%) as was the field capacity (108.5 +- 2.8% and 82.9 +- 1.6% respectively). Mean midday light levels were highest in the intercanopy (1183 +- 149 μmol·m~(-2)·s~(-1)) and were lowest below the canopy (346 +- 99 μmol·m~(-2)·s~(-1) and 219 +- 77 μmol·m~(-2)·s~(-1), canopy and mid-canopy respectively). Mean midday soil temperature varied seasonally, but was highest in the intercanopy (32.6 +- 2.1℃) and lowest at the canopy edge (27.6 +- 1.4℃). Mean soil water content also varied seasonally (with rainfall), and was highest under the canopy (43.4 +- 3.0%) and lowest in the intercanopy (30.3 +- 2.1%). Reduced light levels under the canopy, coupled with high soil organic content may ameliorate high soil temperatures and promote higher soil water content, possibly resulting in reduced water stress and increased J. ashei seedling survival. However, increased growth at the canopy edge may be attributed to increased surface light levels at this location. Low seedling emergence and survival in the intercanopy patch may be due to a combination of factors, in particular seasonal high soil surface temperatures and low soil water content.