Short-Term Effects of Repeated Wildfires in Oak-Juniper Woodlands

Abstract

Fire can shape both the structure and composition of vegetation communities, especially those dominated by species with different regeneration strategies. The dominant species of oak-juniper communities in central Texas include resprouters (oaks [Quercus spp.]) and a reseeder (Ashe juniper [Juniperus ashei Buchholz]). We studied the effects of two overlapping wildfires (1996 and 2009) on oak-juniper woodlands on Fort Hood Military Reservation, Texas, USA. We estimated burn severity and measured vegetation regeneration along permanent transects in once-burned, twice-burned, and nonburned areas. Burn severity was highly variable, but was higher in 1996 than in 2009. In the first growing season after the second fire (2009), understory stem density was 3 to 4 times higher than in 1996, but that difference had disappeared by the third growing season. Overstory density did not differ after the two fires, but density on slopes was 3 to 10 times higher than on mesas. Overall species composition was similar after the two fires, but differed from nonburned areas due to the absence of Ashe juniper. Ashe juniper was still absent from twice-burned areas in 2011, whereas small numbers of junipers (21 ± 34 stems ha−1) had colonized the once-burned areas within three years. Our data suggest that wildfires controlled the historical distribution of Ashe juniper, but not of resprouting species, in the Edwards Plateau, and that long intervals between wildfires are required for Ashe juniper to become a canopy dominant in these woodlands.

The joint effects of fire and herbivory on hardwood regeneration in central Texas woodlands

“…We therefore conclude that juniper seedlings recover more slowly than hardwood seedlings after fires in central Texas, at least in the short-term. Reemts and Hansen (2013) found that juniper recovered more slowly than hardwoods after a high-intensity crown-fire in mixed oak-juniper woodland, finding very few juniper saplings even nine years after the fire. These short and long-term studies demonstrate that fire may be effective at restoring and maintaining low juniper densities.…”

“…Consequently, lower-intensity wildfires become less and less likely. When wildfires do eventually occur in juniper-dominated woodlands, they are crown fires (Reemts and Hansen, 2013). The risk of catastrophic crown wildfires provides an additional reason for prescribed burning in central Texas woodlands.…”

“…However, unlike mesophication, juniperization increases the risk of catastrophic crown fire (Reemts and Hansen, 2013) and greatly reduces herbaceous diversity (Yager and Smeins, 1999). While maples are more mesic-adapted than eastern oak species, junipers are in general more xeric-adapted than the oaks they replace.…”

Crown fires remove a fire-sensitive canopy dominant from oak-juniper woodlands

“…The Palmer Z-index was extreme in February 1996 and mid-range in April 2009. Burn severity in the 2009 re was lower than in the same areas during the 1996 re, perhaps due to a combination of less extreme drought and lower fuel loads (Reemts and Hansen 2013).…”

“…In contrast to relatively low-severity prescribed res, crown res cause major changes in woodland structure and composition. We have previously shown that Ashe juniper was largely absent for the rst 10 years after crown re (Reemts and Hansen 2008) and that repeated crown res slowed juniper recovery even more (Reemts and Hansen 2013). Here we provide a long-term follow up to our original studies (24 years after the rst re and 11 years after the second re) to examine the long-term consequences of crown res in oak-juniper woodlands.…”