Academic journal article The American Midland Naturalist

Loss of Neighbors, Fire, and Climate Effects on Texas Red Oak Growth in a Juniper-Dominated Woodland Ecosystem

Academic journal article The American Midland Naturalist

Loss of Neighbors, Fire, and Climate Effects on Texas Red Oak Growth in a Juniper-Dominated Woodland Ecosystem

Article excerpt

ABSTRACT.-

In this study, we assessed growth response of Quercus buckleyi (Texas red oak), a current codominant deciduous oak species in central Texas woodlands, to changes in competition, fire, and climate over time to evaluate factors related to documented regional decline of this species. For this analysis, we collected 372 tree slabs of Texas red oaks from the woodlands of the Balcones Canyonlands National Wildlife Refuge near Austin, Texas, from which we aged fire scars and measured tree-ring widths to calculate basal area increment and ring-width indices. To determine canopy conditions of these trees over time, we used historical aerial photos from 1937 to 2004 acquired approximately every 15 y to evaluate changes in woody vegetation cover for the locations of the trees sampled. Our results showed that trees affected by loss of local woody vegetation cover, as evaluated by the aerial photographs, and fire had higher average basal area increment than trees without fire evidence and loss of cover. These differences were significant when aspect and slope were added to the analysis. For climate, we found significant correlation between annual Palmer Drought Severity Indices and ring-width indices for the time interval of 1937-1978, but not after, indicative of potential recent decoupling between tree-ring changes and climate. We found drought to potentially be a major driver of community change in this system as it affects tree-ring response, fire, and mortality assessed from sampled trees.

Introduction

Reduced recruitment and growth of oaks in forests and woodlands in North America is a growing concern due to the difficulty in attributing a consistent set of factors for this trend (Abrams, 1992; Dwyer el al., 1995; Thomas et al., 2002). This oak decline has been correlated with herbivory (Russell and Fowler, 2004), canopy closure (Iverson et ai, 2008), reduced fire occurrence and frequency (Abrams et al., 1995), and climate change (Kueppers et al., 2005) or some combination (McEwan et al., 2011). Continued loss of oaks from North American ecosystems may exacerbate projected potential biodiversity decreases with escalated risk to threatened and endangered species (Hansen et al., 2001). In central Texas, further oak decline in remnant woodlands may threaten future critical habitat for the endangered golden-cheeked warbler (Selophaga chrysoparia).

Disturbance in oak forests and woodlands appears to increase oak recruitment and growth through a variety of mechanisms. Oak-dominated vegetation communities may be maintained due to frequent, low-intensity fires (Abrams, 1992; McEwan et al., 2007, Hutchinson et ai, 2008). For example, periodic fire reduces competition from fire-sensitive species and increases growth of oaks by increasing irradiance from openings in the upper canopy (Clinton et al., 1994) and nutrient mineralization (Trammell et al, 2004; Gray and Dighton, 2006). Fire wounding potentially stimulates a tree-ring response and induces basal sprouting in oaks (Smith and Sutherland, 1999; Arevalo et al, 2009; Moreira et al, 2009).

Climate affects oak growth primarily through constraints on carbon assimilation as stomatal conductance, sensitive to plant water potential, is decreased under low site water conditions as an adaptation to reduce xylem cavitation (McDowell et al, 2008). Annual variations in climate often have direct control on variations in tree-ring width, depending on the tree's sensitivity to climate (Douglas, 1920; Fritz, 1966). In addition to year-to-year variation, prolonged climate events such as severe drought, can influence tree growth by changing competition between woody species through direct mortality with feedbacks to disturbance, such as fire, due to increased fuel accumulation, potentially shifting species dominance (Swetnam and Baisan, 2003; Suarez et al, 2004; Mueller et al, 2005). In the 1950s, prolonged and extreme drought was widespread throughout the southwest and central United States resulting in direct mortality of trees, reduced canopy cover, and increased fire occurrence that potentially influenced current species composition of contemporary woodlands (Archer et al, 1988; Allen and Breshears, 1998). …

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