Mound Microclimate, Nutrients and Seedling Survival

Article excerpt

ABSTRACT.-

Mounds formed by pocket gophers (Geomys pinetis) in a longleaf pine-wiregrass savanna had larger daily soil temperature fluctuations than surrounding matrix locations. Microsite temperature differences persisted for more than a year, but were largest during summer months. Mound soil also had lower total carbon, total nitrogen, available ammonium, available phosphate and slightly lower soil moisture than matrix locations. These differences occurred in the context of an ecosystem where frequent fire consumes organic matter and removes herbaceous cover. The distinct microclimate and nutrient characteristics of gopher mounds did not significantly influence wiregrass germination, wiregrass seedling survival or longleaf pine seedling survival. Wiregrass germination and seedling survival in the first growing season was low and did not differ between mound and matrix locations. Longleaf pine seedling survival in the first growing season was 64-77% in the absence of fire, but no more than 15% in blocks that were burned. To see a relationship between mound microsite conditions and biotic response, it may be necessary to focus on microbes or turn to other ecosystems.

INTRODUCTION

Pocket gopher mounds are one example of a disturbed microsite with exposed soil and distinct microtopography produced by a burrowing mammal (Inouye et al, 1997). Raised microtopographic features with little or no shade vegetation are exposed to a high angle of solar insolation and should, therefore, warm up more quickly than their surrounding matrix. For example, Cortinas and Seastedt (1996) observed increased daily soil temperature fluctuations at pocket gopher mounds. Soil temperature should, in turn, be correlated with soil moisture, although Cortinas and Seastedt (1996) and Grant el ai (1980) found no significant soil moisture differences between pocket gopher mounds and the surrounding matrix. Although the potential impact of microtopography on microclimate could be large, soil temperature and moisture have not, to our knowledge, been simultaneously measured year-round at gopher mounds and matrix locations.

The soil nutrients at disturbed microsites such as pocket gopher mounds can differ from the neighboring matrix soil surface as a result of bringing deep substrate to the surface, among other things. Total nitrogen at gopher mounds was lower than at matrix locations in Colorado alpine tundra (Litaor et al, 1996; Sherrod and Seastedt, 2001), California annual grassland (Koide etal., 1987) and Minnesota old fields (Inouye elal, 1987; Huntly and Inouye, 1988). Phosphorus decreased at gopher mounds and burned sites in Texas coastal prairie (Spencer et al., 1985) and Colorado alpine tundra (Sherrod and Seastedt, 2001). In contrast, Grant and McBrayer (1981) reported no significant differences in total nitrogen or phosphorus between gopher mounds and matrix locations in Minnesota old fields.

The availability of nutrients can increase with gopher disturbance. For example, nitrate availability at gopher mounds was higher than at matrix locations in Colorado alpine tundra (Litaor d al, 1996; Sherrod and Seastedt, 2001 ). This reverse trend in inorganic N availability compared to total N may, in part, be caused by altered microclimatic conditions.

Many ecosystems with pocket gopher soil-disturbing activity also experience frequent fires, including tallgrass prairie, coastal prairie, California annual grasslands and longleaf pine savannas. Unlike crown-destroying forest fires (Larsen and MacDonald, 1998; Ogdcn et al., 1998; Baskin, 1999), fires in the ecosystems listed above are commonly low-intensity ground fires that consume just the aboveground component of herbs and/or shrubs. These broad-scale ground fires may either diminish or amplify the microclimatic and nutrient conditions associated with pocket gopher mounds.

There are few data linking microclimatic conditions of microsites with biological responses. …