DISTRIBUTION OF UNDERSTORY PLANT SPECIES ALONG
LIGHT AND SOIL DEPTH GRADIENTS IN AN UPLAND OAK SAVANNA REMNANT IN SOUTHERN
Brian W. Pruka
The Driftless Area is recommended as a priority region for landscape-scale savanna restoration in Wisconsin, given the relative abundance of savanna remnants and species due to the presettlement abundance of prairie-savanna-woodland complexes, the role rugged topography played in discouraging plowing and encouraging light pasturing of former savannas, and the slowing of canopy closure and loss of understory species by the relatively dry, infertile soils in many outcrop areas. Long-term advantages of savanna reserves in the Driftless Area include 1) simplified fire management due to relatively low human population densities and the presence of many topographic fire-breaks, 2) the ability to pursue gradient-based management (as opposed to that based on individual species or communities) given the presence of spatially compact continuous gradients of soil depth and potential canopy coverage/regrowth rates which would allow the simultaneous restoration and management of prairies, oak savannas, and oak woodlands in an oak ecosystem complex; and 3) greater long-term sustainability, based on the ability of species and communities to migrate along sharp soil moisture gradients caused by rugged topography.
The restoration of savannas depends in large part on our understanding of understory plant species response to variation in light and soil depth.
The distribution of 72 herbaceous and 34 woody understory plant species were studied along direct gradients of sunlight and soil depth at one of southern Wisconsin's highest quality savanna remnants, the Parrish oak savanna. Historical photographs and surveyors' records indicated that this site has been vegetated continuously by a savanna mosaic since European settlement. Species distributions at the Parrish site were quantified by presence and percent cover in 250 quadrats. The quadrats were located in a stratified random fashion along 13 transects running from full sunlight to closed canopy. The light regime (direct photosynthetic photon flux density [mol. m-2 d-1], averaged over the growing season) was quantified by computer analysis of hemispheric canopy photographs.
There is a negative correlation between understory light levels and soil depth across the Parrish continuum, with more open vegetation occurring on shallower (presumably drier and less fertile) soils.
The Parrish Oak savanna contained 222 native species distributed along a prairie-savanna-woodland-forest continuum. Nearly all species differed in their distribution along both light and soil depth gradients, with a large proportion of species peaking in the middle of the light gradient which corresponded to savanna or open woodland conditions. Species distributions formed a continuum along both environmental gradients with no distinct plant associations being apparent. Interestingly, species showed the strongest response to regional light regime, computed as the average light regime of a quadrat and those immediately contiguous. Regional light regime was also the environmental variable most strongly correlated with a primary axis of ordinations of the vegetational data using the Bray-Curtis, reciprocal averaging, and local non-metric multi-dimensional scaling techniques.
Species grouped by similar morphological attributes differed in their distribution along the regional light gradient with the observed distributions for each morphological guild paralleling the expected response. The dominant leaf orientation shifted from nearly vertical at the high light (sunny) end of the gradient to nearly horizontal at the shady end; leaf height increased toward sunnier conditions. Most species in the open prairie and savanna had wind-dispersed seeds, whereas most woodland and forest species were ectozoochorous with bur-like disseminules dispersed on fur or feathers. Species richness along the regional light gradient peaked under partial canopy conditions near the high light end of the gradient, perhaps indicating that this has been the predominant local condition over the past several decades.
Species distributions along the regional light gradient of this study strongly parallel those documented by Gilbert (1953) for the compositional gradient from savanna to dry forest to mesic forest derived from the southern Wisconsin forest continuum of Curtis and McIntosh (1951). The fact that many species peaking in frequency under savanna and open woodland canopy levels at the Parrish site were regarded by Gilbert and Curtis (1953) to peak in frequency in dry and dry-mesic forest is hypothesized to be a consequence of fire suppression having created a successional shift from more or less open savannas at settlement to more or less closed, dry-mesic to dry forests by the time Gilbert studied them in the 1950s.
Species distributions along the regional light gradient of this study also parallel those of Bray (1955, 1958, 1960) and Curtis (1959) for the high light end of the sun/shade gradient (i.e., prairie and savanna). However, this study differs in distinguishing species as either open woodland and closed forest species which were uniformly characterized as dry or dry-mesic forest species by Bray and Curtis. I hypothesize that the post-settlement succession of savannas to dry and dry-mesic forests confounded the inferences made by Bray and Curtis and lead them to associate species of more open savannas/woodlands as being native to the less open woodlands/forests in which they persisted following the widespread conversion of savannas to forests due to fire suppression.
The proposed tallgrass savanna species of Packard (1985, 1988a, 1988b, 1993) peak predominantly in the oak woodland portion of the Parrish light gradient. These species may be characteristic of both oak savannas and woodlands and exhibit mesic-to-xeric shifts when invading more closed communities, in a fashion similar to that found by Bray (1958) for wet and wet-mesic prairie species invading drier savannas. In those portions of the Parrish site where canopy cover and soil depth are not strongly correlated, a few species (e.g. Eupatorium purpureum) peak strongly only in areas with deeper, more mesic soils, indicating that canopy coverage alone does not determine their distribution. These species highlight the importance of considering both light and soil depth (i.e. soil moisture) gradients in attempting to characterize responses to canopy gradients, even when there is a generally strong correlation between light regimes and soil depths. Future research on species distributions within savannas would thus benefit from thorough characterization of soil moisture levels and their residual variation about any correlation with local canopy closure.
Bray, J. R. 1955. The savanna vegetation of Wisconsin and an application of the concepts of order and complexity to the field of ecology. Ph.D. dissertation. The University of Wisconsin, Madison.
Curtis, J. T. 1959. The Vegetation of Wisconsin. University of Wisconsin Press, Madison.
Gilbert, M. L. 1953. The phytosociology of the understory vegetation of the upland forests of Wisconsin. Ph.D. Dissertation. The University of Wisconsin. Madison, WI.