Warming and rainfall redistribution effects on linkages between plant functional traits and ecosystem processes in oak savanna.

Introduction

The South Central region of the US is longitudinally bisected by an ecotone that separates the grasslands of the Great Plains and the deciduous forest of the eastern US. In the southern region, this transition is best represented by the Post Oak (Quercus stellata) Savanna Region that occupies approximately 3 million hectares in a zone that extends from south central Texas northward into eastern Oklahoma (Fig. 1). Oak savannas contain the dominant life forms of both adjacent biomes to form a “tension” zone between grasslands and forests which may increase their responsiveness to global change drivers. Global change scenarios can be envisioned where either the tree or grass life forms would gain an advantage and encroach upon the adjacent biome. It is within this context, that tension zones provide a valuable opportunity to explore the responsiveness of these life forms to various global change drivers.

Mean annual precipitation totals 900 to 1100 mm per year with precipitation maxima in the spring and autumn. However, a shallow surface soil combined with high potential evapotranspiration will very likely magnify soil water limitations to plants during the summer and intensify competition for water.

Although several oak species occur in this region, Q. stellata, the major canopy tree dominant, is slow-growing and drought tolerant and may reach an age of 200 to 400 years. The grassland component of the savanna is characteristic of the eastern tallgrass prairie and the major dominant is the C₄ grass, Schizachyrium scoparium (little bluestem). The Post Oak savanna region has undergone a transition from a tree-grass dominated savanna toward a more closed canopy oak and oak-juniper (Juniperus virginiana) woodland over the last century. Woody encroachment has likely resulted from complex interactions among grazing, wildfire suppression, and climate. The invasive nature and rapid growth rate of juniper enable it to attain sufficient size and density to suppress both oak regeneration and grass density in the absence of fire. Given that large contiguous tracts of post oak savanna are rare and those that remain are embedded in an agricultural and suburban land use matrix, the remnant savanna in the region is likely to remain uncoupled from historic, pre-settlement fire regimes. The role of temperature and precipitation distribution in the establishment, growth, and function of tree and grass life forms in this landscape is largely unknown and the subject of our study. We propose that temperature and water availability are the key climatic factors influencing tree-grass interactions during tree seedling establishment, and thus projected global climate change is likely to impact plant community composition and ecosystem function.

The responsiveness of the Post Oak savanna may be especially sensitive to global environmental change because each of the dominant life forms possess contrasting photosynthetic pathways and leaf habit (evergreen and deciduous). The oak and juniper possess the C₃ pathway while the dominant grasses possess the C₄ pathway. These photosynthetic pathways show contrasting responses to temperature and water availability, which may magnify the response of these life forms to various global change scenarios. We propose that:

1. inherent species contrasts in photosynthetic pathway, leaf habit, and phenological plasticity provide life form contrasts that may be useful in predicting ecosystem response to global change drivers, and
2. the expression of phenological and physiological plasticity and the effectiveness of soil water partitioning represent the fundamental ecological mechanisms underpinning species and life form responses to the two global change drivers. The drivers that exceed the limits of plasticity and have the most disruptive effect on soil water partitioning will have the most profound effect on the growth, function, and competitive ability of the various life forms that comprise this savanna.

Our research investigates the effects of global change drivers on contrasting life forms to establish an eco-physiological basis to interpret the response of savanna structure and function to relevant global change scenarios. Experimental simulation of precipitation distribution and warming, both independently and in combination, in a field setting represents a powerful research platform to evaluate the biological mechanisms underpinning the response of terrestrial vegetation to various global change scenarios. This research approach has the potential to produce generalizations describing the responses of contrasting functional groups of tree and grass species to key global change drivers. The use of defined species mixtures permits the resolution of the responses of individual species and species combinations to global change drivers often confounded in studies of intact systems. Consequently, this study has the potential to yield mechanistic insights that, when coupled with ecosystem and community level investigations, in this and other investigations, will complement research on these key global change drivers. A greater understanding of the impact of global change on these dominant plant species will have important implications for understanding the consequences of land-use change, agricultural productivity, and natural resource conservation and management.