Woody Plant Establishment and Spatial Heterogeneity in Grasslands

Abstract

Root exclusion experiments demonstrated the importance of belowground competition between grasses and Prosopis glandulosa (honey mesquite) during the critical seeding establishment phase of the woody plant life cycle. Belowground available volume accounted for 67% and 79% of the variance in first‐ and second‐year Prosopis seedling growth and survival, respectively. Available volume in the vertical dimension was more important than that in the horizontal dimension. Trials spanned years with contrasting annual precipitation, suggesting that root competition occurs in years of near‐average as well as below‐average annual rainfall. Spatial heterogeneity in canopy gaps and belowground biomass was also quantified in a Schizachayriuim–Paspalum grassland matrix and evaluated with respect to Prosopis seedling establishment. Of the 100 grid points encountered in four grass stands, 62% were unoccupied; and 50% of these exceeded 80 cm2 (∼10 cm diameter). Gaps ≥ 10 cm in diameter were sufficient for successful Prosopis germination and survival after one (40%) and two (15%) growing seasons. Herbaceous root biomass was statistically comparable among stands, but point‐specific biomass varied three orders of magnitude (<50 g/m2 to >3000 g/m2). In addition, root biomass was temporally variable, ranging from a mean (±1 se; g/m2) of 768 (86) in a year of below‐average annual rainfall (1996; 721 mm) to 1108 (104) in a year of roughly average annual rainfall (1995; 1032 mm). It is often assumed that grasslands dominated by productive, late seral species will be resistant to woody plant encroachment; and that grazing, by reducing the ability of grasses to competitively exclude woody seedlings, makes grasslands susceptible to tree/shrub invasion. However, given the substantial annual variation in belowground biomass observed in this study, it seems reasonable that ungrazed grasslands may be more susceptible to woody plant encroachment in some years and more resistant in others. Furthermore, given the substantial spatial variation in aboveground gap area and belowground biomass, there may be numerous low‐competition microsites for woody plant seedlings within ungrazed grasslands. A high degree of temporal and spatial variability in gap area and belowground biomass may therefore help explain successful establishment of woody seedlings in ungrazed or lightly grazed, late seral grasslands in the absence of fire. We found no correlation between aboveground structure and belowground biomass at scales of 1–10 m2. Thus, readily quantifiable attributes such as grass basal area or gap area could not be used to infer site susceptibility to woody plant seedling establishment. Corresponding Editor: S. Lavorel.

Positive feedbacks amplify rates of woody encroachment in mesic tallgrass prairie

“…We suspect that clonal stems initially access deep soil water through rhizomatous transfer. Survival rates of woody plants in many types of grassland are usually low until root development extends beyond concentrated grass roots in surface soils (Jackson et al 1996, Partel andWilson 2002), resulting in competitive release (Walter 1971, Weltzin and McPherson 1997, Jurena and Archer 2003, Bond 2008; Z. Ratajczak unpublished data). At KPBS, deep soil water is recharged by winter precipitation and because it is not a primary water source for grasses, this soil-water source is abundant and intra-annually stable (Macpherson 1996, Nippert andKnapp 2007b).…”

“…Specifically, when woody species are young they have a shallow and less robust root system, which overlaps with the extensive grass root systems in the upper soil (Wilson 1993, Jackson et al 1996, Weltzin and McPherson 1997, Partel and Wilson 2002. Because grasses deplete soil moisture in the upper soil, this overlap results in high mortality rates until woody plants develop deeper roots (Scholes and Archer 1997, Jurena and Archer 2003, Bond 2008. Likewise, the thinner bark and lower stature of young woody plants makes them more susceptible to fire than adults (Scholes and Archer 1997, Higgins et al 2000, Bond 2008, Lawes et al 2011).…”

Grass competition suppresses savanna tree growth across multiple demographic stages

“…Empirical studies, however, indicate that the rooting zones of these two guilds can overlap substantially (Knoop andWalker 1985, Smit andRethman 2000), suggesting that even large trees may be competing with grasses for limited soil moisture. A number of studies have shown the inverse, that large trees can suppress grasses (Belsky 1994, Ludwig et al 2004, and others have shown that grasses can suppress tree seedling and sapling establishment (Weltzin and McPherson 1997, Jurena and Archer 2003, Riginos and Young 2007. One previous study examined the effects of grass competition on the growth of larger trees (Knoop and Walker 1985).…”

Spreading speeds for plant populations in landscapes with low environmental variation

“…For example, fecundity may be slightly higher than the mean in some locations and slightly lower in others. Low environmental variation in population measures and environmental conditions within landscapes has been documented by several authors [27,28]. For example, Janišová et al (2012) [29] measured the demographic matrices of the endangered flowering plant Tephroseris longifolia across five locations in a grassland/scrub landscape in the Czech Republic and Slovakia and found their principal eigenvalues to vary between 1.25 and 2.04 (a maximum relative variation of 0.31 from the mean).…”