The California Annual Grassland Ecosystem

Author : Sarah Ann Hoskinson
Publisher :
ISBN 13 : 9781303792021
Total Pages : pages
Book Rating : 4.7/5 (92 download)

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Download or read book Ecosystem Impacts of Plant Species Mixtures in California Grasslands written by Sarah Ann Hoskinson and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Plants can have important impacts on ecosystem processes. Much is known about how plant species impact ecosystem processes when in monoculture, but it is unclear if these monoculture effects can be used to predict the ecosystem impacts of plant mixtures. While several studies have shown that changes in the number of species in mixture can have important impacts on ecosystems, the effects of changes in species relative abundance have been largely overlooked. This is surprising because the relative abundances of species are expected to respond more rapidly to environmental changes than the number of species. Another generally overlooked aspect of mixture effects, especially in grassland systems, is the potential for small- scale spatial heterogeneity in ecosystem effects. For example, in systems with patchy vegetation, the ecosystem effects of plants are traditionally predicted by comparing the soil beneath the base of the plant vs in the interspaces between plants, and many studies consider only one soil depth. However, plant impacts may extend laterally past the base, and these impacts may differ by soil depth. I explored questions that have implications for the predictability of species mixture effects on ecosystem processes in California grasslands, including: 1) in the annual grassland system, do changes in species relative abundance impact ecosystem processes, and is the abundance effect predictable?, and 2) in the perennial bunchgrass system, is there lateral and vertical heterogeneity in the bunchgrass effects on soil, or is the traditional method of comparing soil beneath bunchgrasses vs in the interspace at one depth sufficient? The first main objective of this dissertation was to test how changes in the relative abundances of species affect biomass productivity and soil net N cycling rates. I established experimental plots in a California grassland with two- species mixtures that varied in the proportion of the constituent species. I used two common annual grass species and an annual nitrogen- fixing legume in all possible combinations, resulting in two grass- legume mixtures and a grass- grass mixture. In Chapter 1, I tested the relationship between species relative abundance and aboveground and belowground biomass productivity across four seasons. The primary objective of this study was to determine if the effect of abundance on biomass was additive, and thus predictable, or non-additive (i.e. higher or lower than predicted based on the abundance of the species weighted by their biomass in monoculture). The effect of abundance on aboveground biomass shifted seasonally, from additive in the fall, to non-additive in the winter and spring, back to additive by the end of the season. The non-additive effects on biomass were strong; for example, in one case, the grass species had the same biomass in monoculture but when the species were even, the mixture biomass was 1.4 times greater than the monocultures. While abundance was important for biomass productivity aboveground, it did not have a significant effect belowground. In parallel to Chapter 1, in Chapter 2 I tested the relationship between species relative abundance and net N mineralization and nitrification rates. The objectives of this study were to determine 1) if the abundance effect on net N cycling rates were additive or non-additive, 2) if the abundance effects shifted seasonally, and 3) the mechanisms that may drive the non-additive abundance effects and seasonal shifts. Species relative abundance tended to be more important for net nitrification rates than for net N mineralization rates. In general, in the grass- grass mixture, abundance tended to have non-additive effects on net nitrification rates, while in the grass- legume mixtures, net nitrification rates tended to increase additively with the legume abundance. Like for biomass productivity, there were seasonal shifts in the relationship between abundance and net nitrification rates, but none of the measured mechanisms explained the seasonal shifts. Overall, Chapters 1 and 2 demonstrate that species relative abundance can have important effects on biomass productivity and net N cycling rates, and should be included in biodiversity- function studies along with species richness to advance our understanding of how species mixtures impact ecosystem processes. The second main objective of this dissertation was to explore small- scale spatial variation in ecosystem processes in a perennial bunchgrass stand. I established monocultures of Stipa pulchra (the focal perennial bunchgrass species), where the Stipa were surrounded by bare soil in the interspaces, and mixtures of Stipa and Bromus hordeaceus (an annual grass), where Bromus individuals were in the interspaces. The primary objectives of Chapter 3 were to determine if lateral and vertical variation in Stipa effects on soil depended on whether the interspaces between Stipa were comprised of bare soil vs Bromus, and also if the lateral variation in Stipa effects differed depending on proximity to the nearest Stipa. Soil was sampled continuously from beneath the center of Stipa out towards the interspace at two soil depths (0-15 cm and 15-30 cm below the surface) and analyzed for net N mineralization and nitrification rates, moisture, and organic matter content. Stipa and Bromus had similar effects on soil, so when Bromus was in the interspaces, there was no lateral variation in soil processes. In contrast, Stipa had lower net N cycling rates than the bare soil, so when bare soil was in the interspaces, there was lateral variation. However, Stipa only affected the soil directly beneath and adjacent to its base, so not surprisingly, proximity to another Stipa did not change the lateral soil effect. Compared to the bare soil, Stipa effects on net N cycling rates were stronger in the shallow soil layer than the deeper soil layer. Altogether, this study suggests that the effect of Stipa on soil properties can be predicted by simply comparing bunchgrass vs interspace soil, but that multiple depths should be considered. These results are important because when scaled up to a large area, predictions of soil heterogeneity can affect estimates of the soil characteristics of a system.