Description
The spread of invasive species into natural, native habitats threatens the species diversity and function of many ecosystems globally, and locally. In California the past and future expansion of invasive annual plants is linked to human disturbance and climate change. The invasive annuals differ from native plants in growth form and phenology. Understanding how these differences impact resource dynamics in invaded ecosystems is important for predicting the response of these ecosystems to future climate change. In this dissertation I focused on three aspects of resource dynamics. The carbon (C) cycle and how soil respiration differs between invasive annuals and native coastal sage scrub species; the nitrogen (N) cycle and how altered rainfall patterns mediate N availability and partitioning between invasive annuals, native shrubs and microbes; seed production of two native grasses and how competition from invasive annuals affects their contribution to a future seed bank. In chapter 1, I evaluated how soil respiration (Rs), a critical component of ecosystem carbon (C) storage, differs between native shrubs and inter-shrub patches dominated by invasive annuals. Rs can be partitioned into a root associated, autotrophic component (Ra) and a microbial, heterotrophic component (Rh). Discriminating between Ra and Rh provides insight into the underlying mechanisms which determine seasonal patterns in Rs. I found that phenology played an important role in the difference between invaded and shrub areas. Rs was 40% higher in invaded areas, primarily due to higher Ra early in the season. Overall the shrubs had a shorter respiration season due to more constrained phenology and contributed less C loss to the atmosphere. In invaded areas higher respiration rates were in part due to higher temperature tolerance of Ra and Rh. This suggests that water is more limiting than temperature in invaded areas. If, as expected, rainfall patterns shift in the future then C losses may be even higher from invaded areas. In chapter 2, I experimentally manipulated seasonal rainfall totals and determined the effect on nitrogen (N) partitioning between invasive annuals, a native shrub and the microbial community. Microbes, invasive annuals and native shrubs all competed for early season N, but allocation patterns differed dramatically. Microbes took up N rapidly and turned it over to soil organic matter where the N was stable for the whole growing season. Invasive annuals also allocated N rapidly. Shrubs on the other hand had a mechanism for N storage allowing them to take up N early, when it was available, but allocate it later when they needed it for new leaf growth. Overall the effects of altered rainfall were small. Dry conditions caused slower microbial N turnover early in the season and lower total biomass by the end of the season. At the end of the season N storage differed between invasives and natives. Native shrubs have a conservative nutrient retention strategy and resorbed much of the N from their leaves. In contrast the invasives died and N stayed in the dead leaf litter. When leaf litter decomposes in the following growing season, N becomes available again for uptake. Invasives are able to take advantage of excess N, when it is available, so leaf litter N inputs may benefit invasives in the next growing season. Invasive annual grasses compete more directly with native grasses than with native shrubs, because they have more similar resource demands. In chapter 3, I compared seed production of two native grasses grown in the absence and in the presence of invasive annual competitors. Native abundance was much lower in plots with invasive annuals and therefore seed production for a given area was suppressed in the presence of invasive annuals. However, seed production per unit plant biomass, was higher in the presence of invasives. This indicates that more resources were allocated to reproduction in the environment with invasive competitors. Furthermore, there were no adverse effects on seed quality and in one case seeds taken from plots with invasive competitors produced higher biomass than seeds grown without invasive competitors. Higher seedling biomass could confer an advantage when germinating among other competitors. Together these results indicate that different invasive phenology affects C and N cycling. These differences are particularly pronounced early and late in the growing season. Both the C and N cycle are more rapid and less conserved in invaded areas compared to shrub areas. In the case of C this has important implications for feedbacks to regional and global climate change. In the case of N, it has implications for ecosystem N retention and the possibility to further promote invasion. The evidence I found for early N uptake by shrubs and higher investment to reproductive allocation by native grasses, suggests that natives have resource use strategies which may allow them to evade invasive competition, and persist despite strong invasive pressure.
