Elevated atmospheric CO₂: a nurse plant substitute for oak seedlings establishing in old fields

Abstract

It has been hypothesized that elevated levels of atmospheric CO 2 (eCO 2 ) may facilitate the encroachment of woody plants into grasslands by reducing water stress. In eastcentral Minnesota, sandy soils frequently create drought conditions for plants, and water limitation inhibits the establishment of oaks into old fields situated on these soils. Some have argued that eCO 2 should slow secondary succession by favoring fast-growing early successional species. However, if oak encroachment into old fields is being inhibited by water stress, then eCO 2 could accelerate old-field succession in this region. The purpose of this study was to test the hypothesis that eCO 2 will increase the establishment success of oak seedlings in an old field environment. The study was conducted with CO 2 levels controlled by free air CO 2 enrichment (FACE). In May 1999, four oak (Quercus ellipsoidalis) acorns were planted in each of 24 plots in each of six experimental FACE rings (n 5 576), three of which received elevated levels (550 ppm) of CO 2 . Half the plots in each ring were weeded during the first three summers of the experiment. In summer 2000, water input was manipulated during a 3.5-week period, during which half the plots received regular watering while the other half received no water. Summer 2001 was dry, receiving 35% less rainfall than the mean level. Under hot and dry conditions, eCO 2 increased soil water levels in unweeded plots and enhanced oak establishment (survival and growth) in weeded plots. In 2006, after the eighth growing season following planting, survival was five times greater under elevated than ambient CO 2 . The results showed that under hot and dry conditions, eCO 2 can act like a nurse plant for tree seedlings growing in bare and unshaded areas, increasing seedling survival and growth, and thereby expanding the establishment window for trees encroaching into a grassland environment.

Complex facilitation and competition in a temperate grassland: loss of plant diversity and elevated CO2 have divergent and opposite effects on oak establishment

“…The negative effect of CO 2 enrichment on oak growth is surprising and important in the context of recent work on the singular effect of CO 2 enrichment on woody encroachment (Bond and Midgley 2000;Polley et al 2003;Davis et al 2007;Staver et al 2011). These recent studies suggest that CO 2 enrichment should have positive effects on overall trends in woody encroachment (Davis et al 2007) due to enhanced WUE (Polley et al 2003) and subsequent access to limiting soil moisture (Staver et al 2011), as well as to increased total C availability for woody species that need to invest large amounts of energy towards re-sprouting after fire (Bond and Midgley 2000).…”

“…Our results suggest that as oaks encroach into grasslands, herbaceous species may respond quickly to increased CO 2 enrichment and grow more due to the CO 2 fertilization effect. When oaks arrive into higher biomass grasslands following increases in levels of atmospheric CO 2 , they may be more limited by competition from greater herbaceous biomass than they are facilitated by CO 2 enrichment (e.g., Davis et al 2007). We predict that while possibly this has strong implications for how we interpret future trends of woody encroachment in grasslands, past trends in woody encroachment may have been more strongly controlled by increasing levels of atmospheric CO 2 due to the greater response potential at lower levels of CO 2 (Kgope et al 2009).…”

“…These recent studies suggest that CO 2 enrichment should have positive effects on overall trends in woody encroachment (Davis et al 2007) due to enhanced WUE (Polley et al 2003) and subsequent access to limiting soil moisture (Staver et al 2011), as well as to increased total C availability for woody species that need to invest large amounts of energy towards re-sprouting after fire (Bond and Midgley 2000). Our results suggest that as oaks encroach into grasslands, herbaceous species may respond quickly to increased CO 2 enrichment and grow more due to the CO 2 fertilization effect.…”

“…For example, elevated atmospheric CO 2 appears to benefit woody seedlings grown alone in water-limited environments (Davis et al 2007) and may help explain past woody range expansions (Kgope et al 2009), particularly in the presence of fire (Bond and Midgley 2000). Elevated CO 2 increases plant water use efficiency (WUE), which should increase soil water availability (Polley et al 2003;Reich 2009;Adair et al 2011).…”

Why are there no C 4 forests?

“…Bond et al (2003) demonstrate faster root growth of Acacia trees in elevated CO 2 , which accelerates their ability to escape below-ground competition from C 4 grasses. In the oak-savanna regions of the American Midwest, elevated CO 2 speeds oak seedling establishment in the tall-grass prairie, and hastens woody encroachment (Davis et al, 2007;Wright et al, 2013). These patterns are important to the discussion here because they demonstrate that enhancement of photosynthesis and resource use efficiency by rising CO 2 creates advantages for woody C 3 species in the same manner that C 4 photosynthesis would create advantages for hypothetical C 4 trees.…”