Implications of Climate Change for Future Developments of Phytodiversity

One of the major future challenges affecting phytodiversity is climate change. In the following, we will discuss current knowledge concerning the influence of climate change on plant species richness via effects on nutrient availabilities and invasive species.

Table 6.2 shows an overview of recent studies on implications of the aspects of climate change on P cycling. It becomes obvious that increasing temperatures tend to increase the rate of P cycling, more precipitation seems to decrease P availability, and increased CO2 concentration has no direct effect on P cycling. Simulated increased N deposition increased phosphatase activities and P uptake (Table 6.2). In a study of effects of warming, moisture, CO2 concentration, and N deposition on P cycling, the effect of N deposition was found to have a larger impact than effects of the other tested factors (Menge and Field 2007). This is crucial with respect to global warming, since Rustad et al. (2001) have shown in a meta-analysis that increasing temperatures by 0.3- 6.0°C at 32 research sites (~35-79°N latitude, one at 45°S latitude) increased N mineralization by 46% on average. Turner et al. (2003) showed that soil with a long history of N deposition in northern England had low P concentrations and most P was in the form of relatively stable organic P.

Thus, with respect to global warming, where increases in temperature, the incidence of heat waves and heavy rainfall events are likely (IPCC 2007), and N mineralization is probably increased (Rustad et al. 2001), the availability of P may be increased by higher phosphatase activity and higher plant demand, but losses by leaching or erosion also become more probable. This could at first mean a higher P availability and quicker P cycling, but in the long run lead to mining of soil P, especially if the finding that increased temperatures decreased P in plant litter (Sardans et al. 2006) is valid widely.

The outlined nutrient developments due to global warming would mean that in most unfertilized soils, a development might take place first towards more nutrient-rich conditions (up and to the right on Fig. 6.3), but later towards more nutrient-limited conditions, i.e., down and maybe to the left in Fig. 6.3, depending on the ratio between increased N mineralization and N losses. Increasing nutrient availability would favor R strategists and highly-productive, quickly growing species. Later, decreasing P availability may potentially lead to increasing phytodiversity. If N availability also decreases, e.g., due to larger N losses, an increase in plant diversity could take place, if seed occurrence and other requirements are met. If N availability stays high, so that the N/P ratio increases, graminoids may be favored (Falkengren-Grerup 1998), which may lead to decreased diversity (Gusewell 2004). In a grassland experiment, 3 years of elevated temperatures increased forb production and abundance, but only insignificantly increased species numbers of grasses and forbs (Zavaleta et al. 2003). The soil N and P pools or phosphatase activities were unfortunately not measured.

Climate change leads to shifts of species' distribution ranges towards the poles (Parmesan and Yohe 2003) and influences the success of invasive species.

Table 6.2 Implications of different aspects of climate change for P cycling

Change of

Conditions

Implication for P cycle

Reference

Temperature Mediterranean shrubland, Spain, +1°C over 6 years

Annual grassland, central California, USA, +1°C, 3-5 years

Dry, P limited heathland, NL, incubation of litter at 5°C, 10°C, 15°C, 20°C for 48 days

Incubation experiment with soil from arable field in Denmark, peas and mycorrhizal inocula at 10°C or 15°C

Experimental Forest in New Hampshire, USA, removal of snow cover to promote soil freezing Water Mediterranean shrubland, Spain, 20.6% decrease in availability water availability over 6 years

Annual grassland, central California, USA, 150%

of ambient precipitation, 3-5 years Dry, P limited heathland, NL, incubation of litter at 50%, 100%, 200% moisture content for 48 days

Increasing Annual grassland, central California, USA, 370 or 670

CO, ppm CO,, 3-5 years concentration Incubation experiment with soil from arable field in Denmark, peas and mycorrhizal inocula at 350 or 700 ppm CO, Rice/winter wheat rotation, China, FACE experiment with ambient or +200 ppm CO,, fertilized (NPK) N-deposition/ Annual grassland, central California, USA, +7 g faster N NO^-N nr2 a"1, 3-5 years cycling Calcareous and acid grassland, UK, 0, 3.5, or 14 g

NH4N03-N nr2 a"1, up to 7 years

Increased phosphatase activities and P concentration in Erica multiflora, decreased litter P, soil extractable P and total P in soil No effect on phosphatase activities, plant P

concentrations, or N/P ratios Increased microbial P mineralization with higher temperature, no effect on microbial P immobilization Almost doubled colonization of roots with mycorrhiza

Missing insulation increased leaching of N and P, possibly due to increased fine root mortality No effect on phosphatase activity or soil extractable P

Decreased soil phosphatase activity and total P in aboveground plant biomass Increased microbial P mineralization (approx. 0.8 mg P kg-1 day-1) and immobilization (approx. 6 mg P kg-1 day-1) with higher moisture content No effect on soil phosphatase activities, plant P

concentrations, or N/P ratios No effect on colonization of roots with mycorrhizae

Increased shoot P, decreased P use efficiency in plants

Increased soil phosphatase activity, net plant primary production, total aboveground P Increased phosphomonoesterase activities, increased soil and shoot P

Sardans et al. (2006)

Menge and Field (2007)

van Meeteren et al. (2007)

Gavito et al. (2003)

Fitzhugh et al. (2001)

Sardans et al. (2006)

Menge and Field (2007)

van Meeteren et al. (2007)

Menge and Field (2007)

Gavito et al. (2003)

Menge and Field (2007)

As outlined above, invasive species also have an effect on the P cycle. This will not only affect their own competitive behavior, but also that of the species surrounding them. So far, no studies have investigated indirect effects of invasive species on surrounding vegetation via changes in P cycling. However, increased P availability following establishment of invasive species will favor fast-growing graminoid species and lead to a decrease of plant diversity on the short term.

Thus, the trends due to global changes all lead to a decreased diversity of plant communities in the near future. They also cause a mining of nutrients from the soil, leading to decreased fertility in the long run. This may then allow a recovery of plant diversity, provided that enough species are still present in the seed banks or can colonize from nearby sites. At the same time, decreased soil fertility also reduces productivity. As increased phytodiversity can be positively related with biomass production under experimental conditions at fixed nutrient availability (Hector et al. 1999; Bullock et al. 2001; van Ruijven and Berendse 2003; Dodd et al. 2004), it could become a management tool in the future, especially when resources for mineral fertilizers become limited. Prognoses suggest that the global P reserves would support the current rate of application for about 100 - 470 years (Smil 2000; Syers et al. 2008).

To sum up, changes in N and P availability due to climate change may first lead to decreasing phytodiversity (with increasing productivity). Later, the trend may reverse due to mining of soil P resources. However, long-term studies have shown that biodiversity takes a long time to recover from nutrient additions, especially from P additions (Hejcman et al. 2007b). Thus, measures should be taken to prevent increased P availability in soils of unfertilized systems in relation to other factors influencing plant growth and competitive strength as far as possible.

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