Litter Inputs

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Alterations to the amount and timing of rainfall in tropical forests may significantly affect nutrient-cycling via litter production, although the relationship between climate

Anthropogenic + Increased Nitrogen Deposition Precipitation fT ^^ NH4+ NO3-

Tropical Rainforest Precipitation

Denitrification increases

NO," Nitrification increases

NH4+ + Anions NO3- + Cations Leaching losses increase

Figure 11.4. The potential effects of anthropogenic nitrogen deposition and increased precipitation in tropical forests. Increased inputs of ammonium could stimulate ammonium-leaching in variable charge soils, and lead to associated anion losses. Ammonium deposition could stimulate nitrification, enhancing nitric and nitrous oxide fluxes. Increased nitrate pools via nitrification or direct deposition could stimulate nitrate- and cation-leaching and denitrifica-tion. If soil redox declines with climate change, then nitrification rates could decrease, partially offsetting the effects of nitrogen deposition.

and litter dynamics is likely to be complex. Litter P concentrations have been found to correlate positively with rainfall seasonality and with inter-annual rainfall in moist and dry tropical rainforests (Read and Lawrence, 2003; Wood et al., 2005), but not spatially along larger-scale elevation and rainfall gradients (Silver, 1998). The observed increase in litterfall P content during the wet season or wet years in seasonal environments may be due to increased soil P availability and/or decreased demand for the retranslocation of P during leaf senescence (Wood et al., 2005). Similarly, there may be significant changes in plant P demand associated with seasonal phenological activity (Lal et al., 2001). Although foliar N may increase per unit leaf mass with increasing precipitation (Wright et al., 2001; Wright and Westoby, 2002; Santiago and Mulkey, 2005), leaf litter N concentrations do not appear to vary with precipitation in moist (Santiago and Mulkey, 2005; Read and Lawrence, 2003) or humid (Silver, 1998; Wood et al., 2005) tropical forests. Elevated atmospheric C02 has also been shown in some cases to increase the litter C: N and C: P ratios, though again the effect is not consistent (Kanowski, 2001; Santiago and Mulkey, 2005).

Climate change impacts on litter inputs may also come from shifts in timing or amount of litterfall. In seasonal tropical forests, litterfall and nutrient uptake are synchronized to the annual patterns in precipitation (Jaramillo and Sanford, 1995).

Seasonality of litterfall has been shown to be negatively correlated to mean annual precipitation in gradient studies (Read and Lawrence 2003; Santiago and Mulkey, 2005). Shifts in the duration of the dry season may lead to temporal separation between plant demands and nutrient availability (Silver, 1998). Increased duration and severity of droughts may also lead to an increase in drought deciduousness, or a decrease in the leaf area index of the canopy (Nepstad et al., 2002). This could decrease litter inputs and/or disrupt the synchronicity between nutrient inputs and plant demand (Lodge et al., 1994). Over the long term, a shift to a significantly wetter or drier environment could lead to a species composition shift with associated changes in both litter quality and quantity (Condit, 1998; Santiago et al., 2005).

Below-ground litter inputs are difficult to study and few data are available. Elevated C02 may increase allocation to root tissues in tropical forests if nutrients are limiting (Arnone and Korner, 1995), although this could be offset by increased root mortality and turnover. Thus far, the data from field and greenhouse experiments are very mixed (Norby and Jackson, 2000). At a global scale, there were no strong relationships between root turnover (below-ground NPP/fine root standing stocks) and temperature or precipitation in forested ecosystems (Gill and Jackson, 2000). In temperate forests, fine root growth cycles appear to be regulated by temperature, resulting in strong annual signals of productivity and mortality (Pregitzer etal., 2000). In contrast, tropical soils experience little variation in soil temperature seasonally, suggesting that any patterns in turnover are more likely to be dominantly controlled by soil moisture, nutrient supply, or internal regulation of root: shoot ratios. Dry-season irrigation did not change overall root phenology in a moist tropical forest, but did alter the timing of root growth and mortality by increasing the longevity of new roots while simultaneously increasing the mortality of older roots (Yavitt and Wright, 2001).

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