Plant responses to nutrient addition experiments conducted in tropical forests

Abstract

I present a meta‐analysis of plant responses to 48 nutrient addition experiments conducted with native species in naturally growing tropical forests, exclusive of mangrove forests. The added nutrients include nitrogen (N) in 36 experiments, phosphorus (P) in 33 experiments, calcium and potassium in one experiment each, and various mixtures of essential nutrients in the remaining experiments. I evaluate the hypotheses that nutrients limit tropical forest plants, nutrient limitation is stronger in successional than in old‐growth forests, P but not N is limiting in lowland forests, and N is limiting in montane forests. Responses to the most complete nutrient mix used in each experiment were strong for plant functions that contribute to aboveground production (Hedges’ g averages 0.87) and nonsignificant for fine root biomass. Responses to N addition and to P addition were strong for tissue concentrations of the added element (Hedges’ g averages 0.75 and 1.4, respectively), moderate for fine litter production (0.64 and 0.65, respectively), moderate to weak for plant growth (0.46 and 0.37, respectively) and nonsignificant for fine root biomass. Growth responses were stronger in successional than in old‐growth forests. All responses were unrelated to elevation. The 48 experiments included 30 factorial nitrogen‐phosphorus experiments that enable additional direct tests of the widely cited hypotheses that P limitation is stronger than N limitation in lowland forests and vice versa in montane forests. Both hypotheses were rejected. The N × P interaction effect was nonsignificant across the factorial experiments. In conclusion, nutrients clearly limit tropical forest plants. Limitation by N is widespread in both lowland and montane forests, and the same is true for P. Single experiments identify limitation by calcium and potassium, and correlative studies suggest limitation by calcium, potassium, and magnesium. The available evidence is consistent with the possibility that most macronutrients limit tropical forest plants; however, experiments focus almost exclusively on N and P. The way forward will include taking fuller advantage of existing nutrient addition experiments, siting new experiments strategically, and developing cost‐effective methods to assay responses to all of the essential nutrients soils supply to plants.

Disentangling Drought and Nutrient Effects on Soil Carbon Dioxide and Methane Fluxes in a Tropical Forest

“…The amount of nutrients added in this experiment therefore correspond to twice the annual N input and 35 times the annual P input from leaf litter decomposition (Bonal et al, 2008;Hättenschwiler et al, 2008;Barantal et al, 2012). Our nutrient addition is similar to the amount of nutrients applied in other fertilization experiments in tropical forests (Wright et al, 2018;Wright, 2019).…”

“…Apart from drought, changes in nutrient availability also impact soil gas fluxes in tropical forests. It is generally acknowledged that P rather than N is limiting in lowland tropical forests (Martinelli et al, 1999;Camenzind et al, 2018;Wright et al, 2018;Wright, 2019), and therefore we might expect stronger effects of P addition rather than N addition. In agreement with this, in several tropical forests around the world it has been shown that P addition tends to result in a strong increase in microbial biomass (Cleveland et al, 2002;Kaspari et al, 2008;Liu et al, 2013;Fanin et al, 2015).…”

Competing effects of soil fertility and toxicity on tropical greening

“…We found a strong relationship with soil N only (r 2 = 0.65; p < 0.1) ( Fig. 4a), suggesting that canopy greenness in tropical forests in Panama is predominantly driven by soil N, but not by other nutrients 23,76 . This could reflect the strong association between foliar N and chlorophyll content, with plants taking up additional N even in N-rich soils ("luxury consumption").…”

Species‐specific effects of phosphorus addition on tropical tree seedling response to elevated CO2

“…Although N can limit growth in regenerating forests in eastern Amazonia (Davidson et al, 2004) and there is increasing evidence of nutrient colimitation in the tropics (Wright, 2019), the strongly weathered nature of the tropical landscape means that P is more likely to limit growth (Vitousek, 2004). Indeed, recent evidence suggests that such limitation occurs broadly at the species level (Turner et al, 2018).…”

“…In contrast, the productivity of lowland tropical forests is generally thought to be limited by soil phosphorus (P), rather than by N (Cleveland et al, ; Turner, Brenes‐Arguedas, & Condit, ; Vitousek, ). Although N can limit growth in regenerating forests in eastern Amazonia (Davidson et al, ) and there is increasing evidence of nutrient colimitation in the tropics (Wright, ), the strongly weathered nature of the tropical landscape means that P is more likely to limit growth (Vitousek, ). Indeed, recent evidence suggests that such limitation occurs broadly at the species level (Turner et al, ).…”