Competing effects of soil fertility and toxicity on tropical greening

Abstract

Tropical forests are expected to green up with increasing atmospheric CO 2 concentrations, but primary productivity may be limited by soil nutrient availability. However, rarely have canopy-scale measurements been assessed against soil measurements in the tropics. Here, we sought to assess remotely sensed canopy greenness against steep soil nutrient gradients across 50 1-ha mature forest plots in Panama. Contrary to expectations, increases in in situ extractable soil phosphorus (P) and base cations (K, Mg) corresponded to declines in remotely sensed mean annual canopy greenness (r 2 = 0.77-0.85; p < 0.1), controlling for precipitation. The reason for this inverse relationship appears to be that litterfall also increased with increasing soil P and cation availability (r 2 = 0.88-0.98; p < 0.1), resulting in a decline in greenness with increasing annual litterfall (r 2 = 0.94; p < 0.1). As such, greater soil nutrient availability corresponded to greater leaf turnover, resulting in decreased greenness. However, these decreases in greenness with increasing soil P and cations were countered by increases in greenness with increasing soil nitrogen (N) (r 2 = 0.14; p < 0.1), which had no significant relationship with litterfall, likely reflecting a direct effect of soil N on leaf chlorophyll content, but not on litterfall rates. In addition, greenness increased with extractable soil aluminum (Al) (r 2 = 0.97; p < 0.1), but Al had no significant relationship with litterfall, suggesting a physiological adaptation of plants to high levels of toxic metals. Thus, spatial gradients in canopy greenness are not necessarily positive indicators of soil nutrient scarcity. Using a novel remote sensing index of canopy greenness limitation, we assessed how observed greenness compares with potential greenness. We found a strong relationship with soil N only (r 2 = 0.65; p < 0.1), suggesting that tropical canopy greenness in Panama is predominantly limited by soil N, even if plant productivity (e.g., litterfall) responds to rock-derived nutrients. Moreover, greenness limitation was also significantly correlated with fine root biomass and soil carbon stocks (r 2 = 0.62-0.71; p < 0.1), suggesting a feedback from soil N to canopy greenness to soil carbon storage. Overall, these data point to the potential utility of a remote sensing product for assessing belowground properties in tropical ecosystems. Tropical forests are the 'lungs' of our planet, absorbing and storing the largest carbon stocks of all terrestrial ecosystems 1-4. They are also the source of among the largest uncertainties to projections of Earth's climate, due to their poorly understood responses to changes in CO 2 , climate, land use, and nutrient cycling 5-9. Numerous modeling studies have suggested that tropical forest productivity is increasing 5,6 , and present day observational indicators from space suggest that the tropics are greening likely due to CO 2 fertilization, in the face of increasing pressure from droughts and disturbance 10-16. Despite the apparent greeni...

Savannas are more sensitive to rainfall variability than riparian forests

“…As a general ecosystem function, GPP, is likely to be an emergent property resulting from the interactions of several elements. GPP depends on the water and light availability [27], further on other resources and environmental conditions, such as solar radiation [28], nutrient availability [29], soil toxicity [30], temperature [31], and topography [19].…”

Linking Remotely Sensed Carbon and Water Use Efficiencies with In Situ Soil Properties

“…The larger effects of multiple nutrient additions (N, P, calcium, or Ca) on forest productivity than a single element was, revealed in a recent study of nutrient availability in northeastern US hardwood forests [27]. Additionally, a strong relationship between satellite-derived canopy greenness and soil N, aluminum (Al), and P was observed in tropical forests [52]. Moreover, many modeling experiments considered soil as a mixed column and thus relationships between soil nutrients and vegetation growth cannot be inferred for soil depth.…”

Toward a coordinated understanding of hydro‐biogeochemical root functions in tropical forests for application in vegetation models

“…For nutrient aboveground–belowground coordination, a recent global review indicated greater coordination of leaf with root N : P ratios in tropical forests relative to most other biomes, likely related to widespread tropical soil P scarcity and conservation of P in plant tissues (Wang et al ., 2022). A broad‐scale paper linking remotely sense canopy traits in Panama with soil data found that canopy greenness (a surrogate for NPP) corresponded to variations in soil fertility and toxicity (Fisher et al ., 2020). Also, AMF vs ECM association has been linked to canopy reflectance properties in tropical forests in Hawai'i (as well as in many temperate sites), likely also indicating aboveground–belowground plant nutrition linkages (Sousa et al ., 2021).…”

“…Highlighting the difficulty of using morphological root traits to infer function, fine‐root traits (diameter, SRL, root tissue density, and branching) for 1467 Amazonian tree species had no significant association with landscape‐scale shifts in bulk soil fertility (Vleminckx et al ., 2021). This is in contrast to some aboveground tropical forest traits, like canopy greenness and nutrient content, which often covary with soil fertility and soil texture (Fyllas et al ., 2012; Fortunel et al ., 2014; Fisher et al ., 2020). Root diameter, which is functionally most closely related to water conductivity, has been used as a proxy for AMF colonization rate, even though this relationship has not been consistently demonstrated for tropical forests (Kong et al ., 2014; Lugli et al ., 2020; Yaffar et al ., 2021).…”