Influence of temperature and soil nitrogen and phosphorus availabilities on fine‐root productivity in tropical rainforests on Mount Kinabalu, Borneo

Abstract

We investigated how temperature and nutrient availability regulate fine‐root productivity in nine tropical rainforest ecosystems on two altitudinal gradients with contrasting soil phosphorus (P) availabilities on Mount Kinabalu, Borneo. We measured the productivity and the nutrient contents of fine roots, and analyzed the relationships between fine‐root parameters and environmental factors. The fine‐root net primary productivity (NPP), total NPP, and ratio of fine‐root NPP to total NPP differed greatly among the sites, ranging from 72 to 228 (g m−2 year−1), 281–2240 (g m−2 year−1), and 0.06–0.30, respectively. A multiple‐regression analysis suggested a positive effect of P availability on total NPP, whereas fine‐root NPP was positively correlated with mean annual temperature and with P and negatively correlated with N. The biomass and longevity of fine roots increased in response to the impoverishment of soil P. The carbon (C) to P ratio (C/P) of fine roots was significantly and positively correlated with the P‐use efficiency of above‐ground litter production, indicating that tropical rainforest trees dilute P in fine roots to maintain the C allocation ratio to these roots. We highlighted the mechanisms regulating the fine‐root productivity of tropical rainforest ecosystems in relation to the magnitude of nutrient deficiency. The trees showed C‐conservation mechanisms rather than C investment as responses to decreasing soil P availability, which demonstrates that the below‐ground systems at these sites are strongly limited by P, similar to the above‐ground systems.

Fine root dynamics across pantropical rainforest ecosystems

“…Malhi et al (2011) estimated average tropical forest NPP allocation of 34% to canopy, 39% to woody tissue, and 27% to fine roots, with a considerable variation across sites and regions. In Southeast Asia, NPP allocation to fine roots ranged between 10% and 14% for old‐growth lowland dipterocarp forests (Kho et al, 2013; Riutta et al, 2018), and between 6% and 30% in Kinabalu montane forests (Okada et al, 2017). This small fractional allocation to fine roots in South East Asia may be due to the ectomycorrhizal symbionts of the dipterocarp trees, which enhance nutrient uptake (Brearley., 2012; Robinson et al, 2020).…”

“…In tropical lowland forest, fine root biomass increases with soil sand content (Barbosa et al, 2012; Jiménez et al, 2009; Kochsiek et al, 2013; Priess et al, 1999; Silver et al, 2000) and with precipitation at regional scale (Green et al, 2005; Ibrahim et al, 2020). Fine root biomass increases with elevation (Girardin et al, 2013; Kitayama & Aiba, 2002; Leuschner et al, 2007; Moser et al, 2010; Okada et al, 2017; Sierra Cornejo et al, 2020) in tropical forests. Fine root biomass has also been shown to increase with decreasing mean annual temperature, soil pH, and C:N ratio in Ecuadorian montane forests (Moser et al, 2010) and with decreasing mean annual temperature and soil P availability in Bornean montane forests (Okada et al, 2017).…”

“…Fine root biomass increases with elevation (Girardin et al, 2013; Kitayama & Aiba, 2002; Leuschner et al, 2007; Moser et al, 2010; Okada et al, 2017; Sierra Cornejo et al, 2020) in tropical forests. Fine root biomass has also been shown to increase with decreasing mean annual temperature, soil pH, and C:N ratio in Ecuadorian montane forests (Moser et al, 2010) and with decreasing mean annual temperature and soil P availability in Bornean montane forests (Okada et al, 2017).…”

“…However, in the Peruvian Andes, fine root productivity was high at cloud base zone, and fine root productivity increased with increasing temperature (Girardin et al, 2013). In Bornean montane forests, fine root productivity was reported to increase with increasing temperature and soil phosphorus, but decreased with increasing soil N (Okada et al, 2017).…”

“…This is one of the first analysis to date of a global tropical fine root dataset, and the first to use the exact same protocol across sites; most studies of fine root dynamics in the tropics tend to focus on a particular site or biogeographical region (e.g. Girardin et al, 2013; Okada et al, 2017; Sierra Cornejo et al, 2020), or synthesize results across studies that employ different methodologies (Moser et al, 2010; Röderstein et al, 2005). Hence, it remains unclear whether trends found in any one region are universal features of tropical forest biomes.…”