Exploration capacity versus specific enzymatic activity of ectomycorrhizas in response to primary productivity and soil phosphorus availability in Bornean tropical rainforests.

Ectomycorrhizal (ECM) fungi are functionally important in biogeochemical cycles in tropical ecosystems. Extracellular enzymatic activity of ECM on a ground-area basis is the product of two attributes; exploration capacity (ECM surface-area) and specific enzymatic activity. Here, we elucidated which attribute better explained the ECM enzymatic activity in response to different levels of soil phosphorus (P) and Nitrogen (N) availability in five Bornean tropical rainforests. We determined the surface area of ECM root tips as well as the enzymatic activities per ECM surface area for carbon (C), N and P degrading enzymes in each site. We evaluated the relationship of ECM enzyme activities with the resource availabilities of C (Above-ground net primary production; ANPP), N, and P of ECM by a generalized linear mixed model. The ECM enzymatic activities on a ground-area basis were more significantly determined by specific enzymatic activity than by the exploration capacity. Specific enzymatic activities were generally negatively affected by C (ANPP) and soil P availability. ECM fungi enhance the specific enzyme activity rather than the exploration capacity to maintain the capacity of nutrient acquisition. The less dependence of ECM fungi on the exploration capacity in these forests may be related to the limitation of C supply from host trees. We highlighted the adaptive mechanisms of ECM fungi on nutrient acquisition in tropical ecosystems through the response of enzymatic activity to nutrient availability across the elements.

Phosphorus and nitrogen resorption from different chemical fractions in senescing leaves of tropical tree species on Mount Kinabalu, Borneo.

Nutrient resorption, a process by which plants degrade organic compounds and resorb their nutrients from senescing tissues, is a crucial plant function to increase growth and fitness in nutrient-poor environments. Tropical trees on phosphorus (P)-poor soils are particularly known to have high P-resorption efficiency (PRE, the percentage of P resorbed from senescing leaves before abscission per total P in green leaves). However, the biochemical mechanisms underlying this greater PRE remain unclear. In this study, we determined the P concentration in easily soluble, nucleic acid, lipid and residual fractions for green and senescent leaves of 22 tree species from three sites, which differed in P availability, on the lower flanks of Mt. Kinabalu, Borneo. PRE varied from 24 to 93% and was higher in species from the P-poor site. P-resorption rate was greatest from the lipid fraction, the nucleic acid fraction, and lowest in the easily soluble fraction and the residual fraction when all the species were pooled. For species with higher PRE, P-resorption rate of the residual fraction was relatively high and was comparable in magnitude to that of the other labile fractions. This suggests that tree species inhabiting P-poor environments increased PRE by improving the degradation of recalcitrant compounds. This study suggests that plants selectively degrade organic compounds depending on environmental conditions, which is a key mechanism underlying the variation of PRE.

Nutrient allocation among plant organs across 13 tree species in three Bornean rain forests with contrasting nutrient availabilities.

Allocation of nitrogen (N) and phosphorus (P) among plant organs is an important factor regulating growth rate, which is a key ecological process associated with plant life-history strategies. However, few studies have explored how N and P investment in photosynthetic (leaves) and non-photosynthetic (stems and roots) organs changes in relation to depletion of each element. We investigated nutrient concentrations of plant organs in relation to whole-plant nutrient concentration (total nutrient weight per total biomass) as an index of nutrient status of each individual using the saplings of the 13 species in three tropical rain forests with contrasting N and P availabilities (tropical evergreen forests and tropical heath forests). We found a steeper decrease in foliar N concentration than foliar P concentration with decreasing whole-plant nutrient concentration. Moreover, the steeper decrease in foliar N concentration was associated with relatively stable N concentration in stems, and vice versa for P. We suggest that the depletion of N is associated with a rapid dilution of foliar N because the cell walls in non-photosynthetic organs function as an N sink. On the other hand, these species can maintain foliar P concentration by decreasing stem P concentrations despites the depletion of P. Our results emphasize the significance of non-photosynthetic organs as an N sink for understanding the variation of foliar nutrient concentrations for the tree species in the three Bornean rain forests with different N and P availabilities.

Changes in biomass, productivity and decomposition along topographical gradients under different geological conditions in tropical lower montane forests on Mount Kinabalu, Borneo.

We have examined how the structure and function of a forest ecosystem change with topography (lower-slope versus ridge) and how the changes are modified by nutrient availability depending on geological substrate (Quaternary and Tertiary sedimentary rocks and ultrabasic rock) in the tropical montane rain forests of Mt. Kinabalu (Borneo) where climate is humid and aseasonal. Reflecting the difference in site age and parent rock, the pool size of soluble-P and inorganic-N in topsoil decreased from Quaternary sedimentary >Tertiary sedimentary >ultrabasic rock on the lower-slope, and they decreased from the lower-slope to the ridge on all substrates. Forest structural attributes [stature, above-ground biomass, and leaf area index (LAI)] decreased in the order of Quaternary sedimentary >Tertiary sedimentary >ultrabasic rock in association with soil nutrients on the lower-slopes, and decreased upslope consistently on each of the three substrates. Functional attributes [above-ground net primary productivity (ANPP) and decomposition rate] demonstrated similar patterns to structure. ANPP significantly correlated with LAI among the six sites, while net assimilation rate (ANPP divided by LAI assuming an even productivity between above vs below-ground system) was nearly constant. Therefore, ANPP could be explained primarily by LAI. Topographical change in LAI could be explained by leaf mass per area (LMA) combined with stand-level leaf biomass. LMA increased upslope on all substrates in association with the decrease in individual leaf area. Stand-level leaf biomass decreased upslope on all substrates but the Tertiary sedimentary rock. Our study demonstrated that topography and geological substrates interactively affected forest structure and processes. The effect of topography on forest structure and processes was greater on nutrient-rich substrates than on poor substrates, and the effect of geological substrate was greater on lower-slopes than on ridges.