Vegetation processes are fundamentally limited by nutrient and water availability, the uptake of which is mediated by plant roots in terrestrial ecosystems. While tropical forests play a central role in global water, carbon, and nutrient cycling, we know very little about tradeoffs and synergies in root traits that respond to resource scarcity. Tropical trees face a unique set of resource limitations, with rock-derived nutrients and moisture seasonality governing many ecosystem functions, and nutrient versus water availability often separated spatially and temporally. Root traits that characterize biomass, depth distributions, production and phenology, morphology, physiology, chemistry, and symbiotic relationships can be predictive of plants’ capacities to access and acquire nutrients and water, with links to aboveground processes like transpiration, wood productivity, and leaf phenology. In this review, we identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils. We also identify interesting paradoxes in tropical forest root responses to changing resources that merit further exploration. For example, specific root length, which typically increases under resource scarcity to expand the volume of soil explored, instead can increase with greater base cation availability, both across natural tropical forest gradients and in fertilization experiments. Also, nutrient additions, rather than reducing mycorrhizal colonization of fine roots as might be expected, increased colonization rates under scenarios of water scarcity in some forests. Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between the emphasis in models characterizing nutrient and water uptake rates and carbon costs versus the emphasis in field experiments on measuring root biomass, production, and morphology in response to changes in resource availability. Closer integration of field and modeling efforts could connect mechanistic investigation of fine-root dynamics to ecosystem-scale understanding of nutrient and water cycling, allowing us to better predict tropical forest-climate feedbacks.
more »
« less
This content will become publicly available on August 1, 2025
Calculating Nitrogen Uptake Rates in Forests: Which Components Can Be Omitted, Simplified, or Taken from Trait Databases and Which Must Be Measured In Situ?
Quantifying nitrogen uptake rates across different forest types is critical for a range of ecological questions, including the parameterization of global climate change models. However, few measurements of forest nitrogen uptake rates are available due to the intensive labor required to collect in situ data. Here, we seek to optimize data collection efforts by identifying measurements that must be made in situ and those that can be omitted or approximated from databases. We estimated nitrogen uptake rates in 18 mature monodominant forest stands comprising 13 species of diverse taxonomy at the Morton Arboretum in Lisle, IL, USA. We measured all nitrogen concentrations, foliage allocation, and fine root biomass in situ. We estimated wood biomass increments by in situ stem diameter and stem core measurements combined with allometric equations. We estimated fine root turnover rates from database values. We analyzed similar published data from monodominant forest FACE sites. At least in monodominant forests, accurate estimates of forest nitrogen uptake rates appear to require in situ measurements of fine root productivity and are appreciably better paired with in situ measurements of foliage productivity. Generally, wood productivity and tissue nitrogen concentrations may be taken from trait databases at higher taxonomic levels. Careful sorting of foliage or fine roots to species is time consuming but has little effect on estimates of nitrogen uptake rate. By directing research efforts to critical in situ measurements only, future studies can maximize research effort to identify the drivers of varied nitrogen uptake patterns across gradients.
more »
« less
- Award ID(s):
- 2243586
- PAR ID:
- 10552557
- Publisher / Repository:
- Springer Nature Link
- Date Published:
- Journal Name:
- Ecosystems
- Volume:
- 27
- Issue:
- 5
- ISSN:
- 1432-9840
- Page Range / eLocation ID:
- 739 to 763
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Greater tree density and forest productivity at the tundra–taiga ecotone (TTE) are expected with climate warming, with potential feedbacks to the climate system. Yet, competition for nitrogen (N) may impact TTE dynamics. Greater tree density will likely increase N demand, while reducing N supply through soil shading and slower decomposition rates. We explored whether characteristics of roots and root‐associated fungi important to N acquisition responded to changes in density at the TTE and were related to above‐ground stand productivity and N cycling.
We measured rooting depth, uptake of N forms among soil layers and ectomycorrhizal (EcM) colonization and composition along a natural tree density gradient of monodominant larch
Larix cajanderi in northeastern Siberia. We tested relationships between larch root and fungal characteristics, above‐ground productivity and stand‐level N cycling parameters.Overall, there was preferential uptake of ammonium compared to glycine or nitrate. Nitrogen uptake was greatest in shallow soils of the organic horizon and related to root chemistry, root‐associated fungi and above‐ground N cycling parameters, but the direction of these relationships depended on N form. Uptake of different N forms, rooting depth and EcM colonization and composition were not related to tree density, but fungal composition was correlated with root N chemistry and above‐ground N cycling parameters. In addition to EcM, the abundance of dark septate endophytes and other ascomycetous taxa was positively related to N uptake and above‐ground N cycling parameters.
Synthesis . There was little impact of tree density on root and fungal parameters related to N acquisition suggesting intraspecific larch competition for N was not amplified with increased density. There was, however, a strong impact of root‐associated fungi on N uptake and stand N dynamics regardless of tree density. Together, this suggests an important role of root‐associated fungi on broadscale patterns of N cycling in TTE larch forests independent of changes in tree density expected with climate warming. -
more » « less
Abstract Fine roots constitute a significant component of the net primary productivity (NPP) of forest ecosystems but are much less studied than aboveground NPP. Comparisons across sites and regions are also hampered by inconsistent methodologies, especially in tropical areas. Here, we present a novel dataset of fine root biomass, productivity, residence time, and allocation in tropical old‐growth rainforest sites worldwide, measured using consistent methods, and examine how these variables are related to consistently determined soil and climatic characteristics. Our pantropical dataset spans intensive monitoring plots in lowland (wet, semi‐deciduous, and deciduous) and montane tropical forests in South America, Africa, and Southeast Asia (
n = 47). Large spatial variation in fine root dynamics was observed across montane and lowland forest types. In lowland forests, we found a strong positive linear relationship between fine root productivity and sand content, this relationship was even stronger when we considered the fractional allocation of total NPP to fine roots, demonstrating that understanding allocation adds explanatory power to understanding fine root productivity and total NPP. Fine root residence time was a function of multiple factors: soil sand content, soil pH, and maximum water deficit, with longest residence times in acidic, sandy, and water‐stressed soils. In tropical montane forests, on the other hand, a different set of relationships prevailed, highlighting the very different nature of montane and lowland forest biomes. Root productivity was a strong positive linear function of mean annual temperature, root residence time was a strong positive function of soil nitrogen content in montane forests, and lastly decreasing soil P content increased allocation of productivity to fine roots. In contrast to the lowlands, environmental conditions were a better predictor for fine root productivity than for fractional allocation of total NPP to fine roots, suggesting that root productivity is a particularly strong driver of NPP allocation in tropical mountain regions. -
Functional balance theory predicts that plants will allocate less carbon belowground when the availability of nutrients is elevated. We tested this prediction in two successional northern hardwood forest stands by quantifying fine root biomass and growth after 5–7 years of treatment in a nitrogen (N) x phosphorus (P) factorial addition experiment. We quantified root responses at two different levels of treatment: the whole-plot scale fertilization and small-patch scale fertilization of ingrowth cores. Fine root biomass was higher in plots receiving P, and fine root growth was highest in plots receiving both N and P. Thus, belowground productivity did not decrease in response to long-term addition of nutrients. We did not find conclusive evidence that elevated availability of one nutrient at the plot scale induced foraging for the other nutrient at the core scale, or that foraging for nutrients at the core scale responded to addition of limiting nutrients. Our observations suggest NP co-limitation of fine root growth and indicate complex interactions of N and P affecting aboveground and belowground production in early successional northern hardwood forest ecosystems.more » « less
-
Overstory foliage is collected in late summer from a reference forest to the west of Watershed 6 (also referred to as Bear Brook Watershed). Concentrations of C, N, P, K, Ca, Mg, and the natural abundance of N and C isotopes (delta-15N and delta-13C) in foliage are measured. These measurements, in combination with litterfall estimates of foliar biomass, allow us to estimate the pool of nutrients in foliage. They also allow us to estimate nutrient retranslocation, using measurements of leaf litterfall chemistry. Long-term measurements continue with the aim of detecting disturbances in nutrient cycling and trends in foliar chemistry over long time scales. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.more » « less
