The ability of tropical forests to grow and store carbon is limited, in part, by herbivory. Insects and other animals prefer to feed on nitrogen-fixing trees, reducing the success of fixers and the nitrogen they provide. So reports a new paper out this week in the journal Nature, which recommends accounting for herbivory constraints on nitrogen-fixing trees in climate models and projections of the tropical forest carbon sink.
Herbivory, or the consumption of plants by animals, can pose constraints on the growth and success of nitrogen-fixing trees. Nitrogen-fixing trees are able to convert nitrogen from the atmosphere into a form that can be used by plants, a process known as nitrogen fixation. This is an important process for maintaining soil fertility and supporting the growth of other plants. However, herbivory can limit the ability of nitrogen-fixing trees to fix nitrogen, particularly when the trees are heavily grazed by animals. When animals consume the leaves, stems, and other parts of the trees, they can reduce the trees’ ability to photosynthesize and fix nitrogen.
Some common examples of nitrogen-fixing trees include legumes, such as soybeans and peas, and certain species of acacia and albizia. By partnering with soil microbes, nitrogen-fixing trees turn atmospheric nitrogen gas into a form of nitrogen that is available to plants. When fixers shed their leaves, they enrich soils with nitrogen, benefitting nearby plants. In nitrogen-poor tropical forests, nitrogen-fixing trees are the main source of new nitrogen to soils. In nitrogen-poor tropical forests, plants may struggle to grow and thrive due to a lack of available nitrogen. This can limit the diversity and productivity of the forest ecosystem. Yet they are also rare.
Sarah Batterman, a Tropical Forest Ecologist at Cary Institute of Ecosystem Studies and co-author on the paper, explains, “Tree growth in many tropical forests is limited by lack of nitrogen. Given the substantial benefit of nitrogen to these forests, it has long been a mystery why nitrogen-fixing trees represent just 5-15% of trees. We suspected that herbivores might be preferentially targeting fixers due to their nutritious, nitrogen-rich leaves.”
With colleagues, Batterman set out to reveal if the diets of insects and other herbivores were a constraint.
Their three part study looked at:
(1) whether nitrogen-fixing trees experienced more herbivory than non-fixers,
(2) the carbon cost of herbivory, and
(3) if herbivory was due to herbivore preference for nitrogen-rich leaves.
Fieldwork was performed at Barro Colorado Island in Panama on a 50-hectare plot of mature tropical lowland forest. Seedling trees, a life stage vulnerable to herbivory, were assessed. Leaves were analyzed from 23 fixer species and 20 non-fixer species, representing 350 seedlings and 1,626 leaves. Herbivory on mature leaves was quantified by scanning leaves, and assessing damaged leaf area. For a subset of trees, active herbivory was tracked over a three month period.
Carbon costs of herbivory were determined by looking at the costs of rebuilding lost leaf tissue, and how leaf damage reduced carbon assimilation via photosynthesis. Leaf traits were also collected across species, and included: nutrient concentrations, physical defenses, leaf toughness, and chemical similarity.
It has been observed that nitrogen-fixing trees experience more herbivory than non-fixing trees. Fixers are more vulnerable to herbivory because they invest more resources in nitrogen fixation, leaving them with fewer resources to defend against herbivores. This can make them more attractive to herbivores, who are drawn to the high-nutrient foliage of the trees. As a result, nitrogen fixers trees experienced 26% more herbivory than non-fixers. Their leaves were attacked 21% more than non-fixers, consistent with them being preferentially targeted by insects and other animals.
Fixer seedlings had a higher proportion of leaf area lost than non-fixers, but this number was smaller than expected, indicating that fixers have evolved defense strategies to prevent herbivores from consuming large areas of their leaves. Still, the extent to which this is true will depend on a variety of factors, including the specific species of tree, the type of herbivore, and the ecological conditions of the environment.
Fixers also experience 34% greater carbon opportunity costs due to herbivory than non-fixers, exceeding the metabolic cost of fixing nitrogen. Unexpectedly, high herbivory for fixers was not found to be driven by high leaf nitrogen. The authors note that the only trait that consistently explained all measures of herbivory was the fixation trait itself — which explained up to 24% of variation — suggesting the high herbivory and the trait of fixation are directly linked evolutionarily.
It is also estimated that Nitrogen-fixing trees may experience greater carbon opportunity costs due to herbivory because the process of nitrogen fixation requires a significant amount of energy, which is derived from the tree’s stored carbon. Therefore, if a nitrogen-fixing tree is subjected to herbivory, it may have to use more of its stored carbon to support the nitrogen-fixing process, reducing the amount of carbon available for growth and other processes.
Lead author Will Barker from the University of Leeds explains, “Our findings suggest that nitrogen-fixers bear higher herbivory costs than non-fixers, and that herbivory may be substantial enough to limit the success of nitrogen-fixing trees and their ability to alleviate nitrogen deficits in tropical soils. This has management implications for the species mixes used in reforestation efforts.”
Batterman concludes, “Mature and recovering tropical forests are a large and important carbon sink, yet this sink is weakening due to climate change and potential limitation by nitrogen. The widespread cost of herbivory for nitrogen-fixers should be incorporated in climate change models as a constraint on symbiotic nitrogen fixation and future tropical forest growth.”
Tropical forests play a crucial role in regulating the Earth’s climate by removing carbon from the atmosphere through the process of photosynthesis. In addition to their role as a carbon sink, tropical forests also provide many other important ecosystem services, such as protecting soil and water resources, supporting wildlife habitat, and providing recreational and cultural benefits. However, tropical forests are under threat from a variety of factors, including deforestation, climate change, and fires, which can reduce their ability to sequester carbon and provide other important ecosystem services. It is important to protect and conserve tropical forests in order to maintain their ability to support biodiversity and regulate the Earth’s climate.
Funding for this research was provided in part by the UK Natural Environment Research Council (NE/M019497/1, NE/N012542/1), British Council Grant #275556724, the Leverhulme Trust, and the US National Science Foundation (DEB 1464389).