Chronic insect herbivores accelerate litter decomposition and nutrient recycling rates along an environmental/herbivory gradient in northern Patagonia

Abstract

Canopy insects are the main consumers in forests, and understanding their ecological role includes considering their effects on litter decomposition and nutrient cycling. We experimentally evaluated how litter decomposition is shaped by the decomposition environment and by litter traits shaped by forest moisture origin and chronic insect activity. We selected 4 forest stands at the driest and wettest extremes of beech (Nothofagus pumilio) forests in northwest Patagonia (mean annual rainfall 800 and 3000 mm yr−1 respectively). In two stands from each extreme, we performed three litterbag decomposition experiments. Experiment 1 included a reciprocal litter transplant between dry and wet stands, replicated in 2 different years for 24 and 12 months. Litterbags comprised the “Natural” proportion of undamaged and damaged leaves from the local insect community. Experiments 2 and 3 examined leaf insect damage effects on litter decomposition, respectively “in situ” and under controlled conditions. Here, we included three types of litterbags: 1- “Natural” (described above), 2- only insect-damaged litter, and 3- only healthy (undamaged) litter. Reciprocal transplants showed that litter decomposition was mostly controlled by litter origin and less by the decomposition environment (wet or dry forests). But, challenging predictions based on litter quality produced under water limiting environments, “Natural” litter chemical quality did not differ between forests. Even, when compared under the same decomposition environment (wet or dry forest), “Natural” litter from dry forests decomposed 31% faster and released 109% more N than “Natural” litter from wet forests. “Natural” litter from dry forests had a higher herbivory frequency and higher dominance of endophagous insects than wet stands. The high proportion of damaged litter in dry forests decreased “Natural” litter C:N ratio to match the chemical quality of “Natural” litter from wet forests. Experiment 2 showed that, in both wet and dry forests, damaged litter decomposed 31% faster than undamaged litter (in situ). Insect damage also reduced 90% N retention, but only in dry forests. Our results suggest that differences in insect communities living in contrasting environments could act as key drivers in shaping the recycling of organic matter, counteracting environmental differences between forests.