# Canopy‐Mediated Dynamics of Moss Communities in Primary Succession: Coupling of N2 ‐Fixation and Biomass Accumulation in Subalpine Forests Following Glacial Retreat

## TL;DR

This study explores how moss communities change over 129 years after glaciers retreat, showing how canopy cover and nitrogen inputs affect moss growth and nitrogen fixation in subalpine forests.

## Contribution

The study reveals a strong coupling between moss biomass accumulation and nitrogen fixation, and how canopy structure regulates these processes during primary succession.

## Key findings

- Moss cover and biomass decreased with increasing canopy density.
- Canopy litter and nitrogen inputs influenced moss nitrogen fixation and photosynthesis.
- Moss productivity and ecological functions are regulated by canopy structure and composition.

## Abstract

Accelerated glacial retreat has exposed bare substrates in polar and alpine regions, creating opportunities for investigating primary vegetation succession. Mosses are pioneer species critical for soil development, nutrient cycling, and establishment of subsequent vegetation succession. However, the dynamics of moss communities during primary succession and their responses to canopy‐mediated environmental changes are poorly known. We investigated moss bottom community dynamics along a 129‐year primary successional gradient from barren land to coniferous climax forest on a deglacial foreland in eastern Qinghai‐Tibet Plateau. Additionally, we conducted a reciprocal transplant experiment and a canopy tree litter addition experiment at three succession stages with distinct canopy densities to explore the effects of shifts in canopy composition on the development of the moss bottom layer. Moss biomass and cover in the bottom layer had a nonlinear and fluctuating growth pattern across the primary successional chronosequence, in which successional stages with higher canopy density had lower moss cover and biomass. Transplantation of moss carpets from open to denser canopy stages or canopy litter additions enhanced photosynthetic rates, but suppressed N2‐fixation rates and moss growth. Variations in N2‐fixation and photosynthesis rates were related to daylight hours, relative humidity, and throughfall N levels. Changes in moss bottom layer cover and biomass over the successional chronosequence were positively related to N2‐fixation and regulated by canopy leaf litter and throughfall N inputs. Our results demonstrate a strong coupling between moss biomass and cyanobacterial N2‐fixation, alongside a decoupling of moss photosynthesis from productivity during primary succession following glacial retreat. The effects of canopy cover and composition on moss productivity, photosynthesis, and N2‐fixation rates represent a dynamic set of canopy‐bottom layer interactions that may shape the structure and function of developing subalpine forest.

This study examined the dynamics of moss communities along a 129‐year primary successional gradient on the eastern Qinghai‐Tibet Plateau and explored the effects of canopy composition changes through transplantation and litter addition experiments. Results showed that increasing canopy density led to decreased moss cover and biomass, while canopy litter and nitrogen inputs influenced moss nitrogen fixation and photosynthesis. The findings highlight a strong coupling between moss biomass accumulation and nitrogen fixation, as well as the complex role of canopy structure in regulating moss productivity and ecological functions.