# Novel Patterns of Photosynthetic Activity Within Canopies of Poplar Stands with Different Densities

## TL;DR

This study examines how different planting densities affect photosynthetic activity in poplar canopies, revealing how canopy layers perform under varying conditions.

## Contribution

The study identifies novel patterns of photosynthetic activity in poplar canopies across different planting densities and canopy layers.

## Key findings

- Photosynthetic traits like Pn, Tr, and Gs decrease as canopy layers decline, while Ci increases.
- Leaf traits like LL, LW, LA, and LWC increase with decreasing LL and are negatively correlated with photosynthetic traits.
- Lower planting density (D3) allows all canopy layers to perform efficient photosynthesis, unlike higher densities.

## Abstract

(1) Background: Planting density is an important factor affecting the yield of poplar per unit area. Therefore, determining the optimal height of the photosynthetic canopy layer for different planting densities is critical. (2) Methods: This study takes Populus euramericana ‘N3016’ × Populus ussuriensis as the research object. According to on the average tree height, diameter at breast height, and crown width of the stand, one standard tree was selected from each planting density for the experiment. The canopy of the standard tree was divided into five canopy layers from top to bottom, and the first-order lateral branches of each canopy layer were divided into three sites from outside to inside. The photosynthesis and leaf traits at various positions in different canopy layers were measured. (3) Results: The results revealed significant differences in photosynthetic and leaf traits at different positions of different canopy layers under different planting densities. As the canopy layer gradually declined, photosynthetic traits revealed that instantaneous photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) gradually decreased, while intercellular CO2 concentration (Ci) increased. Moreover, water use efficiency (WUE) initially increased and then decreased under an 825 trees·ha−1 (D3) planting density. Leaf traits revealed that as leaf length (LL) gradually decreased, leaf width (LW), leaf area (LA), and leaf water content (LWC) gradually increased. Under three planting densities, leaf traits were negatively correlated with Pn, Tr, WUE, and Gs, but positively correlated with Ci. (4) Conclusions: As the planting density decreased, the photosynthetic capacity of poplar gradually increased. With a planting density of D3, all canopy layers were able to carry out efficient photosynthesis, and all living branches within the canopy were functional. However, under the planting density of 1650 trees·ha−1 (D1) and 1089 trees·ha−1 (D2), canopy layers 1 to 4 could perform effective photosynthesis, while the photosynthetic capacity of canopy layer 5 was relatively weak. This study reveals the interactive effects of canopy position and stand density on leaf physiological and morphological traits, providing new insights into the photosynthetic efficiency and growth strategies of poplar under different planting densities. It also offers theoretical support for optimizing stand management and enhancing productivity.