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<p>In mountain ecosystems, altitudinal environmental gradients drive morphological and physiological adaptations in plants. A few studies have observed the increased accumulation of nitrogen in leaves at higher altitudes, which enhances the photosynthetic capacity. Assessing canopy nitrogen content along environmental gradients is crucial for understanding plant growth, health, and dry matter quality at the landscape scale and helps understand species adaptability. This study tests the hypothesis that canopy nitrogen content (CNC) of Himalayan forests increases with altitude. We estimated the spatial distribution of CNC of subtropical broadleaved sal (<i>Shorea robusta</i>) and needle-leaved chir pine (<i>Pinus roxburghii</i>) forests in part of North-West Himalaya (India) using in-situ measurements, PRISMA hyperspectral satellite images and partial least squares regression (PLSR) modeling. Spatial variability in CNC along topographic gradients (elevation, slope, aspect and topographic wetness index) was examined using two-way ANOVA and Generalized Boosted Regression Modelling (GBM). PLSR models, cross validated under limited sample size, based on reflectance bands, performed best for sal (<i>R</i><sup>2</sup> = 0.95 and RMSE of 0.02&#xa0;g&#xa0;m<sup>−2</sup>), whereas FDS, highlighting the subtle absorption features performed best for chir pine (<i>R</i><sup>2</sup> = 0.93 and RMSE = 0.05&#xa0;g&#xa0;m<sup>−2</sup>). Genetic Algorithm based variable selection found reflectance at 1726&#xa0;nm and FDS between 1636 to 1687&#xa0;nm as sensitive predictors of CNC across both forest formations. Additionally, wavelengths 929&#xa0;nm and 939&#xa0;nm were found to be the most influential predictors of CNC of sal and chir pine, respectively. Canopy nitrogen maps revealed that CNC increases with elevation, while higher CNC per unit area was observed in needle-leaved chir pine than in broadleaved sal forests, attributed to differences in leaf mass per area (LMA). CNC varied significantly across topographic gradients (<i>p</i> &lt; 0.01), with elevation (49.16%) and aspect (52.49%) showing the highest relative importance for sal and chir pine, respectively. Our study indicates that the CNC of sal and chir pine forests in the subtropical zone of North-West Himalaya exhibits a significant increase with elevation.</p>

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PRISMA Imaging Spectroscopy Reveals Topographic Control Over Canopy Nitrogen Variability in Subtropical Himalayan Forests

  • 期刊论文

  • 发表日期：

    2026年4月13日

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  • Mahima .,
  • Hitendra Padalia,
  • Taibanganba Watham,
  • Ishwari Datt Rai,
  • Subrata Nandy

摘要

In mountain ecosystems, altitudinal environmental gradients drive morphological and physiological adaptations in plants. A few studies have observed the increased accumulation of nitrogen in leaves at higher altitudes, which enhances the photosynthetic capacity. Assessing canopy nitrogen content along environmental gradients is crucial for understanding plant growth, health, and dry matter quality at the landscape scale and helps understand species adaptability. This study tests the hypothesis that canopy nitrogen content (CNC) of Himalayan forests increases with altitude. We estimated the spatial distribution of CNC of subtropical broadleaved sal (Shorea robusta) and needle-leaved chir pine (Pinus roxburghii) forests in part of North-West Himalaya (India) using in-situ measurements, PRISMA hyperspectral satellite images and partial least squares regression (PLSR) modeling. Spatial variability in CNC along topographic gradients (elevation, slope, aspect and topographic wetness index) was examined using two-way ANOVA and Generalized Boosted Regression Modelling (GBM). PLSR models, cross validated under limited sample size, based on reflectance bands, performed best for sal (R2 = 0.95 and RMSE of 0.02 g m−2), whereas FDS, highlighting the subtle absorption features performed best for chir pine (R2 = 0.93 and RMSE = 0.05 g m−2). Genetic Algorithm based variable selection found reflectance at 1726 nm and FDS between 1636 to 1687 nm as sensitive predictors of CNC across both forest formations. Additionally, wavelengths 929 nm and 939 nm were found to be the most influential predictors of CNC of sal and chir pine, respectively. Canopy nitrogen maps revealed that CNC increases with elevation, while higher CNC per unit area was observed in needle-leaved chir pine than in broadleaved sal forests, attributed to differences in leaf mass per area (LMA). CNC varied significantly across topographic gradients (p < 0.01), with elevation (49.16%) and aspect (52.49%) showing the highest relative importance for sal and chir pine, respectively. Our study indicates that the CNC of sal and chir pine forests in the subtropical zone of North-West Himalaya exhibits a significant increase with elevation.