Leaf Physiological and Morphological Responses to Shade in Grass-Stage Seedlings and Young Trees of Longleaf Pine

Lisa J Samuelson,Tom A Stokes

doi:10.3390/f3030684

Abstract

Longleaf pine has been classified as very shade intolerant but leaf physiological plasticity to light is not well understood, especially given longleaf pine’s persistent seedling grass stage. We examined leaf morphological and physiological responses to light in one-year-old grass-stage seedlings and young trees ranging in height from 4.6 m to 6.3 m to test the hypothesis that young longleaf pine would demonstrate leaf phenotypic plasticity to light environment. Seedlings were grown in a greenhouse under ambient levels of photosynthetically active radiation (PAR) or a 50% reduction in ambient PAR and whole branches of trees were shaded to provide a 50% reduction in ambient PAR. In seedlings, shading reduced leaf mass per unit area (LMA), the light compensation point, and leaf dark respiration (RD), and increased the ratio of light-saturated photosynthesis to RD and chlorophyll b and total chlorophyll expressed per unit leaf dry weight. In trees, shading reduced LMA, increased chlorophyll a, chlorophyll b and total chlorophyll on a leaf dry weight basis, and increased allocation of total foliar nitrogen to chlorophyll nitrogen. Changes in leaf morphological and physiological traits indicate a degree of shade tolerance that may have implications for even and uneven-aged management of longleaf pine.

Highlights

Longleaf pine (Pinus palustris Mill.) is a conifer of the southeastern U.S that once dominated the southern landscape, but because of land use changes, difficulties in seedling establishment, lack of fire and conversion to other southern pines, it occupies 3%–5% of the original expanse [1,2]
Shade treatments consisted of a control treatment in which seedlings received ambient levels of photosynthetically active radiation (PAR) in the greenhouse or a 50% shade treatment in which ambient PAR in the greenhouse was reduced by 50%
Leaf mass per unit area has been used as a measure of foliar plasticity to light environment, and lower leaf mass per unit area (LMA) is a common response to shaded environments, in seedlings [31]

Summary

Introduction

Longleaf pine (Pinus palustris Mill.) is a conifer of the southeastern U.S that once dominated the southern landscape, but because of land use changes, difficulties in seedling establishment, lack of fire and conversion to other southern pines, it occupies 3%–5% of the original expanse [1,2]. The open-canopy structure of natural longleaf pine ecosystems is characterized by different age cohorts of longleaf pine in varying gap sizes within a mosaic of diffuse and direct light [3]. There is a renewed interest in restoring the longleaf pine ecosystem but successful establishment of longleaf pine is limited by an incomplete understanding of factors influencing juvenile growth [4]. Rodriquez-Trejo et al [7] observed that shade from adult longleaf pine trees had a nurse tree effect on longleaf pine seedlings along gap edges presumably by decreasing transpirational water losses and mortality from drought. A strong positive relationship between light availability in gaps and seedling growth has been reported by McGuire et al [8]

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