Age-related changes in ecosystem structure and function and effects on water and carbon exchange in ponderosa pine

Abstract

As forests age, their structure and productivity change, yet in some cases, annual rates of water loss remain unchanged. To identify mechanisms that might explain such observations, and to determine if widely different age classes of forests differ functionally, we examined young (Y, approximately 25 years), mature (M, approximately 90 years) and old (O, approximately 250 years) ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) stands growing in a drought-prone region of central Oregon. Although the stands differed in tree leaf area index (LAIT) (Y = 0.9, M = 2.8, O = 2.1), cumulative tree transpiration measured by sap flow did not differ substantially during the growing season (100-112 mm). Yet when water was readily available, transpiration per unit leaf area of the youngest trees was about three times that of M trees and five times that of O trees. These patterns resulted from a nearly sixfold difference in leaf specific conductance (KL) between the youngest and oldest trees. At the time of maximum transpiration in the Y stand in May-June, gross carbon uptake (gross ecosystem production, GEP) was similar for Y and O stands despite an almost twofold difference in stand leaf area index (LAIS). However, the higher rate of water use by Y trees was not sustainable in the drought-prone environment, and between spring and late summer, KL of Y trees declined fivefold compared with a nearly twofold decline for M trees and a < 30% reduction in O trees. Because the Y stand contained a significant shrub understory and more exposed soil, there was no appreciable difference in mean daily latent energy fluxes between the Y stand and the older stands as measured by the eddy-covariance technique. These patterns resulted in 60 to 85% higher seasonal GEP and 55 to 65% higher water-use efficiency at the M and O stands compared with the Y stand.

Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift

“…), assuming that trees had reached equilibrium with the soil during the night, and that ΨPD represents an estimate of soil water potential [45]. The following equation was used:…”

“…Compared to P. sylvestris, Q. ilex showed lower ΨPD and a steeper decline of this variable with decreasing SWC. If we assume that ΨPD is in equilibrium with SWC around roots [45], these results would suggest that P. sylvestris roots have access to wetter soil pockets. This result contradicts the evidence showing that Q. ilex trees tend to be particularly deep rooted and reach deeper (and presumably wetter) soil layers than pines [55]; and authors' personal observations in the study area], as also shown in other pine-oak ecosystems, where higher ΨPD values were found for oak trees compared to co-occurring pines [56].…”

Hurricane impacts on a pair of coastal forested watersheds: implications of selective hurricane damage to forest structure and streamflow dynamics

“…In addition, WS77 had twice as many pine seedlings and more pine saplings per hectare than WS80. Young pines have much higher water use rates than mature pines (Irvine et al, 2004) due to both leaf area and high transpiration per unit leaf area (Delzon and Loustau, 2005). Song et al (2012) analyzed the recovery of South Carolina forests after Hugofour of their 1 ha plots are located in WS80 and were measured from 1994 through 2012.…”

Scaling Up of Leaf Transpiration and Stomatal Conductance of Eucalyptus grandis x Eucalyptus urophylla in Response to Environmental Variables

“…As trees age, their hydraulic properties change, and at the same time, the amount of radiation intercepted by the canopy varies substantially [17]. With the increased size of the tree, water and nutrients must be transported over increasing distances between the root and the apex [7].…”