Forests are the major terrestrial carbon sink and are critically important for the global carbon cycle. Woody plants in forests are the major elements that contribute to the water and carbon cycle between soil, forest and atmosphere. Their roots absorb water from the soil, the stems transport the water to the leaves where >98% of the acquired water transpires back to the atmosphere. Trees fix carbon via photosynthesis in the leaves and use the carbon for maintaining metabolic processes and growth. Plants are supposed to maintain functional balances for water and carbon. For water, they are expected to balance the acquisition of water via the roots with the transport of water in the stem xylem, the storage of water in parenchyma and the loss of water through the leaf stomata. For carbon, they balance the carbon gain in the leaves with the transport of sugars in the phloem within the bark and the storage of carbon in stem parenchyma. In addition, the large woody plants such as trees and lianas are expected to coordinate their stem and crown in such a way that they do not break and maintain a stable plant body. These balances are driven by structural and physiological properties.
In this study, I focus on large woody plants – trees but also lianas – and investigate how they coordinate their functional balances. The main objective of my study is to quantify how canopy co-existing tall woody plants balance the acquisition of carbon and loss of water in the leaves with the transport and storage of water and carbon in the stem. I therefore study their canopy branches, which are probably most important for the carbon gain and water loss of the entire woody plant. I compare deciduous tree species that differ in shade tolerance but co-exist in a temperate Dutch forest, conifer and broadleaved tree species that co-exist in a Chinese temperate forest, and tree and liana species that co-exist in a Chinese tropical rain forest.
In chapter 2, I address the question how trees differ in their functional ratios between leaf area, xylem area and phloem area across deciduous species in a temperate forest. I present a study on 10 deciduous tree species co-existing in an even-aged Dutch forest. I found that the area-based functional ratios did not differ consistently between sun and shade branches, but light-demanding species produced more xylem area and phloem area per leaf area than shade-tolerant species probably to compensate for their higher water loss rates and carbon gain rates in the leaves. This study thus shows that tree species differ in their branch structure to maintain similar functional balances of carbon gain in the leaves versus carbon transport in the phloem, and of water loss in leaves and water transport in the xylem.
In chapter 3, I question how conifer and broadleaved tree species differ in their functional balances between water and carbon related functions in a temperate forest. I compare 5 conifer tree species with 9 deciduous broadleaved tree species in a Chinese temperate forest. Conifers are tracheid-based gymnosperms that have a lower water transport efficiency than vessel-based broadleaved angiosperms. I evaluated if this difference in the water transporting tissue causes a divergence in the functional balances between conifers and broadleaved trees. I therefore studied the ratios in xylem area to leaf area and in phloem area to leaf area between conifers and broadleaved trees. I found that conifers tend to increase xylem area to the amount of leaf area, probably to compensate for the low water transport efficiency in xylem, while phloem area to the amount of leaf area did not differ between conifers and broadleaved trees. Thus, in line with the results of chapter 2, these results indicate that trees tend to enlarge their xylem area to increase their water supply to leaves when those leaves are more active in terms of high water loss rates and high carbon gain rates.
In chapter 4, I question how liana and tree species coordinate possible trade-offs between hydraulic conductivity (water transport efficiency), hydraulic safety (drought resistance) and mechanical safety. I compared 12 liana species with 10 tree species in a tropical evergreen forest in China. Lianas differ from trees by relying on adjacent trees to reach the forest canopy whereas trees support themselves, with possible implications for their mechanical and hydraulic properties. Unexpectedly, I found that lianas have stronger wood but similar wood density compared to trees, and that lianas and trees did not differ in hydraulic traits. Besides, no trade-offs were found between hydraulic traits and mechanical traits, against my expectation. This lack of trade-offs may imply that these adult woody plants, exposed to similar atmospheric conditions, converged in their traits. In contrast, other species communities sometimes show trait differences and trade-offs.
Woody plants thus seem to coordinate their ratios between leaf area, xylem area and phloem area in different ways, since those balances differed in relation to the shade-tolerance of deciduous trees, the tree type (conifers versus broadleaved species), and leaf habit (evergreen vs. deciduous). These differences across species imply that global warming may affect species of temperate forests differently, and thus ultimately change the species composition and related carbon and water cycling in temperate forests. For the tropical forest, I showed that trees and lianas remarkably converged in most hydraulic functions, but not in their mechanical traits. It is not clear whether or not such similarities are shared among many tropical forests, since only few studies report such results. The weak trade-offs between hydraulic and mechanical functions make it hard to speculate on consequences of climate change for the species composition and water and carbon cycle of these forests. In order to answer how these forests will change, I call for studies that link the observed functional differences to overall tree performance properties, such as tree growth rates and survival probability.