Functional Ecology Of Twig Architecture Of Woody Species From A Subtropical Evergreen Broad-leaved Forest In Tiantong, Zhejiang, China

Light interception is important for plant growth, reproduction and survival. Light interception at twig (a current-year shoot) scale may significantly determine the total quantum flux captured by plants, such that twigs are thought of a convenient surrogate for whole plants in studies of plant strategies of light interception. Twig architecture is plastic to surrounding light environments of plants. However, studies have seldom addressed the role of leaf temporal-spatial pattern within-twigs (e.g., leaf distribution patterns along the lengths of twig stems and the leaf-stem growth synchrony) in the optimization of plant light interception.In theory, plants can alter the distribution of leaves along the associated twigs (i.e., within-twig leaf distribution patterns) to optimize light interception at given architectures of their leaves, branches and canopies. Similarly, during twig development, the synchronies between total leaf area and stem length growths and between leaf number and leaf size growths may affect leaf display in time and space and hence influence twig light interception. I used the Y-plant model (a3-dimensional plant architecture model) to estimate the potential effect of the leaf distribution pattern on twig light interception and the potential effect of the synchronous pattern between leaf size and leaf number growths on light interception efficiency. The model simulations show that twigs with even leaf distribution patterns have higher light interception efficiency than those with aggregate leaf distribution patterns. Simulating results also show that twigs with flushing leaf emergence patterns (i.e., leaves emerge as a single flush immediately after bud-break and then expands) show obvious mutual shading among leaves on the twigs and have low light interception efficiency.Based on above model simulations, I hypothesize that plants with different life-forms (e.g., evergreen vs. deciduous species, sun-vs. shade-adapted species) are different in leaf distribution patterns and are different in the synchronous patterns between the growths of leaves and stems during twig development because they differ in light environments and in light demanding. I measured morphological, architectural and physiological characteristics for45canopy tree species (9deciduous species and15evergreen species) and20understory evergreen shrub species (8shade-adapted and13sun-adapted species) in a subtropical evergreen broad-leaved forest Tiantong, Zhejiang China. The results are shown in the following.(1) Leaf distribution evenness (CV, determined as the coefficient of variation of within-twig internode lengths; the higher the CV, the less evenly spaced leaves) differed significantly among life-forms.In the canopy layer, deciduous species (which were characterized by high light-saturated net photosynthetic rates, Amax; light compensation points, LCP; and light saturation points, LSP) had more even leaf distribution patterns than evergreen species (which had low LCP, LSP and Amax); shade-adapted evergreen species had more even leaf distribution patterns than sun-adapted evergreen species. Moreover, CV was positively correlated with large leaf dry mass per leaf area (LMA) and large leaf and twig inclination angles, which collectively specify a typical trait combination adaptive to low light interception, as indicated by both ordinary regression and phylogenetic generalized least squares analyses. These relationships were also valid within the evergreen tree species-group (which had the largest sample size). I propose that the leaf distribution pattern (i.e.,"evenness" CV, which is an easily measured functional trait) can be used to distinguish among life-forms in communities similar to the one examined in this study.(2) There was a significant trade-off between leaf light interception and water shedding. Species in high-rainfall regions have two major alternative approaches to quickly drain off water, i.e. increasing leaf inclination angles relative to the horizontal plane, or developing long leaf drip tips. The study showed that shade-adapted understory species were lower in LMA, Amax, LSP and LCP compared to understory or canopy sun-adapted species, and that their leaf and twig inclination angles were significantly smaller and leaf drip tips were significantly longer than those in sun-adapted species. This suggests that shade-adapted understory species tend to develop pronounced leaf drip tips but not large leaf inclination angles to shed water. The length of leaf drip tips was negatively correlated with the leaf inclination angle and photosynthetic capacity associated traits (including Amax, LSP and LCP). These relationships were consistent between ordinary regression and phylogenetic generalized least squares analyses. The study illustrates the trade-offs between leaf light interception and leaf water shedding and indicates that the length of leaf drip tips can be used as an indicator of adaptation to shady conditions and overall photosynthetic performance of shrub species in subtropical rainforest.(3) The synchronous patterns between the growths of leaves and stems differed significantly among life-forms. Sun-adapted species showed less synchronous patterns in the growths of the total leaf area vs. the stem length (being priority in growth of the stem length) and the leaf size vs. the leaf number (being priority in growth of the leaf number) during twig development compared to shade-adapted species. Moreover, species with less synchronous patterns were generally characterized by higher leaf emergence rates and leaf expansion rates. The different synchronous patterns among life-forms can be attributed to the environmental conditions and the species traits. The study indicates that the temporal dynamic during twig development reflects species ecological strategies for light interception and carbon gains, and that the temporal dynamics of the leaf size-twig size spectrum might be of adaptive significance in plants.I conclude that plants of different life-forms differ in ways to optimize light interception and maximize plant carbon gains. I propose that the temporal dynamics of functional traits deserve further examinations.