Dirk W. Vanderklein

Size-mediated ageing reduces vigour in trees.

There is increasing interest in understanding the costs and benefits of increased size and prolonged lifespan for plants. Some species of trees can grow more than 100 m in height and can live for several millennia, however whether these achievements are obtained at the cost of some other physiological functions is currently unclear. As increases in size are usually associated with ageing, it is also unclear whether observed reductions in growth rates and increased mortality rates are a function of size or of age per se. One theory proposes that reduced growth after the start of the reproductive phase is caused by cellular senescence. A second set of theories has focussed instead on plant size and the increased respiratory burdens or excessive height. We report on experimental manipulations to separate the effects of extrinsic factors such as size from those of intrinsic factors such as age for four tree species of contrasting phylogeny and life history. For each species, we measured growth, gas exchange and leaf biochemical properties for trees of different ages and sizes in the field and on propagated material obtained from the same genetic individuals but now all of small similar size in our common gardens. For all species, evidence indicated that size, not cellular senescence, accounted for the observed age-related declines in relative growth rates and net assimilation rates. Two species exhibited evidence of genetic control on leaf characters such as specific leaf area, although size also exerted an independent, and stronger, effect. We found partial support for the theory of hydraulic limitations to tree growth. The lack of a marked separation of soma and germline, an unlimited proliferation potential of meristem cells and the exponential increase in reproductive effort with size all help explain the lack of a senescence-induced decline in trees. It is possible that trees much older than the ones we sampled exhibit senescence symptoms.

Growth, nutrition and gas exchange of Pinus resinosa following artificial defoliation

SummaryIn three experiments, red pine (Pinus resinosa Ait.) seedlings and trees were subjected to artificial defoliations of varying intensities and subsequent growth, gas exchange and nutritional responses were monitored. In Experiment 1, 2-year-old seedlings received 0, 1 or 2 50% defoliations during a single growing season and were maintained in 1 of 3 low nutrient supply treatments. In Experiment 2, response of 4-year-old seedlings was monitored in the year following 0, 25, 50 or 75% defoliation, while in Experiment 3, response of 11-year-old trees was measured 1 year after being defoliated by 0, 33 or 66%. Regardless of intensity of defoliation, or plant size, clipped plants made qualitatively similar allocational and metabolic adjustments over time. First, leaf diffusive conductance and rates of net photosynthesis were stimulated, especially by light to intermediate defoliation. However, there was no effect of defoliation on foliar nitrogen concentration, and elevated gas exchange rates apparently resulted from altered root-shoot dynamics. Second, allocation of new biomass was preferentially shifted towards foliage at the expense of roots, gradually restoring (but undershooting or overshooting) the ratio of foliage: roots of control plants. During the period when foliage: root balance was being restored, the stimulation of needle gas exchange rates disappeared. Plants defoliated by 25% overcompensated in terms of whole plant growth (were larger at harvest than controls), due to shifts in allocation and enhanced photosynthesis. Defoliated plants also stored a proportionally greater share of their carbohydrate reserves in roots than did control plants, even 1 year after clipping.

Forest response and recovery following disturbance in upland forests of the Atlantic Coastal Plain

Carbon and water cycling of forests contribute significantly to the Earths overall biogeochemical cycling and may be affected by disturbance and climate change. As a larger body of research becomes available about leaf-level, ecosystem and regional scale effects of disturbances on forest ecosystems, a more mechanistic understanding is developing which can improve modeling efforts. Here, we summarize some of the major effects of physical and biogenic disturbances, such as drought, prescribed fire, and insect defoliation, on leaf and ecosystem-scale physiological responses as well as impacts on carbon and water cycling in an Atlantic Coastal Plain upland oak/pine and upland pine forest. During drought, stomatal conductance and canopy stomatal conductance were reduced, however, defoliation increased conductance on both leaf-level and canopy scale. Furthermore, after prescribed fire, leaf-level stomatal conductance was unchanged for pines but decreased for oaks, while canopy stomatal conductance decreased temporarily, but then rebounded the following growing season, thus exhibiting transient responses. This study suggests that forest response to disturbance varies from the leaf to ecosystem level as well as species level and thus, these differential responses interplay to determine the fate of forest structure and functioning post disturbance.

Allometry and photosynthetic capacity of poplar (Populus deltoides) along a metal contamination gradient in an urban brownfield

The value of urban green space has become an increasingly controversial issue. In particular, development of novel vegetative assemblages on vacant lands may have many formerly unrecognized benefits. However, questions remain regarding the functional impairment of these assemblages due to degraded soils. We investigated the effects of elevated concentrations of soil metals on allometry and photosynthetic capacity of naturally colonized poplars (Populus deltoides Michx.) growing at Liberty State Park, New Jersey. We found that allometries of harvested trees did not differ significantly between the low metal load (LML) and high metal load (HML) sites suggesting that metal contamination did not negatively affect carbon allocation patterns of the target species. Likewise, photosynthetic parameters did not differ significantly between the LML and HML sites, suggesting that soil metal contamination did not negatively affect photosynthetic capacity. However, trees from the LML site were significantly younger for a given size than trees from the HML site. Trees from the medium metal load site (MML) differed significantly in allometry and photosynthetic parameters from the other two sites suggesting other edaphic and ecological factors are a stronger driver for carbon allocation patterns and photosynthetic capacity in these trees. Taken together, this research suggests that sapling establishment and growth may be impaired by heavy metals and that trees in HML sites may incur more maintenance costs than trees in LML sites. However, given enough time, poplars can provide considerable rehabilitation of urban brownfields, particularly those that exhibit soil metal contamination.

Plant Architecture and Leaf Damage in Bear Oak II: Insect Usage Patterns

Abstract Bear oak bushes (Quercus ilicifolia) have an inside-outside architecture where there is a set of leaves partially or completely concealed within the bush by an exterior set of leaves. We examine the impact of this architecture on microhabitat differences that are important to insect herbivores. Furthermore, we document the patterns of usage by plant-feeding insects on the inside and outside of the bush in different areas of the pine barrens of Long Island, NY. We show that there is more leaf-chewing damage and more galls of the Cynipid wasp, Amphibilops ilicifolia, on the outside of the plant. This inside-outside pattern for leaf-chewing damage is consistent across different sites, though the degree of difference between the inside and outside varies by location. We suggest that the observed herbivory pattern may be generated by differential larval performance on the inside vs. the outside leaves, differential ovipositional choice between the inside vs. the outside of the bush, or both.

Crown conductance in dwarf, medium, and tall pitch pines in the Long Island Pine Plains

The New York Pine Plains are a unique ecosystem with normal statured and a dwarfed variety of pitch pines (Pinus rigida Mill.). Growing interspersed with the dwarf pines are trees of intermediate height and features. Several hypotheses have been put forward as to why some of the trees are dwarfed, but none have been substantiated. In this study, we tested whether dwarf or medium trees are hydraulically limited compared to normally growing trees. Granier style sap flux sensors were installed in three to six trees of each tree type and sap flux was measured in early August 2004. Sap flux measurements were scaled to crown stomatal conductance using leaf area to sapwood area ratios for each tree. Contrary to expectation, dwarf and medium stature trees had very low leaf area to sapwood area ratios, but high crown stomatal conductances compared to normal trees. Analyses of leaf area, ring widths, and crown stomatal conductance indicate that differences between normal, and dwarf and medium pines are not due to hydraulic limitation, but that stunted growth may be due to other causes.

Plant Architecture and Leaf Damage in Bear Oak. I: Physiological Responses

Abstract Bear Oak (Quercus ilicifolia Wang.) presents a unique architecture in which there is a distinct outer set of leaves on the perimeter of the bush and another set of leaves on the inside. In this paper we examine the relationship between plant architecture, leaf-level physiology, and leaf damage patterns by insects. We measured net photosynthesis, stomatal conductance, and leaf water potential in response to different levels of herbivory or leaf-rolling in both outer- and inner-canopy leaves. Leaf-rolling occurred naturally and was mimicked by hand-rolling leaves to different degrees. Net photosynthesis, stomatal conductance, and leaf water potential varied depending upon a combination of leaf position in the crown and degree of rolling, but not herbivory. Outer-canopy leaves had significantly higher rates of net photosynthesis and stomatal conductance, but lower leaf water potentials and leaf areas than inner-canopy leaves. Leaf rolling had an effect upon photosynthesis and stomatal conductance, which varied depending on whether projected areas of rolled leaves or projected areas of the leaves after they had been unrolled were considered. In both canopy layers, using unrolled projected leaf areas, stomatal conductance and net photosynthesis decreased with increasing leaf rolling. In the outer canopy, using projected areas of rolled leaves, photosynthesis decreased while stomatal conductance increased with increasing leaf rolling. In the inner canopy, photosynthesis increased while stomatal conductance was unchanged with increasing leaf rolling. In all cases, leaf water potential increased with increasing leaf rolling. These results suggest (in order of likelihood) that either rolling creates an area of closed stomata (stomatal patchiness) proportional to the amount of rolling, or leaf specific hydraulic conductance increases, or stomata are responding to changes in the boundary layer of the leaf.

WHITE PINE, JAPANESE LARCH, AND BEAR OAK RESPOND DIFFERENTLY TO PARTIAL DEFOLIATION

Abstract Seedlings (2-0 bare root) of Japanese larch (Larix leptolepis), white pine (Pinus strobus), and bear oak (Quercus ilicifolia) were partially defoliated by clipping half of each leaf per plant. Control plants were not clipped. Photosynthesis, water potential, leaf nitrogen concentration, leaf mass, and total mass were measured one week and nine weeks after treatment. Compensatory responses varied by species and timing. Defoliated Japanese larch had increased photosynthesis rates and foliar nitrogen concentration relative to control plants one week after treatment. Total biomass was also increased relative to controls by the end of the experiment. Defoliated pine seedlings had increased leaf mass and total biomass relative to controls by the end of the experiment. Defoliated oak seedlings showed no compensatory responses. Leaf lifespan did not appear to be a determinant of response amount or type. Instead, we suggest that degree of allocation of resources to aboveground growth, as indicated by root-to-shoot ratio, may be related to degree of compensation to partial defoliation.

How Do Trees Respond to Stress

Trees are a lot like people: they experience stress and they get infected with bugs or diseases and they can be attacked by fire, windstorms, floods, and droughts. We call these things that attack or infect trees disturbances. Trees are not like people, because they cannot go to the doctor to get better and they cannot move away from whatever is disturbing them. This last part is very important because it means that, in order for trees to have existed for hundreds of millions of years, they must have had the ability to cope with disturbances without a doctor. As you may imagine, different kinds of trees have evolved different ways to deal with certain disturbances. This is what we have learned from our research in an area called the Pine Barrens of New Jersey on the East Coast of the United States.

Response of Japanese Barberry to Varying Degrees of Defoliation

Abstract Until recently, it was thought that Berberis thunbergii (Japanese Barberry), a non-native invasive plant that has become particularly widespread in certain regions of New Jersey, benefited from a lack of herbivorous defoliators. However, in 2007 extensive defoliation was documented across a wide geographical distribution in New Jersey, calling this assumption into question. We tested whether Japanese Barberry was negatively affected by partial defoliation by manually clipping 50% or 100% of leaves on current-year stems on small and large plants in the summer of 2008. We found almost no impact of defoliation on growth, carbon storage, or leaf-level physiology for either treatment. We noted some differences between large and small plants, but these were not related to defoliation treatments. Our results suggest that, even in the presence of herbivory, Japanese Barberry is capable of maintaining growth and carbon reserves, thus making it an effective competitor for resources.