This review surveys the large variety of mechanical interactions between wind and plants, from plant organs to plant systems These interactions range from leaf flutter to uprooting and seed dispersal, as well as indirect effects on photosynthesis or insect communication I first estimate the relevant nondimensional parameters and then discuss turbulence, plant dynamics, and the mechanisms of interaction in this context Some common features are identified and analyzed in relation to the wind engineering of manmade structures Strong coupling between plants and wind exists, in which the plant motion modifies the wind dynamics I also present some related biological issues in which the relation between plant life and wind environment is emphasized

Effects of Wind on Plants

Atmospheric carbon dioxide (CO2) and global mean temperature are expected to be significantly higher by the end of the 21st century. Elevated CO2 (eCO2) and higher temperature each affect plant physiology and growth, but their interactive effects have not been reviewed statistically with respect to higher chronic mean temperatures and abrupt heat stress. In this meta-analysis, we examined the effect of CO2 on the physiology and growth of plants subjected to different temperature treatments. The CO2 treatments were categorized into ambient ( 560 ppm) levels, while temperature treatments were categorized into ambient temperature (AT), elevated temperature (ET; AT + 1.4–6°C), or heat stress (HS; AT + >8°C). Plant species were grouped according to photosynthetic pathways (C3, C4), functional types (legumes, non-legumes), growth forms (herbaceous, woody), and economic purposes (crop, non-crop). eCO2 enhanced net photosynthesis at AT, ET, and HS in C3 species (especially at the HS level), but in C4 species, it had no effect at AT, a positive effect at ET, and a negative effect at HS. The positive effect of eCO2 on net photosynthesis was greater for legumes than for non-legumes at HS, for non-crops than crops at ET, and for woody than herbaceous species at ET and HS. Total (W
 T) and above- (W
 AG) and below-ground (W
 BG) biomass were increased by eCO2 for most species groups at all temperatures, except for C4 species and W
 BG of legumes at HS. Hence, eCO2 × heat effects on growth were often not explained by effects on net photosynthesis. Overall, the results show that eCO2 effects on plant physiology and growth vary under different temperature regimes, among functional groups and photosynthetic pathways, and among response variables. These findings have important implications for biomass accumulation and ecosystem functioning in the future when the CO2 level is higher and climate extremes, such as heat waves, become more frequent.

A meta-analysis of plant physiological and growth responses to temperature and elevated CO2

Tree stability in windstorms and tree failure are important issues in urban areas where there can be risks of damage to people and property and in forests where wind damage causes economic loss. Current methods of managing trees, including pruning and assessment of mechanical strength, are mainly based on visual assessment or the experience of people such as trained arborists. Only limited data are available to assess tree strength and stability in winds, and most recent methods have used a static approach to estimate loads. Recent research on the measurement of dynamic wind loads and the effect on tree stability is giving a better understanding of how different trees cope with winds. Dynamic loads have been measured on trees with different canopy shapes and branch structures including a palm (Washingtonia robusta), a slender Italian cypress (Cupressus sempervirens) and trees with many branches and broad canopies including hoop pine (Araucaria cunninghamii) and two species of eucalypt (Eucalyptus grandis, E. teretecornus). Results indicate that sway is not a harmonic, but is very complex due to the dynamic interaction of branches. A new dynamic model of a tree is described, incorporating the dynamic structural properties of the trunk and branches. The branch mass contributes a dynamic damping, termed mass damping, which acts to reduce dangerous harmonic sway motion of the trunk and so minimizes loads and increases the mechanical stability of the tree. The results from 12 months of monitoring sway motion and wind loading forces are presented and discussed.

Mechanical stability of trees under dynamic loads

Summary This paper reviews the current status of mechanistic models for wind damage risk assessment, describing model structure, applicability, validation and current limitations. We focus particularly on the hybrid mechanistic/empirical models GALES and HWIND, which have been designed for calculating wind damage risk at the stand level within uniform forests and which are the most widely adopted models within the research community. These models have been integrated with different methods for predicting the local wind climate in order to calculate the probability of wind damage in a number of different countries. We also discuss ongoing modelling work and proposals for future development in order to deal with complex forest structures and to predict the wind damage risk of individual trees within stands through the integration of mechanistic risk models with forest growth and yield models within a geographical information system framework. This kind of model integration will enable spatial representation of tree lists and damage propagation and allow managers to evaluate the effect of different harvesting and thinning scenarios on the risk of windthrow of both stands and individual trees within a stand.

A review of mechanistic modelling of wind damage risk to forests

In 1981 a storm caused windthrow of 3 million cubic meters of softwood in Denmark, equal to the normal removals of three years. The object of the present analysis is to determine the way in which the probability of windthrow depends on parameters that can be affected by forest management, viz. rotation age, thinning programme, choice of species, spatial distribution of stands and drainage. An empirical data set from 612 stands is used in the estimations. It is found that the windthrow probability is negatively affected by tree diameter, drainage, the time since last thinning and the protection from other stands. The probability increases with tree height, age and relative thinning volume in the latest thinning. Picea is more stable than Abies and Pseudotsuga.

Windthrow probability as a function of stand characteristics and shelter

A closed CO2 and temperature-controlled, long-term chamber system has been developed and set up in a typical boreal forest of Scots pine (Pinus sylvestris L.) near the Mekrijarvi Research Station (62°47′N, 30°58′E, 145 m above sea level) belonging to the University of Joensuu, Finland. The main objectives of the experiment were to provide a means of assessing the medium to long-term effects of elevated atmospheric CO2 concentration (EC) and temperature (ET) on photosynthesis, respiration, growth, and biomass at the whole-tree level and to measure instantaneous whole-system CO2 exchange. The system consists of 16 chambers with individual facilities for controlling CO2 concentration, temperature, and the combination of the two. The chambers can provide a wide variety of climatic conditions that are similar to natural regimes. In this experiment the target CO2 concentration in the EC chambers was set at a fixed constant of 700 µmol mol−1 and the target air temperature in the ET chambers to track the ambient temperature but with a specified addition. Chamber performance was assessed on the base of recordings covering three consecutive years. The CO2 and temperature control in these closed chambers was in general accurate and reliable. CO2 concentration in the EC chambers was within 600–725 µmol mol−1 for 90 % of the exposure time during the "growing-season" (15 April – 15 September) and 625–725 µmol mol−1 for 88 % of the time in the "off-season" (16 September – 14 April), while temperatures in the chambers were within ±2.0 °C of the ambient or target temperature in the "growing season" and within ±3.0 °C in the "off season". There were still some significant chamber effects. Solar radiation in the chambers was reduced by 50–60 % for 82 % of the time in the "growing season" and 55–65 % for 78 % of the time in the "off season", and the relative humidity of the air was increased by 5–10 % for 72 % of the time in the "growing season" and 2–12 % for 91 % of the time in the "off season". The crown architecture and main phenophase of the trees were not modified significantly by enclosure in the chambers, but some physiological parameters changed significantly, e.g., the radiant energy-saturated photosynthesis rate, transpiration rate, maximum photochemical efficiency of photosystem 2, and chlorophyll content.

Controlled Environment Chambers for Investigating Tree Response to Elevated CO2 and Temperature Under Boreal Conditions

A prism-based system for monitoring the swaying of trees under wind loading

Swaying of trees as caused by wind: analysis of field measurements.

This paper describes the technical information and performance of a new multi-objective chamber system enabling the control of environmental variables (e.g., temperature, CO2, air humidity, wind speed, and UV-B radiation) for understanding plant responses to climate change. Over a whole growing season, four different climate scenarios were evenly programmed into the system’s 16 chambers as ambient environment (AMB), elevated temperature (ET), elevated CO2 concentration (EC) and elevated temperature and CO2 concentration (ETC). Simultaneously, the chamber effects were assessed regarding the physiological responses and growth of a boreal perennial grass (reed canary grass, Phalaris arundinacea L.). During the growing season, the chamber system provided a wide variety of climatic conditions for air temperature (T
 a), relative humidity (RH) and CO2 concentration (C
 a) in the AMB chambers following outside conditions. The target temperature (+3.5°C) was achieved to a good degree in the ET and ETC chambers, being on average 3.3°C and 3.7°C higher than ambient conditions, respectively. The target concentration of CO2 (700 ppm) was also well achieved in the EC and ETC chambers, being on average 704 ppm and 703 ppm, respectively. The stable airflow condition inside all of the chambers provided a homogeneous distribution of gases and temperature. The decreases in RH and increases in vapour pressure deficit (VPD) in the elevated temperature chambers were also maintained at a low level. Chamber effects were observed, with some physiological and growth parameters of plants being significantly lower in the AMB chambers, compared to outside conditions. The plant growth was negatively affected by the reduced radiation inside the chambers.

Multi-objective environment chamber system for studying plant responses to climate change

An open-top chamber fumigation system was built in a young Scots pine stand to study the effects of realistic elevated ozone (O3) and carbon dioxide (CO2) concentrations and their combination on trees in natural conditions. Doubled CO2 concentration compared to present ambient concentration, and O3 concentration between 40 ppb and 70 ppb in the first study year (1994) and doubled O3 concentration in years 1995 and 1996 were the target concentrations in the chambers. The O3 concentration in the chambers was successfully maintained close to the target concentration and differences between chambers were small. The mean CO2 concentration in the CO2 treatment was ca. 100 ppm below the target, but was maintained at this level throughout the growing season. Two degrees higher mean air temperature and slightly lower light intensity compared to open air were measured in the chambers. The operation of the fumigation system was satisfactory during the three study years and repeatability of the gas treatments can be regarded good in this low cost exposure system.

Matti Lemettinen

Papers

Controlled Environment Chambers for Investigating Tree Response to Elevated CO2 and Temperature Under Boreal Conditions

A prism-based system for monitoring the swaying of trees under wind loading

Swaying of trees as caused by wind: analysis of field measurements.

Multi-objective environment chamber system for studying plant responses to climate change

Open-top chamber fumigation system for exposure of field grown Pinus sylvestris to elevated carbon dioxide and ozone concentration