Microsite Characteristics Influencing Weed Seedling Recruitment and Implications for Recruitment Modeling W. John Bullied1, Rene C. Van Acker1 and Paul R. Bullock2 1Dept. of Plant Agriculture, University of Guelph, Guelph, ON, N1G 2W1; 2Dept. of Soil Science, University of Manitoba, Winnipeg, MB, R3T 2N2

Interpretive Summary Fig. 2. Soil temperature and soil water potential with depth. Microclimatic Modeling of the Microsite Research on the interactions of microclimate, field topography, soil properties and agro- 40 0 A field microclimate is the local atmospheric conditions (including solar radiation, tem- a ) nomic practices deepens our knowledge base of the seedling recruitment microsite to bet- ) -0.3 perature, precipitation, wind and relative humidity) which exist above the recruitment M P ° C 30 ( (

a l ter understand weed ecology and population dynamics generally, as well as enhance our i zone. Microclimatic modeling of the microsite predicts the microsite environment by u r e -0.6 e n t ability to predict weed seedling recruitment specifically. 20 coupling heat and water flow via the atmosphere-vegetation-soil continuum to produce p e r a t p o t

r -0.9 hydrothermal time within the recruitment zone (Fig. 5). e e m t

a t l Soil depth w o i 10 Surface l

S 0–25 mm 0–25 mm -1.2

o i The influence of microclimate on the microsite is modified by topography. Hillslope as- 25–50 mm 25–50 mm S 50–75 mm 50–75 mm pect and gradient alter the amount of solar radiation received at the soil surface. 0 -1.5 0 15 30 45 60 75 0 15 30 45 60 75 Seedling Recruitment Microsite Days after planting Days after planting Crop residue cover and vegetative growth influence the microclimate-microsite continu- um by reducing solar transmittance, increasing albedo, and reducing evaporative losses. A weed seedling recruitment microsite is the location of a seed in the soil profile which affects germination, time of emergence and seedling establishment. The relationship be- The advantage of microclimatic modeling is that readily available atmospheric data is tween the recruitment of seedlings and their environment (including microclimate, soil, used to predict the microsite environment. Microclimatic information can be used in topography and residue) is essential to understanding the timing of seedling recruitment. weed management decision-making programs to predict seedling emergence timing. Hydrothermal Time Modeling Soil disturbance disperses seeds to diverse microsites throughout the profile of the re- cruitment zone which subjects seeds to gradients of temperature and water with depth. Hydrothermal time modeling describes the progression toward germination by integrat- ing the interactions of temperature and water potential above minimum temperature and The variability that exists in germination and establishment requirements within and water thresholds for a weed species. Hydrothermal time utilizes the lower threshold water Fig. 3. Recruitment depth Fig. 5. Microclimate influence among weed species raises important questions for recruitment research addressing mul- potential of a species to stop thermal accumulation when soil water potential is less than effect on emergence timing. on recruitment microsites. tiple species, as well as regional models of genetic variability within species. the water potential threshold. It also allows thermal accumulation to proceed when soil

water potential is greater than the water threshold. Thus, in variable soil environments, Days after planting 9 14 16 20 22 28 41 45 hydrothermal time more accurately describes germination compared to thermal time. Thermal time 421 493 533 605 653 784 1062 1153

Longwave radiation 0 - Precipitation Evaporation Seedling Recruitment Drivers 20 - Shortwave Albedo 40 - Convection radiation 60 - Soil environmental properties (including temperature, water, compaction, soil texture, Soil cover Recruitment Depth Modeling 80 - light, and soil atmosphere composition) are seedling recruitment drivers which alter the Recruitment zone percentage and timing of seedling emergence. The interaction of these soil properties, The depth at which a seed is buried in the soil profile influences both the probability of Recruitment depth (mm) (microsites) as well as their interaction with depth of recruitment further influences the recruitment seedling emergence and the timing of emergence. Due to the differential response of process. Differences in soil properties such as water retention, relative to a hillslope posi- seeds according to their vertical location in the soil profile, modeling the depth of seed tion across field topography, results in numerous unique microsites (Fig. 1). placement is required to quantify the recruitment process (Fig. 3). The interaction of the soil environment with soil depth influences seedling recruitment. The depth of recruitment interacts with recruitment drivers. The cumulative emergence Vertical distribution of weed seeds in the soil profile subjects seeds to environmental gra- of seedlings in the upper soil profile may not be fully explained by thermal time alone Summary dients. Within a 75-mm profile of the seedling recruitment zone, temperature and water where greater thermal accumulation at the soil surface did not result in earlier emergence fluctuation were greatest at the soil surface and decreased with depth (Fig. 2). for spring wheat distributed as a model weed (Fig. 4). The interaction of soil temperature, The microsite environment within the seedling recruitment zone evolves and fluctuates due to above-ground microclimatic conditions. Heterogeneity of soil properties within Soil environmental conditions favorable for seed germination are usually short lived with water and depth in the hydrothermal model indicates increasing lag in emergence time the recruitment zone creates microsite diversity due to the influence of soil physical prop- germination often limited by temperature or water, particularly at or near the soil sur- with depth in the soil while accounting for periods of extreme dryness at the soil surface. erties on the movement and retention of heat and water. Temperature and water gradients face. exist with fine-scale changes in soil depth in the recruitment zone. Microclimatic information is used to model the environmental properties of the recruit- Fig. 4. Cumulative emergence of spring wheat by thermal and ment microsite through the atmosphere-soil continuum. Fig. 1. Soil water retention across the hillslope. hydrothermal time. Thresholds: T=0°C and ψ=-0.7 MPa. The advancement of seedling recruitment models for field application requires an empha-

0.40 100 100 sis on fine-scale characterization of the recruitment zone to obtain highly detailed tem- Thermal time Hydrothermal ) ) perature and water gradients that drive seedling recruitment. )

% % time 3

Summit Hillslope position (% clay) ( ( − 80 80 m 0.30 Summit (7.0) 3 Backslope (9.5)

t ( m Toeslope (22.2) 60 60 n e r g n c e r g n c

t e 0.20 n Backslope e m e m o e e c

v 40 v 40 i i r t e a t Soil depth a t Soil depth a 0.10 u l 0–25 mm u l 0–25 mm Source

i l w 20 20 Toeslope u m 25–50 mm u m 25–50 mm C C S o 50–75 mm 50–75 mm 0.00 0 0 Bullied, W.J., R.C. Van Acker, and P.R. Bullock. 2012. Review: Microsite characteristics −0.001 − 0 . 0 1 − 0 . 1 − 1 − 10 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Matric potential (MPa) Thermal time (°Cd) Hydrothermal time (MPa°Cd) influencing weed seedling recruitment and implications for recruitment modeling. Can. J. Plant Sci. 92: 627–650. doi:10.4141/CJPS2011-281