Autumn growth of three perennial weeds at high latitude benefits from climate change

In autumn, agricultural perennial weeds prepare for winter and can store reserves into creeping roots or rhizomes. Little is known about influence of climate change in this period. We tested the effect of simulated climate change in autumn on three widespread and noxious perennial weeds, Elymus repens (L.) Gould, Cirsium arvense (L.) Scop. and Sonchus arvensis L. We divided and combined simulated climate change components into elevated CO2 concentration (525 ppm), elevated temperatures (+2–2.5°C), treatments in open‐top chambers. In addition, a control in the open‐top chamber without any increase in CO2 and temperature, and a field control outside the chambers were included. Two geographically different origins and three pre‐growth periods prior to the exposure to climate change factors were included for each species. All species increased leaf area under elevated temperature, close to doubling in E. repens and quadrupling in the dicot species. E. repens kept leaves green later in autumn. C. arvense did not benefit in below‐ground growth from more leaf area or leaf dry mass. S. arvensis had low levels of leaf area throughout the experiment and withered earlier than the two other species. Below‐ground plant parts of S. arvensis were significantly increased by elevated temperature. Except for root:shoot ratio of C. arvense, the effects of pure elevated CO2 were not significant for any variables compared to the open‐top chamber control. There was an additive, but no synergistic, effect of enhanced temperature and CO2. The length of pre‐growth period was highly important for autumn plant growth, while origin had minor effect. We conclude that the small transfer of enhanced above‐ground growth into below‐ground growth under climate change in autumn does not favour creeping perennial plants per se, but more leaf area may offer more plant biomass to be tackled by chemical or physical weed control.


| INTRODUC TI ON
Globally, temperature and concentration of CO 2 are increasing.
We consider Norway as a country at high latitude, characterized by late spring, a relatively warm summer and a short autumn period.
At high latitudes, plant growth is usually restricted by the length of the vegetation period. With global warming, the length of the vegetation period is predicted to increase in northern Europe (Bindi & Olesen, 2011;Trnka et al., 2011).
Human activities shape and steer agro-ecosystems. Altered land use due to climate changes will further alter these systems (Trnka et al., 2011;Wolz et al., 2017). Extreme weather events and changes in climate variability may have large impacts on weeds and other pests (Thornton, Ericksen, Herrero, & Challinor, 2014). While the majority of weed species occurring under arable conditions are annuals, a few species are creeping perennials. In northern Europe, hence at high latitude, Elymus repens (L.) Gould, Cirsium arvense (L.) Scop. and Sonchus arvensis L. are perennial weeds in all cropping systems. These species use the C 3 pathway in photosynthesis, and they have creeping subterranean organs for storage and spreading, these being either roots (C. arvense, S. arvensis) or rhizomes (E. repens; Håkansson, 2003). From these organs, new plants sprout early or late in crops, for example, in cereals. Combine harvest in summer cuts all sprouts at certain heights, but does not erase the plants ( Figure 1, top part). After cutting, sprouts regrow from remaining subterranean plant parts. This re-growth of sprouts is more effective in plants that have had more time to grow and store reserves.
Sprouts translocate resources from above-ground parts to the below-ground parts in autumn; this process stops when sprouts start withering.
The optimum temperature for the species of this study varies from 15 to 30°C (Majek, Erickson, & Duke, 1984;Tiley, 2010;Zollinger & Kells, 1991). E. repens is more important in northern areas with cool to moderately warm summers, and may continue to grow late in the autumn (Boström et al., 2013;Håkansson, 2003). In contrast to E. repens, previous studies under current climate conditions have revealed that S. arvensis is the earliest to wither in autumn, while C. arvense withers more gradually (Tørresen, Fykse, & Rafoss, 2010). While older sprouts produced more biomass, younger sprouts continued to grow later in the season. This extended growth of the above-ground plant parts in young sprouts resulted in increased biomass of the subterranean creeping roots of C. arvense and S. arvensis, while the rhizome biomass of E. repens was less affected (Tørresen et al., 2010).
Studies under controlled conditions and under field conditions at relatively high temperatures show that the three species increase growth under elevated CO 2 concentration: E. repens by 12%-90% (Tremmel & Patterson, 1993;Ziska & Teasdale, 2000), S. arvensis by 50% (Ziska, 2003) and C. arvense by around 70% (Ziska, 2002(Ziska, , 2003. These studies started with seeds (Tremmel & Patterson, 1993;Ziska, 2003), with plants grown in fully controlled environments (Ziska & Teasdale, 2000) or focused on full summer growth of the perennials (Ziska, Faulkner, & Lydon, 2004 While it is common knowledge that both elevation of temperature and CO 2 alone improve plant growth, we further hypothesized (2) that there is a synergistic effect when the two components are combined.
A synergistic effect means that the effect is higher than can be expected when simply adding the effect of each of CO 2 and temperature.
Our third hypothesis focussed on a specific link between the subterranean biomass produced pre-harvest and the autumn growth.
We expected (3) that plants with short pre-harvest growth benefitted relatively more from the climate change components compared to plants with a long pre-harvest growth period. We further expected that this applies to both above-and below-ground biomass production in autumn.
Two geographically different origins of each species, with plant material (rhizomes or roots) collected from different parts of Norway, were included. We expected (hypothesis 4) no differences between the origins in their reaction to the investigated climate change components, since they did not differ strongly under current climate conditions (Tørresen et al., 2010).

| Species and site
The experiments with climatic treatments took place at the Saerheim The size of each chamber was 2.5 m × 3.4 m. The soil was a 60/40 (% by volume) mixture of fertilized fine peat and washed fine sand. For details on the growth medium, see Hanslin and Mortensen (2010). In addition to natural precipitation, water was given when needed, from the day of planting until the end of the experiments. Nutrient supply comparable to that found in autumn stubble fields was given.

| Experimental design
The main experimental factor was simulated climate change (factor CLI). The experiment was arranged as a split plot design with replicates in four blocks in 2004 and three blocks in 2005. Climatic treatments were used as the main plot, and species, origins of species and pre-growth periods as the subplots (Figure 1).
Each climatic treatment within the factor CLI represented different conditions ( Each experimental period started with a pre-growth period (factor PGP), raising the plants before the main period started (Figure 1).
Three pre-growth periods were installed: 31, 63 and 99 days-the short and medium pre-growth period in 2005 and the long pregrowth period in 2004.

PrecipitaƟon (mm) and RH (%)
The plant material was prepared at the beginning of each pre-growth period. In 2004, rhizome fragments of three nodes of E. repens and root fragments of C. arvense and S. arvensis, 5 cm in length and above 3 mm in diameter, were used. Two fragments were planted directly at 5 cm soil depth in 10 L black plastic sacks (in this paper referred to as pots), one origin of one species in each pot. One month later, the plants were thinned to one fragment per pot. In 2005, fragments of roots of C. arvense and S. arvensis, 4 cm long and 3-4 mm thick, and rhizomes of E. repens, two nodes in length, were planted in 5 cm pots at 1.5 cm depth. Each pot contained one fragment. Three to 4 weeks later, the plants were transplanted into 10 L pots filled with the same soil mixture as in 2004.
In 2004

| Observed variables
The above-ground plant parts were cut at the soil surface and separated into green leaves (laminae) and other above-ground plant parts

Elymus repens
Leaf Area (cm 2 per plant ) 981↑ laminae was determined using a Li-3100 Leaf Area Meter (Li-Cor) on the whole material or a representative fraction (>70 cm 2 for E. repens, >120 cm 2 for C. arvense and >160 cm 2 for S. arvensis). The variables Leaf Area (capital letters for variable names), dry mass of leaves (DM Leaves) and dry mass of the total above-ground plant (DM Above Ground) resulted from these measurements. The below-ground plant parts were separated from the growth medium by washing with tap water on a metal mesh of 1.5 cm mesh size. The creeping roots or

Sonchus arvensis
Transformation After each destructive assessment, the rest of the pots were again placed tightly within the central and border pots arrangement.

| Statistical analyses
The species were analysed separately. In both years, the initial as-

| RE SULTS
The variance analyses revealed that the plant growth (DM Plant, DM Leaves, DM Above Ground, DM Below Ground, Leaf Area) was highly influenced by the pre-growth period (PGP) and the experimental growth period (EGP) and much less by the factor climate change in the experiment (Table 2) (Table 4).  Ground) significantly benefitted from elevated temperature, but to a much lesser extent than the leaves (T+; Figure 5). All variables for below-ground parts of C. arvense and E. repens were statistically not different under elevated temperature (T+). These results show that the three species reacted differently to the single effects elevated temperature and CO 2 concentration.
Hence, our first hypothesis was confirmed.

| Interaction elevated temperature and CO 2 concentration
No interaction between temperature and CO 2 was significant for any variable of the three species and this interaction is therefore not shown in Table 2. No synergistic effect of elevated CO 2 (C+) and temperature (T+) occurred, but the combined treatment (CT+) gave just additive effects ( Figure 5). Thus, the second hypothesis was rejected.

| Effect of pre-growth period
The below-ground parts (DM Creeping R, DM Below Ground) and DM Plant of S. arvensis were influenced by an interaction of climate change and pre-growth period ( chamber control for the 31 days pre-growth period (

| Effect of origin
The origins of E. repens and S. arvensis from 63°N had higher Leaf Area and DM Leaves than the origins from 59°N (Table 2; Figure 6). For E. repens, DM Above Ground was higher and RS Ratio was lower for the 63°N origin compared to the 59°N origin. The DM Leaves of C.
arvense reacted in the opposite way (59°N > 63°N), while there was no difference between origins for Leaf Area (Table 2). For C. arvense, DM Creeping R, DM Below Ground, RS Ratio and DM Plant were also higher for the 59°N origin compared to the 63°N origin.
The interaction of pre-growth period and origin was highly significant for many variables of the species, indicating different reactions by origin to each pre-growth period ( Table 2). The leaf areas of E. repens and S. arvensis responded oppositely to pre-growth period and origin ( Figure 6). However, the dry mass of several plant parts' responses to pre-growth period showed a similar pattern for the two origins of each species even if the interaction was significant. The response of the DM Creeping R of the two origins is given as an example ( Figure 6).
Except for DM Leaves of E. repens, no interaction of climate change and origin and no three-factor interaction containing climate change and origin was detected (Table 2). Our fourth hypothesis was thereby confirmed.

| D ISCUSS I ON
Our results indicate that all three investigated species, the monocot E. repens and the dicots C. arvense and S. arvensis, profit from changed climate conditions in autumn, but the detailed reaction of each species was different.
With respect to the lower temperature at high latitudes, the effect of elevated temperature is not surprising. While the effects of temperature on leaves were very strong, this surprisingly did not result in the same strong effects on the rest of the plant. In general, perennials use their photosynthetic activity above ground to extend their below-ground storage system. One could suspect that the experimental growth period (EGP) in autumn was simply too short to effectively do the latter. For S. arvensis and C. arvense, the decrease in almost all variables from short (32 days) to long experimental growth period (61 days) clearly speaks for the opposite. These species lose leaf area and dry mass above and to a lesser extent below ground in the longer autumn period-climatic treatments did not stop or turn around this process. The reaction of E. repens was different: In the same period, Leaf Area indicating above-ground growth did not decrease significantly ( Figure 4). Elevated temperature (T+, CT+) increased Leaf Area significantly and kept it growing and green irrespective of experimental growth period. Hence, E. repens used higher temperature in autumn to keep green leaves above ground and we cannot rule out that the long experimental growth period with 61 days might have been too short for successful transfer from above-to below-ground biomass.
The effects of pure elevated CO 2 were not significant for any variables, except for an increase in RS Ratio of C. arvense, when contrasted to the open-top chamber control ( Figure 5). Hence, an increase in CO 2 alone would not allow any of the three species investigated to profit in their autumn growth. This is in contrast to other studies with larger increase in biomass of these species (12%-90%, largest range in E. repens, Ziska & Teasdale, 2000) and a higher increase in root:shoot ratio of both C. arvense and S. arvensis due to projected future elevated CO 2 concentrations (reviewed by Ziska et al., 2011).
Although the leaf variables increased in a range of doubling to quadrupling in the treatment with both enhanced temperature and CO 2 (CT+), the effect was just additive. No synergistic effect of temperature and CO 2 in comparison to the open-top chamber control occurred for any of the species.
Our findings that in all three investigated species, the origins (more southern or more northern origin) did not differ in their reaction to climate change factors mean that we can generalize our results about the influence of climate change on these species.
However, the various reactions of the measured variables to the interaction between origin and the length of the period before harvest (PGP) and the length of the autumn growth period (EGP) indicate complex reactions of creeping perennials to this interplay. A small or no 'chamber effect' is promising and shows that the control in opentop chambers is close to field conditions, and that the future effect of elevated temperature and CO 2 can be indicated based on these data.
To sum up, similar reactions of the species show that under climate change in autumn mainly leaf growth profited. Elevated temperature was much more important than elevated CO 2 .
The overall massive effect of the pre-growth period shall be accounted for, before characterizing each species. Plants were grown in this period without any modification of climate; thus, it is just the length of the period that differed. The period in early to high summer is important for arable perennial weeds, because they need to perform both shoot competition in dense crop stands as well as translocating nutrients into the vegetative survival organs. The longer the pre-growth period, the more below-ground dry mass was produced.
It is an experimental weakness that different pre-growth periods in different years do not allow separating the two effects 'year' and 'pre-growth period'. However, the influence of the three pre-growth periods regarding dry mass partitioning is consistent (Figure 3).
To what extent the pre-growth period (PGP) triggers the plant growth in the experimental growth period (EGP) under the factor climate change is strongly species specific. The shorter the pregrowth period, the more above-ground growth was increased by the CT+ treatment relative to below-ground growth (decreased RS Ratio) in autumn for E. repens (Table 2), while for S. arvensis especially more DM Creeping R (and DM Below Ground and DM Plant) occurred at the CT+ treatment (Tables 2 and 4). In the settings of the experiments, it appeared that the length of pre-growth period was more important for autumn plant growth than the length and the conditions of experimental growth period. We speculate that these effects may have been more pronounced if the pre-growth period had happened under climate change, too.
Elymus repens is the only monocot of the three species. Compared to dicots, monocot plants have many shoots. The absolute leaf area and leaf biomass at ambient conditions were high throughout autumn. In our trial without competition, the growth of green leaves continued until the end of the experiment. At locations with warmer winters, as in the United Kingdom, E. repens shoots (green leaves) may even survive the winter (Palmer & Sagar, 1963). In colder climates, most of the above-ground biomass dies during winter (Håkansson, 1967). All above-ground parts of E. repens benefitted more equally from enhanced climate change conditions than the other two species. This species can grow and produce rhizomes as long as the temperature is above 5-6°C (Håkansson, 1969). However, in our study, rhizome dry mass and the whole below-ground part did not increase under climate change. Our interpretation of the observed growth pattern is that E. repens utilizes the altered autumn growth conditions to produce only a moderate amount of above-ground biomass which, however, was kept green without withering longer than the two other species.
Cirsium arvense responded to climate change in the leaf variables (Leaf Area, DM Leaves) and RS Ratio only. The response in the leaf variables was huge. In other studies, with future estimated CO 2 levels, plants established from seeds increased their biomass by 69% (Ziska, 2002), while in studies under field conditions, plants established from root fragments responded even more strongly: 2.5-3.3 times more below-ground parts and 1.2-1.4 times more shoots with elevated CO 2 (Ziska et al., 2004). In our study, we did not find such an effect in neither DM Below Ground nor in DM Above Ground. For optimal root growth, Tiley (2010) described this species as requiring 15°C. Our experiments met these temperatures; thus, the temperatures would have allowed for more reaction in the below-ground parts. Thomsen, Brandsaeter, and Fykse (2013) found that C. arvense plants profited from an undisturbed root system but could stand disturbance as soon as the roots had reached a minimum depth. The root systems in the pots were not disturbed and could reach the full pot depth. Hence, we suspect that even under the ambient climate, the C. arvense plants in the experiments were enough prepared for the coming winter. Better conditions dramatically increased green leaves but were either not necessary or not usable for more below-ground growth.
Sonchus arvensis had the lowest levels of leaf area throughout the experiment; the species withered earlier than the other two (Tørresen et al., 2010). Benefits in above-ground leaves from climatic treatments (T+, CT+) were even greater than in C. arvense. Moreover, there was translocation into below-ground dry mass. Hence, S. arvensis seems to start preparations for winter earlier than the other two species. This is regulated by photoperiod and temperature, indicating that short photoperiod in combination with warmer autumns may suppress sprouting from root buds (Liew et al., 2012;Taab, Andersson, & Boström, 2018).
According to Munné-Bosch (2008), the onset of withering of leaves is influenced by photoperiod. We speculate that higher temperature may slightly delay withering of leaves in S. arvensis. The summer growth period (PGP) already influenced these processes with more leaves in autumn if the summer growth period has been short. More leaves mean that the plant can respond more to the climate change factors resulting in more translocation of assimilates into the below-ground parts as a result of climate change in autumn (CT+) and a short pre-growth period.
The below-ground parts for the medium and long pre-growth periods were already much larger at the start of the experimental period in autumn and could already be prepared enough for winter. We assume more active preparations in S. arvensis for the next year, which make the reaction to the experimental factors more complex in this species than in the other two. Although S. arvensis responded most to the simulated climate change, the strong periodicity of the withering processes did not allow for direct and simple reaction in autumn growth.
Our results indicate short-term implications for arable farming: the small transfer of enhanced above-ground growth into below-ground growth under climate change in autumn does not favour creeping perennial plants per se. Reduced control of E. repens and C. arvense by glyphosate under elevated CO 2 is observed in other studies (Ziska et al., 2004Ziska & Teasdale, 2000). For C. arvense, the reason for this could be that more roots were developed with elevated CO 2 causing a dilution of glyphosate. In our study, the root biomass was almost unaffected by elevated CO 2 -this can result in less effect on herbicide efficacy than observed by Ziska and co-workers. However, herbicide efficacy depends on various conditions, and different herbicides may cause different reactions (Patterson et al., 1999;Waryszak, Lenz, Leishman, & Downey, 2018;Ziska, 2016). Physical and chemical treatments will not necessarily become more difficult as climate change can give a longer time period in autumn suitable for both types of weed control (top part of Figure 1) and elevated temperatures during autumn may in general increase efficacy of herbicides. In autumn, more above-ground leaf biomass of perennials under climate change means bigger and hence more competitive perennial weeds.
A following cover crop or main crop such as winter wheat in autumn can change above-ground growth via competition. However, it is very likely that the cover crop or main crop benefit in the same way as the perennial weed species under climate change (cf. winter wheat; Hanslin & Mortensen, 2010). So far it is unknown how the plants will react to various winter kill factors, and this may influence the overwintering of the species and hence the spread/competitive ability in the next year. Warmer winters may increase winter survival and distribution of perennial weeds (McDonald et al., 2009;Østrem, Folkestad, Solhaug, & Brandsaeter, 2017).
Long-term implications for arable land use under climate change will be even more complex. All three species reacted positively to temperature for leaf area and leaf dry mass-measured on plant level. Longterm implications must consider the population level. In general, weeds can react to climate change through different processes and at different scales (Peters, Breitsameter, & Gerowitt, 2014). Range and niche shifts cannot occur in a pot experiment, as used in our study. Applying the concept of trait shifts to the perennials in our experiment is also crucial, because perennials stay the same plants before and after the simulated harvest. Perennials can become several years old without successful sexual reproduction and no obvious possibility to genetically adapt to changing conditions. Hence, our experiments observed the scope of immediate reactions of plants, which indicate their future opportunities or necessities to perform trait shifts. Even without considering genetic adaptations, all three species will not suffer but profit under climate change, giving them a good position in the long-term race for resources on arable fields. At high latitude, we expect E. repens to profit most via longer growth in autumn. C. arvense is successful in most arable systems-under ambient current and elevated conditions. S. arvensis is a candidate to profit from climate change, but for fully understanding the complicated internal regulation of dormancy, sprouting and withering in this species further research are required.

ACK N OWLED G EM ENTS
We

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.